Leishmania donovani infection in scid mice: lack of tissue response and in vivo macrophage activation correlates with failure to trigger natural killer cell-derived gamma interferon production in vitro.

Infection of immunocompetent mice with Leishmania donovani is characterized by the development of a tissue granulomatous response, in vivo macrophage activation, and a predominantly Th1-type CD4+ T-cell response. To determine whether a recently described T-cell-independent pathway of gamma interferon (IFN-gamma) production involving the collaboration of macrophages and natural killer (NK) cells contributed to this pattern of events, we have investigated the responses of scid mice to L. donovani infection. The multiplication of parasites in the livers of scid mice progressed at a rate equivalent to that seen in BALB/c mice over the first 14 days of infection, but by day 28 scid mice had a fivefold-higher parasite burden. This infection was not, however, accompanied by any demonstrable histological response in the liver or by elevated major histocompatibility complex class II expression on splenic macrophages. In vitro, L. donovani was unable to trigger IFN-gamma production from scid spleen cell cultures under conditions which allowed efficient triggering by bacterial stimuli. Although L. donovani also failed to stimulate the release of tumor necrosis factor, an important macrophage-derived cofactor for IFN-gamma secretion by NK cells, exogenous recombinant tumor necrosis factor alpha could not restore the IFN-gamma response. Even with the potent synergistic effect of exogenous interleukin-2, L. donovani was unable to stimulate this pathway to the same extent as Listeria monocytogenes. Indeed, L. donovani inhibited the response to L. monocytogenes in a dose-dependent fashion. Experiments involving the transfer of supernatants and the use of neutralizing monoclonal antibodies have failed to find evidence that interleukin-10 is involved in this inhibition. These data suggest that NK cell-derived IFN-gamma is unlikely to participate in the early regulation of visceral leishmaniasis in the mouse.