Blok 8 - VU-Heleen - Immuunologie

8/3/2019 Blok 8 - VU-Heleen - Immuunologie

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Samenvatting hoofdstuk 1 5 uit:

Basic and clinical immunology

Peakman M, Vergani D

1e druk 1997

Chapter 1: Anatomy and cells of the immune system

CELLS OF THE IMMUNE SYSTEM

Bone marrow is source of precursor cells (fetal life; liver too).

Haemopoiesis = process by which all cells that circulate in the blood arise and mature.

Pluripotent haemopoietic stem cell = is capable of giving rise to all blood cell lineages.

White blood cells: granulocytes, monocytes and lymphocytes.

Granulocytes = 65% of white cells

0,5-1% basophils (blue staining)

3-5% eosinophils (red-staining)

90-95% neutrophils (unstained)

Polymorphonuclear cell = multilobed nuclei of granulocytes (= neutrophils/eosinophils).

G. circulate in blood and migrate into the tissues. Mast cells are fixed in the tissues and have an unidentified

precursor.

Monocytes and dendritic cells = 5-10% of white cells

Monocytes stay for +/- 24 hrs in the blood, after which they enter the tissues and stay there as macrophages.

Characteristics: bigger, single nucleus en abundant granular cytoplasm.

Specialised forms: alveolar macrophages (lung), Kupffer cells (liver), mesangial cells (kidney), microglial

cells (brain) and osteoclasts (bone)Dendritic cells: bone marrow derived. Function in activation and priming of lymphocytes. Specialised form =

follicular dendritic cell

Lymphocytes = 25-35% of white cells

Bone marrow derived. Two subtypes: B and T (ratio 1:5). They have highly specialised glycoprotein molecules

on their surface. Location: blood, lymphoid organs and sites ofchronic inflammation.

B lymphocytes: differentiate within the bone marrow (also liver during fetal life). Function = recognition ofantigens through antibodies (= surface receptors). Fixed in the tissues they mature in plasma cells and

secrete antibodies.

T lymphocytes: their involvement with the thymus is early in life and critical for acquiring the ability todistinguish self and non-self (= protection against infection) => graft rejection. They also make a telling

contribution to B lymphocyte function.

Thymus = lymphoid organ

Natural killer cells

Capable of lysing virus-infected cells and tumour cells. A population of cells defined morphologically as large

granular lymphocytes (LGL) also have NK function.

ORGANS OF THE IMMUNE SYSTEM

Lymphocytes recirculate on average in 1-2 days. It is a highly regulated process ofimmunesurveillance,

controlled according to cell type and anatomy.

Primary lymphoid organs = bone marrow and thymus: development and maturation.

Secondary lymphoid organs = lymph nodes and spleen: maturation and development of immunity. MALT =mucosa-associated lymphoid tissue (respiratory tract). GALT = gut-associated lymphoid tissue (gut).

Primary lymphoid organs


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Both microenvironment within the marrow and the influence of soluble mediators that act as colony stimulation

factors determine the celltype in which the pluripotent stem cell develops.

The thymus is an immunological black box. It develops itself in the sixth week of fetal life. By exposure to the

immature cells of thymic epithelial and macrophage-derived, they develop into immature T lymphocytes.

Secondary lymphoid organs

Lymph nodesLymphocytes enter the lymph nodes either through the lymphatics or from the blood:

Afferent lymphatics -> subcapsular marginal sinus -> cortex -> medulla -> efferent lymphatics -> thoracicduct -> venous system.

Blood -> large cuboidal endothelial cells on high endothelial venules (HEV)Primary follicles (in cortex) are at rest.

Secondary follicles arise following stimulation of a local immune response: germinal centre enlarges and B

lymphocytes proliferate and differentiate.

Paracortex = T lymphocytes + interdigitating dendritic cells

Medulla has medullary cords of lymphoid cells (plasma cells)

Lymphos have specific homing receptors for the HEVs -> organisation

GALT = Peyers patches + isolated lymphoid follicles = lack a capsule and afferent lymphatics.

SpleenWhite pulp = lymphoid tissue

Red pulp = reticular tissue and sinuses bathed in blood

Arteries entering the white pulp are surrounded by the periarterial lymphatic sheath (PAL; T zone), which

contains follicles and germinal centres (B zone).

