Interleukin 12 Primes Macrophages for Nitric Oxide

Interleukin-12 (IL-12) is a recently described iminunoregulatory cyto kine with potent therapeutic activity in various preclinical models of infectious or malignant disease. As part of our ongoing evaluation of potential mechanisms accounting for the potent antitumor activity of IL-12,wehaveinvestigatedtheinfluenceof IL-12administrationontotal serum nitrate/nitrite (NO@)levels and the production ofnitric oxide (NO) by peritoneal macrophages from normal and tumor-bearing mice. We report here that IL-12 administration to either normal or tumor-bearing mice for periods of time ranging from 7-19 days induced progressive increases in serum NO@ levels and primed pentoneal macrophages for NO production on subsequent exposure to lipopolysaccharide or IL-2 ex vivo. Treatment of resident peritoneal macrophages or the macrophage cell line ANA-1 with IL-12 alone or IL-12 in combination with various other sthnuli failed to induce NO production, suggesting that the effects of IL-12occurredvia an indirectmechanism. Furthermore,wehaveshown that not only was the production of NO by macrophages from untreated long-term, tumor-bearing mice suppressed compared with control mice treated with vehide or IL-12, but also that IL-12 administration overcame this suppression and delayed tumor growth. Lastly, we have shown that administration of weekly pulses of IL-2 in combination with IL-12 addi tively enhanced the priming of macrophages for NO production ex vivo and delayed tumor growth far more effectively than either agent alone. These observations and reports in the literature regarding the potential influence of NO on development of the immune response and on the regulation of tumor growth and vascularization suggest that NO may play a significant role in the antitumor activity of LL-12and IL-2.