Cytokines in Allergic Inflammation

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CONTENTS

Introduction: Definition, Evolution of CytokineBiology, and Nomenclature 65

Cytokine Production in Innate Immunity 65

Cytotoxic Immunity 69

Humoral Immunity 69

Cellular Immunity 70

Allergic Immunity 71

Antiinflammatory Cytokines 73

T Helper Lymphocyte Families 75

Cytokines Involved in Th1 Differentiation 75

Cytokines Involved in Th2 Differentiation 77

Cytokines and Immune Responses to Allergens 80

Summary 80

5Cytokines in Allergic InflammationJOHN W. STEINKE | LANNY J. ROSENWASSER | LARRY BORISH

Introduction: Definition, Evolution ofCytokine Biology, and Nomenclature

Cytokines are secreted proteins with growth, differentiation,and activation functions that regulate and direct the nature ofimmune responses. They are involved in virtually every facet ofimmunity and inflammation, including innate immunity,antigen presentation, B and T lymphocytemediated adaptiveimmunity, hematopoietic stem cell differentiation, cellularrecruitment and activation, and adhesion molecule expression.Which cytokines are produced in response to an immune insultdetermines whether an immune response develops and subse-quently whether that response is cytotoxic, humoral, cell-mediated, allergic, or tolerogenic. For this chapter, cytokines aregrouped as follows: those that are associated primarily withinnate or adaptive immune responses, those that predominantly

mediate cytotoxic (antiviral and anticancer), humoral, cell-mediated, or allergic immunity, and those that are immunosup-pressive. A discussion of the complementary family of secretedimmune proteins, the chemokines, is presented in Chapter 7.

Cytokine Production in InnateImmunity

Cytokines derived primarily from dendritic cells (DCs), mono-nuclear phagocytic cells, and other antigen-presenting cells(APCs), including epithelial cells, are effective in promotingformation of the cellular infiltrate and corresponding damage

to resident tissue characteristic of inflammation (Fig. 5-1)processing of antigens after they are taken up by APCs, melized, and presented to T helper (Th) lymphocytes providepathway for this class of cytokine production. Alternativelinnate immune system can be triggered to produce cytovia pattern recognition receptors (see Chapter 1) that recostereotypic components specific to pathogens. These recepsuch as the Toll-like receptors (TLRs), contribute to the aof the immune system to distinguish pathogenic from pathogenic proteins. The cytokines predominantly producAPCs include tumor necrosis factor (TNF), IL-1 and

members of the IL-1 family, IL-6, CXCL8 (IL-8) and members of the chemokine family, IL-12, IL-15, IL-18, Iand IL-27.

TUMOR NECROSIS FACTOR

The TNF superfamily (Table 5-1) represents more thahomologous proteins involved in inflammation and adaimmunity. The active form of members of TNF superfacytokines, as well as their various receptors, usually is homotrimer. Receptors are largely membrane-bound signmolecules with occasional soluble decoy receptors, wherealigands can be soluble (cytokines) or membrane-boundtwo members primarily associated with the functions origi

ascribed to TNF are two proteins derived mainly from mnuclear phagocytes, in the case of TNF-, and lymphocytthe case of lymphotoxin-(LT-; formerly TNF-).1The potent inducer of monocyte-derived TNF is lipopolysacch(LPS), acting through TLR4, Myd88, and CD14.

TNF- is processed as a membrane-bound protein which the soluble active factor is derived through cleavage uTNF-converting enzyme (TACE).2 LT- can be syntheand processed as a typical secretory protein but usually is lito the cell surface by forming heterotrimers with a tmembrane-associated member of this family, LT-. TNF-LT-bind to the same cell surface receptors, TNFR1 (p75

SUMMARY OF IMPORTANT CONCEPTS

Cytokines are families of secreted proteins that mediate immand inflammatory reactions at local or distant sites.

Each cytokine within a cytokine family has a unique set of retors on cells that respond to the cytokine. Genetic factors consome aspects of cytokine biology.

Tissue expression of cytokines provides targets for treatmendisease and disease modification.

A new set of biotherapeutic agents is being developed umonoclonal antibodies and antagonists directed against ckines and cytokine receptors.


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66 SECTION A Basic Sciences Underlying Allergy and Immunology

type II receptors, which are expressed on B cells, neutrophiland bone marrow cells and have a minimal intracelluladomain. The capture and sequestration of IL-1 by these inactivdecoy receptors serve an antiinflammatory function. The capaity of IL-1Ra to bind to the type I (proinflammatory) IL-1without transducing biologic activities, which is due to failurto recruit IL-1RacP, is the basis for its capacity to function a

a cytokine antagonist.8

IL-1 is produced primarily by APCbut also by numerous other cell types. IL-1 production stimulated by a variety of agents, including LPS, other cytokines, microorganisms, and antigens. IL-1, IL-1, IL-18, anIL-36 are synthesized as minimally active precursors withoutsecretory leader sequence. The mechanism for their secretiodepends on their cleavage by a specific converting enzymtermed interleukin-1 converting enzyme (ICE), or caspase-which cleaves the procytokines into their active secreteforms.9The activation of IL-1 and IL-18 takes place within specialized cellular multimeric protein complex termed thinflammasome.Identification of the natural antagonist IL-1R(anakinra) suggested that it could be used as a treatment foa variety of inflammatory conditions, including rheumatoi

arthritis and severe asthma, but especially in diseases characterized by constitutive overactivation of the inflammasomsuch as gout, Muckle-Wells syndrome, familial Mediterraneafever, familial cold urticaria, and other inherited periodic fevesyndromes.

With production of IL-1 during the early immune responsa spectrum of changes associated with clinical illness has beedescribed: IL-1 interacts with the central nervous system tproduce fever, lethargy, sleep, and anorexia. An IL-1hepatocyinteraction inhibits production of so-called housekeepinproteins (e.g., albumin) and stimulates the synthesis of acutphase response peptides (e.g., amyloid peptide, C-reactiv

TNFR2 (p55), with comparable affinities that produce distincteffects.3TNFs induce antitumor immunity through direct cyto-toxic effects on cancerous cells and by stimulating antitumorimmune responses. TNF interacts with endothelial cells toinduce intercellular adhesion molecule-1 (ICAM-1), vascularcell adhesion molecule-1 (VCAM-1), and E-selectin, permittingthe egress of granulocytes into inflammatory loci. TNF is a

potent activator of neutrophils, mediating adherence, chemo-taxis, degranulation, and the respiratory burst. TNF is respon-sible for the severe cachexia that occurs in chronic infectionsand cancer.1 Furthermore, TNF induces vascular leakage, hasnegative inotropic effects, and is the primary endogenousmediator of toxic shock and sepsis. Other TNF superfamily(TNFSF) members have been recognized, as described else-where.4,5Of particular interest are cytokines involved in B cellmaturation and differentiation including TNFSF13 (APRIL)and TNFSF14 (BAFF/BLyS), because mutations in one of theirshared receptors, TNFSF13B (TACI), are recognized as the mostcommon genetically identifiable cause of common variableimmune deficiency. Monoclonal antibodies to TNFSF14 (BLyS)were recently approved for the treatment of systemic lupus

erythematosus (SLE).

INTERLEUKIN-1

The IL-1 family comprises 11 members: IL-1, IL-1, the IL-1receptor antagonist (IL-1Ra), IL-18, IL-33, IL-36, IL-36,IL-36, IL-36Ra, IL-37, and IL-38 (Table 5-2).6IL-1and IL-1have similar biologic activities, and both of these proteins,along with IL-1Ra, interact with affinities similar to those ofthe IL-1 receptors IL-1RI and IL-1RII. IL-1RI in combinationwith IL-1RacP forms a heterodimer to transduce the biologiceffects attributed to IL-1and IL-1.7This is in contrast with

Figure 5-1 Actions of mononuclear phagocytic (monocyte/macrophage) cell-derived cytokines. These cytokines are uniquely potent in generaing the symptoms and initiating the immune responses associated with infection and inflammatory disorders. CNS,Central nervous systemICAM,intracellular adhesion molecule; IFN,interferon; IL,interleukin; NK,natural killer; TNF,tumor necrosis factor.

CNS

Eosinophil activation

Immune cells

NK cell activation

Lymphocyte (T&B)activation

PMN activationLiver

Monocyte/macrophage

Muscles

Muscle catabolism

Fibroblasts, epithelium

Acute phaseresponse

Fever,lethargy,anorexia

Blood vessels

Joints

Leukocyte adhesion;

ICAM and E-selectinexpression; vascular leakage

Osteoclast andchondroclast activation;

collagen deposition;synoviocyte proliferation

ICAM expression;cytokine production;

arachidonate metabolism

IL-1,IFN-TNF-

IL-1,IFN-TNF-

IL-1,IFN-

IL-1,IL-6

,TNF-

IL-1,IL-6IL-

1,IL-6

IL-1,IL-6IL-12,IL-15,IL-18,IL-23

TNF-,IF

N-

IL-12,IL-15,IL-18

IFN-,TNF-

IL-1,IFN

-

IL-1,TNF-

IL-18


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5 Cytokines in Allergic Inflammation

Tumor Necrosis Factor Superfamily (TNFSF) and Receptors (TNFRs)

TNFSF Ligand Common Name(s) Receptor(s) Receptor Common Name(s) Activities

TNFSF1 LT-(TNF-)

TNFRSF1ATNFRSF1BTNFRSF14

TNFR1TNFR2

See text

TNFSF2 TNF- TNFSF1ATNFSF1B

TNFR1TNFR2

See text

TNFSF3 LT- TNFRSF3 LTR See textTNFSF4 OX40L/CD252 TNFRSF4 OX40/CD134 T cell activation; Th2 immune deviation

TNFSF5 CD40L/CD154 TNFRSF5 CD40 T cell activation, B cell proliferation andifferentiation (isotype switch)

TNFSF6 FasL (CD95L) TNFRSF6TNFRSF6B

Fas (CD95)DcR3

Apoptosis

TNFSF7 CD27L/CD70 TNFRSF7 CD27 T and B cell activation and differentiatalso activation of mast cells, NK cells

TNFSF8 CD30L/CD153 TNFRSF8 CD30/Ki-1 T cell activation; also B cell and NKactivation

