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 cell s within the first week of systemic infection of mice (P. A. Gulig and T., J. Doyle, Infect, Immun. 61:504-511, 1993), The presently describ ed study was aimed at identifying the host cells associated,with Spy-m ediated virulence by manipulating the mouse host and the salmonellae. To test the effects of T cells and B cells on the Spy phenotype, salmo nellae 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 stra ins at 5 days postinoculation, When mice were pretreated with cyclopho sphamide to induce granulocytopenia, there was a variable increase in total salmonella infection, but the relative splenic CFU of Spv(+) ver sus Spv(-) S. typhimurium was not changed after oral inoculation. In c ontrast, depletion of macrophages from mice by treatment with cyclopho sphamide plus liposomes containing dichloromethylene diphosphate resul ted in equivalent virulence of Spv(+) and Spv(-) salmonellae. To exami ne if the spy genes affected the growth of salmonellae in nonphagocyti c cells, an invAnaphT mutation was transduced into Spv(+) and Spv(-) S ., typhimurium strains, InvA(-) Spv(+) salmonellae were not significan tly affected for splenic infection after subcutaneous inoculation comp ared with the wild-type strain, and InvA(-)Spv(-)salmonellae were only slightly attenuated relative to InvA(+) Spv(-)salmonellae, Invasion-d efective salmonellae still exhibited the Spy phenotype, Therefore, inf ection of nonphagocytes is not involved with the Spy virulence functio n. Taken together, these data demonstrate that macrophages are essenti al for suppressing the infection by Spv(-)S. typhimurium, by serving a s the primary host cell for Spy-mediated intracellular replication and possibly by inhibiting the replication of salmonellae within other ma crophages.