Analysis of host cells associated with the Spv-mediated increased intracellular growth rate of Salmonella typhimurium in mice

The 90-kb virulence plasmid of Salmonella typhimurium encodes five spy genes which increase the growth rate of the bacteria within host cells within the first week of systemic infection of mice (P. A. Gulig and T., J. Doyle, Infect, Immun. 61:504-511, 1993), The presently described study was aimed at identifying the host cells associated,with Spy-mediated virulence by manipulating the mouse host and the salmonellae. To test the effects of T cells and B cells on the Spy phenotype, salmonellae were orally inoculated into nude and SCID BALB/c mice, Relative to normal BALB/c mice, nude and SCID BALB/c mice were unaffected for splenic infection with either the Spv(+) or Spv(-) S. typhimurium strains at 5 days postinoculation, When mice were pretreated with cyclophosphamide to induce granulocytopenia, there was a variable increase in total salmonella infection, but the relative splenic CFU of Spv(+) versus Spv(-) S. typhimurium was not changed after oral inoculation. In contrast, depletion of macrophages from mice by treatment with cyclophosphamide plus liposomes containing dichloromethylene diphosphate resulted in equivalent virulence of Spv(+) and Spv(-) salmonellae. To examine if the spy genes affected the growth of salmonellae in nonphagocytic cells, an invAnaphT mutation was transduced into Spv(+) and Spv(-) S., typhimurium strains, InvA(-) Spv(+) salmonellae were not significantly affected for splenic infection after subcutaneous inoculation compared with the wild-type strain, and InvA(-)Spv(-)salmonellae were only slightly attenuated relative to InvA(+) Spv(-)salmonellae, Invasion-defective salmonellae still exhibited the Spy phenotype, Therefore, infection of nonphagocytes is not involved with the Spy virulence function. Taken together, these data demonstrate that macrophages are essential for suppressing the infection by Spv(-)S. typhimurium, by serving as the primary host cell for Spy-mediated intracellular replication and possibly by inhibiting the replication of salmonellae within other macrophages.