TNFSF9 4-1BBL TNFRSF9 4-1BB/CD137 T cell activation, apoptosis

TNFSF10 TRAIL/Apo2-L TNFRSF10ATNFRSF10BTNFRSF10C

TNFRSF10DTNFRSF11B

TRAILR1/DR4TRAILR2/DR5TRAILR3,DcR1

TRAILR4, DcR2OPG

Apoptosis

TNFSF11 RANKL/TRANCE/OPGL TNFRSF11ATNFRSF11

RANK/TRANCE-ROPG

Inhibition of osteoclasts

TNFSF12 TWEAK TNFRSF12A TWEAKR Apoptosis

TNSFS13 APRILTALL-2 TNFRSF13BTNFRSF17

TACIBCMA

B cell activation and differentiation

TNFSF14 BAFF/BLyS/TALL-1 TNFRSF13BTNFRSF13CTNFRSF17

TACIBAFFRBCMA

B cell activation and differentiation

TNFSF15 VEGI TNFRSF6BTNFRSF12

DcR3TRAMP/DR3/APO-3

Apoptosis

TNFSF18 GITRL/AITRL TNFRSF18 GITR/AITR T cell activation; mediator of Treg-induimmune suppression; M2 macrophagdifferentiation

None EDA1EDA2

None EDA4XEDAR

Embryogenesis

LIGHT TNFRSF14 HVEM/HveA/ATARLTR

T cell and DC activation; apoptosis;herpes simplex virus cell entry

None NGF TNFRSF16 LNGFR/p75NTR

None TNFRSF19TNFRSF19L

TROY/TAJRELT

None TNFRSF21 DR6 ApoptosisTNFRSF25 TRAMP/DR3/LARD Apoptosis

AITL/R,Activation-inducible TNFR family ligand/receptor; APRIL, A proliferation-inducing ligand; ATAR,another TRAF (TNF receptorassociafactor)associated receptor; BAFF,B cellactivator factor; BCMA,B cell maturation; BLyS,B-lymphocyte stimulator; DcR,decoy receptor;DC,dendritic cell; DR,death receptor; EDA,ectodysplasin; GITR,glucocorticoid-induced TNFR familyrelated; HVEM(also HveA), herpesventry mediator; LARD,lymphocyte-associated receptor of death; LIGHT,homologous to lymphotoxins; LNGFR,low-affinity nerve growth freceptor; LT,lymphotoxin; OPG,osteoprotegerin; NGF,nerve growth factor; NTR,neurotrophin receptor; RANKL,receptor activator of Nligand; RELT,receptor expressed in lymphoid tissue; TACI,transmembrane activator and calcium modulator of cyclophilin ligand; TALL,

TNF- and ApoL-related leukocyte-expressed ligand; TRAIL,TNF-related apoptosis-inducing ligand; TRAMP,TNF receptor apoptosis-medprotein; TRANCE,TNF-related activation-induced cytokine; Treg,regulatory T lymphocyte; TWEAK,TNF-like weak inducer of apoptosis; Vvascular endothelial cellderived growth inhibitor; XEDAR,X-linked ectodysplasin receptor

TABLE5-1

peptide, complement). IL-1 also stimulates endothelial celladherence of leukocytes through the upregulation of ICAM-1,VCAM-1, and E-selectin. IL-1 contributes to the hypotensionobserved in septic shock. TNF-and IL-1 share numerous bio-logic activities, the major distinction being that TNF-has nodirect effect on lymphocyte proliferation. IL-1Ra is secretednaturally in inflammatory processes, and its production is

upregulated by many cytokines, including IL-4, IL-6, IL-13transforming growth factor beta (TGF-). Production of ILmodulates the potentially deleterious effects of IL-1 innatural course of inflammation by means of a feedback that blocks IL-1 activity. IL-1 also plays important roladaptive immunity including augmenting B cell proliferand increasing immunoglobulin synthesis. The role of


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it functions as an intracellular antiinflammatory cytokine. Aftcleavage, mature IL-37 translocates to the nucleus and assocates with Smad3, where it inhibits transcription of proinflammatory genes and thereby shares many immune suppressivproperties with TGF-. Other members of the IL-1 superfamiare summarized in Table 5-2.

INTERLEUKIN-6

IL-6 is a multifunctional cytokine capable of regulating thimmune response, acute-phase response, hematopoiesis aninflammation. Mononuclear phagocytic cells are the moimportant source of IL-612; however, IL-6 is also produced bnumerous other cells including T and B lymphocytes, granulo

cytes, eosinophils, fibroblasts, endothelial cells, keratinocytemast cells, and bone marrow cells. The IL-6 receptor consists oIL-6receptor and gp130 and can exist as either a membranebound or a soluble form. Upon binding, the active IL-complex is composed of two molecules each of IL-6, IL-6(CD126) receptor, and gp130. Under the influence of IL-6, lymphocytes differentiate into mature plasma cells and secreimmunoglobulins. As discussed later on, IL-6 is the majodeterminant of the differentiation of nave T cells into Th1lymphocytes. In addition to lymphocyte activation, IL-6 shareseveral activities with IL-1, including the induction of pyrexiand it is an even more important inducer of hepatocyte synthesis of acute phase proteins. In contrast with these proinflammatory effects, IL-6 mediates several antiinflammatory effect

Whereas both IL-1 and TNF induce synthesis of each other, awell as IL-6, IL-6 terminates this upregulatory inflammatorcascade and inhibits IL-1 and TNF synthesis while stimulatinsynthesis of IL-1Ra.

INTERLEUKINS IL-12, IL-23, AND IL-35

IL-12 is derived most importantly from DCs, but also fromLangerhans cells, mononuclear phagocytic cells, B cells, PMNand mast cells.13The biologically active form is a heterodimeThe larger subunit (p40) is homologous to the soluble receptofor IL-6, whereas the smaller subunit (p35) is homologous t

superfamily members in driving T cell differentiation andestablishing immune deviation is discussed later on.

INTERLEUKIN-18

IL-18 is produced by numerous inflamed tissues, but not lym-phocytes or natural killer (NK) cells.10Like IL-1, IL-18 requiresa specific converting enzyme (i.e., ICE, or caspase-1) to permitsecretion and activation. In contrast with most cytokines, IL-18is constitutively expressed, and release of its active form is regu-lated through activation of this converting enzyme. However,its biologic activity is more similar to that of IL-12 than tothat of IL-1. IL-18 has an important role in cellular adhesion,driving ICAM-1 expression. IL-18 binds to a unique heterodi-

meric receptor that is composed of members of the IL-1Rfamily (see Table 5-2). IL-18R expression is upregulated byIL-12, so that these two cytokines synergize to stimulate inter-feron gamma (IFN-) release. IL-18 is thus involved in not onlythe innate immune response but also in promoting the adaptiveresponse by driving Th1 immune deviation (as discussed lateron). Soluble IL-18 receptors also are secreted and act as inhibi-tory decoy receptors.11

INTERLEUKINS IL-36 AND IL-37

IL-36 (see Table 5-2) comprises three separate proteins, IL-36,IL-36, and IL-36, that all bind the same receptor complexcomposed of IL-1RAcP and IL-1Rp2.6Binding of IL-36 leads to

activation of inflammation similar to that produced by IL-1and IL-1. IL-36is expressed in the skin, and overexpressionproduces an inflammatory phenotype similar to that seen inpsoriasis. Like IL-1Ra, IL-36Ra acts as a naturally occurringantagonist for IL-36, IL-36, and IL-36by binding to IL-1Rp2,but not recruiting IL-1RAcP and thus preventing signalingthrough this receptor. IL-37 (formerly IL-1F7) has five differentsplice variants that are produced in a precursor form, all ofwhich are cleaved by caspase-1 (i.e., ICE) to produce matureIL-37. Transcripts for IL-37 are found in numerous humantissues including lymph nodes, thymus, bone marrow, placenta,lung, testis, and uterus. IL-37 is unique in the IL-1 family in that

Interleukin-1 (IL-1) Superfamily and Receptors

Family Name Common Name Receptor Coreceptor Activities

IL-1F1 IL-1 IL-1RI IL-1RacP Stimulates synthesis of acute phase proteins, cytokine production, cellularadhesion, T and B cell proliferation, and Ig synthesis

IL-1F2 IL-1 IL-1RI IL-1RacP Same as for IL-1

IL-1F3 IL-1Ra IL-1RI NA Inhibitory for IL-1and IL-1activities

IL-1F4 IL-18 IL-18R IL-18R Increased proinflammatory cytokine production including IFN-, Th1 immunedeviation, and increased cellular adhesion through increased ICAM-1

IL-1F5 IL-36Ra IL-1Rrp2 NA Inhibitory for IL-36, IL-36, and IL-36activities

IL-1F6 IL-36 IL-1Rrp2 IL-1RacP Induction of IL-1, IL-1, and other proinflammatory cytokines

IL-1F7 IL-37 IL-18R* Unknown Antiinflammatory

IL-1F8 IL-36 IL-1Rrp2 IL-1RacP As for IL-36

IL-1F9 IL-36 IL-1Rrp2 IL-1RacP As for IL-36

IL-1F10 IL-38 Unknown Unknown Unknown

IL-1F11 IL-33 ST2 IL-1RacP Increased Th2 cytokine production including IL-5 and IL-13; Th2 immunedeviation; increased IgE levels; eosinophilic inflammation

ICAM,Intercellular adhesion molecule; IFN,interferon; Ig,immunoglobulin; NA,not applicable.*Not confirmed.

TABLE5-2


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remodeling and synergizing with other growth factoproduce erythrocytes and, especially, platelets. ProductioIL-11 has been documented in a variety of stromal cells ining fibroblasts, epithelial cells, endothelial cells, and osteobExpression of IL-11 is induced by IL-1, TGF-, and TNas well as IL-13 in Th2-dominated diseases, whereas it is inited by IL-6 and IL-4. IL-11 binds to a receptor complex sisting of gp130 and IL-11R. The receptor complex dime

to amplify the signaling cascade leading to increases inproduction of acute phase proteins and cytokines. In particIL-11 enhances T cell production of IL-4 and IL-10 while initing IFN- and IL-2 production. A role for IL-11 in astremodeling is suggested by studies demonstrating increexpression of IL-11 from eosinophils in severe asthma16ancapacity of this cytokine to stimulate fibroblast proliferand collagen deposition in mouse models of asthma.

INTERLEUKIN-32

IL-32 was discovered in a search for IL-18inducible genesnot homologous to any other known cytokine and exists iisoforms.17The highest levels of expression are observed in

and T cells; however, expression also was observed in epithand endothelial cells in response to proinflammatory stsuch as IFN- and IL-1. In addition to the role of IL-3apoptosis, its biologic activities include induction of seproinflammatory cytokines (such as TNF-and IL-6) andmokines (including CXCL8 [i.e., IL-8]) and CXCL2 (mphage inflammatory protein 2 [MIP-2]) from differentmacrophages and amplification of IL-1and IL-6 releaseNOD receptor activation.

