Enhancement of nitric oxide release in mouse inflammatory macrophages co-cultivated with tumor cells of a different origin.

In the present study we investigated whether synthesis of nitric oxide (NO) by macrophages is affected by contact with tumor cells. Although it is well known that NO generated by macrophages influences different activities related to tumor progression, there is limited information on the modulatory role of tumor cells on NO release by macrophages. The experimental protocol used in our study consisted in the determination of NO secreted by macrophages, either resident or inflammatory, co-cultivated with tumor cells (B16 melanoma and L929 fibrosarcoma cells) at different cell densities and macrophage:tumor cell ratios. This experimental in vitro protocol simulates the different interactions between macrophages and tumor cells that occur during the development of a tumor mass. We found that the co-cultivation with tumor cells induced an increased secretion of NO in macrophages provided that they express an inflammatory phenotype, and they were challenged with LPS or IFNgamma/LPS. Two more variables were found to be critical in the increase of NO generation in inflammatory macrophages cultivated with tumor cells: a high cell density and a prevalence of tumor cells over macrophages. The enhancement of NO secreted in inflammatory macrophages stimulated by tumor cells was not observed in normal murine fibroblasts co-cultivated with tumor cells.