Immunological characterization of Vi antigens
Whiteside, Roberta Emerson
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Vi antigen vms first discovered as a component of the typhoid bacillus in 1934. Since that time, serologically identical or very closely related Vi antigens have been found to occur in almost all freshly isolated strains of Salmonella tyohosa, in some strains of Salmonella paratyphi C and in one strain each of Paracolobactrum ballerup and Escherichia coli. The purpose of the present investigation was to immununologically characterize Vi antigens which were purified by the method of Baker (Baker, E. E., To be published). Serologic properties of Vi antigens were studied by the agar diffusion technique. It was found that zones of specific precipitate could be identified in the agar which were due to Vi antigen and its antibody. Vi antigens prepared from P. ballerup, E. coli, S. typhosa, strain Ty2 and S. typhosa, strain 61 were serologically identical by this method. This precipitin technique presented itself as a highly sensitive method for detection of small amounts of antigens and as little as 1.0 microgram of purified Vi antigen per milliliter of 0.15 M sodium chloride solution formed a visible zone of precipitation. In order to be certain that the properties of Vi antigen under consideration were attributed solely to Vi antigen, this sensitive agar diffusion technique was applied to determine purity of antigen preparations. It was noted that Vi antigens derived from P. ballerup and S. typhosa, strain 61 were serologically pure substances. However, Vi antigen preparations were isolated from E. coli and S. typhosa, strain Ty2 which contained a very small amount of another substance. A serologic study of Vi antigen was made in which several concentrations of Vi antigens were reacted in agar diffusion precipitin tests with different dilutions of antisera. The antisera were prepared by immunizing rabbits with acetone killed and dried organisms containing Vi antigen. It was observed that at a certain ratio of Vi antigen to antibody, two precipitate zones were visible. Since the position of the precipitate zone in the agar was determined by the concentration of reactants when other factors remained constant, one antibody concentration was reacted with decreasing concentrations of Vi antigen. The two zones of specific precipitate became superimposed and further dilution of Vi antigen did not result in separation of the two zones. It may be that acetylated and a small amount of deacetylated Vi antigen were present in the purified preparations. Like the capsular polysaccharide of Pneumococens Type I, acetylated Vi antigen may be capable of absorbing all the specific antibody while the deacetylated antigen may have been capable of absorbing only antibody reactive with the deacetylated antigen leaving the antibody titer for the acetylated antigen unchanged. This hypothesis was strengthened by agar diffusion precipitin studies using Vi antigen which was deacetylated by hydrolysis with sodium hydroxide. Vi antigens were characterized further biologically. Acetone killed and dried Vi containing organisms were excellent immunizing agents in the rabbit. However, purified Vi antigens from these organisms were very poor immunizing agents in rabbits. Rabbits were apparently very sensitive to intoxication by the Vi containing organisms under study. Therefore, purified Vi antigens were tested for toxic effects in these animals. It was found that the pure Vi antigens were relatively non-toxic even when administered in large doses. Purified Vi antigens were employed as preparatory injections in an effort to demonstrate the Shwartzman phenomenon of local skin reactivity. It has been shovm that pure Vi antigens were incapable of preparing rabbit skin for the local Shwartzman reaction. The agar diffusion technique was employed in comparative studies of antigens from several strains of S. typhosa. Antigenic analyses of the various strains of S. typhosa are still under investigation. A technique of absorption directly in the agar diffusion plate was proposed and is presently under study.
Thesis (Ph.D.)--Boston University
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