INTERFERONS

There are three members of the IFN family, , , and their nomenclature is based on their ability to interfere

viral growth. Type I interferonsIFN- and IFN-derived from monocytes, macrophages, B lymphocytes, ancells. However, the most important source of IFN-is the macytoid DC (pDC), typically reflecting their activatioviral RNA through TLR7 or by immunostimulatory bacDNA through TLR9. Type I IFNs have significant antactivity mediated through their ability to inhibit viral reption within infected cells, protect uninfected cells from ition and stimulate antiviral immunity by CD8+ lymphoand NK cells. IFN- has other important biologic actincluding upregulation of class I major histocompaticomplex (MHC) antigens and mediation of antitumor actOther IFN-like factors with antiviral immunity include Iand IL-29. These type III interferons are members of the I

family and are discussed further on.IFN- (the sole type II interferon) is made primari

T cells and NK cells. The biologic properties of IFN-inonly modest antiviral activity, and its prominent derivfrom T lymphocytes, combined with its wide-ranging functsuggest that it is more of an interleukin than an interferoroles in cellular and allergic immunity are discussed later.

Humoral Immunity

Two cytokines important for B lymphocyte maturation ibone marrow are the lymphoid stem cell growth factors

IL-6. Homodimers and monomers of the p40 peptide act ascompetitive antagonists by binding to IL-12Rs without trans-ducing activating signals. IL-12 stimulates IFN- productionand activates and induces proliferation, cytotoxicity, and cyto-kine production of NK cells. Other activities include prolifera-tion of T helper and cytotoxic lymphocytes. Its roles in Th1helper immune deviation are discussed later on.

IL-23 is a heterodimer consisting of a unique IL-23chain

and the p40 chain of IL-12.

14

IL-23 is secreted primarily byactivated DCs, with additional amounts coming from macro-phages. Its receptor consists of a heterodimer comprisinga unique IL-23R chain and the IL-12R1 chain. IL-23 actson T cells to enhance proliferation and activates NK cells. Incontrast with IL-12, recent studies suggest that IL-23 hasmore important influences on Th17 differentiation (see laterdiscussion).

The newest member of the IL-12 family is IL-35. It is a het-erodimer composed of the p35 subunit of IL-12 and the IL-27B(EBI3) subunit. IL-35 is a product of regulatory T cells, and itsactivities include promoting their proliferation while alsodirectly mediating suppression of effector T cells.

INTERLEUKIN-15

IL-15 has many activities similar to those of IL-2 and is distin-guished from IL-2 through its use of a unique IL-15Rchainas part of its receptor-signaling complex, in addition to shareduse of the IL-2R and the common chain.15 Mononuclearphagocytic cells, epithelium, and fibroblasts are sources ofIL-15, whereas T lymphocytes, the most important source ofIL-2, do not express IL-15. IL-15 is produced in response tosignals that induce the innate immune system, including, inparticular, viral infections, and promotes antiviral immuneresponses. Like IL-2, IL-15 is a T cell growth factor, differenti-ates NK cells, and stimulates B cell growth and differentiation.

INTERLEUKIN-27

IL-27 is a heterodimeric protein composed of the IL-27A (p28)protein and the IL-27B (EBI3) subunit that it shares with IL-35.The primary cells responsible for IL-27 production are macro-phages, DCs, and endothelial cells. IL-27 is one of several cyto-kines that uses gp130 as part of its receptor along with theunique WSX-1 protein. Proinflammatory functions includeits ability to synergize with IL-12 to induce IFN-productionfrom T helper and NK cells. IL-27 thus contributes to IL-12mediated Th1 immune deviation.

Cytotoxic Immunity

Immune responses directed against virus-infected and neoplas-tic cells are mediated primarily by CD8+cytotoxic lymphocytesand NK cells. Cytokines that activate cytotoxic immunityinclude IL-2, IL-4, IL-5, IL-6, IL-7, IL-10, IL-12, IL-15, and IL-27which are discussed elsewhereas well as by IL-11, IL-32,and, most important, TNF and the interferons.

INTERLEUKIN-11

Originally described as a stimulatory factor for hematopoieticprogenitor cells, IL-11 also functions in promoting the cyto-toxic immune response, along with being involved in bone


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involved in activation of NK cells, B cells, cytotoxic T cells, anmacrophages.

INTERFERON-

The most important cytokine responsible for cell-mediateimmunity is IFN-.19It is produced primarily by T helper lymphocytes but also is derived from cytotoxic T cells and NK cell

IFN- directly stimulates antigen processing by APCs anfurther stimulates antigen presentation via increased MHclass I and II expression and cytokine production. On mononuclear phagocytes, IFN-stimulates effector functions drivintheir differentiation from largely inactive monocytes into poteneffector macrophages characterized by enhanced adherencphagocytosis, degranulation, and production of reactive oxygeand nitrogen molecules. Thus, IFN- is responsible for theaccumulation at the site of cellular immune responses as celnewly capable of killing intracellular pathogens (e.g., mycobateria) and cancers. These classically activated M1 macrophagin general are high producers of IL-1, TNF-, IL-6, IL-12, anIL-23, but not IL-10, and as such are proinflammatory anparticipate in Th1 polarization. These M1 macrophages con

trast with the tolerogenic M2 macrophages, discussed latewhich display the opposite profile, thereby promoting immuntolerance, and are induced primarily by IL-4. M1 macrophageare induced by IFN-but can be induced by microbial stimu(e.g., LPS) or other cytokines (e.g., TNF-, granulocytemacrophage colony-stimulating factor [GM-CSF]). In additioto mononuclear phagocytes, IFN- stimulates killing by Ncells and neutrophils. It stimulates adherence of leukocytes tendothelial cells through induction of ICAM-1, an activishared with IL-1, IL-17, IL-18, IL-26, and TNF. IFN-inhibiviral replication, albeit weakly in comparison with other IFNand, as discussed later, is an inhibitor of allergic responsethrough its capacity to suppress IL-4mediated effects.

INTERLEUKIN-16

IL-16 is an atypical cytokine in that it lacks the classical structural motifs found in other cytokines. It is translated into a prform that is cleaved by caspase-3, giving rise to the activprotein. IL-16 is produced by T cells, eosinophils, mast cellmonocytes, and DCs but also by fibroblasts and epithelicells. Production of IL-16 is increased by IL-1, TGF-, IL-GM-CSF, IL-13, and histamine. The CD4 molecule is its primarreceptor, although the chemokine receptor CCR5 enhanceIL-16 binding. IL-16 is chemotactic for T cells, eosinophilmast cells, and monocytes. Among the CD4+ T cells subsetIL-16 preferentially induces migration of Th1 and Foxp3+regulatory T lymphocytes (Tregs). In addition, IL-16 promotes Th

and dampens Th2 responses through modulation of cytokinactivity.20

INTERLEUKIN-17

IL-17 comprises a structurally related family of six proteinIL-17A through IL-17F, having no known sequence similaritto any other cytokines.21 (Because of its unique spectrum oactivities, IL-17E is now termed IL-25 and is discussed later onIL-17 family members have overlapping but not identical biologic activities, and their cell sources are distinct. IL-17A (geneally referred to as IL-17) is expressed mainly in CD4+T cells an

and IL-11. IL-7, produced by stromal tissue of the bone marrowand thymus, is essential to the development of nave andmemory B and T lymphocytes through interactions with lym-phoid precursors. In addition, IL-7 stimulates proliferation anddifferentiation of cytotoxic T and NK cells and stimulates thetumoricidal activity of monocytes and macrophages.

After the egress of B cells from the bone marrow, isotypeswitching, activation of mature B cells into immunoglobulin-

secreting B cells, and their final differentiation into plasma cellsare processes that are under T cell control. Cytokines that triggerisotype switching include IL-4 and IL-13, which induces the IgEisotype; TGF-, which triggers IgA; and IL-10, involved in thegeneration of IgG4. Other cytokines that influence B cell matu-ration include IFN-, IL-1, IL-2, IL-5, IL-6, IL-12, IL-15, and,most important, IL-21.

INTERLEUKIN-21

IL-21 shares homology with IL-2, IL-4, and IL-15 and, like IL-2,is produced predominantly by activated T lymphocytes and NKT cells.18The signaling complex for IL-21 consists of a uniqueIL-21R and the common c. IL-21 receptors are expressed on

activated B, T, and NK cells. It shares biologic activities withIL-2 and IL-15, including the capacity to activate NK cells andpromote the proliferation of T lymphocytes. IL-21 is increas-ingly recognized as being central to the proliferation and sur-vival of B cells within germinal centers and their differentiationinto immunoglobulin-producing plasma cells. As discussedfurther on, this activity is mediated by the production of IL-21by the resident T cell of secondary lymphoid tissue, termed thefollicular T lymphocyte. Activation of B cells by IL-21 includesinduction of the transcription factor BCL6, which drives ger-minal center functions. However, BCL6 is an inhibitor of STAT6and specifically blocks the STAT6-mediated heavy chain classswitch, making IL-21 an inhibitor of IgE production. In addi-tion to this production of IL-21 by follicular T cells, as discussed

later, IL-21 is produced by Th17 cells and has an important rolein their differentiation.

Cellular Immunity

INTERLEUKIN-2

T cell activation involves their stimulation by antigen-MHCcomplexes (signal 1) in the presence of accessory signals pro-vided by the cognate interaction of CD80 or CD86 on APCswith T cell CD28 (signal 2) and soluble cytokinederivedsignals, provided especially by members of the IL-1 super-family (signal 3). This activation of T cells induces their simul-taneous secretion of IL-2 and their expression of IL-2R. The

complete IL-2 receptor consists of this newly induced IL-2R(CD25), combining with IL-2R (CD122) and the commonc (CD132). Subsequent binding of IL-2 to these IL-2R-positive T cells induces clonal T cell expansion. The require-ment for both IL-2 production and IL-2Rexpression ensuresthat only T cells specific for the antigen inciting the immuneresponse become activated. Whereas newly activated effector Tlymphocytes are induced to express the CD25, CD25 is consti-tutively expressed on regulatory T lymphocytes. Therefore, asdiscussed later, IL-2 also has antiinflammatory activities throughits ability to activate and induce proliferation of regulatory Tcells. In addition to its role as a T cell growth factor, IL-2 is


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Induction of Immunoglobulin E

Interleukin-4. In addition to T helper lymphocytes, ILderived from basophils, NK T cells, mast cells, and eosino(Table 5-3). In both eosinophils and basophils, IL-4 existpreformed, granule-associated peptide that can be ra

released during inflammatory responses. IL-4 stimulates Mclass II, B7 (CD80/CD86), CD40, surface IgM, and low-afIgE receptor (CD23) expression by B cells, thereby enhantheir antigen-presenting capacity. IL-4 induces the immglobulin isotype switch from IgM to IgE.25IL-4 is readily tifiable in allergic inflammatory tissue such as that inasthmatic lung or allergic nasal passages, defining it as thetotypic allergic cytokine.

