IMMUNE SUPPRESSION BY RECOMBINANT INTERLEUKIN (RIL)-12 INVOLVES INTERFERON-GAMMA INDUCTION OF NITRIC-OXIDE SYNTHASE-2 (INOS) ACTIVITY - INHIBITORS OF NO GENERATION REVEAL THE EXTENT OF RIL-12 VACCINE ADJUVANT EFFECT

Recombinant interleukin 12 (IL-12) can profoundly suppress cellular im mune responses in mice. To define the underlying mechanism, recombinan t murine (rm)IL-12 was given to C57BL/6 mice undergoing alloimmunizati on and found to transiently but profoundly suppress in vivo and in vit ro allogeneic responses and in vitro splenocyte mitogenic responses. U se of neutralizing antibodies and genetically deficient mice showed th at IFN-gamma (but not TNF-alpha) mediated rmIL-12-induced immune suppr ession. Splenocyte fractionation studies revealed that adherent cells from rmIL-12-treated mice suppressed the mitogenic response of normal nonadherent cells to concanavalin A and IL-2. Addition of an inhibitor of nitric oxide synthase (NOS) restored mitogenic responses, and indu cible (i)NOS-/- mice were not immunosuppressed by rmIL-12. These resul ts support the view that suppression of T cell responses is due to NO produced by macrophages responding to the high levels of IFN-gamma ind uced by rmIL-12. When a NOS inhibitor was given with rmIL-12 during va ccination of A/J mice with irradiated SCK tumor cells, immunosuppressi on was averted and the extent of rmIL-12's ability to enhance inductio n of protective antitumor immunity was revealed. This demonstrates tha t rmIL-12 is an effective vaccine adjuvant whose efficacy may be maske d by its transient immunosuppressive effect.