ABSTRACT
CB-17 scid mice exhibit a T-cell-independent but gamma interferon (IFN-gamma)-dependent immunity to Listeria monocytogenes. In this study, we analyzed the specific cellular interactions involved in this process. scid mouse-derived natural killer (NK) cells cultured with heat-killed (HK) L. monocytogenes and macrophages secreted IFN-gamma. No IFN-gamma was produced in cultures containing HK L. monocytogenes but lacking macrophages. However, medium derived from macrophages incubated with HK L. monocytogenes or other microorganisms stimulated IFN-gamma production by isolated NK cells. Treatment of macrophage-conditioned supernatants with neutralizing monoclonal anti-tumor necrosis factor (TNF) significantly reduced their capacity to stimulate NK cells to produce IFN-gamma. Yet, purified recombinant TNF-alpha by itself was unable to stimulate NK cells. Thus, TNF was necessary but not sufficient to induce maximal IFN-gamma production by NK cells. Sonicated L. monocytogenes stimulated production of IFN-gamma by NK cells that was resistant to anti-TNF. Stimulation was markedly enhanced by the addition of recombinant TNF-alpha. These studies demonstrated that activation of scid NK cells for secretion of IFN-gamma requires two signals: TNF-alpha and a second product which may be of bacterial origin and may require processing by mononuclear phagocytes. We suggest that the T-cell-independent production of IFN-gamma by NK cells provides the host with a rapid mechanism to temporarily heighten nonspecific resistance to infection until such time as T-cell-dependent sterilizing immune responses can be generated.
