1. 4th SEMINAR THE ADAPTIVE IMMUNE RESPONSE: ANTIGENS AND ANTIGEN-SPECIFIC RECEPTORS

2. SENSINGRECOGNITION SIGNALING RESPONSE INNATE IMMUNITY CellsReceptors Signaling pathways Cell-Cell collaboration Effector functions DEFENSE SYSTEMS ADAPTIVE IMMUNITY SENSINGRECOGNITION SIGNALING RESPONSE

3. RECOGNITION BY CELLS OF THE ADAPTIVE IMMUNE SYSTEM Antigen-specific receptors: B cell receptor (BCR) and T cell receptor (TCR) The basic structure (90%) of the receptors (BCR or TCR) is common Each cell expresses a receptor that is unique in specificity (the 10% difference means different specificity) These differences in antigen-specificity are achieved during maturation in the central lymphoid organs (bone marrow and thymus)

4. ANTIGEN Any structure that can be recognized by the adaptive immune system (BCR, TCR). Antigenicity: ability of a chemical structure to bind specifically to a TCR or a BCR/antibody According to the results of the specific binding an antigen may be either: Immunogenic: recognition induces an immune response Tolerogenic: recognition induces tolerance (specific immune non- responsiveness)

5. FACTORS INFLUENCING IMMUNOGENICITY Size (the bigger the better) haptens: antigens that can not provoke an immune response because of their small size unless they are attached to a carrier molecule (e.g. a self peptide) Genetics Species (evolutionary the farther the better) Individual (e.g. transplantation antigens) Age (young: immature, old: decreasing number of lymphocytes) Dose Route (vaccination) subcutaneous > intravenous > oral / intranasal Not true for live vaccines (e.g. oral polio vaccine) Adjuvant (vaccination) substances that enhance the immune response to an antigen (aluminum salts, LPS, Freund’s adjuvant, TLR ligands) depot effect – slower biodegradation, prolonged antigen intake by antigen presenting cells activation of innate immunity Physical status corpuscle (cell, colloid) or soluble denatured or native Degradability antigen presentation by APCs

6. ANTIGENIC DETERMINANT (EPITOPE) Part of the antigen that directly interacts with the antigen binding site of the TCR or BCR/antibody.

7. B CELL EPITOPET CELL EPITOPE Recognized by B cells proteins polysaccharides lipids DNA steroids etc. (many artificial molecules) cell- or matrix-associated or soluble Recognized by T cells proteins mainly (8-23 amino acids) requires processing and presentation by APCs

8. ANTIGEN RECOGNITION ≠ CELL ACTIVATION

9. HH LL HH LL Secreted Ig Antigen-specific soluble protein EFFECTOR MOLECULE Membrane-bound Ig Antigen-specific Receptor (BCR) RECOGNIZING MOLECULE  signaling B CELL PLASMA CELL Antigen binding BCR AND ANTIBODY

10. IMMUNOGLOBULINS Definition: Glycoprotein molecules that are present on B cells as part of the BCR or produced by plasma cells as antibodies in response to an immunogenic antigen. Membrane-bound immunoglobulin (mIg) - BCR Secreted immunoglobulin (sIg) – antibody serum antibodies = gamma globulin fraction

11. STRUCTURE 2x Heavy chain (light blue) 2x light chain (dark blue) Variable regions  antigen binding Constant regions hinge region carbohydrate disulfide bond C H1 VLVL CLCL VHVH C H2 C H3

12. Epitope CDR1 CDR2 CDR3 CDR1 CDR2 CDR3 Light chain Heavy chain CDR3 FR1 FR2 FR3 FR4 CDR1 CDR2 variability index 25 75 50 100 aminoacid sequence N – C terminal 150 100 50 0 HYPERVARIABLE REGIONS CDR = Complementarity Determining Region – those amino acids of the variable regions that directly interact with the epitope FR = frame – those amino acids of the variable regions that do not interact directly with the epitope (stabilizer function)

13. DIFFERENT VARIABLE REGIONS  DIFFERENT ANTIGEN-BINDING SITE  DIFFERENT SPECIFICITY

14. (Classes/subclasses) Sequence variability of H/L- chain constant regions Sequence variability of H and L-chain variable regions (individual, clone- specific) Allelic variants isotype idiotype allotype

