Soluble and membrane-bound TNF-α are involved in the cytotoxic activity of B cells from tumor-bearing mice against tumor targets

Splenic B cells from BALB/c mice bearing mammary adenocarcinomas are capable of performing Ab-dependent cellular cytotoxicity. Effector-target conjugation after 18 h results in minimal cytoplasmic damage, whereas extensive nuclear disintegration is observed. To determine whether splenic B cells from tumor-bearing mice can effect direct cytotoxicity against tumor cells, L929 and WEHI 164 cells were used as targets. B lymphocytes from tumor-bearing mice, but not from normal animals, were capable of lysing these two types of tumor cells. However, only a low level of cytotoxicity could be detected when the nontumorigenic 3T3 cells were used as targets. To elucidate the mechanism of cytotoxicity of these killer B cells, RNase protection assays were performed using perforin, granzyme A, TNF-α, and lymphotoxin probes. No perforin, granzyme A, or lymphotoxin RNA could be detected in purified preparations of B cells from normal and tumor-bearing mice. B cells from normal mice did not have TNF-α RNA. In contrast, B cells from tumor bearers expressed TNF-α RNA. TNF-α could be detected in supernatants from both unstimulated and stimulated tumor bearers' splenic B cells, as measured by ELISA, and its lytic activity was neutralized by anti-TNF-α Ab. Western blots revealed the presence of TNF-α on the surface of the killer B cells. Paraformaldehyde-fixed B cells from tumor-bearing mice but not from normal animals were able to lyse TNF-α-sensitive tumor targets. This cytotoxicity was neutralized by anti-TNF-α Ab. These results suggest that TNF-α in soluble and membrane-bound forms may be involved in the mechanism of cytotoxicity exerted by B cells from tumor-bearing mice.