Tetanus toxin fragment C expressed in live Salmonella vaccines enhances antibody responses to its fusion partner Schistosoma haematobium glutathione S-transferase

Tetanus toroid has been used widely as an adjuvant, The atoxic fragment C f rom tetanus toxin (TetC) is potently immunogenic when expressed in Salmonel la vaccine strains and has been used as a fusion partner for antigens (Ag). However, there has been no formal comparison of the immunomodulatory impac t of TetC on its fusion partners. In this study, we have addressed this imp ortant issue. The protective 28-kDa glutathione S-transferase (GST) from Sc histosoma haematobium (Sh28GST) was expressed either as a fusion to TetC or as the full-length Sh28GST alone in a nonvirulent aroA-attenuated strain o f Salmonella enterica serovar Typhimurium. The Sh28GST proteins were solubl e and stably expressed in Salmonella, as evaluated by Western blotting with TetC and/or Sh28GST antisera, Mice were immunized orally with a single dos e of the live recombinant Salmonella. The constructs were stable in mice bu t, dramatically, only the strain expressing the TetC-Sh28GST fusion elicite d significant antibody (Ab) responses directed against Sh28GST as determine d by enzyme-linked immunosorbent assay. An analysis of the isotype profiles showed that these mice also produced anti-Sh28GST immunoglobulin A and GST -neutralizing assays revealed high levels of neutralizing Abs in sera. Thes e are important correlates of protection in schistosomiasis. In addition, s timulation of spleen cells from immunized mice with Sh28GST Ag showed that both strains, expressing Sh28GST alone or the TetC-Sh28GST fusion, were abl e to stimulate the secretion of Th1-related cytokines (gamma interferon and interleukin 2) to comparable levels. Thus, TetC has modulated the immune r esponses generated against its fusion partner, Sh28GST, by markedly enhanci ng the Ab responses elicited. These results have important implications in the rational development of live vaccines.