IL-4 has important influences on T lymphocyte growthferentiation, and survival. As discussed later, IL-4 establishedifferentiation of nave Th0 lymphocytes into the Th2 phtype. IL-4 also is important in maintaining allergic immresponses by preventing T lymphocytes apoptosis.26,27Theduction of IL-4 by Th2 lymphocytes renders these cells re

tory to the antiinflammatory influences of corticosteroidsAs with its action on B cells, IL-4 enhances the expressi

MHC molecules and low-affinity IgE receptors (CD23) on rophages. The influence of IL-4 on mononuclear phagochowever, is primarily antiinflammatory, including its tendto downregulate antibody-dependent cellular cytotoxinhibit expression of Fcreceptors, thereby reducing theirduction of reactive oxygen species and nitric oxide, and inhing production of IL-1, IL-6, IL-12, and TNF-while stimulproduction of IL-1Ra and IL-10.28This profile defines thenotype of the tolerance-promoting M2 macrophage. M2ferentiation also occurs in response to other cytokines TGF-, IL-10, IL-13, IL-21), as well as corticosteroids, immcomplexes, and a member of the TGF-family termed ac

A.29 M2 macrophages also are characterized by high levescavenger and mannose receptors.

Another important activity of IL-4 is its ability to inexpression of VCAM-1 on endothelial cells. This enhaadhesiveness of endothelium for T cells, eosinophils, basoand monocytes, but not neutrophils, as is characteristic of gic reactions.30IL-4 receptors are present on mast cells, wthey function to stimulate IgE receptor expression, alongexpression of the enzyme leukotriene C4 (LTC4) syntthereby stimulating their capacity to produce cysteinyl letrienes.31IL-4 can stimulate mucin production and contrto the excessive mucus production in the asthmatic air

to a lesser extent in neutrophils, eosinophils, and CD8+T cells.The selective production of IL-17 by clonal T helper lympho-cytes has led to the recognition of the Th17 cell as a distinctlymphocyte subset, as discussed subsequently. In contrast withIL-17A, IL-17B is not expressed by immune cells, but its expres-sion is observed in the spinal cord, testis, ovaries, small intes-tine, pancreas, stomach, prostate, and colon. IL-17C is producedby inflamed tissue and interacts with IL-17RE to selectively

expand Th17 cells.

22

As with IL-17A, its most closely structur-ally related family member, IL-17F, also is expressed by activatedCD4+T cells but also can be produced by activated basophilsand cord bloodderived mast cells, suggesting a wider tissuedistribution.21 In contrast with IL-17A and IL-17F, IL-17D isexpressed by resting CD4+T and B cells.

Both IL-17 (IL-17A) and IL-17F induce expression of avariety of cytokines and chemokines from epithelial and vascu-lar endothelial cells, including IL-6, granulocyte colony-stimulating factor (G-CSF), GM-CSF, and CXCL10 (i.e.,interferon-inducible protein-10 [IP-10]). IL-17 has a particu-larly important role in activation of fibroblasts that contributeto a central role for this cytokine in fibrotic autoimmune dis-eases. IL-11 and the neutrophil-activating factors IL-6 and

CXCL8 (IL-8) are secreted by fibroblasts in response to IL-17(and IL-17F). In addition to direct activation of fibroblasts,IL-17 induces expression of the profibrotic cytokines IL-6and IL-11, whereas IL-17F induces TGF-expression, a cyto-kine also associated with fibrosis. IL-17 activates macrophages,fibroblasts, and stromal cells, inducing their expression ofICAM-1 and secretion of prostaglandin E2 and nitric oxide.As a result of these activities, IL-17 is increasingly recognizedto have important roles in autoimmune diseases, especiallythose involving fibrosis and tissue remodeling, such as inflam-matory bowel disease and multiple sclerosis.23 In immunity,the ability of IL-17 to recruit and activate neutrophils makesit uniquely important to the T cell activation of protectiveimmunity against extracellular bacterial pathogens, such as

Klebsiella.IL-17 family members are expressed in patients withasthma.24 The ability of these cytokines to induce neutrophilbut not eosinophil migration makes it plausible that they playroles in severe persistent asthma, in which accumulation ofneutrophils is a hallmark of disease. Both IL-17 and IL-17Finduce goblet cell hyperplasia and mucus hypersecretion.Through these effects and its ability to promote fibrosisand remodeling, IL-17 is thought to contribute to the develop-ment of airway hyperreactivity, remodeling, and subepithelialfibrosis.

Allergic Immunity

One possible outcome of proinflammatory T cell activation is

the development of allergic (and presumably antiparasite)immunity. Several features specifically associated with the asth-matic state are regulated by cytokines, including IgE produc-tion, eosinophil activation, and mast cell proliferation.

REGULATION OF IMMUNOGLOBULIN E

The inappropriate production of IgE in response to allergendefines atopy. Induction of IgE is primarily a function of theactivities of IL-4 and IL-13, with synergistic influences providedby IL-9. By contrast, IFN-and IL-21 are important inhibitorsof IgE isotype switch.

Sources of Interleukins IL-4 and IL-13

Cell Source IL-4 IL

T helper lymphocytes: Nave T cells Yes Y T follicular helper (Tfh) cells Yes N Th2 cells Yes Y Natural killer (NK) T cells Yes Y

Basophils Yes NEosinophils Yes Y

Mast cells Yes Y

Innate lymphoid type 2 (ILC2) cells (nuocytes) No Y

TABLE5-3


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signature is supported by the efficacy of IL-13targeting therapies in this endotype.34,35

Interleukin-9. The primary source of IL-9 is the T helper lymphocyte population, including Th2 cells, with additionamounts coming from mast cells and eosinophils. Within helper populations, IL-9 appears to be primarily produced byunique subfamily termed the Th9 lymphocyte (discussed later

characterized by production of abundant IL-9 and little IL-4Originally described as a mast cell growth factor, IL-9 also contributes to mast cellmediated allergic responses through iability to stimulate production of mast cell proteases, inflammatory cytokines, and chemokines. Additionally, IL-9 primemast cells to respond to allergens by increasing their expressioof FcRI. IL-9 synergizes with IL-4 to enhance productioof IgE and memory B cell differentiation.36The same synergleads to enhanced IL-5 production resulting in greater numbeand maturation of immature eosinophil precursors. On iown, IL-9 increases expression of IL-5R on eosinophilthereby providing a positive feedback loop for increasineosinophil numbers and activity. IL-9 acts on airway epithelicells by inducing T cell and eosinophil chemotactic factors suc

as CCL11 (eotaxin), CCL2 (MCP-1), CCL3 (MIP-1), anCCL7 (MCP-3). After lung allergen challenge, IL-9 plays aimportant role in inducing goblet cell hyperplasia and expresion of the mucin genesMUC2andMUC5ACfrom bronchiepithelial cells.

Inhibition of Immunoglobulin E Production:Interferon-and Interleukin-21

IFN- functions as an inhibitor of allergic responses througits capacity to inhibit many IL-4 functions, including IL-4induction of the IgE isotype switch and expression of lowaffinity IgE receptors. As discussed, IL-21 is essential to germnal center formation and activity, effects mediated through iinduction of the transcription factor BCL6. However, BCL

recognizes and competes for the same DNA sequences as thofor STAT6, thereby constraining STAT6-mediated induction ogerm line transcripts. The implication of this observation that IgE production can occur only in a milieu not containinIL-21induced BCL6 (i.e., not in the germinal center). Of notit is speculated that the absence of Th17 cells in hyper-Igsyndrome and, correspondingly, the reduced expression of IL21induced BCL6, leads to unopposed activity of STATthereby driving the robust elevation of IgE that defines thsyndrome.

EOSINOPHILIA

Another characteristic feature of allergic diseases is the presenc

of activated eosinophils.

Interleukin-5

IL-5 is the most important eosinophilopoietin37 and also cainduce basophil differentiation. In addition to stimulatineosinophil production, IL-5 is chemotactic for eosinophils anactivates mature eosinophils, inducing secretion and enhancintheir cytotoxicity. IL-5 promotes accumulation of eosinophithrough its ability to upregulate responses to chemokines and2integrins on eosinophils, thereby promoting their adheence to VCAM-1expressing endothelial cells. IL-5 prolongeosinophil survival by blocking apoptosis. Administration o

however, IL-13 appears to be more important in this process.Functional IL-4 receptors are heterodimers consisting of theIL-4Rchain interacting with either the shared chain or theIL-13R1 chain25 (Fig. 5-2). This shared use of the IL-4Rchain by IL-4 and IL-13 and the activation by this chain of thesignaling protein STAT6 serve to explain many of the commonbiologic activities of these two cytokines.

Interleukin-13. IL-13 is homologous to IL-4 and shares manyof its biologic activities on mononuclear phagocytic cells, endo-thelial cells, epithelial cells, and B cells. Thus, IL-13 induces theIgE isotype switch and VCAM-1 expression.25Functional IL-13

receptors are a heterodimer containing the IL-4Rchain and aunique IL-13Rchain. The two IL-13Rchains that have beendescribed include the active form of the receptor IL-13R1 anda decoy receptor, IL-13R2, which lacks the motif requiredfor initiating intracellular signaling cascades (see Fig. 5-2).32IL-13R1 expression is more limited than that with IL-4 recep-tors and includes endothelial cells, B cells, mononuclear phago-cytes, and basophils. Expression is observed on some mast cellsand Th17 cells33 but in no other T cell lineage. The limiteddistribution of IL-13R1 explains the unique ability of IL-4 toinduce Th2 lymphocyte differentiation and mast cell activation.Biologic activities of IL-4 and IL-13 also are distinguished bytheir distinct cellular sources (see Table 5-3). In contrast withIL-4, IL-13 is produced by innate lymphoid type 2 (ILC2) cells

(nuocytes), although IL-4, but not IL-13, is produced by T fol-licular helper (Tfh) cells and basophils (see Table 5-3). IL-13 ismore widely secreted and can function in a hormonal fashion,in contrast with IL-4, which is largely retained within theimmune synapse, thereby limiting its activities to a more para-crine fashion. Accordingly, IL-13 is more readily identified inallergic inflammatory tissue.25 IL-13, acting through this hor-monal mechanism, causes mucus hypersecretion and nonspe-cific airway hyperreactivity (AHR), and its expression results inthe characteristic airway metaplasia of asthma, with the replace-ment of epithelial cells with goblet cells.25 The importance ofIL-13 in presentations of asthma associated with a robust IL-13

Figure 5-2 IL-4 and IL-13 receptors. Type 1 IL-4 receptors are het-erodimers of IL-4Rinteracting with the shared chain and bind onlyIL-4. Their unique expression on most T helper cells and mast cellsrenders these cells only responsive to IL-4. Type 2 receptors can bindboth IL-4 and IL-13. They are more widely expressed and consist ofheterodimers of IL-4Rand IL-13R1. In addition, IL-13 can bind tothe IL-13R2, which lacks a cytoplasmic domain and thereby functionsas a decoy receptor. IL,Interleukin.


