Natural Killer cell Stimulatory Factor (NKSF) or interleukin-12 (IL-12) is a heterodimeric cytokine of 70 kDa formed by a heavy chain of 40 kDa (p40) and a light chain of 35 kDa (p35). Although it was originally identified and purified from the supernatant of Epstein-Barr virus-transformed B cell lines, it has been shown that among peripheral blood cells NKSF/IL-12 is predominantly produced by monocytes, with lower production by B cells and other accessory cells. The most powerful inducers of NKSF/IL-12 production are bacteria, bacterial products and parasites. In addition to the biologically active p70 heterodimer, the cells producing NKSF/IL-12 also secrete a large excess of monomeric p40, a molecule with no demonstrable biological activity. NKSF/IL-12 is active on T lymphocytes and NK cells on which it induces production of lymphokines, enhancement of cytotoxic activity and mitogenic effects. NKSF/IL-12 induces T and NK cells to produce IFN-gamma and synergizes with other IFN-gamma inducers in this effect. In vitro, and probably in vivo, NKSF/IL-12 is required for optimal IFN-gamma production. When human lymphocytes are stimulated with antigens in vitro, addition of exogenous NKSF/IL-12 to the culture induces differentiation of T helper type 1 (Th1) cells, whereas neutralization of endogenous NKSF/IL-12 with antibodies favors differentiation of Th2 cells. IFN-gamma, a product of Th1 cells, enhances NKSF/IL-12 production by mononuclear cells, whereas IL-10 and IL-4, products of Th2 cells, efficiently inhibit it. Therefore, NKSF/IL-12 appears to be an important inducer of Th1 responses produced by accessory cells during early antigenic stimulation and its production is regulated by a positive feedback mechanism mediated by Th1 cells through IFN-gamma and a negative one by Th2 cells through IL-10 and IL-4. The balance of IL-12 production versus IL-10 and IL-4 production early during an immune response might therefore be instrumental in determining Th1-type versus Th2-type immune responses. Because of this potential role of IL-12 during immune responses, our results demonstrating the impaired ability of HIV seropositive patients to produce NKSF/IL-12 in response to bacterial stimulation suggest that this defect in NKSF/IL-12 production might be a factor contributing to their immune depression.