Immunologic Comparison of Westphal-Type Lipopolysaccharides and Free Endotoxins from an Encapsulated and a Nonencapsulated Avian Strain of Pasteurella multocida

SUMMARY The immunogenicity and the gel diffusion-precipitin reactions of a free endotoxin and a Westphal lipopolysaccharide (lps) from cells of virulent encapsulated Pasteurella multocida were compared to the same antigens from cells of an avirulent nonencapsulated mutant. The losses of capsule and virulence had almost no effect on the serologic reactions. Differences in rates of induction of active immunity in mice were obtained with the free endotoxins of the 2 cell forms. The free endotoxin from the encapsulated cells induced good immunity in 3 weeks, but that from the nonencapsulated cells induced an equivalent degree of immunity only by the 6th week. Major differences were observed between the lps and the free endotoxins. The free endotoxins readily induced antibody formation and active immunity, but the lps did not. Although lps and endotoxins both contained components usually found in endotoxins, including nitrogen, phosphorus, heptose, and hexose, the free endotoxin had more nitrogen. Gel diffusion-precipitin reactions of partial identity were obtained between the free endotoxin and the lps. The antigen, presumably lps, obtained from the free endotoxin by phenol extraction has been found to give a line of identity with the lps from the cells. Both the lps and the free endotoxin were reactive with homologous antiserum prepared against whole cells, but the free endotoxin was cross-reactive with some of the 15 other serotypes, whereas lps was not cross-reactive. Vigorous mixing of the free endotoxin or whole cell preparation with 50% phenol at room temperature and subsequent dialysis to remove the phenol resulted in complete loss of ability to induce active immunity. Since most polysaccharides are stable to phenol and most proteins are irreversibly denatured by it, it would appear that some form of protein is necessary for the induction of active immunity.