Cytotoxicity of lymphocytes activated by superantigen toxic-shock-syndrome toxin-1 against colorectal cancer LoVo cells

Toxic-shock-syndrome toxin-1 (TSST-1), a superantigen, can stimulate T cell activation and be used for immunotherapy. In this study, we employed the carcinoembryonic antigen (CEA)-positive LoVo cells to test whether retrovirus-mediated TSST-1 expression could activate human T cells and promote cytotoxicity against tumor cells. We first generated plasmids of pLEGFP-N1-5HRE-CEAp-TSST-1-linker-CD80TM containing a fusion gene of the CEA promoter, 5 copies of the hypoxia-response elements (HRE) as an enhancer, the fragments for TSST-1, and the transmembrane domain of CD80 (CD80TM) and control pLEGFP-N1-5HRE-CEAp (without TSST-1) and generated retroviruses of 5HCTC and 5HC, respectively. After infection with 5HC and 5HCTC retroviruses to establish cell lines, the high levels of TSST-1 expression were observed on the membrane and cytoplasm of the 5HCTC-infected LoVo cells, particularly culture under a hypoxic condition, but not on CEA(-) HeLa cells. Furthermore, the TSST-1-expressing LoVo cell lysates, but not the control cell lysates, stimulated human T cell proliferation, and the co-culture of the TSST-1-expressing LoVo, but not control cells, with human peripheral blood mononuclear cells (PBMC) induced a high frequency of TNF-alpha- and IL-2-secreting T cells in vitro, particularly under hypoxic conditions. More importantly, co-culture of the TSST-1-expressing LoVo cells, particularly under hypoxic conditions, but not control cells, with different numbers of PBMC promoted potent cytotoxicity against LoVo cells in a dose-dependent manner in vitro. These data provide proof of the principle that selective induction of TSST1 expression in CEA(+) colorectal cancer (CRC) cells activates T cells that destroy tumor cells, particularly under a hypoxic condition. Therefore, our findings may aid in the design of new immunotherapy for the intervention of CRC at clinic.