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The R5 (chemotype Rb) but not the R10 (chemotype Rd) mutant of murine pathogen Salmonella typhimurium 395MS was extremely virulent in intraperitoneal infections of C57BL/10ScCr mice carrying the ityS and lpsD alleles. C57BL/6J (ityS lpsN) and C3H/HeJ (ityR lpsD) mice showed a much higher resistance to the R5 mutant. Further studies were performed with peritoneal macrophages in vitro in order to elucidate susceptibility in lipopolysaccharide (LPS)-hyporesponsive mice carrying ItyS. The intracellular killing capacity of the ItyS LpsD macrophages was lower than that of the ItyS LpsN macrophages for the R5 mutant and may partly explain the increased susceptibility of the ItyS LpsD mice. The deep rough mutant, R10, was rapidly killed intracellularly by the ItyS LpsD macrophages. Processing of the bacteria in macrophages that had phagocytosed R5 or R10 bacteria was followed for up to 18 days by endotoxin measurements (limulus assay) and immunostaining, with monoclonal antibodies to various parts of the LPS molecule being used. Only 0.1% or less of the macrophage-associated bacteria remained alive after 48 h of incubation, and none were alive on day 7. Although immunostaining showed that LPS was present in both the LpsD and LpsN macrophages during the whole incubation period of 18 days, endotoxin activity in the LpsD macrophages on day 7 was lower than that in the LpsN macrophages, indicating that qualitative modifications of the chemical composition or physical state of the LPS molecule occurred. The interleukin-6 response in the ItyS LpsD macrophages was delayed and of shorter duration compared with that in the ItyS LpsN macrophages. The results suggest that the difference between the LPS-hyporesponsive and -responsive ItyS mice in susceptibility to infection with the R5 mutant was due to the lower activation state of the LpsD macrophages during infection, leading to a lower intracellular bactericidal systems of the macrophages. A rapid killing of the bacterium should restrict the infection and may partly compensate for a diminished inflammatory response. The persistence of LPS within the cells is discussed.

Susceptibility of lipopolysaccharide-responsive and -hyporesponsive ItyS Mice to infection with rough mutants of Salmonella typhimurium