PROTECTION AGAINST ORAL CHALLENGE 3 MONTHS AFTER IV IMMUNIZATION OF BALB C MICE WITH LIVE ARO SALMONELLA-TYPHIMURIUM AND SALMONELLA-ENTERITIDIS VACCINES IS SEROTYPE (SPECIES)-DEPENDENT AND ONLY PARTIALLY DETERMINED BY THE MAIN LPS O-ANTIGEN/

The role of the main LPS O antigen in the specificity of protection as mediated by systemic mechanisms following immunization with live atte nuated Aro Salmonella vaccines was studied in mice. Innately Salmonell a-susceptible (Ity(s)) BALB/c mice were immunized intravenously with a single dose of either Salmonella typhimurium SL3261 aroA (LPS 04,5,12 ) or Salmonella enteritidis Se795 aroA (LPS 01,9,12), and challenged o rally 2-3 months Inter with either S. typhimurium C5 or S. enteritidis Thirsk. Nearly isogenic transductants of the two challenge strains ex pressing either their own LPS or that of the other serotype (S. typhim urium C5 04 or 09, and S. enteritidis Thirsk 09 or 04) were also used Both vaccines conferred similar high protection against the virulent s train of the homologous serotype expressing its own LPS. There was no protection against the heterologous serotype expressing ifs own LPS. H owever, when vaccinated mice were challenged with either the same sero type as the vaccine but expressing the heterologous LPS, or with the h eterologous serotype expressing the LPS of the vaccine, protection was always lower than protection against the fully homologous serotype. A nti-smooth LPS antibodies showed higher titres against the homologous LPS, but with significant cross-reactivity with the heterologous LPS. Antibodies to O-rough S. typhimurium and S. enteritidis LPS were prese nt following immunization with either of the two vaccine strains. The LPS alone cannot fully account for the specificity of protection in th is model; other (protein) antigens may be responsible. It remains to b e seen whether there is a T-cell mediated component to the specificity of protection conferred by live Salmonella vaccines. Copyright O 1996 Elsevier Science Ltd.