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OTHER Th2 CELLDERIVED CYTOKINESINVOLVED IN THE DEVELOPMENT OFALLERGIC INFLAMMATION: IL-31

Interleukin-31

IL-31 is another cytokine that was identified on the basis sequence homology to other cytokines.42It is a member oIL-6type cytokine family. Signaling occurs via a heterod

consisting of the IL-31RA and a second chain also utilizethe related superfamily member oncostatin M (the OSMRis expressed primarily by Th2-like lymphocytes, but low lalso are observed in CD8+T cells. IL-31 targets keratinocepithelial cells, eosinophils, basophils and monocytes. Itslogic activities include induction of chemokines thatinvolved in recruitment of neutrophils, monocytes, and T Overexpression of IL-31 in mice produces a skin inflamminfiltrate suggestive of atopic dermatitis, and it is found inbiopsy specimens from patients with atopic dermatitis.43,4

Antiinflammatory Cytokines

In addition to cytokines that stimulate cytotoxic, cel

humoral, and allergic inflammation, several cytokines havedominantly antiinflammatory effects, including, as previdiscussed, IL-1Ra, IL-35, but also TGF-and members oIL-10 family.

TRANSFORMING GROWTH FACTOR-

The TGF-superfamily comprises more than 30 proteinsregulate cell growth, having both stimulatory and inhibeffects on different cell types.45TGF-itself exists as threferent isoforms: TGF-1, TGF-2, and TGF-3. It is prodby many cells, including osteocytes, epithelial cells, fibrobplatelets, monocytes, eosinophils, and T cells. TGF-produT helper lymphocytes represent distinct lineages term

repressor (Tr1) or type 3 T helper (Th3) cells (Table 5-4). Aity of TGF-is controlled at the posttranslational level, beTGF- is synthesized as a latent molecule complexed wlatency-associated peptide, and it requires proteolytic cleaupon secretion from the cell to become active. BindinTGF-results in signaling predominantly through the SMpathway. Its name is derived from its initial recognition important stimulant of fibrosis, inducing formation of extrlular matrix and the promotion of wound healing andformation. In immunity, TGF-, in general, inhibits immglobulin secretion by B lymphocytes and also cytotoxicimononuclear phagocytes and NK cells. Production of Tby apoptotic cells creates an immunosuppressive milieu; tone explanation for the absence of inflammation and aut

munity as a consequence of apoptotic cell death.46In conwith these antiinflammatory effects, TGF- is a chemoattant for macrophages and supports the isotype switch toby B cells.45 TGF- expressed in gut lymphoid tissue bycells is responsible for secretory IgA production and alcritical for the maintenance of immune nonresponsiveneotherwise benign gut pathogens and food allergens. TGis constitutively produced in the healthy lung and can promote B and T cell nonresponsiveness. TGF-productioregulatory cells lessens allergic inflammation through a capto inhibit IgE synthesis and mast cell proliferation. By conhowever, eosinophils are an important source of TGF-,4

IL-5 to humans causes mucosal eosinophilia accompanied byan increase in bronchial hyperreactivity. The importance ofIL-5 in asthma was questioned as a result of the disappointingresults in early trials using IL-5 antagonists. These results reflectboth IL-5independent pathways toward persistent eosinophil-mediated inflammation (e.g., especially GM-CSF), but also theincreased recognition of noneosinophil-mediated contribu-tions to asthma. However, in patients whose asthma was deter-

mined to be associated with persistent eosinophilia despite theuse of inhaled corticosteroids, anti-IL-5 proved to be an effec-tive therapy.38,39 In addition to Th2-like lymphocytes, othersources for IL-5 include mast cells and NK T cells. IL-5 interactswith a specific IL-5R that consists of a heterodimer containinga unique IL-5R and a chain (CD131) that is shared withGM-CSF and IL-3 receptors.37

Interleukin-3 and Granulocyte-MacrophageColony-Stimulating Factor

In addition to IL-5, IL-3 and GM-CSF contribute to the activ-ity of eosinophils in allergic inflammation through their capac-ities to prolong eosinophil survival and promote eosinophilactivation. IL-3 supports the growth of precursors for a variety

of hematopoietic cells, including DCs, erythrocytes, granulo-cytes (especially basophils), macrophages, mast cells, and lym-phoid cells. The major source of IL-3 is T lymphocytes, but inallergic inflammation, it also is derived from eosinophils andmast cells.

GM-CSF supports the maturation of DCs, neutrophils, andmacrophages and can synergize with other cytokines to supportthe production of platelets and erythrocytes. Mature granulo-cytes and mononuclear phagocytic cells are activated in responseto GM-CSF. GM-CSF activates mature eosinophils, increasingtheir degranulation, cytotoxicity, and response to chemoattrac-tants. The role of GM-CSF in allergic immunity derives fromits shared ability with IL-3 and IL-5 to inhibit eosinophil apop-tosis and to prolong eosinophil survival at sites of allergic

inflammation. GM-CSF is particularly important in the allergicairway, because activated eosinophils lose their expression ofIL-5R and responsiveness to IL-5, instead upregulatingGM-CSF receptors. Thus, GM-CSF, and not IL-5, may beresponsible for the persistent survival and function of eosino-phils in the asthmatic airway. As noted, these observationsprovide one explanation for the earlier reported failure of IL-5antagonism in asthma.

MAST CELL PROLIFERATION AND ACTIVATION

Increased numbers of mast cells characterize allergic diseases,and as with elevated IgE concentrations and eosinophilia, thisis a T celldependent process. The most important cytokine

responsible for mast cell growth and proliferation is stem cellfactor (SCF) (formerly termed c-kit ligand).40 SCF is derivedfrom bone marrow stromal cells, endothelial cells, and fibro-blasts. SCF induces histamine release from human mast cellsand remains the only cytokine known to have this property.Thus, systemic administration of SCF is associated with cutane-ous mast cell proliferation and urticaria. In addition to beingessential for mast cell differentiation, SCF interacts with otherhematopoietic growth factors to stimulate myeloid, lymphoid,and erythroid progenitor cell growth. Additional cytokinesincluding IL-3, IL-5, IL-6, IL-9, IL-10, IL-11, and nerve growthfactor also contribute to mast cell proliferation.41


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inhibits production of IFN- by Th1 lymphocytes; IL-4 anIL-5 by Th2 lymphocytes48; IL-1, IL-6, IL-8, IL-12, and TNF-by mononuclear phagocytes; and IFN-and TNF-by NK cellIn addition, IL-10 inhibits MHC class II, CD23, ICAM-1 anCD80/CD86 expression by DCs and other APCs. Reduction oCD80/CD86 expression inhibits the ability of the APC tprovide the accessory signal necessary for T helper activation,resulting in inhibition of Th1 and Th2 cytokine production

Constitutive expression of IL-10 by immature DCs anM2-differentiated mononuclear phagocytic cells in the respiratory tract of normal subjects has a central role in the inductioand maintenance of tolerance to allergens and otherwise benigbioaerosols. By contrast, asthma and allergic rhinitis are assocated with diminished IL-10 expression in the airway, whiccontributes to the development of an inflammatory milieuand creates a permissive influence on expression of mature DCSupport for a modulating role for IL-10 in human allergdisease is further derived from observations that IL-10 inhibieosinophil survival and IL-4induced IgE synthesis. IL-1TNF-, and other proinflammatory cytokines stimulate IL-1secretion, suggesting a homeostatic mechanism whereby ainflammatory process stimulates IL-10 secretion, leading to a

inhibitory feedback process. These inhibitory effects of IL-1are in contrast with its effect on B lymphocytes, in which functions as an activating factor to stimulate cell proliferatioand immunoglobulin secretion. IL-10 enhances isotype switching to IgG4 and functions as a growth cofactor for cytotoxic cells. Thus, IL-10 inhibits cytokines associated with cellulaimmunity and allergic inflammation, while stimulating humorand cytotoxic immune responses. As with TGF- and manother cytokines, the distinct and apparently conflicting actionof IL-10 are a function of which cells are producing anresponding to it, and of the stage of the immune responsduring which it is acting.

More recently recognized members of the IL-10 familinclude IL-19, IL-20, IL-22, IL-24, IL-26, IL-28, and IL-29. 51

These cytokines and their receptors share structural homologand intron-exon structure with IL-10 and IL-10 receptors bualso loosely share homologies with interferons and interferoreceptors, and many display antiviral activity. In contrast witIL-10, none of these cytokines significantly inhibit cytokinsynthesis, an activity that remains unique to IL-10.

IL-19 is expressed primarily by monocytes51 but also habeen found in epithelial and endothelial cells. Signaling by IL-1occurs by means of a receptor complex composed of the IL-20Rand IL-20R2 chains, leading to activation of STAT1 and STATIL-19 activates monocytes in autocrine and paracrine fashioto release the cytokines IL-6 and TNF-and reactive oxygespecies. Monocyte IL-19 production is upregulated by IL-4 andownregulated by IFN-. IL-19 can promote Th2 immune dev

ation, suggesting a role in allergic diseases.53 Consistent witthis concept, serum levels of IL-19 in asthma are elevated ancorrelate with higher levels of IL-4 and IL-13.54

IL-20 is expressed predominantly by monocytes, DCs, epthelial cells, endothelial cells, and skin keratinocytes. Similar tIL-19, IL-20 signals through an IL-20R1/IL-20R2 heterodimehowever, IL-20 also binds to a receptor complex composed oIL-22R1 and IL-20R2. Both of these heterodimer receptor complexes mainly signal through the STAT3 and STAT1 pathwayIL-20 induces keratinocyte proliferation, and in mice, its overexpression is lethal secondary to defective skin formation. Consistent with suggested influences in inflammatory skin disease

this has been ascribed as promoting the fibrosis and remodelingobserved in asthma. These conflicting pro- and antiinflamma-tory influences reflect the distinctive actions of TGF- as afunction of which cells are producing it, the stage of the immuneresponse during which it is acting, different signaling pathwaysit engages, and other divergent influences.

