MICR 304 Immunology & Serology

1. MICR 304 Immunology & Serology Lecture 9 TCR, MHC molecules Chapter 3.10 – 3.19, 4.9- 4.11, 5.1 – 5.19

2. Overview of Today’s Lecture • Generation of T cell receptor (TCR) • MHC molecules • Antigen presentation via MHC molecules

3. Key Players in Immunology

4. 2 chains Connected by disulfide bond Variable region Constant region Short cytoplasmic tail Mostly a and b chain Some specialized T-cells have g and d chain (gd T cells) T Cell Receptor

5. T-Cell Receptor Belongs to the Immunoglobulin Super Family

6. Gene Segments Coding for the T-Cell Receptor • Variable region of a-chain is composed of V and J gene segments • Variable region of b-chain is composed of V, D and J gene segments

7. Generation of the TCR by Gene Rearrangement and Recombination No secondary modification of TCRs

8. Diversity of the Lymphocyte Antigen Receptors P- and N- nucleotides: nucleotides added during initial gene rearrangement and recombination

9. Unique Properties of TCR Compared to BCR • Only one antigen binding site • Never secreted • Recognize processed antigen presented through specialized molecules

10. TH CTL TCR Recognizes Antigen Presented by MHC Molecules • MHC: major histocompatibility complex • First identified in transplantation immunology • T cells recognize antigen bound to an MHC molecule • Two types of MHC molecules • MHC I: presents endogenouspeptides • Virus encoded • Produced by intracellularly replicating microorganisms • Tumor antigens • MHC II: presents exogenous peptides • Uptake through phagocytosis and degradation in phagolysosome MHC I MHC II

11. CD3: signal transduction CD4: Interaction with MHC II CD8: Interaction with MHC I Antigen Recognition through TCR Requires Additional Molecules TH CTL Cytotoxic granules Cytokines Ag presenting cell Any nucleated cell

12. Contrast TH Cells and CTL

13. gd T cells are Distinct • Minority of T cell population • Bind heat shock proteins and nonpeptide ligands • Mycobacterial lipid antigen • Phosphorylated ligands • Not restricted by classical MHC I or MHC II molecules • May bind to free antigen

14. Expression of MHC Molecules Differs Between Tissues • MHC I positive: any nucleated Cell • MHC II positive: only antigen presenting cells (APC) • IFNg can MHC II in other cells • APC: take up antigen, degrade it, load it onto MHC II and present it at their cell surface • Human activated T cells • Microglia in brain

15. Exogenous peptides are bound to MHC II along the groove 13 - 17 aa Endogenous peptides are bound to MHC I by their ends Ionic interaction 8 – 10 aa Structure of MHC Molecules MHC I MHC II

16. Intracellular Compartment Exogenous peptides Endogenous compartment

17. Subcellular Location of Pathogens and their Products Endogenous Ag Exogenous Ag Engage CTL Engage TH cells

18. Proteasomes TAP1, 2 (Transporters associated with antigen processing) CD8 Lysosomal proteases CLIP (class II associated invariant chain peptide) B7 CD4 CD28 Key Molecules in Antigen Presentation MHC I On target cell On CTL On APC MHC II On TH

19. Loading of Endogenous Peptides onto MHC I (1) • Chaperones guide in ER nascent MHC I to TAP • Endogenous proteins are degraded in proteasome and enter ER through TAP • Peptide loading occurs in the ER • If peptides are too long they can be trimmed the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP) • MHC I with peptide loaded is sent to cell surface

20. Loading of Endogenous Peptides onto MHC I (2)

21. Active Learning Exercise: By which mechanisms could viruses interfere with the presentation of viral peptides on MHC I at the cell surface? Viral Strategies to Interrupt Presentation of Viral Peptides • Blocking entry of viral peptides into ER • Retention of MHC I in ER • Degradation of MHC I via transport of MHC I into cytosol • Blocking access of CTLs to surface expressed peptide loaded MHC I

22. Loading of Exogenous Peptides onto MHC II

23. Loading of Exogenous Peptides onto MHC II (1) • MHC II leaves ER with CLIP • CLIP • Class II associated invariant-chain peptide • Binds to peptide groove • Prevents premature peptide loading • MHC II vesicle fuses with phagosome containng degraded exogenous peptides • HLA-DM removes CLIP from MHC II peptide groove and exogenous degraded peptide can bind • HLA-DM is MHC II like

24. Loading of Exogenous Peptides onto MHC II (2)

25. Limitations in MHC Binding Pose a Problem • How can so many different pathogen derived peptides be presented? • Introduce variability in MHC molecule • MHC is polymorphic • MHC is polygenic

26. Polymorphism and Polygeny Increase Variability • Polymorphism • Numerous variants (alleles) for each gene • MHC genes are the most polymorph genes known • Polygeny • Several different genes for MHC I and MHC II • A set of genes with a broader range of peptide binding is expressed

27. 3 genes and gene products for MHC I A (a) B (a) C (a) b2 microglobulin is monomorphic 3 genes and 4 gene products for MHC II DR (a, b1, b2) DP (a,b) DQ (a,b) Over 1000 alleles Alleles are co-dominant expressed Genes Coding for MHC Molecules Polygeny Polymorphism

28. Polymorphism of MHC Genes Number of Different Alleles … a growing list!

29. Allelic Variation Occurs at Specific Sites with in the MHC Molecules

30. The Expression of MHC Alleles is Co-Dominant

31. Intra- and Interpersonal Variability of MHC Molecules • Within a person multiple MHC molecules are expressed • 3 MHC I genes x 2 (father, mother) = 6 MHC I • 4 sets of MHC II genes x 2 (father, mother) = 8 MHC II • Cells within a person are uniform • Cells from another persons carry different sets of MHC molecules!

32. T Cell Recognition of Antigens isMHC Restricted MHC molecules participate in antigen recognition.

34. Superantigens • Antigens that are not processed • Crosslink TCR with MHC • Can simultaneously stimulate 2 - 20% of all T cells.

35. Non-Classical MHC Genes • Resemble MHC class I genes in structure • Many associate with b2microglobulin • Also called MHC Ib • Comparatively little polymorphism • Some bind to activating NK cell receptors (NKG2D) • Example: MIC-A • Induced in response to cellular stress • Trigger cytotoxicity • Some bind to NK inhibitory receptors (NKG2A) • Example: HLA E • Inhibit cytotoxicity • Found on fetus derived placental cells • Some present lipid antigen to T cells • Example CD1

36. Refresher: NK Cell Mediated Killing

37. Today’s Take Home Message • The TCR consists of two chains, a and b, and is similar to the arm of an antibody molecule with the TCR a-chain representing the light chain and the b-chain the heavy chain. • T cell recognize digested peptide presented through MHC molecules. • T helper cells recognize peptide on MHC II and utilize CD4 to ensure proper binding to MHC II. • CTL recognize peptide on MHC I and utilize CD8 to ensure proper binding to MHC I. • MHC are polymorphic and polygenic. • Non-classical MHC I molecules (MHC Ib) interact with inhibitory and activating NK cell receptors.