Role of T cells, TNFα and IFNγ in recall of immunity to oral challenge with virulent salmonellae in mice vaccinated with live attenuated aro^- salmonella vaccines

The SL3261 Salmonella typhimurium aro A live vaccine strain confers solid protection against oral challenge with virulent salmonellae, immunity persisting long after the vaccine has been cleared from the tissues. BALB/c mice immunized with SL3261 and later subjected to in vivo depletion of both CD4⁺ and CD8⁺ T cells had impaired recall of immunity to oral challenge with the virulent S. typhimurium C5, with increased mortality and higher bacterial loads in the reticuloendothelial system (RES). Selective depletion of CD4⁺ cells alone significantly impaired resistance both 8 and 14 weeks after vaccination as determined by estimation of bacterial numbers in organ homogenates. Depletion of CD8⁺ cells alone had less effect on immunity when performed at 8 weeks than at 14 weeks after immunization. Administration of anti-IFNγ or anti-TNFα antibodies also impaired recall of immunity, exacerbating a secondary infection in vaccinated mice. Challenge of T cell-depleted immune mice with virulent salmonellae caused hepatosplenomegaly with minute grossly visible focal lesions, and a marked increase in the number and severity of necrotic foci in spleen, liver and lymph nodes. A widespread mononuclear cell infiltrate was present. The histopathology in anti-IFNγ-treated mice was qualitatively similar to that seen in T-cell depleted mice. In contrast, in the anti-TNFα-treated mice splenomegaly was much less than in T cell-depleted mice. Granulomas were absent, no mononuclear infiltration was observed and there was severe necrosis; the lesions appeared similar to or worse than those seen in naïve mice. Surprisingly, IFNγ was detectable in sera of both controls and T cell-depleted mice on day 8 of the secondary infection, as well as in sera of anti-TNFα-treated mice on day 6 of infection. The results indicate that T cells, IFNγ and TNFα are all important in the specific recall of immunity to virulent salmonellae conferred by immunization with live vaccines, with the effect of T cell and IFNγ depletion (marked macrophage infiltration) being qualitatively very different from that of TNFα neutralization (no mononuclear infiltrate or granuloma formation).