T‐helper‐ and accessory‐cell‐independent cytotoxic responses to human tumor cells transfected with A B7 retroviral vector

As a means to increase the immunogenicity of tumor cells, we have developed a retroviral vector to transfect human B7, a molecule capable of delivering co‐stimulatory signals to T cells. Three different tumors, a melanoma, an ovarian carcinoma and a myelomonocytic leukemia, were transfected with high efficiency. When compared for their capacity to stimulate allogeneic T cells, B7 + but not B7 − tumor cells were able to stimulate strong proliferative and cytotoxic responses. The effector CTL generated recognised B7 + and B7 − cells as well as untransfected tumor cells, indicating that B7 is required in the inductive but not the effector phase of the response. Remarkably, B7 + tumor cells were able to induce cytotoxic responses both by CD4‐depleted and by CD8‐purified T cells, demonstrating that expression of B7 is at the same time necessary and sufficient to induce a cytotoxic response in the absence of T‐helper cells and accessory cells.