STUDIES ON THE LOADING AND RECOGNITION OF ANTIGENS ON MHC MOLECULES Don C. Wiley, Dept. of Molecular and Cellular Biology, Harvard University and Howard Hughes Medical Institute, Cambridge, Massachusetts, 02138.

In the cellular immune response, antigen specific cell-cell recognition results from the binding of an antigen receptor (TCR) to the complex of an antigenic peptide bound to a class I or class II major histocompatibility complex (MHC) glycoprotein. The TCR is a glycoprotein on the membrane of T lymphocyte and the MHC molecule is on the surface of a target cell. Specific receptor binding triggers signals within T cells that are central to the development of the T-cell repertoire, regulation of the immune response, and activation of cytolytic T cells (CTL). Generalizations about the mechanisms of peptide recognition by class I and class II MHC molecules derived from X-ray crystal structures and biochemical analyses will be reported^1,2,3,4.

The loading of antigenic peptide on class II MHC molecules involves an escort protein called the Invariant chain (Ii). This molecule stabilizes class II molecules and escorts them to a cellular compartment where they bind antigenic peptides. Our NMR and biochemical experiments indicate that Ii may be partially unfolded so that a segment of it can bind into the peptide binding site^5,6. The X-ray structure of a fragment of this peptide-loading intermediate has been determined by X-ray crystallography⁷.

The recognition of peptide/MHC molecule complexes is being studied by assembly of a ternary complex of TCR/peptide/MHC class I molecule. The ectodomains of human TCR class I MHC have been expressed as insoluble inclusion bodies in bacteria and refolded. The antigen specific cell-cell interaction complex has been assembled and shown to retain the binding specificity of the in vivo intercellular interaction. This complex has been crystallized⁸ and its structure is being determined.

1Bouvier, M. and Wiley, D.C., PNAS in press, 1996.

²Guo, H-C et al., in preparation.

³Stern, L.J. et al., Nature 368, 215-221, 1994.

⁴Jardetzky, T.S. et al., PNAS 93, 734-738, 1996.

⁵Jasanoff, A. et al., PNAS 92, 9900-9904, 1995.

⁶Park, S.-J. et al., PNAS 92, 11289-11293, 1995.

⁷Ghosh, P. et al., Nature 378, 457-462, 1995.

⁸Garboczi et al., in preparation.