TNF-alpha mediated selection of macrophage-resistant gene-regulatory tumor variants

In vitro macrophage- or TNF-alpha-mediated selection procedures on 3LL tumor cells have led to the selection of 3LL variants manifesting a highly reduced sensitivity towards the cytotoxic effects of both TNF-alpha and tumoricidal macrophages, while retaining the parental sensitivity to the cytolytic activity of (i) H2O2, (ii) macrophage-ADCC reactions and (iii) NK cells. A correlation was observed between the TNF-alpha binding capacity of the 3LL cell lines and their susceptibility towards macrophage- and TNF-alpha-mediated cytotoxicity, indicating that macrophage and TNF-alpha sensitivity may partially be regulated at the TNF-alpha receptor level. Further, the selected 3LL variants are gene-regulatory variants rather than cellular mutants, as upregulation of the TNF-alpha receptor by interferon-gamma (IFN-gamma) or 5'-azacytidine treatment resulted in an increased vulnerability of the selected 3LL variants to the killing activity of macrophages and TNF-alpha. The resistance of the 3LL variants to macrophage- and TNF-alpha-mediated cytotoxicity in vitro was reflected by a higher tumorigenic and metastatic potential in vivo. Therefore, the generation of TNF-alpha- and macrophage-resistant variants through immunoselection may contribute to the basic mechanisms of tumor progression and metastasis.