Fine-specificity analysis of antibodies directed to the C-terminal peptides of cytochrome c recognized by T-lymphocytes.

Recognition of cytochrome c by T-lymphocytes seems to involve the amino acid residues in the C-terminal region of the molecule. Lys-99 has particularly been identified as one of the critical residues in the recognition process. We have now raised antibodies against the C-terminal region of the cytochrome c molecule to map the residues that may be recognized by B-lymphocytes. These antibodies were generated in high-responder B10.A mice against either the 81-104 CNBr fragment of pigeon cytochrome c or against the synthetic spliced fragment (86-90)-(94-103) of the tobacco hornworm moth cytochrome c. A good antibody response was obtained for both fragments as measured by solid-phase radioimmunoassay. A series of peptides related to these fragments were synthesized for competitive inhibition to assess the antigenic sites on these molecules. In spite of substantial homology between the moth (86-90)-(94-103) and pigeon (81-104) fragments, the antibody populations raised against each fragment differed in their recognition patterns. Residues 99 (Lys), 103 (Ala) and 104 (Lys) were found to be crucial for binding of the anti-pigeon antibody to the pigeon 81-104 fragment. The fine specificity mapping of the antigenic sites on the moth (86-90)-(94-103) fragment indicated that along with some of the residues in the N-terminus (86-90), residue 99 (Lys) was involved in recognition of the moth (86-90)-(94-103) fragment by its antibody. This residue (Lys-99) also acts as a T-cell receptor contact site for both pigeon and moth cytochrome c. We therefore conclude that common patterns of recognition must exist between T and B-cells that recognize the C-terminal region of cytochrome c. Since Lys-99 is also present in mouse cytochrome c, the antigenic site must involve both self and non-self residues.