Immunology 13 - Immunology 13 Exogenous Pathway of Antigen...

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Immunology 13- Exogenous Pathway of Antigen Processing and Presentation MHC II: 2 chains: alpha and beta; each encoded by separate genes (for each gene there are 2 possible alleles, and there is a wide variety of alleles within the population); B pleated sheet forms floor of cleft; 2 alpha helices on top; open conformation (can bind longer peptides) Peptide Binding to MHC Class II molecules is stabilized by numerous contact points Not constrained by peptide length; sequences at ends of peptides are not as important Rely on contact points throughout peptide via hydrophilic/hydrophobic interactions The average peptide binding to class II MHC is 13-17 amino acid residues The ends are unimportant for peptide-protein contact Anchor residues lie at various distances from the ends of the peptide Lengths are completely different given one MHC molecule’s binding repertoire; length plays little role in binding Location of anchor residues differs between different MHC molecules Peptides for a given molecules have similar anchor residues The binding pockets of MHC II molecules accommodate a greater variety of side chains than those of MHC I molecules No upper limit, in principle, to the length of the peptide that can bind Throughout the peptide’s length bonds are made with residues that are highly conserved in MHC class II molecules Anchor residues lie at various distances from the ends of the peptide V alpha domain makes contact primarily with the amino-terminal half of the bound peptide, and the V beta domain contacts primarily the carboxy terminal half Binding results in some degree of conformational change particularly within the V alpha CDR3 loop Interactions between the TCR and the MHC molecule might predominate at the start of contact, guiding the receptor into the correct position, where a second, more detailed interaction with the peptide as well as the MHC molecule dictates the final outcome Inherent specificity for MHC molecules is encoded in the TCR genes, and there is selection during T cell development for a repertoire of receptors able to interact appropriately with the particular MHC molecules present in that individual The structure of CD4 and CD8 Co-receptors CD4: 4 Ig like domains CD8: 2 separate proteins (alpha and beta) joined via disulfide bond Interaction between T cells and MHC molecules is determined by coreceptors Both are transmembrane proteins, short cytoplasmic tails; extracellular Ig-like domains T-cells express either CD4 or CD8, not both Each T cell has a T-cell receptor which recognizes the peptide/MHC complex and a CD4/8 coreceptor which binds NON COVALENTLY to the side of MHC (doesn’t block the peptide binding site) Co-receptors stabilize interactions between T-cell and Ag presenting cell Interactions between TCR and MHC molecules is very weak! So it must be stabilized on the side by a co-receptor MHC binding region on CD4 is located on the lateral face of the D1 domain, and CD4 binds to a hydrophobic crevice
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Immunology 13 - Immunology 13 Exogenous Pathway of Antigen...

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