Activation of the human immature natural killer cell subset by IL-12 and its regulation by endogenous TNF-alpha and IFN-gamma secretion.

Interleukin-12 (IL-12) is a newly described cytokine that can activate NK cell cytotoxic function. The present study investigated the activation pathway of human peripheral blood-derived immature NK cells by IL-12 and its comparison to activation by interleukin-2 (IL-2). Immature NK cells were prepared by flow cytometry cell sorting techniques whereby the nonbinding Free NK subset was separated from the NK target conjugate subpopulation. Coculture of Free cells with IL-12 overnight resulted in enhanced cytotoxicity and recruitment of cytotoxic Killer cells. The IL-12-mediated activation of cytotoxicity was dependent on endogenous TNF-alpha secretion and was suppressed by endogenous (interferon-gamma (IFN-gamma) secretion. However, IL-2-mediated activation was inhibited by anti-TNF-alpha antibody, but was not affected by endogenous IFN-gamma secretion. There was an upregulation of the expression of CD69 and CD25 surface antigens by IL-12-treated cells and this upregulation was diminished in the presence of anti-TNF-alpha antibody. IL-12 also upregulated the expression of the 75-kDa TNF receptor (TNF-R p75) and the upregulation was inhibited by anti-IFN-gamma antibody. The 55-kDa TNF receptor (TNF-R p55), however, was not affected by IL-12, but in the presence of anti-IFN-gamma antibody, its expression was enhanced. Thus, these results suggest that IFN-gamma selectively upregulates the expression of TNF-R p75 by IL-12 but inhibits the expression of TNF-R p55 by IL-12. Unlike IL-2, IL-12, does not stimulate the proliferation of Free cells. Furthermore, unlike IL-2, IL-12 does not stimulate the secretion of TNF-alpha by Free cells, but significantly stimulates IFN-gamma secretion. These findings demonstrate that IL-12 share some, but not all, properties of IL-2 in the activation pathway of immature NK Free cells. Furthermore, the pathway of activation of Free cells by IL-12 is under the regulation of both endogenous TNF-alpha and IFN-gamma secretion.