Mechanism of the protective action of anti-Salmonella IgM in experimental mouse salmonellosis

The kinetics of mouse salmonellosis caused by Salmonella typhimurium was studied in mice preinjected with the IgM or IgG fraction prepared from a rabbit anti-Salmonella serum. Compared on the basis of antibody units determined by an enzyme immunoassay, IgM was ten times more effective than IgG in promoting removal of the bacteria from blood after intravenous (IV) injection and their uptake in the reticuloendothelial system (RES). The subsequent killing of the bacteria was, however, only minor, in accord with the negligible protective effect of serum antibodies in IV infection. IgM was over 1000 times more effective than IgG in promoting killing of the bacteria after intraperitoneal (IP) challenge. Neither antibody had an effect on the multiplication of the bacteria in the RES. The protective action of antibody was thus almost entirely mediated by peritoneal-cavity cells acting in the very early phase of infection. The greater effect of IgM is suggested to be a special feature of Salmonella infections, connected with the capacity of these bacteria for intracellular survival and multiplication in the RES