MICR 304 Immunology & Serology Lecture 9 TCR, MHC molecules Chapter 3.10 – 3.19, , 5.1 – 5.19 Lecture 9 TCR, MHC molecules Chapter 3.10 – 3.19, , 5.1 – 5.19

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

Key Players in Immunology InnateAdaptive Cells Phagocytes Epithelial Cells NK Cells Lymphocytes (B-Ly, T-Ly) Effector molecules Complement Antimicrobial (Poly)Peptides Antimicrobial Lipids? Antibodies

T Cell Receptor 2 chains –Connected by disulfide bond –Variable region –Constant region –Short cytoplasmic tail Mostly  and  chain Some specialized T- cells have  and  chain (  T cells)

T-Cell Receptor Belongs to the Immunoglobulin Super Family

Gene Segments Coding for the T-Cell Receptor Variable region of  -chain is composed of V and J gene segments Variable region of  -chain is composed of V, D and J gene segments

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

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

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

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 endogenous peptides Virus encoded Produced by intracellularly replicating microorganisms Tumor antigens –MHC II: presents exogenous peptides Uptake through phagocytosis and degradation in phagolysosome MHC I CTL MHC II TH

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

Contrast TH Cells and CTL THCTL CD on surfaceCD4CD8 MHC Interaction MHC IIMHC I Target cellsAPC (macrophages, DC, B cells, others) Any nucleated cell Response upon activation Cytokine releaseCytotoxic granule release (and some cytokines)

 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

Expression of MHC Molecules Differs Between Tissues MHC I positive: any nucleated Cell MHC II positive: only antigen presenting cells (APC) –IFN  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

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

Intracellular Compartment Exogenous peptides Endogenous compartment

Subcellular Location of Pathogens and their Products Endogenous Ag Exogenous Ag Engage CTLEngage TH cells

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

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

Loading of Endogenous Peptides onto MHC I (2)

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 Active Learning Exercise: By which mechanisms could viruses interfere with the presentation of viral peptides on MHC I at the cell surface?

Loading of Exogenous Peptides onto MHC II

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

Loading of Exogenous Peptides onto MHC II (2)

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

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

Genes Coding for MHC Molecules 3 genes and gene products for MHC I –A (  ) –B (  ) –C (  ) –  2 microglobulin is monomorphic 3 genes and 4 gene products for MHC II –DR ( ,  1,  2) –DP ( ,  ) –DQ ( ,  ) Over 1000 alleles Alleles are co- dominant expressed PolygenyPolymorphism

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

Allelic Variation Occurs at Specific Sites with in the MHC Molecules

The Expression of MHC Alleles is Co-Dominant

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!

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

Superantigens Antigens that are not processed Crosslink TCR with MHC Can simultaneously stimulate % of all T cells.

Non-Classical MHC Genes Resemble MHC class I genes in structure Many associate with  2 microglobulin 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

Refresher: NK Cell Mediated Killing

Today’s Take Home Message The TCR consists of two chains,  and , and is similar to the arm of an antibody molecule with the TCR  -chain representing the light chain and the  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.