The relationship of receptor occupancy to the kinetics of cell death mediated by tumor necrosis factor.

An analysis of the relationship between TNF receptor occupancy and the kinetics of cell death has been conducted in the sensitive L929.10 murine fibrosarcoma cell line. The receptor affinity and number, measured by competitive and saturation binding assays with human 125I-TNF, is consistent with a single class of binding sites having a Kd = 1.5 nM and 7,000 sites/cell. Human lymphotoxin competed equally well with TNF for binding to the receptor. Determination of the kinetics of L929.10 cell death as a function of TNF concentration revealed three sequential phases to the cytolytic process: An induction phase, that lasted 4 to 9 hrs after the addition of TNF, in which no cell death occurred; the burst phase in which cells died at a rate of 12 to 14% per hr; and the third phase, termed the slow lytic phase, was characterized by a rate of lysis of 1 to 2% per hr. Two parameters, the length of the induction phase and the percentage of cells killed in the burst phase, were dependent on TNF receptor occupancy. Both of these receptor dependent processes were maximized at 10% receptor occupancy. Half maximal lysis of the L929.10 clone was achieved at 10 pM or 0.67% receptor occupancy. In contrast, the rates of cell death during the burst phase and the slow lytic phase were independent of the concentration of TNF. Mitomycin C, an antitumor drug that alkylates and crosslinks DNA, enhanced the sensitivity of the L929.10 cells to TNF mediated lysis by modulating both of the TNF receptor dependent processes and by doubling the (apparent) rate of cell death (22 to 25%/hr) during the burst phase. These results suggest a direct relationship exists between the strength of the TNF-receptor signal and nuclear events, perhaps those associated with processes involved in DNA replication or cell division, during the cytotoxic reaction induced by these inflammatory cytokines.