INTERLEUKIN-10 FAMILY: IL-10, IL-19, IL-20,IL-22, IL-24, IL-26, IL-28, AND IL-29

IL-10 is a product of numerous cells, including Th1 and Th2lymphocytes,48 cytotoxic T cells, B lymphocytes, mast cells,immature DCs, and mononuclear phagocytic cells (M2 cells).The primary T cell source for IL-10 is regulatory T lymphocytes,described further on. IL-10 binds to a heterodimer receptorcomplex composed of molecules of the IL-10 receptor type 1and type 2 (IL-10R1 and IL-10R2) chains. IL-10R1 is uniquelyinvolved in IL-10 binding, whereas IL-10R2 is a commonsubunit shared with other members of the IL-10 superfamily.Upon binding of IL-10, signaling is initiated through activationof JAK1 that is bound to IL-10R1 and subsequent activation ofSTAT3, leading to activation of IL-10responsive genes. IL-10

Cytokine Production Patterns of T CellSubtypes

T Helper Family Cytokine Profile

Th1* IFN-LT-(TNF-)

Th2* IL-4IL-5

IL-9IL-13IL-25IL-31

Th9 IL-9IL-10

Th17 IL-17 (IL-17a)IL-17fIL-21IL-22IL-23

Th22 IL-22TNF-

Tfh IL-21IL-4

ILC2 (nuocytes) IL-5IL-13

nTreg mTGF-(IL-10)

Th3 TGF-(IL-10)

iTreg IL-10IL-35(TGF-)

GM-CSF,Granulocyte-macrophage colony-stimulating factor; IFN,interferon; iTreg,inducible regulatory T lymphocyte; LT,lymphotoxin; nTreg,natural regulatory T ymphocyte; mTGF-,membrane TGF-; Tfh,T follicular helper (cell); TGF,transforminggrowth factor; TNF,tumor necrosis factor.

*Both Th1 and Th2 lymphocytes produce TNF-, GM-CSF, IL-2, IL-3,

and, under certain circumstances, IL-10.

TABLE5-4


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responses.56 IL-28 and IL-29 also shift immature DCs toa program characterized by the ability to produce Foexpressing CD4+CD25+ regulatory T cells, thereby genertolerance-inducing DCs.

T Helper Lymphocyte Families

TH1, TH2, AND TH17-LIKE LYMPHOCYTES

Subclasses of T helper lymphocytes can be identified onbasis of their repertoire of cytokines (see Table 5-4). Navecells produce primarily IL-2 but also may synthesize cytocharacteristic of both Th1 and Th2 lymphocytes. In humtype 1 helper cells produce IFN-and LT- (TNF-), buIL-4 and IL-5. Type 2 helper cells produce IL-4, IL-5, IL-9, Iand IL-25, but not IFN- or LT-. Both classes can proGM-CSF, TNF-, IL-2, IL-3, and, to a lesser extent, IAlthough distinct Th1/Th2 cytokine profiles are seldom apent in human cells, there remains an inverse relationbetween the tendency of T lymphocytes to produce IFNopposed to IL-4 or IL-5. Type 1 Th1 lymphocytes activcells and M1 macrophages, promote cell-mediated imm

responses and are important in antibody-dependent immuTh2 lymphocytes produce IL-4, IL-5, and IL-13 to induce parasite and allergic immune responses. A third class helper lymphocyte, the Th17 cell, has been described onbasis of its unique cytokine repertoire (especially IL-17, buIL-21 and IL-22), its distinct differentiation pathwaymaster transcription regulatory factor and its counterretory influences on Th1 and Th2 differentiation. Many tional T helper families continue to be described, the prominent of which are discussed subsequently.

Cytokines Involved in Th1Differentiation

One of the most important questions in understandingcause of allergic disorders is to determine the basis for Th1differentiation as opposed to generation of either immignorance or active immune tolerance in response to alleexposure. In general, a stereotypic response is shared inprocess of Th1/Th2/Th17 deviation (Table 5-5). This respinitially involves a tilt toward differentiation that is drivethe cytokine milieu in which the nave T lymphocyte is vated. These cytokines induce activation of unique JAK/STspecific pathways, leading to the appearance of family-spmaster regulators (Fig. 5-3). These transcription faproduce an initial, low-level production of the defi

its expression is increased in keratinocytes at sites of psoriaticlesions. Additionally, IL-20 has angiogenic properties.

IL-22is derived primarily from T lymphocytes, in particularTh17 and Th1 cells, as well as activated NK cells. The IL-22receptor complex is a heterodimer consisting of IL-22R1 andIL-10R2 chains with signaling thought to occur via STAT3. Thepattern of IL-22R1/IL-10R2 expression suggests that the mostimportant cellular targets of IL-22 are found within the skin,

kidney, respiratory, and digestive systems, but not immune cells.These target tissues are in contact with the environment, andIL-22 is recognized to mediate the production of antimicrobialpeptides and innate immune defenses; accordingly, IL-22 is pre-sumed to play a role in innate pathogen defense. The otherprominent biologic activity described for IL-22 is induction ofacute phase responses in hepatocytes. Consistent with theinterferon-like activities of these cytokines, IL-22 induces MHCclass I antigen expression.

IL-24 is a fourth member of the IL-10 family and is pro-duced by monocytes as well as by Th2 lymphocytes in an IL-4inducible fashion. IL-24 signals through a heterodimerconsisting of the IL-20R1 and IL-20R2 chains leading to activa-tion of STAT3. IL-24 promotes secretion of TNF- and IL-6

from human monocytes. It was originally discovered as a factorexpressed by melanoma cells that were fully differentiated intononproliferating states. Consistent with this putative antineo-plastic activity, it also inhibits proliferation, induces tumorapoptosis, and engages an antitumor immune response.

IL-26is generated primarily by T memory cells, Th17 cells,and NK cells and is important in the transformation of humanT cells after their infection by herpesvirus.55The IL-26 receptorconsists of a heterodimer of the IL-20R1 chain and the IL-10R2chain, which activates both STAT1 and STAT3. Biologic activi-ties of IL-26 include induction of CXCL8 (IL-8) and expressionof ICAM-1. It is located in the q15 cytokine gene clusteron chromosome 12, along with IFN- and IL-22an areathought to contribute to allergic and autoimmune diseases. By

contrast, IL-19, IL-20, and IL-24 cluster in the IL-10 region onchromosome 1.

Finally, IL-28Aand IL-28B, alternatively named IFN2 and3, belong, with IL-29(IFN1), in a cytokine family that shareswith type I interferons the same JAK/STAT signaling pathway.IL-28 and IL-29, at the amino acid level, also are closely relatedto the type I IFNs, but their genomic organization more closelyresembles that of members of the IL-10 family. All three cyto-kines bind to a heterodimer consisting of a unique IL-28Rchain (alternatively named IFN-R1) combined with IL-10R2.These cytokines have antiviral properties, and their presenceprevents viral infection and enhances innate immune

T Helper Family Differentiation Pathways

T HelperFamily Initial Cytokine Milieu

InitialSTAT

Master TranscriptionRegulator Cytokine STAT IL-1 Superfa

Th1 IL-12(IL-27)

STAT4 T-bet IFN- STAT1 IL-18

Th2 Lowstrength T cellreceptor engagement;

IL-4

STAT5 GATA-3 IL-4 STAT6(MAF, NFAT)

IL-33

Th17 IL-6 STAT3 RORC2 IL-21, IL-23, TGF- STAT3 IL-1

IFN,Interferon; IL,interleukin; MAF,macrophage-activating factor; NFAT,nuclear factor of activated T cells; TGF,transforming growth facto

TABLE5-5


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for Th2 development, along with a decrease in production oIL-4.59 Once Th1 cells undergo their initial differentiation

newly synthesized IFN-, acting through STAT1, furtheincreases expression of T-bet. The IL-1 superfamily membeinvolved in optimizing Th1 cytokine secretion is IL-18. In addtion, IL-18 upregulates IL-12R expression and thereby synergizes with IL-12 in Th1 production.

The initial cytokine signal driving Th17 immune deviationprovided by IL-6. This Th17-inducing action of IL-6 on navT cells is antagonized by IFN- and IL-4. In the appropriamilieu, IL-6 engagement leads to phosphorylation of STATthat, in turn, drives expression of RORC2, a transcription factothat is the master regulator for differentiation of human Th1cells.60 RORC2 drives expression of IL-17 and IL-21. IL-2functioning in autocrine fashion, acts to establish Th17 diffeentiation and promote proliferation. IL-21 increases IL-23

expression and binding of IL-23 secreted from APCs to furtheincrease expression of RORC2, IL-17, and IL-22, thus establishing committed Th17 cell differentiation. Although TGF-is norequired for human Th17 development, it plays a pivotal roin expansion of Th17 cells by inhibiting Th1 differentiatiopathways.61TGF-plays a pleiotropic role in T helper famidifferentiation: In the absence of IL-6, it promotes differentiation into regulatory T cell pathways but also can synergize witIL-4 to promote Th9 differentiation. Finally, the IL-1 superfamily member maximizing cytokine secretion from Th17 cells (seTable 5-5) is IL-1 itself, and the presence of either IL-1 oIL-1enhances Th17 cytokine production.62Th17 cells are th

cytokine(s) for each of these families. The newly synthesizedcytokines then act in an autocrine fashion to fully activate the

differentiation pathway, in general, via a distinct signalingpathway. Finally, each of these three family members usesengagement by distinct members of the IL-1 superfamily tofully drive maximal cytokine secretion.57Each of these mecha-nisms is summarized in the following discussion; see also Figure5-4and Table 5-5.

Th1 differentiation initially can be induced through theinfluences of several cytokines, but IL-12 provides the mostimportant role.58DCs constitute the most important source ofthe IL-12 that drives Th1 immune deviation. In addition,insofar as mononuclear phagocytes are an additional source ofIL-12, this suggests a mechanism whereby antigens more likelyto be processed by macrophages, including bacterial antigens(e.g., mycobacteria), produce Th1 responses. IL-12 interacts

with nave T helper lymphocytes to activate STAT4, leading toexpression of T-bet. T-bet is a nuclear transcription factor thatis the master regulator responsible for the differentiation ofTh1 cells. Actions of T-bet include production of IFN- andupregulation of the IL-12 receptor. Simultaneously, it blocksalternative T helper differentiation pathways by suppressingexpression of Th2 cytokines, such as IL-4, and acting to nega-tively regulate Th17 differentiation. Similar to IL-12, IL-27 alsoactivates STAT4, leading to increased expression of T-bet andIFN-. The addition of recombinant IL-27 to T cells under Th2-polarizing conditions similarly results in decreased expressionof GATA-3, the transcription factor that is the master regulator

Figure 5-3 Representative signaling via the JAK/STAT (Janus kinase/signal transducer and activator of transcription) pathway. Signaling througmany cytokine receptors initially leads to the recruitment and phosphorylation of members of the JAK family, consisting of JAK1 to JAK3 anTyk2. The two chains of the interleukin-12 (IL-12) receptor bind JAK2 and Tyk2, respectively, whereas the IL-4 type 1 receptor binds JAK1 anJAK3. This use of JAK3 by the shared chain is consistent with sharing of the severe combined immunodeficiency (SCID) phenotype by JAKdeficiency and chain deficiency. The next step in signal transduction involves the tyrosine phosphorylation of the STAT proteins. After theactivation, these proteins dimerize and migrate to the nucleus, where they bind to specific regulatory sequences in the promoters of cytokinresponsive genes, thereby initiating gene transcription. P,Phosphate.

