Studies on Immunization Against Plague

Abstract

Summary A water soluble carbohydrate-protein immunizing antigen, Fraction IA isolated from Pasteurella pestis, was studied for its electrophoretic and serological homogeneity. Determination of the 50% immunizing dose of Fraction IA in mice showed that Fraction IA was a potent immunizing agent against infection with P. pestis. Twelve to 22 micrograms of the antigen were sufficient to protect 50% of the mice against 100 MLD of a highly virulent strain of the organism. The antigen preparations were electrophoretically inhomogeneous. Quantitative precipitin reactions showed that although the Fraction IA preparations were homogeneous on the basis of the analysis of the supernates, a serologically active minor component could be detected by absorption studies. However, the portion of the curves of the reaction for the zones of antibody excess and equivalence are believed to be the results essentially of the precipitation of Fraction IA and its antibody. The lack of complete inhibition of precipitation in the extreme antigen excess zone in the Fraction IA system was probably due, in part, to the serological activity of the minor component. However, removal of this component resulted in an increased but still incomplete inhibition of precipitation. This suggests that the carbohydrate moiety may be serologically active and may act in a manner similar to that of the pneumococcus polysaccharide. It is believed that the quantitative precipitin test, using a standard serum, can be used to assay preparations of P. pestis antigen by determining the amount of antigen required to bring the system to equivalence.