In the red pulp, blood in the splenic venous sinuses interact with the marginal zone (macrophages), next to the

PAL.

Function: lymphoid organ (infectious agents + antigen-antibody complexes -> immune response), filter bed

(removal of effete and defective red and white blood cells -> macrophages eat them) and reclamation site

(resources are recycled -> green pulp)

Chapter 2: Innate immunity 1: physical and humoral protection

FREEDOM FROM THE BURDEN OF DISEASE

Definition of the immune system = identify selfand recognise non-self.

We are born with some immunity and the rest may be acquired during life. Some specific responses may be

retained in an immunological memory.

TYPES OF IMMUNITY

Innate immunity= immunity present at birth = present for life, has no specificity and no memory.

Useful in protection against:

-pyogenic organisms

-fungi

-multicellular parasitesThree components:

physicochemical: barriers like skin, mucosae, secretions and cilia.

humoral = immunologically active factors: complement, opsonins (C-reactive protein) and proteolyticenzymes (-> chapter 2)

cellular: neutrophil, eosinophil, mast and NK cell (-> chapter 3)

Acquired immunity

Absent at birth, has specificity and memory = adaptive (-> chapter 4 & 5)

COMPLEMENT

An antibody is specific forantigens on a target (acquired immunity). A complement complements the action of

the antibody in destruction of organisms.

Complement is a (40-) protein cascade. Components are made in the liver.

Three pathways: alternative, classical and common/membrane attack pathway.


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Complement proteins

Most are soluble. They circulate in an inactive state. After activation they catalyse the conversion of the next

component -> amplification -> cell/bacterial lysis + production ofpro-inflammatory mediators +

solubilisation of antigen-antibody complexes.

Theres a standard nomenclature (Table 2.1, page 12).

Complement activation

Occurs through two distinct pathways: classical and alternative and stimulate the final common pathway. The

first two have a distinct enzyme cascade that culminate in the cleavage of C3 and C5. They are triggered by

different substances and through different initiation mechanisms.

COMPLEMENT PATHWAYS

Alternative pathway = primitive + part of the innate immune system.

Classical pathway = recent + combines with antibodies -> acquired immune system.

The classical pathway

Activated by an immune complex (= antigen + antibody) (and by aggregated immunoglobulins, DNA and C-

reactive protein) which binds with complement C1. C1 = C1qr2s2. This cleaves C4 and C2 to form the classical

pathway C3 convertase C4b2a. Following C3 cleavage, the C5 convertase is formed, which cleaves C5.Biologically active fragments C4a and C3a are generated.

The alternative pathway

Consists of a tick-over mechanism: a continuous, slow reaction sequence, accelerated by activators. C3 'tick-

over generates C3b, C3bB and C3bBb (by factor D) = alternative pathway C3 convertase, which in turn

cleaves C3. The tick-over is accelerated if the active enzymes are stabilised on bacterial cell walls, or if more

C3b is produced from the classical pathway, or by impetus offactor P (properdin). The alternative pathway

C5 convertase C3bBb3b is generated.

The membrane attack pathway

The C5 convertases generate C5b and the pro-inflammatory C5a. C5b67binds the target cell membrane and with

addition ofC8 and a C9 polymer the membrane attack complex (MAC) is formed. Holes are made into the

membrane and death results through osmotic lysis.

Control mechanisms

By lability of the active molecules (C3a, C4a and C5a). Dilution of activated molecules lessens their potential

impact.

Circulating proteins:

-C1-inhibitor: blocks C1

-Factor I: degrades C3b, destroys C4b (enhanced if bound with C4-binding protein = C4bp)

-Factor H: binds with C3b to accelerate factor I.

-Protein S and SP-40,40: bind C5b67 complex -> inactivity.

-Carboxypeptidase N: inactivates C3a, C4a and C5a.

Membrane bound proteins:

-Membrane attack complex inhibitory factor = CD59 = cluster of differentiation 59: avoids

accidental insertion by MACs into a lympho or host cell.

