RECOGNITION BY T CELLS

IMMUNOLOGY AG RECOGNITION BY T CELLS 1. OVERVIEW -T cells recognize Ag using receptors similar to Ab molecules -TCR receptor diversity generated by V(D)J recombination -Recognizes Ag as a complex associated with MHC (polymorphic) -Optimal Ab response needs B cells and T cells Helper T cells (Th) -activate B cells to make Ab Cytolycic T cells (Tc)- kill infected cells and tumor cells T cells obey the Clonal Selection Theory -Ag receptors distributed on T cells in a clonal fashion, Ag stimulates activation and clonal expansion -T cells do not secrete Ag receptors T cells develop in the thymus, (derived from bone marrow pluripotential hematopoietic stem cells) T cell Recognize native and denatured Ag -T cells see antigenic determinants = short stretches of linear sequence (10 AA) =epitope -Recall: B cell Ab recognize conformational determinants lost in denaturation T cells recognize Ags only on the surface of other cells -T cells don’t recognize free antigen, has to be presented on the surface of APC (ex: macrophage, DC) Proof of Order: AntigenAPC T cells MHC Restriction -T cells from someone for a given Ag cant recognize the Ag on an APC from someone else b/c of MHC -Genetic polymporphism of MHC = different individuals express different MHC alleles -MHC are the major target Ags recognized in graft rejection (Mice = HHumans=HLA) Proteins encoded by MHC (Now wild type = “balance polymorphism” -complex of over 3000 kb of DNA with rich array of genes CLASS HUMAN MOUSE LOCI Composition Expression Example LOCI I ABC KDL α2β2m All cells but A27 (α2 is heavy chain), neurons (β2m on diff. chrom.) II DP, DQ, DR I-E, I-A Αβ B cells K d (heterodimers) Mac, DC Class III = complement proteins and cytokines (soluble serum proteins, C2, C4, factor B) CLASS I CLASS II (Raymond removed the image showing MHC Class I vs Class II composition. See above table.) Note: In murine MHC, class Ib molecules encoded by chrom. Segment telomeric to H2-L 1)Non polymorphic 2)Tissue specific expression Humans CLASS 1b molecules = HLA, E, F, G Note: Class II expression induced in endothelial cells and fibroblasts by cytokine interferon-γ -Variation clustered in class I = α1 and α2 domains class II = α1 and β1 T cell Ag receptor is dual specific, recognizing both Ag and MHC of APC Individual T cells have specificity for particular MHC gene product Locus used depends on (1)functional T cell subset involved(2)Ag being recognized To define which locus is used for a particular T cell 1. Test ability of APCs with MHC to present Ag to Ag-specific T cells -test on various individuals, shared allele will lead to clonal expansion! 2. Use monoclonal Ab specific for distinct MHC encoded products how to make? -Add T cell,, antigen, different Ab = Ab that blocks proliferation means molecule used on surface of APC 2 theories for T cell recognition of Antigen 1. Dual Recognition Model -T cell expressed one receptor for Ag and one for MHC 2. Single Receptor Model -one receptor reads Ag and MHC PROOF: Fusion of 2 T cell lineshybrid T cells (exhibit parental specificity, but not mixed or crossed patterns) Ag and MHC specificity did not behave independently but as a unit Discovery of T cell Ag receptor 1. Generation of monoclonal Ab specific for individual T cell clone (ANTI-IDIOTIPIC AB TO TCR) -Ab recognize unique determinants on the variable regions of TCR  purify receptors at protein level 2. Subtractive hybridization to identify genes uniquely expressed by T cells -look for Ig like that underwent gene rearrangementTCR TCR Structure (like Fab part of IgR) -immunoglobulin gene superfamily, composed of two polypeptide chains α and β linked by a disulfide bond -variable N terminal domain, constant globular domain in a β pleated sheet, membrane D, cytoplasmic D -stabilized by intrachain disulfide bond btw cysteines (Rayond removed the image showing TCR and CD3 chain structure.) -Ag combining site formed by association of variable domains (hypervariable regions) -α/β TCR assembled from gene segments by gene rearrangement β chain = (VDJ + C)  diversity from V(D)J [like IgR heavy chain] α chain = (VJ +C) lacks D [like IgR light chains) Identical rearrangement to Immunoglobulin genes (but initiated in thymus)10 possible TCR structure RAG1/RAG2 : 12-23 spacer role : junctional diversity (N) -but no somatic mutation in hypervariable region, no class switching Differences Btw T and B cell Ag Receptors (1) Univalent (B cell is divalent) (2) Only in membrane firm (BCR can be secreted) (3) Noncovalent attachment with CD3 (invariant, play role in signal transduction) (BCR: Igα and Igβ) (4) Ligand is processed Ag + MHC (Ag ligand is native Ag) CD3 -surface expression used a s a marker for T cells -Ab to CD3 used therapeutically to deplete T cells -minor subset of CD3+ T cells don’t express α/β heterodimer (instead γδ) α/β heterodimer receptor responsible for T cell specificity for both Ag and MHC!!! MECHANISMS OF AG RECOGNITION 1. Ags must be processes prior to recognition -lag time from when Ag is taken up by APC processed and presented Ag to T cell -processing exposes appropriate linear epitopes on MHC -fst macs before lag timeno T cell response afer lag time T cell response -1 specific rxn: Binding of peptide fragment of protein Ag to MHC product on APC Saturated with a valence of 1 2. MHC molecules function as immune response genes Immune response gene=genetic element that controlled response to synthetic immunogen -inherited as a single gene and localized to the H-2 complex on chromosome #17 (mouse) -Only certain peptide fragments of a given protein Ag will bind to a given MHC molecule -Most individuals respond to most protein Ags, b/c proteins usually have some linear AA sequence epitope that can bind to at least one of the several class I or class II MHC molecules expressed by an individual -Some autoimmune diseases correlate with certain HLA alleles! -Many different peptides derived from different protein Ags can potentially bind to given MHC gene product. -variety of peptides can fit into the groove of a given MHC molecule -only certain AA tolerated at key “anchor positions” Ex: HLA-A2 binds nonameric peptides with leucine at position #2 and valine at position #9 -different MHC alleles have different anchor residues Class I = interactions with polar main chain atoms of peptide termini provide binding energy -longer peptides tolerated by bulking in middle 3DStructure= 2 walls of binding site (helix) +floor of binding site (beta sheet) for 9aa Class II = binding site open at either end to allow larger peptides to bind A MODEL FOR AG RECOGNITION BY T CELLS 1.Ag uptake by APC (macrophage) 2 22. Partial Proteolysis of Ag in endocytic compartments