Homologous and Heterologous Resistance in Mice Given Bacterial Endotoxins

The single injection into mice of a sublethal amount of endotoxin from Brucella melitensis confers partial resistance to subsequent lethal doses for at least 4 mo. Three weekly injections confer a greater protection, which persists for at least 10 mo. Three weekly injections of endotoxins from Salmonella typhosa, Escherichia coli, or Shigella sonnei result in homologous resistance for 2 mo. Resistance to heterologous endotoxins in mice immunized with either brucella, typhoid, coli, or shigella endotoxins was inconstant and persisted less than 1 mo. A striking exception was that heterologous protection against brucella endotoxin was consistently present for at least 4 mo. Serum precipitins for heterologous endotoxins were formed in only 4 of 60 mice tested; in contrast, intradermal reactions to heterologous endotoxins occurred in 16 of 46 animals. Resistance to infection with Brucella melitensis was maximal in mice immunized with brucella endotoxin, but partial resistance occurred in mice immunized with heterologous endotoxins. The mechanism of resistance to endotoxin was discussed but no complete explanation was advanced.