-Decay accelerating factor (DAF): competes for C4b, thus inhibiting formation of the classical

pathway C3 convertase

Complement receptors = CR

CR-1 binds C4b and enhances factor I (like C4bp). There are two main groups of CRs: CR-1 to CR-4 and

receptors for C4a, C3a and C5a.

BIOLOGICAL ACTIVITIES GENERATED BY COMPLEMENT ACTIVATION

Opsonisation

= coating of bacteria or other pathogens and facilitating their removal. C3b has an opsonic activity.

Cell recruitment and activationC4a, C3a and C5a are anaphylatoxins -> role in anaphylaxis in activating mast cells and basophils.

C5a and (lesser) C3a are also chemotactic = the ability to attract cells (neutrophils).


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Cell lysis

Through complete complement activation.

Removal of immune complexes

Immune complexes are potentially harmful because of inciting complement activation. Large complexes can

become insoluble and fixed in the tissues.Complement can maintain immune complexes in solution and can expedite their removal from the circulation.

C3b-coated complexes attach to cells CR-1 (immune adherence) on erythrocytes, who transport them to the

liver and spleen, where they are released and taken up by macrophages.

OTHER FACTORS IN HUMORAL IMMUNITY

C-reactive protein (CRP) is produced in the liver and binds certain bacterial cell wall components and then

activates complement through the classical pathway, independently of antibody. Its blood levels rise within

hours of the start of an infective or inflammatory process -> useful in monitoring this and their response to

treatment.

Fibronectin: binds staphylo- and streptococci + macrophages and monocytes -> clearance. Especially used in

disease monitoring in premature babies.

Lysozyme: bactericidal enzyme secreted in saliva, tears and other body fluids and present in neutrophil granules.

Chapter 3: Innate immunity 2: cellular mechanisms

Neutrophils: kill and remove bacteria and fungi.

Eosinophils: role in controlling infection with multi cellular parasites, such as worms.

Mast cells: are involved in allergy.

Basophils: unknown.

All of them do something by releasing their granules.

Division between innate and acquired immune system is artificial.

NEUTROPHILS

The multilobed nucleus is important to make rapid transit from the blood through tight gaps in the endothelium.

Concentration: 2-7 x 10

9

/litre. The half-life in blood is 6 hrs and in tissues 1-2 days.

Neutrophil granules

Two main types: primary or azurophilic granules (appear during development in the bone marrow) and

secondary or specific granules (appears later; 3x more common).

Releasing granules intra- or extracellularly follows in response to several stimuli:

The products of bacterial cell walls: N-formylated peptides (FMLP) -> bind to neutrophil receptors.

Complement proteins: iC3b -> bind to receptors.

The leukotriene (LTs) group of lipid mediators (from the lipooxygenase pathway of arachidonatemetabolism): LTB4.

Cytokines: NAP-1 (neutrophil activation protein-1; = IL-8), TNF-alfa (tumour necrosis factor-alfa) andGM-CSF (granulocyte-monocyte colony stimulating factor).

Primary granules:

Myeloperoxidase: microbicidal enzyme. Collagenase + elastase: break down fibrous structures in extracellular matrix -> progress through the tissues.

Cathepsin G: cidal to Gram+ and Gram- organisms and some Candida species.

Cathepsin B, D and E: also bactericidal.Secondary granules:

contain pre-synthesised receptors for some of the molecules capable of activating them -> enhancing thedirectional nature of the response -> important in chemotaxis

assortment of proteins with unknown function

Others: Lactoferrin: decreases cell surface charge -> enhancing cell adhesion; it also helps in generating the*OH, a microbicidal toxin.

Neutrophil activation

Stimuli: C5a, LTB4, FMLP and IL-8 -> are released at sites of infection and inflammation.Neutrophils must migrate to the relevant site in the tissues. This requires organisation:


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Tissue damage results in release of mediators with profound effects on vessel walls (endothelial cells lining the

post-capillary venules) -> dilatation with increase leakiness and reduction of blood flow rate.

Neutrophils have a tendency to adhere lightly to the endothelium: rolling.

Substances from damaged tissues or bacteria have a direct effect on endothelial cells: become sticky ->

adhesion of neutrophils.