IL-12R

c

IL-4R

IL-12R

IL-12

IL-4Cytoplasm

Tyk2

JAK3

JAK2

P

P

PP P

PP

JAK1

P

P

STAT4

P

STAT6

STAT4homodimer

PP

PP

STAT6homodimer

STAT4 element

STAT6 element

Nucleus

TranscriptionIL-12responsive

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TranscriptionIL-4responsivegenes


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Cytokines Involved in Th2Differentiation

INTERLEUKIN-4

In addition to nascent activated Th2 cells, external sourcIL-4 can contribute to the milieu that establishes Th2 difftiation, including activated eosinophils, basophils and cells. One robust source for IL-4 is NK T cells.64,65NK Tare CD4+lymphocytes that express an invariant rearrangeof the T cell receptor (V24-J18 in humans) and are speized to recognize glycolipid antigens presented in the conteCD1d, which is closely related structurally to the MHC c

molecule.

INTERLEUKIN-25

IL-25 is a member of the IL-17 family (IL-17E), but becof its unique spectrum of activities, it has been given thistinct nomenclature. Binding of IL-25 occurs via a heterodcomplex composed of IL-17RB and IL-17RA.66Similar to IL-5, IL-9, and IL-13, it is derived from Th2 lymphocytesadditional sources include mast cells, eosinophils, basopand, most important, epithelial cells. The production of Iby injured epithelial cells is an important innate immune s

only T cell lineage that expresses the IL-13R1 chain.33Bindingof Th2-derived IL-13 to Th17 cells attenuates Th17 cellactivity.

The initial signal driving Th2 immune deviation includessignals provided by the cytokines IL-4, IL-19, IL-25, and thymicstromal lymphopoietin (TSLP).63This early signal may reflectthis cytokine milieu but is also thought to act through low-signal-strength T cell receptor engagement, leading to activa-tion of the transcription factor STAT5, which then promotesexpression of GATA-3, the master regulator of Th2 cells. GATA-3suppresses expression of T-bet to suppress Th1 differentiationand IFN- production, leading to the initial appearance oflow levels of IL-4. IL-4 and GATA-3 similarly inhibit differen-

tiation of Th17 lymphocytes. This initial production of IL-4then acts in an autocrine fashion to activate STAT6, whichfurther amplifies GATA-3 production to establish Th2 immunedeviation. Other transcription factors, including especiallymacrophage-activating factor (MAF) and nuclear factor of acti-vated T cells (NFAT), contribute to IL-4 and other Th2 cytokineproduction once Th2 differentiation has been established. TheIL-1 superfamily member that maximally drives cytokine secre-tion by Th2 cells is IL-33, discussed in further detail later on.Many cytokines are responsible for creating the milieu thatpermits Th2 deviation, including especially IL-25, IL-33, andTSLP (Fig. 5-5).

Figure 5-4 T helper cell differentiation pathways. T helper cell differentiation is driven by numerous factors, including the affinity of histocompatibility complex (MHC)-antigen binding to the T cell receptor, but, most important, reflects the cytokine milieu in which that intion takes place. Differentiation often involves activation of a specific STAT protein that drives the expression of a master regulating trantion factor. Differentiation is established through a positive feedback loop involving expression of one of the nascent differentiated T hcells signature cytokines, acting in an autocrine fashion. For Th1, Th2, and Th17 cells, expansion and cytokine secretion are potentiated additional presence of several cytokines of the IL-1 superfamily. APCs,Antigen-presenting cells; DCs,dendritic cells; IFN,interferon; IL,leukin; iTreg,inducible regulatory T lymphocyte; NK,natural killer; TGF,transforming growth factor; TSLP,thymic stromal lymphopoietin

Cell sources:APCs (DCs, B cells,mononuclear phagocytes)Epithelial cells and otherstromal cellsNave T cells, NK T cells,innate lymphoid type 2(ILC2) cells (nuocytes)

IL-12, IL-27

STAT4/1 - T-bet

mastertranscriptionfactor

Autocrinecytokine driving

expansion

IL-1 superfamilymember driving

expansion

Cytokinesignature

STAT(s)

STAT5/6 - GATA-3

STAT3 - RORc2

? - BCL6

? - IRF4/

pu .1

? - Foxp3

Th1

TSLP, IL-33, IL-25

IL-6, TGF-

INF-

LTIFN-

Th2

IL-4

IL-18

IL-1

IL-33

Th17

IL-21, IL-23

Tfh

IL-21

Th9

IL-9

iTreg

IL-10, TGF-

Th0

Th0

Th0

Th0

Th0

Th0IL-10IL-35

IL-9IL-10

IL-21IL-4

IL-17IL-17FIL-21IL-22

IL-4

IL-5IL-9IL-13

IL-4, TGF-

IL-10, TGF-

Initial cytokine milieu

??


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IL-5 and IL-13 secretion by ILC2 cells (nuocytes). Accordinglits administration increases IgE production and generaterobust mucosal eosinophilic inflammation in the lungs.67,69

THYMIC STROMAL LYMPHOPOIETIN

TSLP is another important contributor to Th2 immune deviation.70 Chemicals, microbes and allergens are implicated a

stimuli for TSLP upregulation in inflamed tissue (see Fig. 5-5TSLP is expressed by epithelial cells of the skin, gut and lunand primes resident DCs in such a way as to promote Th2 cytokine production by their subsequently engaged effector T cellTSLP-primed DCs also produce high levels of Th2-attractinchemokines including CCL17 (thymus and activationregulatechemokine [TARC]) and CCL22 (macrophage-derived chemokine [MDC]). High levels of TSLP are found in the keratinocyteof patients with atopic dermatitis and in the lungs of asthmatpatients.71The TSLP receptor is a heterodimer composed of unique TSLP-specific receptor and the IL-7Rchain (CD127TSLP receptors are expressed primarily by DCs, but their expresion by mast cells also promotes secretion of Th2 signaturcytokines. TSLP receptor expression on circulating T cells is rar

but probably is involved in amplification of Th2 responses sites of allergic inflammation.

The role of IL-25, IL-33, and TSLP in promoting a Th2associated milieu is summarized in Figure 5-5. In this modeinjured epithelium has a central role in driving allergic inflammation through its ability to produce these cytokines. IL-25 anIL-33 act directly on nave T cells to promote their differentiation into Th2-like cells. By contrast, epithelium-derived TSLacts primarily on DCs to drive them to induce a Th2-likprocess. In addition, both IL-25 and IL-33 act directly on macells to drive their repertoire of Th2-associated cytokines. Moimportant, IL-25 and IL-33 act on ILC2 cells (nuocytes) t

driving Th2 immune deviation in the subsequent adaptiveimmune response. IL-25 stimulates release of IL-4, IL-5, andIL-13 from Th2 lymphocytes but, of note, also drives IL-5 andIL-13 secretion from nonlymphoid cells, specifically ILC2 cells(nuocytes) as discussed later on. Through this ability, IL-25administration in mice induces IgE secretion and eosinophilicinflammation. IL-25 increases expression of CCL5 (RANTES)and CCL11 (eotaxin 1), which further contributes to the

homing of eosinophils to the lungs. Mature Th9 cells alsoexpress IL-17RB and respond to IL-25 by increasing their IL-9expression.

INTERLEUKIN-33

IL-3367is a member of the IL-1 superfamily (in which it is des-ignated IL-1F11) (see Table 5-2) that signals through an IL-1receptorrelated protein (originally termed ST2) and its core-ceptor IL-1RAcP.68As with IL-1 and IL-18, IL-33 is cleaved bycaspases; however, in contrast with IL-1 and IL-18, which areactivated by caspase-1 in the inflammasome, IL-33 is moreactive in its native configuration and is partially inactivatedaftercleavage by caspase-3 and -7. The model for IL-33 action is that

it is released from cells undergoing necrosis in response toinfection or inflammation and then acts as a proinflammatoryTh2-promotingendogenous danger signal.

In addition to Th2 immune deviation, IL-33 is a chemoat-tractant for Th2 cells. IL-33 is primarily expressed by bronchialepithelial cells, with additional sources including fibroblasts andsmooth muscle cells and it is also inducible in lung and dermalfibroblasts, keratinocytes, activated DCs, and macrophages.IL-33 receptors are expressed on T cells (specifically, Th2-likecells), macrophages, hematopoietic stem cells, eosinophils,basophils, mast cells, and fibroblasts. As discussed, IL-33enhances cytokine secretion by Th2 cells and, like IL-25, induces

Figure 5-5 Epithelium-derived cytokines in Th2 differentiation and allergic inflammation. The interleukins IL-25 and IL-33 and thymic stromlymphopoietin (TSLP) are produced by injured epithelium and play critical roles in driving expression of Th2 cytokines. TSLP acts on dendritcells to direct them to promote the differentiation of nave T cells into Th2 cells. By contrast, IL-25 and IL-33 act directly on the nave T cells tpromote Th2 immune deviation. In addition, these three cytokines can generate a Th2 cytokine milieu independent of the adaptive immun

system. TSLP and IL-33 directly induce the full repertoire of Th2 cytokine secretion from mast cells. Similarly, IL-25 and IL-33 act on type 2 innatlymphoid (ILC2) cells (nuocytes) to drive their more restricted secretion of IL-5 and IL-13.

Th2

Mastcell

ILC2

Th0 IL-4

IL-5IL-9IL-13

Epithelialdamage

Allergen

Microbes

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Table 5-4).82 These CD4+ T regulatory cells are subclasaccording to their unique expression profiles and functproperties and include thymus-derived natural regulatocells (nTreg) cells, the TGF-producing Th3 cells of thmucosa, inducible IL-10producing lymphocytes, termedlymphocytes, and other presentations of inducible Treg (icells. Unlike other CD4+T cells, Tregs do not proliferate wresponse to T cell receptor stimulation, and they suppres

proliferation of other effector T cells. Tregs are regarded as etial for preventing autoimmune disease, controlling chinflammatory disease, and modulating allergic inflammati

Natural regulatory T lymphocytes (i.e., nTregs) are gated in the thymus and are characterized by constitutive exsion of IL2-R (CD25), low-level expression of IL(CD127), and expression of intracellular Foxp3. These differentiate in the thymus in response to thymus-preseautologous antigens and mediate tolerance to self antiFoxp3 is regarded as the master regulator of Treg differentiaand its expression is induced by STAT5. Foxp3 associatesother transcription factors including NFAT and NF-Bthese complexes can bind directly to more than 700 tgenes, driving Treg transcriptional machinery, cytokine pro

tion, and surface protein expression. IL-2, acting througconstitutively expressed IL-2 receptors, is regarded as essefor their development and expansion.

