We have recently demonstrated that the superantigen staphylococcal enterotoxin A (SEA) conjugated to colon-carcinoma-reactive monoclonal antibodies (MAbs) directs T cells to lyse human colon-carcinoma cells, representing a potential novel tumor therapy. To further analyze the mechanism of antitumor effects of superantigen-activated T cells, we compared the activity of free and MAb-conjugated SEA in a long term in vitro co-culture assay of human peripheral-blood mononuclear cells (PBMC) and colon-carcinoma cell lines. Activation of resting T lymphocytes with SEA conjugated to the colon-carcinoma-reactive MAb C215 or free SEA resulted in strong inhibition of the growth of all studied colon-carcinoma cell lines. The growth of WiDr colon-carcinoma cells was unaffected by the presence of unactivated mononuclear cells, whereas addition of pM concentrations of SEA or C215-SEA conjugate completely suppressed tumor-cell growth. The suppressive effect was mediated by both CD4+ and CD8+ T cells and required the presence of MHC-Class-II+ monocytes. The inhibition of tumor-cell growth was to a large extent mediated by soluble factors present in supernatants from SEA- or C215-SEA-activated mononuclear cells. Quantitation of cytokine mRNA in SEA-activated mononuclear cells by the reverse transcriptase-polymerase chain reaction (RT-PCR) revealed strong induction of mRNA encoding the cytokines IL-1alpha, IL-1beta, IL-2, IL-6, TNF-alpha, TNF-beta and IFN-gamma. The use of cytokine-specific MAb demonstrated that IFN-gamma was of major importance for the tumor-growth-inhibitory activity in supernatants of SEA-activated lymphocytes. Addition of recombinant cytokines to WiDr colon-carcinoma cells showed that TNF-alpha was able to act synergistically with IFN-gamma to suppress tumor-cell growth. The local production of tumor-suppressive cytokines induced by MAb-targeted superantigens is likely to be of particular relevance for inhibition of the growth of tumor cells not expressing the targeted tumor-associated antigen.
