Nitric oxide synthesis contributes to inhibition of graft-versus-tumor-effects against intraperitoneal Meth A tumor

The role of nitric oxide (NO) in graft-versus-tumor-effect (GVT) was evaluated in the present study. GVT was induced by intravenous injection of C57BL/6J (H-2b) mouse splenocytes to {C57BL/6J (H-2b) × BALB/c (H-2d)} F1 mice bearing Meth A (H-2d) ascites tumors. Induction of GVT increased nitrite production and expression of inducible NO synthase by ascites cells. The increased nitrite production was inhibited by NG-monomethyl-l-arginine (MLA). Experiments employing immunomagnetic depletion of Mac-1+ cells from ascites indicated that macrophages were a major cellular source of the nitrite production. Interferon-γ levels were increased in both serum and ascites fluid during GVT. Induction of GVT prolonged survival of ascites-bearing mice, and increased urinary nitrate excretion. MLA administration inhibited GVT-induced increase in urinary nitrate excretion, and further prolonged GVT-induced increase in survival. These results indicate that NO synthesis is induced in tumors during GVT, and the NO acts as an inhibitor of GVT.