EXPRESSION OF THE TUMOR-NECROSIS-FACTOR GENE IN TUMOR-CELLS CORRELATES WITH REDUCED TUMORIGENICITY AND REDUCED INVASIVENESS INVIVO

We investigated whether a constitutive production of low amounts of tumor necrosis factor (TNF) by neoplastic cells affects their in vivo tumorigenicity. TNF-resistant derivatives were isolated from the TNF-sensitive murine fibrosarcoma cell lines L929s and WEHI164cl13s. L929r1-type TNF-resistant subclones were found to constitutively produce TNF in vitro, in contrast to non-TNF-producing but TNF-resistant L929r2 and WEHI164cl13r2 cell clones. The TNF-sensitive parental cell lines as well as the L929r2 and WEHI164cl13r2 cell lines similarly induced fast-growing tumors upon s.c. inoculation into nude mice (Swiss-nu/nu). In contrast, the TNF-producing L929r1-type cells showed reduced tumorigenicity and in vivo growth rate, which both inversely correlated with the level of in vitro-produced TNF. Tumor take incidence but not the in vivo growth rate of L929r1-type cells was greatly facilitated by prior whole body gamma-irradiation (350 rads) of the recipient, implying the involvement of host mechanisms at least in the lower take incidence of L929r1 tumors. These host mechanisms, possibly activated by tumor-produced TNF, acted only locally, inasmuch as the growth of an inoculum of L929s cells was not influenced either by a simultaneous distant inoculum of L929r1 cells or by established, distant L929r1 tumors. Efforts to eliminate these host mechanisms by prior local UV irradiation of the skin were unsuccessful. All L929 cell types were found to be similarly susceptible to killing by host cytotoxic effector cells (macrophages and natural and lymphokine-activated killer cells). Histological investigation did not reveal clear differences in tumor-associated inflammatory cells but revealed that tumors induced by L929r1-type cells, in contrast to L929s and L929r2 tumors, did not show invasiveness in host tissues. Moreover, L929r1 tumors were frequently encapsulated, which was never observed for tumors induced by L929s and L929r2 cells. Taken together, our results suggest that tumor-derived TNF locally activates host antitumor activities. Possible effector mechanisms are discussed.