T-cell hybridomas reveal two distinct mechanisms of antileishmanial defense

Using lymph node lymphocytes of Leishmania major-infected mice, we constructed and cloned two T-cell hybridomas that could activate macrophages to exert antileishmanial defense in vitro. One clone, 1D5, produced lymphokines (including gamma interferon) that induced these effects. Production of the macrophage-activating lymphokines and the protective effect of 1D5 were suppressed by the addition of cyclosporine A to cultures. The other clone, 1B6, produced no detectable macrophage-activating lymphokines, and its protective ability was not suppressed by cyclosporine A. Granulocyte-macrophage colony-stimulating factor (a lymphokine also known to induce antileishmanial effects in macrophages) was not detectable in culture supernatants of either clone. Furthermore, neither clone was cytotoxic to infected macrophages. Antileishmanial defense induced by 1B6 was genetically restricted; that is, infected macrophages and hybridoma cells had to be syngeneic for an antileishmanial effect to occur. In contrast, such restriction was not a property of clone 1D5, a clone that was responsive to alloantigens as well as leishmanial antigens. When incubated at a temperature (34 degrees C) at which lymphokines are relatively ineffective for antileishmanial defense, 1B6 but not 1D5 retained its antileishmanial properties. These observations provide clear evidence for the existence of two distinct mechanisms of macrophage activation: one that is lymphokine dependent, and one that is apparently lymphokine independent. The expression of these two mechanisms by cloned cells strongly suggests that they are properties of different T-cell subpopulations, extending our prior conclusions based on studies of heterogeneous T-cell populations. We hypothesize that the latter macrophage activation process involves a cell contact-dependent mechanism which might involve the interaction of a lymphocyte membrane-associated macrophage-activating factor (such as tumor necrosis factor) with its receptor on the macrophage, resulting in activation of antileishmanial effects but not host cell cytotoxicity.