Antibody Formation in Mice Infected with Salmonella typhimurium by Primary Immunization with Sheep Erythrocytes or Bacterial Cells

Infection of mice with a strain of Salmonella typhimurium , known to multiply in macrophages, resulted in a marked alteration of their antibody response to further antigenic stimulation with sheep red blood cells (sRBC) or killed bacterial cells. The mice infected with a wild‐type S. typhimurium and simultaneously immunized with sRBC, showed a significantly stimulated immune response to sRBC as revealed by the increased number of hemolytic plaque‐forming cells in their spleens during the period following such immunization. Whether the animals were infected intraperitoneally or intravenously did not affect the stimulant effect of the infection on the immune response to sRBC. Injection of killed bacteria in amounts up to 10 6 organisms, instead of infection with live organisms, did not elicit any effective enhancement of antibody formation to sRBC. In cases of attenuated S. typhimurium infection, the stimulant effect on antibody formation to sRBC was obvious in the mice infected with the organisms simultaneously with or 1 week prior to the sRBC injection, while a slight suppression of antibody formation was observed in mice infected 3 weeks before the injection with sRBC. A similar effect of the infection was observed when the serum anti‐bacterial antibody response was examined after stimulation with bacterial antigens. The altered immune responses after Salmonella infection are discussed in relation to the adjuvant effect of bacterial endotoxins and also to the acquired immunity to mouse typhoid.