Accumulation of a Soluble Antigen in the Peritoneal Fluid of Guinea Pigs Infected with Salmonella Typhi

Soluble Vi antigen linked with host globulins accumulated in the peritoneal and pleural exudates of guinea pigs infected with Salmonella typhi strain$Ty_2$. It was precipitated together with globulin and was purified by exposure of the precipitated globulins to pepsin and precipitation with 68% ethanol. The final product did not produce precipitation bands with anti-guinea pig globulin antibodies but gave 1 band with an anti-Citrobacter bailerup immune serum. The electrophoretic mobility was identical with that of the Vi antigen of C. ballerup, and quantities smaller than 1 µg immunized mice against virulent S. typhi. When added in subeffective quantities it enhanced the immunizing potency of a conventional acetone-dried typhoid vaccine. A subeffective dose of the purified product with 105 bacteria of an acetone-dried vaccine from the in vivo grown$Ty_2$effected a 50% protection in mice, while$1.5X10^7$acetone-dried microorganisms of the in vitro grown$Ty_2$were needed to achieve the same effect. The$LD_50$of the in vitro grown strain$Ty_2$decreased 14 times when the bacteria were injected intracerebrally along with the purified product.