Rolling and adhesion involves adhesion molecules. Ligand and counter-ligands are up-regulated on both rolling

neutrophils and endothelium -> adhesion.

Adhesion molecules

Three main families of surface proteins: selectin, integrin and intercellular adhesion molecules (ICAMs).

Many are named using the CD nomenclature.

Selectins

Their N-terminal domain resembles various lectins (molecules with affinity for sugar moieties).

P-selectin (on a platelet/ endothelium) -> bind leukocytes

E-selectin (on endothelium) -> bind leukocytes

L-selectin (on a leukocyte) -> bind endothelium

Integrins

Heterodimers with an alpha and beta chain. Two families: beta-1 (T cell function) and beta-2 (adhesion allleukocytes) integrins.

Beta-2 integrins have the same beta chain (CD18) and three different alpha chains (CD11a-c).

CD11a/CD18 = Leukocyte function associated antigen-1 (LFA-1) on lymphos, monocytes and neutrophils.

CD11b/CD18 = Macrophage-1 (Mac-1) or complement receptor-3 (CR-3) on granulocytes and monocytes.

CD11c/CD18 = p150/95 or complement receptor-4 (CR-4) on tissue macrophages.

Intercellular adhesion molecules (ICAMs)

ICAMs are singlechain molecules that form part of the immunoglobulin gene superfamily of molecules. It is

expressed on a variety of cells involved in immune processes, as well as on endothelium. Its a ligand for

integrins and also the receptor for rhinoviruses (cause the common cold).

Neutrophil migrationConstitutive expression = permanent expression. The interaction between selectins and their ligands is a weak

adhesive force, but it provides the first step in neutrophil migration -> slow down and move along the margin of

the endothelium = margination.

Rolling = process of proceeding of the marginating pool along the endothelium.

Now further adhesive mechanisms are needed -> activating signals for neutrophils and endothelium: iC3b,

FMLP, LTB4 and cytokines as IL-8 (= stimuli for releasing granules too).

Neutrophil activation has two effects:

Functional upregulation of adhesion molecules (LFA-1 and Mac-1).

Secondary granules release fresh adhesion molecules onto the neutrophil surface.On the endothelium ICAM-2 and 1 is constitutively expressed and induced by various mediators (interferon-

gamma, IL-1 and TNF).

The induction of the integrin/ICAM system locally in inflamed tissues increases neutrophil-endothelium affinity

by many times -> neutrophils stop and pass through the endothelium = diapedesis. The neutrophil is drawn tothe point of highest concentration of the activating signals.

Chemotaxis

= the directed movement of a cell along a gradient of increasing concentration of the chemoattractant. It is used

by neutrophils to migrate. Chemoattractants: C5a complement component, FMLP, IL-8 and LTB4.

When a chemotactic factor binds at one pole of the cell:

the cell releases granules, to up-regulate receptors

the cell extends its membrane and cytoplasm into pseudopodiaThe leading pseudopodium anchors itself and the remainder of the cytoplasm is drawn up. Etc.

Phagocytosis

Neutrophils can ingest more than one bacterium or fungus at once. An abscess filled with pus (dead or dying

neutrophils) can be formed. A sterile abscess can be formed if the process is inflammatory but not infective(foreign body).


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For phagocytosis opsonins are needed. They coat the targets and enhance the neutrophil in engulfment.

Neutrophils have receptors for opsonins on their surface and are also activated by the opsonins.

Opsonins: C3b, C-reactive protein and antibody (specific immune system).

Phagocytosis: pseudopodia extend to surround the target and meet and fuse to form a phagosome. The

phagosome fuses with neutrophil granules, releasing their digestive and toxic contents. Occasionally, granule

contents may be discharged into the external milieu, leading to tissue damage = reverse phagocytosis.

Neutrophil killing

When the killing of bacteria and fungi is impaired, the individual suffers recurrent, often fatal infections.

Killing is the result of two microbicidal routes:

oxygen-dependent route:- Respiratory burst (use of O2): results in toxic metabolites: O2-, H2O2, 1O2 and *OH. The

main constituents, found in the secondary granules, are a membrane bound complex of three

components: flavin adenine dinucleotide (FAD), quinolone and cytochrome b558.

