Expression of co-stimulatory molecules by tumor cells decreases tumorigenicity but may also reduce systemic antitumor immunity

Many tumor cells do not express co-stimulatory molecules, and this may account, in part, for their poor ability to stimulate T cells directly. One strategy to enhance immune recognition would be to express such molecules on the tumor cell. Here, we show that expression of a member of the B7 family of co-stimulatory molecules by CMT93 murine colorectal tumor or 1735 murine melanoma cells resulted in a local antitumor response in immunocompetent mice. The antitumor effect was diminished in athymic nude mice, indicating that T cells played an important part in this response. The ability of the B7-expressing tumor cells to generate systemic protective immunity was investigated by excision of tumors that developed from the initial inoculation followed by rechallenge with parental tumor cells. CMT93 is a poorly immunogenic tumor and no significant systemic immunity was elicited by the expression of B7-1 in these cells. 1735 melanoma is a mildly immunogenic tumor. Unexpectedly, the systemic immunity obtained with 1735 tumors expressing B7-1 or B7-2 was weaker than that generated by parental 1735 cells (p < 0.001, stratified logrank test), even when co-expression of interferon-γ in the B7-1 cells produced high levels of surface MHC class I expression. These results suggest that some caution is appropriate when considering the use of these molecules in the gene therapy of cancer.