CYTOTOXICITY IN L929 MURINE FIBROSARCOMA CELLS AFTER TRIGGERING OF TRANSFECTED HUMAN P75 TUMOR-NECROSIS-FACTOR (TNF) RECEPTOR IS MEDIATED BY ENDOGENOUS MURINE TNF

We compared the biological function of the human tumor necrosis factor receptors p55 (hTNF-R55) and p75 (hTNF-R75) expressed in the murine ( m) fibrosarcoma cell line L929. Receptor-specific triggering of hTNF-R 55 in transfected L929 cells by agonistic monoclonal antibodies or hTN F-R32WS86T, a hTNF-R55-specific mutant of hTNF, resulted in cytotoxici ty. Specific clustering of hTNF-R75 in transfected L929 cells by agoni stic monoclonal antibodies or hTNF-D143F, a hTNF-R75-specific mutant o f hTNF also induced cytotoxicity, albeit at low level. In both cases, the cytotoxic activity of receptor clustering could be synergized by a ddition of 20 mM LiCl. Remarkably, cytotoxicity induced after R75 trig gering in transfected L929 cells could be completely abolished by addi tion of neutralizing anti-mTNF antibodies, in contrast to cell killing seen after specific R55 clustering. No soluble mTNF could be demonstr ated using a sensitive biological assay, although L929 cells were expr essing low levels of mTNF-specific mRNA as shown by PCR. These data cl early demonstrate that minute amounts of endogenously produced TNF can be a key mediator in R75-mediated cytotoxicity. Presumably. the latte r efficiently traps the ligand and transfers it to TNF-R55, and/or by binding it, protects the endogenously made TNF from inactivation. (C) 1995 Academic Press Limited.