Normal Immune Responses, Hypersensitivities, and

Allograft Rejection

Normal immune response• Protection against infectious disease

Innate immunity• epithelial barriers • phagocytic neutrophils and macrophages• natural killer (NK) cells• circulating plasma proteins (complement, clotting)

Adaptive immunity— “the immune response”• cellular

– B-cells and plasma cells– helper (Th) and cytotoxic (CTL) T-cells

• humoral– antibodies– cytokines

Lymphocytes and Receptors• Tcells—TCRs recognize MHC-Ag complexes

Class I MHC universally expressed• HLA-A, HLA-B, HLA-C single 2 microglobulin dimer • Ag is peptide processed in cytoplasm by proteasome from

intracellular microbe (B,V) or tumor-associated protein• TCR dimer complexed with , CD3 and CD8, CD28

Class II MHC restricted to DC, M, B-cells• HLA-DP, HLA-DQ, HLA-DR – dimer• Ag is lysozome-processed peptide from extracellular microbe

(B,E) protein• TCR dimer complexed with , CD3 and CD4, CD28

• Bcells—IgM complexes with Ig, Ig and CD40 or CD21 coreceptor

• NK cells—class I MHC inhibitory receptors with CD16 (IgG Fc receptor) and NKG2D coreceptors

Antigen-Presenting Cells• Dendritic Cells

Dendritic Cells (DCs)• located in T-cell zones of lymphoid tissues to present antigens to

circulating T cells• express high levels of class II MHC and T-cell costimulatory

molecules• resident in tissues, e.g. Langerhans cell of the epidermis and in the

interstitium of many nonlymphoid organs, e.g. heart and lungs Follicular Dendritic Cells (FDCs)

• located in the germinal centers of lymphoid follicles in the spleen and lymph nodes to display antigens to activated B cells

• express IgG Fc and complement receptors to efficiently trap antigen bound to antibodies and complement

• Macrophages phagocytosed microbes and protein antigens presented as

peptide fragments to T cells• B cells

present peptides to Th cells and receive signals that stimulate antibody responses to protein antigens

Pattern recognition receptors• DCs and macrophages respond to pathogen-associated

and damage-associated molecular motifs• Toll-Like Receptors (10 in human)

membrane receptors on outer plasma membrane and vessicle membranes

TLR2 for Gram positives, TLR3 for fungi, TLR4 for Gram negatives, TLR9 for viral and bacterial DNA (CpG)

• NLRs (NOD-like receptors) (at least 20) Cytosolic proteins bind a variety of microbial products NLRC-type have caspase activation domain NLRP-type have pyrin domain

• Activate caspases and NF-kB• Affect gene expression via NF-kB and MAP kinase

cascades• Release IL-1 and other proinflammatory cytokines

DCs in culture and epithelium

Tissues of the Immune System

• Generative Lymphoid Organs thymus, where T cells develop bone marrow, where all blood cells are produced and

where B cells mature

• Peripheral Lymphoid Organs lymph nodes, where lymph-borne antigens are

trapped by FDCs and where DCs concentrate spleen, where blood-borne antigens are trapped by

FDCs and where DCs macrophages concentrate T lymphocytes and B lymphocytes are segregated

into different regions

B cell activation and effects

Th activation and effects

MHC is HLAHLA and Disease Association

DiseaseHLA Allele

Risk (%)

Ankylosing spondylitis B27 90–100

Postgonococcal arthritis B27 14

Acute anterior uveitis B27 14

Rheumatoid arthritis DR4 4

Chronic active hepatitis DR3 13

Primary Sjogren syndrome

DR3 9

Type 1 diabetes DR3 5

DR4 6

DR3/DR4 20

Hypersensitivity Reactions

• Exposure to antigen results in sensitivity• Repeat exposure may result in pathologic

hypersensitivity• Both exogenous and endogenous

antigens may elicit hypersensitivity • Hypersensitivity is an imbalance between

effector and control mechanisms of immune responses

• Development of hypersensitivity is often associated with the inheritance of particular susceptibility gene

Types of hypersensitivity reactions

• Type I — immediate immunologic reaction occurs within minutes of antigen

binding to antibody bound to mast cells in individuals with prior sensitization

• Type II — Ab reaction to bound Ag caused by antibodies that react with antigens present

on cell surfaces or in the extracellular matrix

• Type III — Ab complex with circulating Ag antigen-antibody complexes deposited on vessel

walls cause inflammation and tissue damage

• Type IV — delayed-type initiated by antigen-activated (sensitized) T cells

Immediate hypersensitivity

• Presentation of antigen to naive Th cells• Naive cells differentiate into Th2 cells• Th2 cells produce cytokines upon

subsequent encounter with the antigen IL-4 stimulates B cell class switching to IgE

and promotes additional Th2 cell development IL-5 promotes development and activation of

eosinophils IL-13 enhances IgE production and stimulates

mucus secretion by epithelial cells

• Mast cells and basophils bind IgE

IgE cross-linking activates mast cells

• Hypersensitivity mediated by IgE-dependent activation of mast cells upon re-expsure to antigen

