Summary:Interleukin-12 (IL-12) is a heterodimeric cytokine that enhances the cytolytic activity, proliferation, and interferon-γ (IFN-γ ) production by T lymphocytes and natural killer cells, and has significant antitumor activity in a variety of murine tumor models. The induction of interferon (IFN)-γ by IL-12 in tumor-bearing mice plays an important role in its antitumor activity. We therefore examined the effects of IL-12 on IFN-γ production by T cells derived from patients with renal cell carcinoma (RCC), including freshly isolated tumor infiltrating lymphocytes (T-TIL), matched peripheral blood T cells (T-PBL), and RCC-specific TIL lines. IL-12 alone induced IFN-γ secretion by T cells from normal individuals and appeared to act synergistically with either IL-2 or anti-CD3 antibody. In contrast, it failed to stimulate significant IFN-γ secretion by T-PBL and T-TIL from RCC patients. This unresponsive state in T-PBL appeared selective because IFN-γ was produced when cells were stimulated with either phytohemagglutinin or anti-CD3 antibody. Moreover, costimulation through the T-cell receptor (TCR)/CD3 complex or with IL-2 made T-PBL from RCC patients responsive to IL-12, possibly secondary to the upregulation of IL-12R (β chain). A selective loss of IL-12-dependent production of IFN-γ was also consistently observed in two of three established RCC-specific TIL lines. Although these cell lines did not respond to any concentration of IL-12, they did produce IFN-γ after ligation of the TCR/CD3 or stimulation with IL-2. IL-12 also acted either syngeristically or additively with IL-2, anti-CD3 antibody, or autologous tumor cells to induce IFN-γ production. The observed decreases in IFN-γ production in response to IL-12 may have a negative effect on the development of T-cell immunity. The clinical importance of these findings during in vivo administration of IL-12 remains to be determined.