B cell responses to a peptide epitope: IV. Subtle sequence changes in flanking residues modulate immunogenicity.

We examined modulation of primary humoral responses to a model synthetic peptide immunogen, peptide PS1CT3, as a consequence of single amino acid substitutions. Two analogues were employed, one in which the amino-terminal histidine (His1, peptide G28CT3) and another in which an internal proline (Pro14, peptide G41CT3) were replaced with glycine residues. Peptide G28CT3 displayed markedly enhanced immunogenicity relative to peptide PS1CT3 in BALB/c mice, whereas peptide G41CT3 was only poorly immunogenic. Nevertheless, in all three cases the mature polyclonal IgG response was predominantly directed against a tetrapeptide segment of sequence Asp-Pro-Ala-Phe between positions 4 and 7 of the sequence. While all three peptides proved equally capable of priming Ag-specific Th cells, they, however, displayed significant differences in their abilities to recall T cell responses. Regardless of the priming immunogen, in vitro challenge with either PS1CT3 or its analogues consistently gave a hierarchy of potencies as G28CT3 > PS1CT3 > G41CT3. This could also be correlated with B cell recall responses in which an identical hierarchy was obtained on restimulation of G41CT3-primed B cells in adoptive transfer experiments. Subsequent studies revealed that peptide-mediated modulation of Th cell recruitment by Ag-primed B cells was probably due to differences in on-rates for engagement of B cell Ag receptor by these analogues. This was despite the fact that all three peptides displayed equally randomized conformations in solution. These studies indicate that even subtle variations in the flanking sequences can markedly influence the immunogenicity of B cell epitopes.