15. IgG - gamma ( γ ) heavy chains IgM - mu ( μ ) heavy chains IgA - alpha ( α ) heavy chains IgD - delta ( δ ) heavy chains IgE - epsilon ( ε ) heavy chains light chain types kappa ( κ ) lambda ( λ ) HUMAN IMMUNOGLOBULIN CLASSES encoded by different structural gene segments (isotypes)

17. Fab antigen binding valence = 1 specificity determined by V H and V L Fc effector functions IMMUNOGLOBULIN FRAGMENTS STRUCTURE/FUNCTION RELATIONSHIPS papain Fc Fab VHVH VLVL

18. F(ab’) 2 - Bivalent! IMMUNOGLOBULIN FRAGMENTS STRUCTURE/FUNCTION RELATIONSHIPS pepsin Fc peptides F(ab’) 2

19. ANTIBODY FUNCTION Role of the Fab part: Binds the antigen May form crosslinks between antigens (precipitation / agglutination – see later) Neutralization: binding can block the enzyme or toxin or other virulence factors of pathogens and can avoid damage to host cells Role of the Fc part: Activate cells carrying Fc- receptors on their surfaces: Phagocytic cells – opsonized phagocytosis NK cells – antibody-dependent cellular cytotoxicity (ADCC) Activates the complement system via the classical pathway

20. NEUTRALIZATION OPSONIZATION

21. ADCC

22. (A) High-affinity FcRs on the surface of the cell bind monomeric Ig before it binds to antigen. (mast cell) (B) Low-affinity FcRs bind multiple Igs that have already bound to a multivalent antigen. (macrophage, NK cell)

23. COMPLEMENT ACTIVATION

24. PRODUCTION OF IMMUNOGLOBULINS

25. Epithelial cell J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss pIgR and IgA are internalised ‘Stalk’ of the pIgR is degraded to release IgA containing part of the pIgR (the secretory component) J C C S S S S C C S S S S C C ss IgA and pIgR are transported to the apical surface in vesicles B cells located in the submucosa produce dimeric IgA B Polymeric Ig receptors are expressed on the basolateral surface of epithelial cells to capture IgA produced in the mucosa SECRETORY IgA AND TRANSCYTOSIS MUCUSMUCUS J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss J C C S S S S C C S S S S C C ss

26. TCR Alpha and beta chains instead of light and heavy (innate subgroup of T cells express gamma-delta chains as TCR) Both chains are membrane-bound Monovalent interaction with the antigen Antigen recognition requires presentation by antigen presenting cells via MHC molecules Recognize peptide antigens (mainly) No secreted form

27. ANTIGEN PRESENTATION T cells that express CD8 as co-receptor (cytotoxic T cells) recognize peptides presented via MHC class I molecules T cells that express CD4 as co-receptor (helper T cells) recognize peptides presented via MHC class II molecules For activation both cell types require the help of APCs Antigen presentations that lead to T cell activation take place in the secondary lymphoid tissues (e.g. lymph nodes) MHC I is expressed by every nucleated cell  RBCs don’t express them Professional antigen presenting cells express MHC class I and class II molecules: »macrophage (innate) »DC (innate) »B cell (adaptive)

28. MHC RESTRICTION OF T CELL RECOGNITION 1.A given TCR recognizes a defined MHC – peptide complex 2.The same peptide presented by another MHC is not recognized by the same TCR 3.Another peptide bound to the same MHC is not recognized by the same TCR 1.2. 3.

29. α β ε δ ε γ ζ ITAM Immunoreceptor Tyrosine-based Activation Motif ACTIVATION ACTIVATION OF TCR AND BCR

30. SUPERANTIGENS Microbial proteins that bind to and activate all the T cells that express a particular set or family of TCR molecules. The activation is independent from the presented antigen. Leads to polyclonal T cell activation that causes life threatening inflammatory responses.

31. conventional antigen monoclonal/oligoclonal T cell response 1:10 4 - 1:10 5 superantigen polyclonal T cell response 1:4 - 1:10 10 7 – 10 8 / 10 11 10 10 / 10 11 activated T cells SUPERANTIGENS