Structural aspects of a protein epitope and their role in the major histocompatibility complex control of T cell responsiveness

We have previously shown that immunization of C57BL/10 (H-2 b) mice with the tobacco mosaic virus protein (TMVP) or with its tryptic peptide number 8, representing residues 93–112 of TMVP, induces T cells which proliferate in vitro in response to TMVP and peptide 8. In contrast, immunization of congenic B10.BR (H-2 k) mice with either TMVP or with peptide 8 induces T cells which respond in vitro to the homologous but not the heterologous antigen. The capacity to exhibit cross-reactivity between TMVP and peptide 8 on the T cell level has been shown to be under major histocompatibility complex (MHC)-linked genetic control. The lack of cross-reactivity has been attributed to the inability of the H-2 k APC to present the appropriate epitope to T cells. In the present paper, we report results of a comparative analysis of the role of structural aspects of the epitope on the proliferative T cell responses from TMVP and peptide 8-immune C57BL/10 (H-2 b) and B10.BR (H-2 k) mice. Utilizing a panel of synthetic peptides representing portions of peptide 8 and a panel of peptide-protein conjugates, we have determined that peptide 8-immune T cells of the H-2 k strain appear to recognize a single epitope within peptide 8, located at its N-terminus. In contrast, in the H-2 b strain, both TMVP and peptide 8-immune T cells appear to recognize two overlapping epitopes within peptide 8; one located in the middle region and the other toward the N-terminus. Experiments with H-2 b T cells revealed that random amino acids added to the carboxyl or amino-terminus of nonstimulatory peptides can confer activity to these peptides, demonstrating limited specificity of interaction between antigen and Ia b. Results of experiments dealing with fixation of antigen-presenting cells suggest that TMVP requires processing in order to be recognized by peptide 8-immune H-2 b proliferative T cells whereas peptide 8 does not. Taken together the results suggest that the T cell responsiveness to TMVP and peptide 8 exhibited by these two congenic strains H-2 b and H-2 k is not only controlled by the strains MHC but is also influenced by antigen processing. Antigen processing may eliminate a potential epitope for the primary induction and the secondary stimulation of B10.BR T Cells.