Genetic control of the innate resistance of mice to Salmonella typhimurium: expression of the Ity gene in peritoneal and splenic macrophages isolated in vitro.

The mouse Chromosome 1 locus Ity regulates the extent to which Salmonella typhimurium replicates within the reticuloendothelial cell system (RES) during the first days of infection. If animals are homozygous for the Itys susceptibility allele, the Gram-negative bacterium undergoes rapid net multiplication, and mice die of a typhoid fever-like disease by day 10 of infection. Animals that are homozygous or heterozygous for the resistance allele, Ityr, control net bacterial replication and survive the first phase of salmonellosis. Indirect studies have implicated the resident macrophage as the effector cell for regulation of early in vivo salmonellae growth. To verify this supposition and to evaluate the phenotypic expression of Ity, we developed an in vitro assay to compare kinetics of S. typhimurium growth within Ityr and Itys macrophages. Resident peritoneal and splenic macrophages were used from inbred Ityr and Itys mice and from Ity congeneic mice. With these mice and through the use of radiolabeled S. typhimurium and an avirulent temperature-sensitive mutant of the bacterium, we found that: phagocytosis of S. typhimurium by Ityr and by Itys macrophages was the same; S. typhimurium grew to a greater extent in Itys peritoneal and splenic macrophages than in Ityr cells; Ityr macrophages killed intracellular salmonellae more efficiently than did Itys macrophages. Thus, we have demonstrated directly that Ity is expressed by the macrophage and have shown for the first time with Ity congeneic mice that the basis for differential net growth of virulent S. typhimurium in Ityr and Itys macrophages is a variation in the degree of bacterial kill.