Immunological Evidence for Peptide-Carbohydrate Mimicry with a Group A Streptococcus Polysaccharide-Mimetic Peptide

The immunogenicity of a peptide-protein conjugate developed by linking a peptide mimic DRPVPY of the Group A Streptococcus cell-wall polysaccharide (GAS-CWPS), to tetanus toxoid (TT) was examined. BALB/c mice were immunized three times subcutaneously following homologous or heterologous prime/boost strategies at 4- or 6- week intervals in two different experiments. DRPVPY-TT, CWPS-TT, heat-killed, pepsin-treated GAS bacteria (with exposed polysaccharide) and TT, were used as immunogens with alum as adjuvant. Antibody titers were determined by ELISA with GAS bacteria (with exposed polysaccharide) and DRPVPY-linked to bovine serum albumin (BSA, DRPVPY-BSA) as solid phase antigens. All mice primed with DRPVPY-TT developed high IgG anti-peptide and anti-GAS titers. The binding of polyclonal anti-peptide antibodies to GAS could be inhibited by purified CWPS, synthetic oligosaccharides corresponding to CWPS, DRPVPY-BSA, DRPVPY and DRPVP, as assessed by competitive-inhibition ELISA. Anti-oligosaccharide titers were also obtained upon titration of anti-peptide sera with synthetic oligosaccharide-BSA conjugates. All mice primed with CWPS-TT and mice primed and boosted with GAS developed IgG anti-peptide titers. These data demonstrate conclusively the cross-reactivity of the immune responses and support the hypothesis of antigenic mimicry of the GAS-CWPS by the hexapeptide DRPVPY. However, mice boosted with DRPVPY-TT, after 6-8 weeks, showed a decrease in IgG anti-GAS titers, but an increase in IgG anti-peptide titers, suggesting carrier-induced suppression of the response to polysaccharide. Strategies are outlined for further refinement of a DRPVPY conjugate as a surrogate of the cell-wall polysaccharide for use in vaccines against Group A Streptococcus.