GM1 and tumor necrosis factor-α, overexpressed in rena cell carcinoma, synergize to induce T-cell apoptosis

The ability to induce T-cell apoptosis is one mechanism by which tumors evade the immune system, although the molecules involved remain controversial. We found that renal cell carcinoma (RCC)-induced T-cell apoptosis was inhibited by >50% when cocultures were performed with ganglioside-depleted tumor cells, caspase-8-negative lymphocytes, or anti-tumor necrosis factor-alpha (TNF alpha) antibodies, suggesting that tumor gangliosides synergize with signals delivered through TNF alpha death receptors to mediate T-cell killing. The synergy between tumor-derived TNF alpha and the RCC-overexpressed ganglioside GM1 for killing resting T cells is corroborated by studies using purified GM1 and rTNF alpha, which indicate that a 48-hour pretreatment with the ganglioside optimally sensitizes the lymphocytes to a TNF alpha-induced apoptotic death. However, activated T cells, which synthesize TNT alpha themselves, can be killed by exogenous GM1 alone. RelA-overexpressing lymphocytes are protected from GM1 plus TNF alpha-mediated apoptosis, a finding consistent with our previous studies indicating that gangliosides inhibit nuclear factor-kappa B activation. These results are clinically relevant because, similar to T-cells cocultured with GM1-overexpressing RCC lines, T cells isolated from the peripheral blood of patients with metastatic RCC are also heavily coated with that tumor-shed ganglioside. This population of patient cells, unlike T cells isolated from normal donors, is highly susceptible to apoptosis induced by rTNF alpha or by metastatic patient sera, which contain elevated levels of the cytokine. This report thus extends our previous studies by demonstrating that tumor-derived TNF alpha enhances RCC apoptogenicity not only by inducing ganglioside synthesis but also by initiating receptor-dependent apoptosis in T cells in which the nuclear factor-kappa B activation pathway has been inhibited by GM1.