Mechanism of action differences in the antitumor effects of transmembrane and secretory tumor necrosis factor-alpha in vitro and in vivo.

The proinflammatory cytokine tumor necrosis factor-alpha (TNFalpha) exists naturally in two forms, a 26 kDa transmembrane form (TM-TNFalpha), and a 17 kDa secretory form (S-TNFalpha). The biological roles for each of these forms of TNFalpha in tumor killing have not been completely elucidated. Therefore, in this study, three different recombinant retroviral vectors, wild-type TNFalpha, solely secretable TNFalpha mutant, and uncleavable transmembrane TNFalpha mutant, were constructed by molecular techniques and stably transfected into a murine hepatic carcinoma cell line (H22). TNFalpha, either secreted in cell culture supernatants by secretable TNFalpha mutant- or wild-type TNFalpha-producing tumor cells, or as a treansmembrane form expressed on the cell surface of uncleavable TNFalpha mutant- or wild-type TNFalpha-synthesizing tumor cells, was demonstrated to be cytotoxic against the TNF sensitive L929 cell line. The H22 cells transfected with the three different forms of TNFalpha were shown to kill parental H22 cells in an in vitro cytotoxicity assay [effect/target (E/T) ratio-dependent manner], and their maximal killing rates were approximately 38-43% at E/T ratio of 5:1. The injection of total 2.5 x 10(5) mixed cells containing transfected and parental H22 tumor cells at different ratios into syngeneic mice resulted in the inhibition of tumor growth with a maximal inhibition rates of approximately 57 approximately 72% at E/T ratio of 5:1. A transient weight loss was found in mice bearing solely secretable TNFalpha mutant producing tumors, whereas no obvious side effects were seen in mice bearing uncleavable TNFalpha mutant or wild-type TNFalpha expressing tumors. Finally, we demonstrate that tumors secreting S-TNFalpha promoted the subsequent infiltration of CD4(+) T cells, and to a lesser extent CD8(+) T cells, to the tumor site. The TM-TNFalpha expressing tumors up-regulated Fas (CD95) expression and inhibited the expression of tumor metastasis associated molecule CD44v3. These results suggest that S-TNFalpha and TM-TNFalpha kill cancer cells in vivo through different mechanisms of action. We conclude that the non-secreted form of TNFalpha may be an ideal candidate for cancer gene therapy due to its therapeutic potential and lowered side effect profile.