Blockade of transforming growth factor-β signaling in tumor-reactive CD8+ T cells activates the antitumor immune response cycle

Transforming growth factor-β (TGF-β) is a potent immunosuppressant. Overproduction of TGF-β by tumor cells leads to evasion of host immune surveillance and tumor progression. Results of our early studies showed that adoptive transfer of tumor-reactive, TGF-β-insensitive CD8+ T cells into immunocompetent mice was able to eradicate lung metastasis of mouse prostate cancer. The present study was conducted with three objectives. ( a ) We tested if this technology could be applied to the treatment of solid xenograft tumors in allogeneic immunodeficient hosts. ( b ) We determined relevant variables in the tumor microenvironment with the treatment. ( c ) We tested if immune cells other than CD8+ T cells were required for the antitumor effect. Mouse prostate cancer cells, TRAMP-C2 of the C57BL/6 strain, grown in immunodeficient allogeneic hosts of BALB/c strain, were used as a xenograft model. Tumor-reactive CD8+ T cells from C57BL/6 mice were isolated, expanded ex vivo , and rendered insensitive to TGF-β by introducing a dominant-negative TGF-β type II receptor vector. Seven days following s.c. injection of TRAMP-C2 cells (5 × 105) into the flank of male BALB/c-Rag1−/− mice, tumor-reactive, TGF-β-insensitive CD8+ T cells (1.5 × 107) were transferred with and without the cotransfer of an equal number of CD8-depleted splenocytes from C57BL/6 donors. Naive CD8+ T cells or green fluorescent protein-empty vector–transfected tumor-reactive CD8+ T cells were transferred as controls. Forty days following the transfer, the average tumor weight in animals that received cotransfer of tumor-reactive, TGF-β-insensitive CD8+ T cells and CD8-depleted splenocytes was at least 50% less than that in animals of all other groups ( P < 0.05). Tumors in animals of the former group showed a massive infiltration of CD8+ T cells. This was associated with secretion of relevant cytokines, decreased tumor proliferation, reduced angiogenesis, and increased tumor apoptosis. Based on these results, we postulated a concept of antitumor immune response cycle in tumor immunology. [Mol Cancer Ther 2006;5(7):1733-43]