- Hydrogen peroxide-myeloperoxidase-halide system: results in toxic metabolites: chlorine

(Cl2) and hydroxyl ions (OH-).

oxygen-independent route: microbicidal enzymes such as lysozyme and cathepsins (->from primarygranules).

EOSINOPHILS3-5% of granulos in blood. But there are many more in the tissues at epithelial surfaces for several weeks.

Staining is result of cationic proteins with affinity for eosin. Main role: protection against multicellular parasites

such as worms (helminths) by the release of toxic, cationic proteins. They also contribute to allergic disease

(asthma).

Eosinophil granules

Two main types:

Specific: predominate. Contain four types of cationic proteins: major basic protein (MBP; toxic tohelminths), eosinophil cationic protein (ECP; toxic to helminths and bactericidal), eosinophil neurotoxin

(toxic to helminths) and eosinophil peroxidase (catalyses a similar reaction as myeloperoxidase).

Primary: mostly unknown.

Other: lipid vesicles which can swiftly release LTC4 and LTD4. LTC4 and platelet activating factor (PAF)can produce changes in airway smooth muscle and vasculature -> allergic reactions.

Eosinophil activation

By a variety of mediators. They have specific receptors for C3b/C4b (CR-1), iC3b (CR-3), C5a and LTB4, for

different classes of antibody: IgE (allergy) and IgA (protection of mucosal surfaces) and for cytokines: IL-3, IL-

5 (growth factor for eosinophils) and GM-CSF. After binding they are activated.

Eosinophil adhesion

Similar to neutrophils, but eosinophils alone appear to possess very late antigen-1 (VLA-1 = integrin), which

binds vascular cell adhesion molecule-1 (VCAM-1) on the endothelium.

Eosinophils in host defence

They employ complement and antibody-guided local release of toxic cationic proteins onto the surface ofhelminths (no phagocytosis and intracellular degranulation!).

They overlap with the acquired immune response (interaction) and synthesise and express CD4 and HLA-DR

(associated with antigen-specific cell-mediated responses) -> this cell is more sophisticated than the neutrophil

(together with longer life, cytokines secretion and cell activation state).

MAST CELLS AND BASOPHILS

Two main features: they have histamine-containing granules and they have high-affinity receptors for IgE (i.c.w.

eosinophils).

Mast cell and basophil granules

Two categories:

pre-formed mediators: histamine: blood half-life of


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- increased vascular permeability -> leakage of plasma fluid -> swelling (wheal).

- acting directly on local axons with inducement of more widespread vascular changes (flare).

synthesised de novoEffect differs according to the stimulus and site, from a localised wheal and flare to anaphylactic shock.

Mast cell and basophil activation

By antibody class IgE (allergic disease), anaphylatoxins C3a, C4a and C5a (basophils and lung mast cells) andFMLP (basophils).

Chapter 4: Acquired immunity: antigen receptors

During first encounter between host and microbe, the immune system identifies and learns the distinctive

structural features of it -> specific recognition. Effector responses can be initiated and a memory bank can be

formed, so that next time the reaction is more quickly and with a greater initial force.

=> specificity, memory and variable intensity.

The innate and acquired immune responses can complement each other (antibodies).

ANTIGENS AND ANTIBODIES

Antigens

= structures which generate an anti- response by the immune system. Three elements that are used in binding

and recognition of antigens:

Antibodies: generated by B-lymphos and plasma cells. Have recognition sites for part of an intact antigen.The interaction is shape and conformation dependent. Can bind soluble antigens. Epitope = part of the

antigen with which the antibody interacts = antigenicdeterminant (determines which antibody will bind).

T cell receptors: differences: interacts with a short segment of amino acids derived from the intact antigenby proteolysis = peptide antigen = T cell epitope. Cannot bind soluble antigens directly -> they must be

held and presented by MHCs.

Major histocompatibility complex (MHC) molecules: hold the peptide antigen enclosed within a groove-> is recognised by the T cell receptor.

