1klg
From Proteopedia
Crystal structure of HLA-DR1/TPI(23-37, Thr28-->Ile mutant) complexed with staphylococcal enterotoxin C3 variant 3B2 (SEC3-3B2)
Structural highlights
FunctionDRA_HUMAN Binds peptides derived from antigens that access the endocytic route of antigen presenting cells (APC) and presents them on the cell surface for recognition by the CD4 T-cells. The peptide binding cleft accommodates peptides of 10-30 residues. The peptides presented by MHC class II molecules are generated mostly by degradation of proteins that access the endocytic route, where they are processed by lysosomal proteases and other hydrolases. Exogenous antigens that have been endocytosed by the APC are thus readily available for presentation via MHC II molecules, and for this reason this antigen presentation pathway is usually referred to as exogenous. As membrane proteins on their way to degradation in lysosomes as part of their normal turn-over are also contained in the endosomal/lysosomal compartments, exogenous antigens must compete with those derived from endogenous components. Autophagy is also a source of endogenous peptides, autophagosomes constitutively fuse with MHC class II loading compartments. In addition to APCs, other cells of the gastrointestinal tract, such as epithelial cells, express MHC class II molecules and CD74 and act as APCs, which is an unusual trait of the GI tract. To produce a MHC class II molecule that presents an antigen, three MHC class II molecules (heterodimers of an alpha and a beta chain) associate with a CD74 trimer in the ER to form a heterononamer. Soon after the entry of this complex into the endosomal/lysosomal system where antigen processing occurs, CD74 undergoes a sequential degradation by various proteases, including CTSS and CTSL, leaving a small fragment termed CLIP (class-II-associated invariant chain peptide). The removal of CLIP is facilitated by HLA-DM via direct binding to the alpha-beta-CLIP complex so that CLIP is released. HLA-DM stabilizes MHC class II molecules until primary high affinity antigenic peptides are bound. The MHC II molecule bound to a peptide is then transported to the cell membrane surface. In B-cells, the interaction between HLA-DM and MHC class II molecules is regulated by HLA-DO. Primary dendritic cells (DCs) also to express HLA-DO. Lysosomal miroenvironment has been implicated in the regulation of antigen loading into MHC II molecules, increased acidification produces increased proteolysis and efficient peptide loading. Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedWhile most immunotherapies for cancer have focused on eliciting specific CD8+ cytotoxic T lymphocyte killing of tumor cells, a mounting body of evidence suggests that stimulation of anti-tumor CD4+ T cell help may be required for highly effective therapy. Several MHC class II-restricted tumor antigens that specifically activate such CD4+ helper T lymphocytes have now been identified, including one from a melanoma tumor that is caused by a single base-pair mutation in the glycolytic enzyme triosephosphate isomerase. This mutation results in the conversion of a threonine residue to isoleucine within the antigenic epitope, concomitant with a greater than five log-fold increase in stimulation of a CD4+ tumor-infiltrating lymphocyte line. Here, we present the crystal structures of HLA-DR1 in complex with both wild-type and mutant TPI peptide antigens, the first structures of tumor peptide antigen/MHC class II complexes recognized by CD4+ T cells to be reported. These structures show that very minor changes in the binding surface for T cell receptor correspond to the dramatic differences in T cell stimulation. Defining the structural basis by which CD4+ T cell help is invoked in an anti-tumor immune response will likely aid the design of more effective cancer immunotherapies. Minor structural changes in a mutated human melanoma antigen correspond to dramatically enhanced stimulation of a CD4+ tumor-infiltrating lymphocyte line.,Sundberg EJ, Sawicki MW, Southwood S, Andersen PS, Sette A, Mariuzza RA J Mol Biol. 2002 May 31;319(2):449-61. PMID:12051920[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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