The T lymphocyte response to cytochrome c. V. Determination of the minimal peptide size required for stimulation of T cell clones and assessment of the contribution of each residue beyond this size to antigenic potency.
The B10.A T cell proliferative response to pigeon cytochrome c is mainly directed against a single antigenic determinant located at the carboxy-terminal end of the molecule. In the present experiments, we used synthetic peptide analogs of the carboxy-terminal sequence of moth cytochrome c to explore the structural requirements for antigenic potency. The minimum-sized peptide capable of stimulating a full response varied with the T cell clone, but within the limits of the biological systems studied, was shown to be moth fragment 97-103. Addition of more amino acids at the amino terminal end increased the antigenic potency in uneven increments, with a large contribution being made at residue 95. Analysis of amino acid substitutions at this position provided no evidence that it contained a residue that directly contacted the T cell receptor. Instead, good agreement with an analysis that made use of helix-coil transition theory suggested that this residue, as well as others, increased antigenic potency by contributing to the stabilization of the secondary structure of the molecule in an alpha-helical configuration. The maximum effect of chain length on antigenic potency appeared to stop at residue 93, in agreement with the theoretical analysis. However, addition of several more amino-terminal residues to residue 93 showed one additional significant increment of increased potency. This was almost entirely accounted for by a single lysine located four amino acids beyond the glutamic acid at residue 93 (approximately one turn of an alpha-helix away). To experimentally test whether alpha-helix-forming tendencies could account for the increased potency of the larger analogs, the degree of helix formation in trifluoroethanol was assessed by circular dichroism measurements. A good correlation was found between antigenic potency and percentage of alpha-helix for peptides of increasing chain length from moth 95-103 up to moth 86-90; 94-103. These results suggest that secondary structure may play an important role in determining the potency of antigenic determinants involved in the activation of T lymphocytes.
R H
RH
Schwartz
B S
BS
Fox
E
E
Fraga
C
C
Chen
B
B
Singh
D000818:Animals; D000941:Antigens / Q000276:immunology*; D002999:Clone Cells / Q000276:immunology; D010856:Columbidae / Q000276:immunology; D003574:Cytochrome c Group / Q000276:immunology*; D004306:Dose-Response Relationship, Immunologic; D000939:Epitopes / Q000276:immunology; D008213:Lymphocyte Activation / Q000187:drug effects; D009036:Moths / Q000276:immunology; D010455:Peptides / Q000276:immunology*; D011487:Protein Conformation / Q000187:drug effects; D013601:T-Lymphocytes / Q000145:classification / Q000276:immunology*; D014269:Trifluoroacetic Acid / Q000494:pharmacology
D000941:Antigens; D003574:Cytochrome c Group; D000939:Epitopes; D010455:Peptides; D014269:Trifluoroacetic Acid