THE SALMONELLA-DUBLIN VIRULENCE PLASMID DOES NOT MODULATE EARLY T-CELL RESPONSES IN MICE

The virulence plasmid in Salmonella dublin mediates systemic infection in mice and cattle, The role of gamma delta T cells or hepatic extrat hymic T cells has recently been reported to be important in the contro l of the early stage of Salmonella choleraesuis infections of mice, He re we report on T-cell responses in conventional mice after challenge with a virulent strain of S. dublin carrying a virulence plasmid or wi th a strain cured of the plasmid. Over a period of 4 days postinfectio n, when both strains could be compared, similar changes in alpha beta and gamma delta T-cell subsets in peritoneal cavities, livers, and spl eens were recorded, demonstrating no clear role of the virulence plasm id in modulation of early T-cell responses. To investigate further the role of the virulence plasmid in pathogenesis, the growth of the plas mid-cured strain was assessed in SCID, SCID bg, and irradiated mice. D uring the first 6 days after infection, there was no statistically sig nificant difference in the net growth of Salmonella cells in the liver s and spleens of SCID and SCID bg mice compared with conventional BALB /c mice, This observation excludes a key role for a T- or B-cell-media ted immune response in controlling the initial growth of the plasmid-c ured S. dublin strain, Thereafter, the immunocompromised mice were no longer able to control infection, although SCID mice were more efficie nt at controlling net bacterial multiplication than SCID bg mice, pote ntially implicating NX cells in the control of infection in SCID mice, The early control of net bacterial multiplication in the spleens and livers of BALBlc mice was ablated by whole-body X-irradiation. Both wi ld-type and plasmid-cured strains multiplied significantly more rapidl y in irradiated than in conventional BALB/c mice, However, the numbers of wild-type bacteria still increased more rapidly than the numbers o f the cured strain, These results are consistent with a role of the S, dublin virulence plasmid in promoting in vivo growth of Salmonella ce lls.