Immunoglobulins, immune response

1. Immunoglobulins, immune response Martin Liška

2. 1. The structure of immunoglobulins

4. 2. Isotypes • (in principle) classes of antibodies distinguished on the basis of H chain structure differences • 5 types: m (IgM), d (IgD), g (IgG), a (IgA) and e (IgE) • in addition, we can distinguish subtypes of antibodies within some classes (IgG, IgA) based on their H chain differences

5. 3. Domains and their biological function • in principle: domains of V regions form a recognizing unit and domains of C regions determine secondary biological functions of antibody (i.e. biological half life, distribution in the body, binding complement, binding to cells through Fc-receptor)

6. 4. Variable region of Ig molecule • hypervariable loops are concentrated at the spikes of variable regions where antigen binding sites are localized • the binding site specificity is determined by aminoacid sequences and both by morphology and shape of the loop

7. 5. The biological features of distinct Ig classes IgG • the most abundant serum Ig • the most important Ig of secondary immune response • the only Ig which passes through the placenta • the main opsonizing Ig • activates complement via classical pathway • biological half life 21 day

8. IgA • found both in serum and seromucinous secretions • defense of mucosa • opsonization • does not activate complement

9. IgM • in pentamer form is found in serum; in monomer form is bound on membrane of B cells • prevailing antibody of primary immune response • high-effective agglutinant and cytolytic agent • usually isohaemagglutinins and natural antibodies

10. the best classical way complement activator • does not bind phagocytes Fc receptor, but substantially enhances phagocytosis through complement activation • biological half life 6 days

11. IgD • free form in serum, boundon B cells membrane • antigen receptor on B cells

12. IgE • in normal conditions low amounts in serum • mainly bound on mast cells (binds through FceR) • anti-helminth defense • immediate type allergic reactions

13. Ig subclasses • differences in H chain structure and biological properties • IgG1 a IgG3 participate in defence against viral and bacterial protein antigens • IgG2 ensures defence against antigens which does not require help from T lymphocytes

14. 6. Allotypic and idiotypic variations • allotypes = allelic variants of isotypes • idiotypes = structural determinants localized in variable region having connection with the ability of antigen binding • idiotopes = epitopes in variable region (idiotype is the sum of idiotopes) • anti-idiotypic antibodies = in principle reflect the antigen

15. 7. Genetic basis of Ig production a/ L chains genes K chain – genes located on chromosome 2 - V, J and C segments • chain – encoded in similar complex of genes on chromosome 22

17. b/ genes encoding H chain • more complicated • localized on chromosome 14 • V, D, J, C segments (genes encoding individual segments contain more regions compared with L chains) • during completion of V/D/J exon, gene rearrangement occurs

18. Development of B lymphocytes Lymphoid progenitor → pro-B cells During maturation from pro-B cells into pre-B cells: Ig genes of the heavy chain recombine; pre-B cells express pre-BCR During maturation from pre-B cells into B cells: Ig genes of the light chain recombine Immature B cells express membrane IgM Mature B cells express membrane IgM and IgD = BCR and are able to respond to antigen in peripheral lymphoid tissues

19. Mechanisms contributing to antibody diversity: • chance recombinations • imprecise joining of V, D, J genes • N-region additions • extensive mutations involving variable-region genes after antigen exposure

20. Isotype switching • during the immune response, plasma cells switch from producing IgM to IgG or to another Ig class (IgA, IgE) • the switch involves a change in the H-chain constant domains (CH) • no change in antigen-binding specificity ! (no alteration in the L chain or in the variable portion of H chain)

21. Allelic exclusion • once the process of rearrangement on one of chromosomes is successful, then all attempts on second chromosome are stopped • the same rule governs both for H- and L-chains • every single B cell produces only one type of H- and one type of L-chain

22. Clonal restriction • each B cell expresses identical copies of an antibody that is specific for single epitope • when a B cell divides, the chromosomes in its progeny cells bear the selected allelic genes, and these genes do not undergo any further V/J or V/D/J rearrangements • immunoglobulins produced by given B cell and its progeny are identical in epitope specificity and in k- or l-chain isotype

23. Clonal expansion • proliferation of lymphocytes activated by reaction with an antigen • all lymphocytes of generated clone have the identical antigenic specificity