These nTregs are distinguished from less well-charactefamilies of nonthymically generated regulatory T lympho(i.e., iTregs) that develop in response to peripherally exprself antigens and externalideally nonpathogenicantiAlthough IL-2 is regarded as essential for developmennTregs in the thymus, a variety of conditions are capabgenerating both Foxp3+ and Foxp3 iTregs from peripCD4+ T cells, and these cells may or may not express CFurther confounding the characterization of iTregs is recotion that activated T effector cells depend on induced exsion of CD25 to drive their proliferation. Accordi

discrimination between nTregs and peripherally induced latory T cells (iTreg) is difficult on the basis of Foxp3, IL-and CD25 alone. Self-reactive T cell receptor repertoireunique to nTregs. however, as is their use of distinct noncoDNA sequences within the Foxp3 locus that serves to suptheir lineage stability.

The proposed mechanisms for Treg-mediated suppreinclude cell contact and cytokine-dependent (IL-10, TFIL-35) mechanisms. Natural Tregs primarily utilize cell condependent mechanisms (e.g., membrane-bound TGwhereas the inducible type iTregs primarily utilize seccytokines, especially IL-10.

Oral administration of antigens, including food-deallergens, produces an iTreg family of lymphocytes chara

ized by variable IL-10 but prominent TGF-expression, wmediate the suppression associated with oral tolerance. gut-associated subset of CD4+T helper lymphocytes is terTh3 cells. In addition to their association with tolerance,cells are relevant to antigen-specific IgA production.

Other forms of iTregs can be generated by numerous pways including low dose antigen delivery in the presenTGF-and an absence of IL-2. Induction of Tregs is promby antigen presentation by immature (IL-10producing as well as by the immunosuppressive (IL-10hi, IL-12low) M2of macrophages. Nave CD4+ T cells exposed to repeantigen stimulation and IL-10 differentiate into a fami

increase their selective production of IL-5 and IL-13. Theseactions on ILC2 and mast cells can occur independent ofongoing allergen exposure, suggesting a mechanism for allergen-independent perpetuation of allergic inflammation.

Other T Helper Families: Nuocytes, Th9, Th22,and T Follicular Helper Cells

ILC2 lymphocytes (nuocytes) represent one of several novel

innate lymphocyte populations. These other family membersare described elsewhere.72ILC2 cells are particularly importantin allergic inflammationthey are prevalent, for example, inthe asthmatic airway.73They are characterized by their expres-sion of high levels of IL-5 and IL-13 (and not IL-4).74,75Similarto NK T lymphocytes, ILC2 cells are non-B/non-T cells, moreclosely related to NK cells, that express IL-17BR and IL-33R andthereby respond to IL-25 and IL-33 with enhanced expressionof IL-5 and IL-13. Depletion of IL-25 and IL-33 therefore resultsin defective Th2-like immune responses to helminth infection,which can be reversed by adoptive transfer of wild-type but notIL-13deficient ILC2 cells.

Th9 lymphocytesare a subfamily of Th2 cells characterizedby prominent production of IL-9, moderate levels of IL-10 and

relatively less IL-4. They result from the differentiation of Th2cells in the presence of TGF-. IL-9 expression in these cells isincreased by the transcription factors PU.1 and IRF4, possiblyacting as heterodimers, indicating that these are likely to serveas the master transcription regulator for this cell type.76,77IL-25further enhances IL-9 production on Th9-polarized cells.78

As predicted from their nomenclature, Th22 lymphocytesarecharacterized by their prominent expression of IL-22 and, inaddition, TNF-. Of note, psoriasis, Crohn disease, interstitiallung diseases, and rheumatoid arthritis all are associated withelevated levels of IL-22 that correlate with disease severity.79Theproduction of IL-17 by Th22 cells infiltrating psoriatic skin isthe basis for the efficacy of IL-17targeting modalities in thisdisorder.80,81Th22 cells are polarized by IL-6, TGF-, and TNF-

, typically being produced by keratinocytes or epithelial cells.As noted, the highly pleiotropic cytokine TGF- is thereforedominantly involved in the differentiation of Treg cells (as dis-cussed further on) but can be switched to induce Th17, Th9, orTh22 cells in the additional presence of IL-6, IL-4, or TNF-,respectively.

Finally, a specialized population of CD4+T cellsthe afore-mentioned T follicular helper (Tfh) cellinteracts with B cellsin the germinal center to support affinity maturation andisotype switching, and to produce memory B and long-livedplasma cells. Tfh cells are defined by their follicular locationand, in addition to expressing IL-21, are characterized by highexpression of CXCR5 and the T cell inhibitory receptor PD-1.As previously discussed, the transcription factor BCL6 is the

master regulator of Tfh cell development (see Fig. 5-4). BCL6increases expression of CXCR5 and PD-1 while decreasingexpression of the master regulators for other T cell helper lin-eages, thus ensuring the Tfh cell fate.

INTERLEUKIN-10 AND REGULATORYT LYMPHOCYTE FAMILIES: TREG, TR1,AND Th3 CELLS

In addition to traditional subclasses of helper lymphocytes,several families of regulatory T lymphocytes (Tregs) have beenidentified, devoted to control of other effector T lymphocyte (see


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cellular inflammation. The absence of inflammation in normsubjects is maintained by influences that promote the development of tolerance. Immune responses to allergens do develoin nonatopic subjects, but these responses generally are of lower order of magnitude than those observed in allergic sub

jects. Thus, nonallergic subjects demonstrate decreased allergeninduced T cell proliferation and lower allergen-specific Igantibody responses compared with their allergic counterparts.

One influence contributing to immune nonresponsiveness diminished accessory cell function. In the healthy lung, in contrast with asthmatic lungs, DCs are immature and characterizeby reduced or absent expression of CD80/CD86, are unable tpresent allergen to T helper lymphocytes, cannot stimulate celular activation and proliferation, and are characterized by aIL-10hi/IL-12lowcytokine profile.87These immature DCs, whepresented with antigen, promote the development of IL-10producing tolerogenic T cell responses (Tr1 cells). The cellulamilieu of the nonasthmatic respiratory tract is characterized bthe presence of M2 macrophages, characterized by elevateconcentrations of IL-10 (and TGF-), which also helps mitigadevelopment of inflammatory responses.

SummaryNumerous cytokines are important in the pathophysiology oallergic disorders (summarized in frontispiece). The IgE isotypswitch results from the activities of IL-4 and IL-13 and is inhibited by IFN-and IL-21. Numerous cytokines, including IL-synergize with IL-4 and IL-13 to enhance IgE secretion. IL-4 responsible for the differentiation of IL-4-producing lymphocytes and other cytokines, including epithelial cellderiveIL-25, IL-33, and TSLP, contribute to this process. IL-12, IL-1and IFN- inhibit the differentiation of IL-4-producing Th2like cells, instead promoting Th1 differentiation, whereas IL-with TGF-, IL-1, and IL-23, serves an analogous role of promoting the Th17 phenotype. IL-5 is the most important eosin

ophilopoietin and, with GM-CSF and IL-3, prolongs the survivof mature eosinophils. These three cytokines are primariresponsible for generating the activated eosinophils that chaacterize allergic inflammatory processes. Mast cell differentiation and proliferation result from the activity of SCF, and othecytokines, including IL-3, IL-6, IL-9, IL-10, IL-11, and nervgrowth factor, contribute to these processes. Many cytokinecontribute to the inflammatory state of allergic inflammatordisorders. Induction of VCAM-1 by TNF-, IL-4, and IL-1promotes the selective recruitment of eosinophils, basophiland lymphocytes. Once in the airways, these leukocytes can bactivated by numerous cytokines and chemokines. Cytokinimportant in promoting fibrosis and airway remodeling aIL-6, IL-11, IL-17, and TGF-.

Treg cells termed Tr1 cells, characterized by their dominantproduction of IL-10 (and lower levels of TGF-). The inductionof iTreg cells, especially these Tr1 cells, is likely to play a keyrole in reducing allergen-specific T cell responsiveness afterimmunotherapy and helping to maintain the state of immunenonresponsiveness in healthy individuals.50,82Local productionof functional Tr1 lymphocytes, identified by nasal biopsy,was enhanced by grass pollen immunotherapy and associated

with clinical improvement. Similarly, allergen-specific Tregswith a suppressive capacity have been identified and expandedfrom cat and pollen immunotherapytreated subjects. Thephenotype of these iTreg cells does not always fit with theclassic descriptions of Treg families as just described. Specifi-cally, IL-10producing T cells from patients receiving immuno-therapy variably express CD25, Foxp3 and other markers.83,84This often-confusing field is discussed in more detail inChapter 4.

Cytokines and Immune Responsesto Allergens

Bronchial biopsy specimens from patients with allergic asthma,

skin test challenge sites from allergic patients, and the nasalmucosa in patients with allergic rhinitis all are characterized byT helper lymphocytes displaying a Th2-like cytokine profile.Although there may be a reduced presence of the Th1 cytokines,allergic inflammatory tissue remains characterized by theprominent presence of IFN-. This IFN- exacerbates allergicinflammation through its ability to activate accessory cell func-tion, stimulate cytokine secretion, induce adhesion moleculeexpression, and activate both eosinophils and neutrophils. Theconcept that IFN- promotes allergic inflammation is sup-ported by evidence that IFN-producing Th1 lymphocytesexacerbate murine asthma.85Th1-like processes are particularlyprevalent in patients with severe asthma, especially those withthe concomitant presence of irreversible obstruction and neu-

trophilic inflammation.The pattern of cytokine response to allergens observed in

nonallergic individuals is even more complex. Nonatopic indi-viduals are exposed to the same concentrations of allergens asthose tracked for their allergic counterparts living in the sameenvironment. Remaining healthy requires active systems thatprevent the development of inflammation. It is frequently statedthat the immune response to allergens in nonallergic subjectsis characterized by Th1-like lymphocyte responses. However,functional Th1 responses, as discussed, stimulate the recruit-ment and activation of mononuclear phagocytes and are associ-ated with cellular immunity and granuloma formationfeaturesnot present in healthy subjects. If present in vivo, these Th1-likecells must therefore be present in a milieu that constrains

REFERENCES

Cytokine Production in Innate Immunity

1. Ware CF. The TNF Superfamily-2008. CytokineGrowth Factor Rev 2008;19:183-6.

2. Perez C, Albert I, DeFay K, et al. A nonsecretablecell surface mutant of tumor necrosis factor(TNF) kills by cell-to-cell contact. Cell 1990;63:251-8.

3. Tartaglia LA, Goeddel DV. Two TNF receptors.Immunol Today 1992;13:151-3.

4. Vinay DS, Kwon BS. TNF superfamily: costimu-lation and clinical applications. Cell Biol Inter

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Cytokines in Allergic Inflammation