Antigens interact with these three through non-covalent forces (reversible). Theres a dynamic equilibrium

between the dissociated and complex form. Affinity = the concentration (= dissociation constant (Kd)) ofantigens allowing half the antibodies to be complexed and half to remain in solution. The smaller the number the

higher the affinity. Order: antibody > MHC molecules > T cell receptor. Antibodies have at least 2 sites for

binding and also a flexible hinge -> increases the overall strength of attachment = avidity.

Antibodies

= soluble glycoproteins that belong to immunoglobulins.

Functions and uses:

1. In host defence:

targeting infective organisms

recruitment of damaging host effector mechanisms

neutralisation of toxins

removal of foreign antigens from circulation2. In clinical medicine:

diagnosis/monitoring a disease

administration of pooled antibodies passively for host therapy/protection3. In laboratory science: diagnostic and research applications.

Activation of the following immune effector functions:

complement activation

stimulation of phagocytosis and killing by polymorphonuclear cells.

recruitment of killer cells

activation of mast cells

The many roles and uses of antibodies

Antibodies act as a guidance system, focusing effectors onto appropriate targets. So antibodies provide target

specificity for the innate immune system.Characteristic features of antibodies:

1. Two modes of expression:


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- synthesised and displayed on B lymphos as receptors.

- exported as soluble proteins by plasma cells.

2. Each antibody monomer has two identical antigen-binding sites.

3. The gene that codes for antibodies enables antibody structure to vary enormously -> 1011 different antigen-

binding sites.

4. Apart from the antigen-binding site, other parts can be modified to give the antibody molecule different

effector functions.

IMMUNOGLOBULINS

Structure

Four polypeptide chains: 2 identical heavy (H) and 2 identical light (L) chains. Formula: H2L2.

Light chains: 2 or 2 chains.

Heavy chains: 2 , 2 , 2 , 2 or 2 chains.Each H or L chain has a stable segment = C region -> hold effector functions; and also a variable segment = V

region -> include the antigen-binding site.

The 2 H chains are linked together by 2 interchain disulphide bonds. Each H chain has a light chain attached to it

by an interchain disulphide bond. Within the H and L chains, there are intrachain disulphide bonds, creating

domains, which contain two layers of -pleated sheet with 3 or 4 strands of antiparallel polypeptide chain.Many molecules in the immune system share a similar domain structure = immunoglobulin supergene family.

The domains are named according to which chain they are in, and numbered: VL, CL domain and VH, CH1-4. VLand VH domains = antigen-binding site. CH domains = major effector functions.

Hinge region = in the middle of the molecule for maximising the chances of binding 2 antigens at one time.

Cleaving the Ig molecule with papain produces 2 Fab fragments (fragment antigen binding) with antigen-

binding ability and 1 Fc fragment (fragment cystallisable) with effector functions. Fc receptors = surface

receptors for Ig molecules, that interact with the Fc fragment.

Cleaving the Ig molecule with pepsin produces a F(ab)2 fragment with 2 antigen-binding sites, but no effector

functions remaining.

The effector function part of the antibody can be varied by variation in the genes, producing the 5 different H

chains -> giving rise to the majorclasses of Igs (Ig = whole immunoglobulin molecule): IgA, IgD, IgE, IgG and

IgM and to the subclasses: IgG1-4 and IgA1-2.

Immunoglobulin classes and subclassesImmunoglobulin G (IgG)

Most abundant (10g/l). Has 3 CH domains; monomer; 4 subclasses with variability in the hinge region and

functional domain. The subclasses vary in their relative ability to perform some of the effector functions (IgG2

for response to bacterial polysaccharide) and in their serum concentration (Table 4.2; page 39). IgG is a major

activator of the classical complement pathway (IgG1 and IgG3) and binding to C1q is greatly increased when

IgG is complexed to antigen.

Three cellular receptors: Fc R1, 2 and 3 -> used to bind, recruit and activate cells; also important for placental

transfer of IgG (specific protection on the newborn)

Immunoglobulin A (IgA)

Next most abundant (1-4g/l). Distinctive in 2 ways:

1. Can occur as a monomer, but also as a dimer (2 IgA molecules joined by an J chain).2. It is the major Ig secreted onto the external surfaces -> presence in secretions (saliva, bronchial fluid, gut

secretions, tears, etc.) ofsecretory IgA. Achieved by attachment to poly-IgR (synthesised by epithelial cells

lining mucosal surfaces) -> endocytosis, transport and secretion (of IgA and the secretory chain (= remnant

of poly-IgR)). IgAs plays an important role in protection against bacterial, viral and protozoal infections of

mucosae -> can activate the alternative pathway and is an opsonin, reacting with Fc R.