24. Monoclonal antibodies • immunoglobulins arising from a single clone of B cells, or more precisely cells artificially created by hybridisation of B lymphocytes of specific antigenic specificity (= produced Ig have the same antigenic specificity) with tumor cell (= cells are „immortal“)

25. The utilization of monoclonal antibodies: • Diagnostics (flow cytometry, ELISA, autoantibodies etc.) • Treatment (anti-IgE, anti-TNF-a, anti-CD3)

26. Humoral immune response • The recognition of antigen by specific Ig on the surface of naive B lymphocyte • The binding of antigen cross-links Ig receptors of specific B cells and then activation signals are delivered inside the B cell; the necessary second signal is provided by a breakdown product of the complement protein C3 • Clonal expansion of B cell and secretion of low levels of IgM

27. Humoral immune response • Protein antigens activate antigen-specific T helper cells which stimulate B cell; antigen presentation of these antigens to T helper cells is required • T helper cells exprime CD40L on their surface and secrete cytokines → proliferation and differentiation of antigen-specific B cells, isotype switching • Affinity maturation = affinity of antibodies for protein antigens increases with prolonged or repeated exposure to the antigens (B cells migrate into follicles and form germinal centers → proliferate rapidly and their Ig V genes undergo extensive somatic mutations; at the same time, the antigen complexed with secreted antibody is displayed by FDC → B cells that recognize the antigen with high affinity are selected to survive)

28. Phases of humoral immune responses

29. Primary immune response • First antigen exposure • The amounts of antibody produced is smaller 2 types of antigens: • T-dependent – help from antigen-specific T helper cells is required; protein antigens • T-independent – antibody production is induced directly, without the involvement of T helper cells; typically polysaccharides, lipids

30. Secondary immune response • Subsequent antigen exposure • Higher amount of antibodies is produced • With protein antigens, secondary responses show increased isotype switching and affinity maturation (= production of antibodies with increased affinity to antigen) • Memory cells involvement

32. Affinity and avidity of antibodies • affinity = the strength of the binding between a single binding site of a molecule (e.g.antibody) and a ligand • avidity = expresses the strength of interaction of polyvalent antibody with a polyvalent antigen

33. Ontogenesis of immune response a/ prenatal

34. Hematopoiesis • Mesoblast – from 2nd (3rd) week of gestation • Liver – from 6th (8th) week of gestation, in liver hematopoiesis persists whole prenatal period • Bone marrow – from 10th (12th) week of g., from 20th week the main organ of hematopoiesis

35. T lymphocytes • Precursors from week 7, from week 8-9 lymphocytes move into thyme, where they differentiate • TCR gene segments rearrangement, expression of TCR on the surface of T lymphocytes • Selection

36. B lymphocytes • Precursors from day 8 • Fetal B lymphocytes express IgM on their surface • Synthesis of specific antibodies start at week 20-24, but IgA+M levels are in fact undetectable, IgG production starts after birth

37. Monocytes-macrophages • Macrophages can be detected by week 3-4 • Mature monocytes appear by month 5 in fetal circulation

38. Neutrophils • Mature cells are detectable from week 12-14

39. Postnatal B lymphocytes • relative counts decrease after the birth • respond to immunization presumably by IgM production, switching to other isotypes is slower • slow increase of child’s own IgG connected with decrease of maternal IgG levels (by month 3-6) • IgM reaches levels common i adults at the age of 1-3 yr., IgG+A between the age of 10-15 yr. • Humoral response to polysaccharide antigen arises by the age of 2 yr.

40. T lymphocytes • More than 90% are naive, but their numbers decrease in adult age • Proliferation under mitogen stimulation similar to adults X response to specific antigens only after contact with them • Lower cytotoxic activity of T lymphocytes

41. Innate imunity • Newborns´phagocytes have generally decreased functional ability, activity of NK-cells is decreased • Decreased total complement activity (concentration of its compounds is of 35-70% of adults)

42. c/ Old age • decreased cytotoxicity of NK-cells and macrophages • decreased resistance against viral infections, decreased anti-tumour immunity • switching from Th1 to Th2 • weaker humoral response under new stimuli • increased production of autoantibodies