• mast cells are bone marrow–derived• abundant near blood vessels, nerves and subepithelial

tissues• cytoplasmic membrane-bound granules contain active

mediators and acidic proteoglycans that bind basic dyes • activated by cross-linking of high-affinity IgE Fc receptors• triggered by complement C5a and C3a

• Basophils also have cell surface IgE Fc receptors and cytoplasmic granules

• circulate in the blood in extremely small numbers but can be recruited to inflammatory sites

Mast cell degranulation

• Preformed mediators released from granules Vasoactive amines

• histamine causes intense smooth muscle contraction, increased vascular permeability, and increased mucus secretion by nasal, bronchial, and gastric glands

Enzymes• neutral proteases (chymase, tryptase) and several

acid hydrolases Proteoglycans

• Heparin (anticoagulant) and chondroitin sulfate package and store the amines in the granules

Mast cell lipid mediators• Synthesized after activation of PLA2 releases

AA from plasma membrane Leukotrienes

• LTC4 and LTD4 - several thousand times more active than histamine in increasing vascular permeability and causing bronchial smooth muscle contraction

• LTB4 is highly chemotactic for neutrophils, eosinophils, and monocytes

Prostaglandin D2• causes intense bronchospasm as well as increased mucus

secretion Platelet-activating factor (PAF)

• causes platelet aggregation, release of histamine, bronchospasm, increased vascular permeability, vasodilation and is chemotactic for neutrophils and eosinophils

• PLA2 dependent, not AA product

Mast cell cytokines

• TNF, IL-1, and chemokines (eotaxin, CXCL8) attract neutrophils, eosinophils, basophils,

monocytes

• IL-4 amplifies the Th2 response

• IL-3, IL-5, and GM-CSF support survival of eosinophils

• Cell recruitment and survival supports the late-phase response

Systemic Anaphylaxis• Subsequent exposure to minute amounts of antigens in

previously sensitized individuals Hospital acquired

• administration of foreign proteins (e.g. antisera), hormones, enzymes, polysaccharides, or drugs (e.g. penicillin)

Community acquired• food allergens (e.g. peanuts, shellfish) or insect toxins (e.g. bee

venom)

• Within minutes after exposure: itching, hives, skin erythema, contraction of respiratory

bronchioles and respiratory distress

• Followed shortly by: vomiting, abdominal cramps, diarrhea, and laryngeal obstruction

due to edema

• Within an hour circulatory shock due to massive edema and hypovolemia

Atopy

• Chronic, localized, anaphylactic-like responses to antigens Asthma Eczema (dermatitis) Allergic rhinitis Urticaria (hives and wheals)

• Mediated by mast cells and eosinophils with similar symptoms bronchoconstriction, inflammation, itching,

edema

Summary of Type I hypersensitivityAction Mediators

Vasodilation, increased vascular permeability

Histamine

PAF

Leukotrienes C4, D4, E4

Neutral proteases that activate complement and kinins

Prostaglandin D2

Smooth muscle spasm Leukotrienes C4, D4, E4

Histamine

Prostaglandins

PAF

Cellular infiltration Cytokines (e.g., chemokines, TNF)

Leukotriene B4

Eosinophil and neutrophil chemotactic factors

Type II hypersensitivity

• Opsonization and Phagocytosis• Mechanism involves IgG or IgM

Cells opsonized by IgG antibodies are recognized by phagocyte Fc receptors

Opsonization activates the complement system by the classical pathway

• Complement activation forms the membrane attack complex Osmotic lysis of cells

• Antibody-dependent cellular cytotoxicity (ADCC) cell lysis proceeds without phagocytosis NK cells

Ab-mediated cellular injury

Destruction of blood cells• Transfusion reactions

cells from incompatible donor are opsonized by preformed antibody in the host

• Erythroblastosis fetalis hemolytic disease of the newborn mother-fetus antigenic difference (e.g. Rh factor) maternal IgG crosses the placenta to cause

destruction of fetal red cells

• Autoimmune hemolytic anemia, agranulocytosis, and thrombocytopenia individuals produce antibodies to their own blood cells

• Drug reactions drug binds to RBC, acts as hapten for Ig activation

Disease Target Antigen Mechanisms of DiseaseClinicopathologic Manifestations

Autoimmune hemolytic anemia

Red cell membrane proteins (Rh blood group antigens, I antigen)

Opsonization and phagocytosis of red cells

Hemolysis, anemia

Autoimmune thrombocytopenic purpura

Platelet membrane proteins (Gpllb: Illa integrin)

Opsonization and phagocytosis of platelets

Bleeding

Pemphigus vulgaris

Proteins in intercellular junctions of epidermal cells (epidermal cadherin)

Antibody-mediated activation of proteases, disruption of intercellular adhesions

Skin vesicles (bullae)

Goodpasture syndrome

Noncollagenous protein in basement membranes of kidney glomeruli and lung alveoli

Complement- and Fc receptor–mediated inflammation

Nephritis, lung hemorrhage

Acute rheumatic fever

Streptococcal cell wall antigen; antibody cross-reacts with myocardial antigen

Inflammation, macrophage activation Myocarditis, arthritis

Myasthenia gravis Acetylcholine receptor Antibody inhibits acetylcholine binding, down-modulates receptors