Immunoglobulin M (IgM)

Pentamer: 5 IgM monomers joined by the J chain. It is the first immunoglobulin synthesised in an antibody

response (primary response) -> it has multiple functional domains (complement activator) and 10 antigen-

binding sites. Theres not a high affinity for antigens, but the many sites compensate this.

Immunoglobulin D (IgD)

Serum concentrations are extremely low -> function unclear: surface expression on B cell lymphos at animmature stage and the signalling on interaction with antigen in a lymphoid follicle is a critical part of B lympho

activation.


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Immunoglobulin E (IgE)

Largest: 4 CH domains. Serum concentration rises in response to parasitic infections, atopic individuals and type

1 hypersensitivity patients (allergy). Function: bind and activate mast cells. Mast cells have high affinity

receptors: Fc R1. Antigen + IgE + Fc R1 on mast cells causes vascular effects. B lymphos and eosinophils

have low-affinity receptors: Fc R2 -> for regulation of IgE production.

IMMUNOGLOBULIN EXPRESSION

B cell surface expression

Binding of antigen to surface IgD is an early event in the B lympho cell cycle. In the mature B lympho antigen

interaction with other Ig classes leads to internalisation -> antigen is broken down and presented to T lymphos

-> T lymphos are activated -> immune response.

Isotypes, allotypes and idiotypes

Immunoglobulins can be characterised by their own ability to act as antigens: isotypes are epitopes that are

present on all molecules of a class (e.g. IgE) or chain (e.g. CH) type in a species; allotypes vary betweenindividuals, idiotypes reflect variation in the antigen-binding sites of the immunoglobulin. The antigenic epitope

within the antigen-binding site = idiotope. A collection of idiotopes = idiotype.

Primary and secondary antibody responses

Primary antibody responses are slow, mainly IgM and decline to low levels (IgG appears towards the end of the

primary response). Repeated exposure to the antigen at a later time elicits a more rapid response to a higher peak

level that declines to a higher baseline level; this secondary response is predominantly IgG. The secondary

response is antigen-specific and demonstrates acquisition of memory. Theres also a higherintensity. IgG in

the secondary response has a higher affinity for antigen: positive selection.

Haptens

= molecules that are too small to generate an antibody response. But coupled to a carrier, haptens may elicit

production of antibodies that are able to bind the hapten directly, without the carrier.

Clonality in antibody responses

Each plasma cell produces a single antibody = monoclonal antibody; tumours formed from a single plasma cell

will produce large quantities of this monoclonal antibody. A population of plasma cells will respond to different

epitopes on macromolecular antigens giving rise to many clones of plasma cells and many antibody types: a

polyclonal response. Some areas of antigens are targeted more frequently than others = dominant epitopes.

IMMUNOGLOBULIN GENES

The genetic system for antibodies have to generate a diversity for antigen binding and a limited choice of

functional characteristics.

Generation of antibody diversity: a limited number of genes are available, which combine randomly.

Organisation: each chain is encoded by a gene complex. Within the complex are segments: Vgenes (variableregion), Cgenes (constant region),Jgenes (joining these two) andD genes (generating diversity).

Light chain genesTheres noD segment for light chains. Number of combinations = Vgenes xJgenes x Cgenes. chain: 1000

different combinations. chain: 150.

Heavy chain genes

Two differences:

1. It hasD genes.2. The different Cgenes, encoding the H chain isotypes, are located together, downstream from the other

genes.

There are 250-1000 VH genes, 12DH and 4JH gene segments (= 48 000). When combined with L chains: more.Forming different antibody specificities through combining different genes randomly = combinatorial

diversity.

Generation of immunoglobulin diversityTwo other additions to diversity:


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-Junctional diversity = the random imprecision in the joining of the segments caused by loss or retention of

nucleotides or codons, which results in frame-shifts.

