Interleukin-12 (IL-12) can promote tumor
regression via activation of multiple lymphocytic and myelocytic
eVectors. Whereas the cytotoxic mechanisms employed
by T/NK/NKT cells in IL-12-mediated tumor kill are well
deWned, the antitumor role of macrophage-produced cytotoxic
metabolites has been more controversial. To this end,
we investigated the speciWc role of nitric oxide (NO), a major
macrophage eVector molecule, in post-IL-12 tumor regression.
Analysis of tumors following a single intratumoral
injection of slow-release IL-12 microspheres showed an
IFN-dependent sevenfold increase in inducible nitric oxide
synthase (iNOS) expression within 48 h. Flow cytometric
analysis of tumor-resident leukocytes and in vivo depletion
studies identiWed CD11b+ F4/80+ Gr1lo macrophages as the
primary source of iNOS. Blocking of post-therapy iNOS
activity with N-nitro-L-arginine methyl ester (L-NAME)dramatically enhanced tumor suppression revealing the
inhibitory eVect of NO on IL-12-driven antitumor immunity.
Superior tumor regression in mice receiving combination
treatment was associated with enhanced survival and proliferation
of activated tumor-resident CD8+ T-eVector/memory
cells (Tem). These Wndings demonstrate that macrophageproduced
NO negatively regulates the antitumor activity of
IL-12 via its detrimental eVects on CD8+ T cells and identify
L-NAME as a potent adjuvant in IL-12 therapy of cancer.