Muscle weakness, paralysis

Graves disease (hyperthyroidism)

TSH receptor Antibody-mediated stimulation of TSH receptors

Hyperthyroidism

Insulin-resistant diabetes

Insulin receptor Antibody inhibits binding of insulin Hyperglycemia, ketoacidosis

Pernicious anemia Intrinsic factor of gastric parietal cells

Neutralization of intrinsic factor, decreased absorption of vitamin B12

Abnormal erythropoiesis, anemia

Type III Hypersensitivity• Formation of immune complexes

abundant, novel protein antigen triggers immune response resulting in antibodies

secreted antibodies react with the antigen still present in the circulation forming antigen-antibody complexes

• Deposition of immune complexes organs where blood is filtered at high pressure to form

other fluids, like urine and synovial fluid, are most affected

• Tissue injury acute inflammatory reaction with complement-fixing

antibodies (i.e., IgG and IgM) leukocyte Fc receptor-antibody complexes induce pathologic lesions

Immune-complex hypersensitivity

Immune complex injury

• Principal morphologic manifestations acute necrotizing vasculitis necrosis of the vessel wall immune complexes, complement, and plasma protein

produce eosinophilic, fibrinoid necrosis

• Chronic serum sickness repeated exposures to antigen SLE persistent antibody responses to autoantigens

• Local Immune Complex Disease Arthus reaction localized area of tissue necrosis resulting from acute

immune complex vasculitis in the skin

Immune complex deposition in SLE

Immune complex deposition in SLE

Type IV hypersensitivity

• Initiated by antigen-activated (sensitized) T cells

• Delayed-type hypersensitivity (DTH) CD4+ Th1 cell cytokines recruit macrophages induced by environmental and self-antigens

• Direct cell cytotoxicity CD8+ CTLs cause tissue damage frequently follow viral infections

• Many autoimmune diseases are type IV hypersensitivities

Tuberculin reaction

• Classic example of DTH intracutaneous injection of purified a protein-

containing antigen of the tubercle bacillus (tuberculin)

reddening and induration of the site appear in 8 to 12 hours, reach a peak in 24 to 72 hours

characterized by the accumulation of CD4+ T cells and macrophages around venules, producing perivascular “cuffing”

Within 48 to 72 hours, a positive TB skin test is marked by an area of reddish induration greater than 10 mm. It is the induration (firm bump) that is gently palpated that determines the size, not the area of redness. This reaction is slightly larger than the average positive test 17 mm in size. The positive reaction shown here was obtained with a TB skin test performed 20 years after the initial infection.

Tb skin test

Mechanism of DTH

DiseaseSpecificity of Pathogenic T Cells

Clinicopathologic Manifestations

Type 1 diabetes mellitus

Antigens of pancreatic islet β cells (insulin, glutamic acid decarboxylase, others)

Insulitis (chronic inflammation in islets), destruction of β cells; diabetes

Multiple sclerosis Protein antigens in CNS myelin (myelin basic protein, proteolipid protein)

Demyelination in CNS with perivascular inflammation; paralysis, ocular lesions

Rheumatoid arthritis

Unknown antigen in joint synovium (type II collagen?); role of antibodies?

Chronic arthritis with inflammation, destruction of articular cartilage and bone

Crohn disease Unknown antigen; role for commensal bacteria

Chronic intestinal inflammation, obstruction

Peripheral neuropathy; Guillain-Barr syndrome

Protein antigens of peripheral nerve myelin

Neuritis, paralysis

Contact sensitivity (dermatitis)

Various environmental antigens (e.g., poison ivy)

Skin inflammation with blisters

Type 1 DMAn islet of Langerhans demonstrates insulitis with lymphocytic infiltrates in a patient developing type I diabetes mellitus. This lesion precedes clinical onset of diabetes mellitus and is rarely observed.

Mechanism of CD4+ T cell DTH• Naive CD4+ T cells recognize peptides displayed by

dendritic cells• Differentiation of antigen-stimulated T cells to Th1 or

Th17 cells is driven by the cytokines produced by APCs IL-12 induces differentiation of CD4+ T cells to the Th1 subset

which makes IFN-γ IL-1, IL-6 and IL-23 with TGF-β stimulate differentiation of T cells

to the Th17 subset

• Previously activated T cells respond to subsequent exposure Th1 cells secrete cytokines, mainly IFN-γ IFN-γ–activated macrophages express more class II MHC,

secrete TNF, IL-1, and chemokines, and produce more IL-12 Activated Th17 cells secrete IL-17, IL-22, chemokines, and other

cytokines that recruit neutrophils and monocytes to the reaction

• Macrophages promote fibrosis and damage

Mechanism of CD8+ cytotoxicity

• Naïve CD8+ recognize Ag in the context of MHC-class I on cell surfaces

• Differentiated CTLs contain perforin-granzyme protease complexes that induce apoptosis in target cells

• This mechanism plays a role in T1DM and transplant rejection