Cooperation between staphylococcal enterotoxin B and low dose melphalan in the cure of mice bearing a large MOPC-315 tumor and extensive metastases.

We have recently demonstrated the importance of V beta 8+/CD8+ cells for the curative effectiveness of a suboptimal low dose (0.75 mg/kg) of melphalan (L-phenylalanine mustard; L-PAM) for mice bearing a large s.c. MOPC-315 tumor and extensive metastases. Here we show that staphylococcal enterotoxin B (SEB), which is known to selectively stimulate T cells expressing members of the TCR-V beta 8 gene family, substantially improved the curative effectiveness of the suboptimal dose of L-PAM for mice bearing a large MOPC-315 tumor. Moreover, treatment of mice with mAb F23.1 (anti-V beta 8) abrogated the in vivo therapeutic effect of SEB for low dose L-PAM-treated MOPC-315 tumor bearers (L-PAM TuB mice). Analysis of the effect of SEB on the tumor-infiltrating lymphocytes (TILs) demonstrated that the SEB-mediated therapeutic effect was associated with a significant increase in: 1) the percentage of V beta 8+ cells among the CD8+ T cells that accumulated in the s.c. tumor nodules, 2) the total number of V beta 8+/CD8+ cells present per tumor on day 4 after low dose L-PAM therapy, and 3) the ability of the TILs to lyse MOPC-315 tumor cells in vitro in a short term assay. Furthermore, treatment of mice with mAb F23.1 abolished the ability of SEB to render the TIL population of L-PAM TuB mice more cytotoxic in vitro for MOPC-315 tumor cells. Thus, the SEB-mediated improvement in the therapeutic outcome of low dose L-PAM therapy for mice bearing a large MOPC-315 tumor may be due in part to SEB-mediated increase in the contribution of the V beta 8+ T cells to tumor eradication through enhancement in the magnitude of the anti-MOPC-315 lytic activity exhibited by the TIL population.