Synergistic effects between Staphylococcal enterotoxin type B and Monophosphoryl lipid A against mouse fibrosarcoma

Staphylococcal enterotoxin B (SEB) is a potent inducer of cytotoxic T-cell activity, cytokine production and necrosis induction in vivo. Monophosphoryl lipid A (MPL) is an adjuvant derived from the lipopolysaccharide of E. coli, Salmonella Minnesota Re595 and other gram negative bacteria. We investigated the possibility of the therapeutic application of SEB+ MPL in mice with fibrosarcoma.

The antitumor effect of SEB+MPL, SEB and MPL in mice with inoculated fibrosarcoma tumor (WEHI-164) was examined by intravenous (i.v.) and intratumoral (i.t.) injection and the sizes of the inoculated tumors, IFN-gamma production, and CD4(+)/CD8(+) T cell infiltration were determined. The inoculated tumors were also examined histologically.

In the i.v.-injected group of mice with SEB+ MPL, reduction of tumor size showed a significant difference compared with mice in the i.t., the i.v. (MPL)-injected groups and the negative control group (p < 0.02). Moreover, the mice in the i.v. (SEB+ MPL)-injected group showed significantly higher levels of IFN-gamma (p < 0.009) and CD4(+)/CD8(+) T cell infiltration when compared with the other groups (p < 0.02). A significantly higher frequency of necrosis in tumor tissues was also observed in mice in the i.v. (SEB+ MPL)-injected group in comparison with other groups (p < 0.009).

Our findings suggest that tumor cell death is caused by increased cytotoxic T-cell activity, cytokine levels, in response to IV injection of SEB+MPL. They also suggest that tumor cell death by synergistic effect of one of the strongest bacterial superantigens (SEB) with monophosphoryl lipid A and SEB+MPL may be a good option for use as a novel therapy in patients with fibrosarcoma.