-Somatic hypermutation = the occurrence of a single mutational change after the initial gene rearrangements,

probably during class switching (from primary to secondary response).

Class switching

In class switching in B cells, the early expression of IgM gives way to the mature, activated expression of otherclasses of Igs -> achieves modulation of the functional capabilities determined by the antibody produced.

Affinity maturation

B cells with the highest affinity (by somatic hypermutation during class switching) will be stimulated to

proliferate further = affinity maturation.

Genetic basis for immunoglobulin gene rearrangement and class switching

Theres an order in choosing the different genes. So that some genes are more often chosen than others.

T CELL RECEPTORS FOR ANTIGEN

T cell receptors = TCRs: have a basic molecule composed of 2 chains: these may be / (90%) or / . Thechains of each type of TCR are divided into variable and constant domains. The T cell expresses only one form

of TCR. The / TCR appears first on primitive T lymphos in the thymus = TCR1. / TCR = TCR2.

Structure and function of the T cell receptor

A disulphide-bonded heterodimer of an and a chain. Theyre also part of the immunoglobulin supergenefamily. They have hypervariable regions of the variable domains, forming CDRs 1-3. CDRs 1 and 2 interact with

the 1 and 2 domains of the MHC molecule, whilst CDR3 interacts with the peptide within the groove.

Structure and function of the T cell receptor

Three types: a disulphide-linked heterodimer of and , a non-disulphide-linked heterodimer and a

disulphide-linked homodimer

The T cell receptor genes

Theres a similar depth of diversity as that for antibodies. The genes are divided into two separate groups: for thevariable an for the constant domain.

Genes for the and chains

Diversity for the chain = VxJx C= 10 000. For the chain = VxD xJx C= 4800 => diversity for an

chain = 48 million.

The C and C genes do not appear to encode segments typical of secreted proteins -> secretion is functionalnot important.

There is no somatic hypermutation.

Genes for the and chains

Diversity for chain = 32 and for chain = 60 => total diversity = 1920.

Generation of T cell receptor diversity

TCRs vary in structure through the use of a large pool of gene segments for the receptor chains, random

recombination of variable, diversity and joining genes and highly random imprecision in the joining process.

Chapter 5: The human leukocyte antigens

Collection of genes in general = major histocompatibility complex (MHC): contains a group of genes that code

for proteins expressed on the surface of a variety of cell types. In humans = human leukocyte antigens = HLA

system ( human MHC).

HLA molecules are involved in antigen recognition by T lymphos by presenting antigen as a short peptide

embedded within a physical groove.

Differences in HLA molecules between individuals are responsible for organ graft rejection. Possession of

certain HLA genes was linked to greater susceptibility to particular diseases (MS, DM1 and ankylosingspondylitis).

The new HLA nomenclature is based on genotyping. But the old is also used.


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THE MAJOR HISTOCOMPATIBILITY COMPLEX

The immune response

Law of MHC restriction: a cytotoxic T cell will only kill target cells that present both a specific antigen and the

correct MHC molecule.

Structure of the HLAHLA comprises 3 major classes (I, II and III ) of genes. Genes: CAPITAL ITALIC LETTERS; antigen molecules

produced by them: CAPITAL ROMAN LETTERS; polypeptide chains: Greek letters and .

Class I region

Class I HLA genes:HLA-A toHLA-J, withHLA-A,HLA-B andHLA-Cas best known. They encode the amino

acid sequence of the class I chains. The class I chain is an invariant molecule = 2-microglobulin.Function: presenting peptide antigens to T lymphos.

Class II region

The class II genes have 3 major subregions: DP, DQ and DR -> for presentation of peptide antigens to T

lymphos. HLA-DM plays a critical role in loading peptide into the other class II HLA molecules.

There are different haplotypes for an individual, governing the number of DR molecule types produced.

Gene polymorphism = a large number of different alleles for DRB genes -> there is enormous potential forindividuals to differ in the HLA genes they possess.

Molecules encoded by the MHC act further as:

- peptidases to cleave large protein antigens

- transporters, carrying antigenic peptides to the correct intracellular compartment.

Class III region

Contains several genes coding for complement components.

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Blok 8 - VU-Heleen - Immuunologie