Interleukin-7 (IL-7) and the membrane molecule B7 are both able to provide proliferation and activation signals for T cells. However, tumor cells transfected to express either molecule alone are not reliably rejected in syngeneic hosts or are not sufficiently immunogenic to serve as potent tumor vaccines. Since IL-7 and B7 have shown synergistically to induce activation and proliferation of T cells in vitro, we have expressed B7.1 by means of a retrovirus in the mammary adenocarcinoma TS/A which arose spontaneously in a BALB/c mouse and in the plasmacytoma J558L and their IL-7-transfected sublines to improve vaccine efficacy. Expression of IL-7 or B7.1 alone in tumor cells decreased tumorigenicity, but nevertheless tumors grew in a substantial number of mice. In contrast, IL-7/B7.1 cotransfected cells did not grow as tumor in a single case. This inhibition of tumor growth was completely T cell dependent, because TS/A-IL-7/B7.1 cells retained their full tumorigenic potential in T cell-deficient mice. Analysis of tumor-infiltrating T lymphocytes revealed increased numbers of T cells in B7, IL-7 and IL-7/B7 transfected compared to parental tumors. In IL-7/B7 transfected tumors, T cell numbers were not further increased compared to that in single-gene-transfected tumors. However, T cells in B7 and IL-7 transfected tumors differed phenotypically with respect to activation markers. In B7 transfected tumors, T cells were predominantly CD28(+) and CD25(-), while in IL-7-transfected tumors, T cells were mainly CD28(-) and CD25(+). In IL-7/B7 cotransfected tumors, the majority of T cells was CD28(+) and CD25(+). Thus, IL-7 and B7 induced an anti-tumor immune response by complementary T cell directed pathways in a cooperative fashion. Importantly, immunization of mice with the transfected cells and subsequent contralateral challenge with parental tumor cells showed that IL-7/B7 co-expressing cells induced the most strongly protective immunity, which is superior to that induced by single-gene transfectants and to the adjuvant Corynebacterium parvum. Vaccine efficacy was abrogated when irradiated cells were used for vaccination. Together, our results show that IL-7 and B7.1 transfected tumor cells induce strong T cell activation and tumor immunity.
