Structural studies on MHC-I antigen presentation and on the costimulatory molecule B7-H3

Abstract

T cell activation occurs as a consequence of antigen receptor recognition of antigen presented by major histocompatibility complex (MHC) molecules. T cell responsiveness is necessarily modulated by the CD28/B7 family of costimulatory molecules, which are present on the surface of T cells and antigen presenting cells. We focused on the structural basis of peptide presentation by the MHC class I molecule H-2Db and on the characterization of the recently identified costimulatory molecule B7-H3. Here the crystal structures of allelic H13 minor histocompatibility antigen peptides in complex with Db revealed a novel means of presenting noncanonical peptides by burying water molecules in the Db peptide binding groove. Additionally, imposition of an anchor residue in anchorless H13 peptides to increase binding affinity was shown to decrease activity of a H13-reactive T cell by altering the peptide conformation in the Db cleft. Further, the structural effects of allelic beta-2 microglobulin (b2m) isoforms on peptide presentation by Db were examined. The crystal structures of Db/H13 with allelic b2m isoforms revealed differences in the relative rigidity of the b2m subunit, although these structural changes were not visibly propagated to reveal differences in peptide conformation. This suggests that the b2m allelic effect on antigen presentation is likely in peptide binding to MHC rather than peptide presentation. Lastly, the newly identified costimulatory molecule B7-H3 was characterized and shown to exist as a dimer in solution. The possibility of disulfide-linked homodimerization was ruled out, although it is possible to exist as a noncovalent dimer on the cell surface. In addition, endogenous human B7-H3 was shown to exist as a four domain isoform rather than the two domain form previously suggested. These results suggest that B7-H3 operates differently than its B7 family members, potentially reflecting a novel mechanism for the initiation of signal transduction.