Guinea pig T lymphocyte responses to a decapeptide antigen (NH1-Asp5-Ans6-Glu7-Glu8-Gly9-Phe10-Phe11-Ser12-Ala13-Arg14-OH) of human fibrinopeptide B (hFPB) were examined using various synthetic peptide analogues containing single residue substitutions. Each analogue was examined for antigenicity as determined by an in vitro proliferative responses of hFPN-immune strain 2 guinae pig T cells. In addition, both strain 2 and strain 13 animals were immunized with each analogue and immunogenicity assessed by in vitro T cell-proliferative responses with the homologous immunizing analogue and the parent peptide. Replacement of arginine14 with lysine formed an immunogenic analogue which showed no antigenic cross-reactivity with the native peptide in strain 2 T cell responses. In addition, substitution of arginine14 with blocked lysine again produced a unique immunogenic analogue that showed little or no antigenic identity with the intact lysine analogue or the native peptide. In similar fashion, substitution of resideu phenylalanie10 with tyrosine or Phe(4-NO2) created unique immunogenic analogues with little or no antigenic identity to the native peptide with strain 2 T cells. By contrast, replacement of phenylalanine11 with either tyrosine or Phe(4-NO2) resulted in analogues with a total loss of immunogenicity and antigenicity in strain 2 T cell responses. An analogue in which glutamic acid7,8 were replaced with glutamine retained a small degree of antigenicity with hFPB-immune T cells, but T cells from strain 2 animals immunized with the Gln analogue responded only marginally to the Gln analogue while producing good proliferative responses with the native peptide. On the other hand, an analogue in which asparatic acid5 was replaced with asparagine retained most of the antigenic identity with hFPB for strain 2 T cell responses. None of thee analogues were immunogenic for strain 13 guinea pigs. These observations are discussed with respect to the contribution of each substituted residue to T cell respones, mechanism of Ir gene function, and a model for T cell recognition of small peptide antigens.

One approach to investigating the nature of T lymphocyte recognition of exogenous antigens and mechanisms of Ir gene control has used small, well-defined peptide antigens. In previous studies we used synthetic homologues and analogues of human fibrinopeptide B to examine strain 2 and strain 13 guinea pig T cell responses and found that Ir gene control correlated with the presence or absence of the carboxyl terminal residue, and that responsiveness was determined by macrophage Ia antigens (1, 2). In addition, several peptide residues were identified that were responsible for the specificity of the T cell responses, and most likely served as contact residues for clonally distributed T cell antigen-combining receptors (3). However, these immune responses were solely cell mediated, and we were unable to generate detectable antibody by a variety of approaches. It was therefore difficult to compare T and B cell recognition of the same peptide antigen to determine whether the antigencombining repertoire of both cell types was similar. For this reason, we have employed the octapeptide hormone angiotensin II (AII) 1 as an antigen system to investigate T and B cell recognition. AII has been used previously to investigate the specificity and spatial constraints of antibody binding in several species (reviewed in reference 4). Moreover, Dietrich (5) found that free AII elicited both immediate and delayed-type hypersensitivity reactions in guinea pigs. In this study we have extended the findings of Dietrich to examine Ir gene control and the specificity of T cell responses to a variety of synthetic homologues and analogues of AII in strain 2 and strain 13 guinea pigs. Evidence is presented that demonstrates the exquisite specificity of Ir gene control of T cell responses and indicates that the diversity of the antigen recognition repertoire in strain 2 and strain 13 animals is generally nonoverlapping.

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