The long term goal of these studies is to determine constraints in the T cell recognition of protein which might be imposed by the structure of the antigenic moiety. To further this goal, in the next funding period of this grant, studies on the structural constraints of class II restricted T helper cell recognition of peptide antigens will be continued, and extended to the T cell recognition of a structurally rigid component of hemoprotein antigens, the heme prosthetic group, will also be examined. The first specific aim of these studies is to continue elucidating the role of electrostatic interactions in peptide/class II binding. The second specific aim is to examine the conformational constraints of class I MHC presentation essentially using the principles of peptide design and synthesis that were used in our studies on class II recognition during the last two funding periods. This will require the establishment of a class I restricted model antigen system. This will be done by producing class I restricted T cells to hen egg lysozyme by in vitro peptide priming and in vivo priming with transfected cell lines. Finally, molecular differences between the T cell recognition of peptide antigens, which can adopt multiple conformations, and an antigen, the heme moiety of hemoproteins, which has strong conformational constraints, will be explored. The response to heme is of high precursor frequency in the peripheral T cell repertoire of I-As and I-Ak bearing naive mice, thus the cause of this high precursor frequency will also be examined with regard to (a) the mechanism by which heme binds to MHC and stabilizes the interaction of APC with naive T cells and (b) possible frequency in the peripheral repertoire of responder strains.