Tumor-derived chemokine CCL5 enhances TGF-β-mediated killing of CD8 ⁺ T cells in colon cancer by T-regulatory cells

Chemokine CCL5/RANTES is highly expressed in cancer where it contributes to inflammation and malignant progression. In this study, we show that CCL5 plays a critical role in immune escape in colorectal cancer. We found that higher levels of CCL5 expression in human and murine colon tumor cells correlated with higher levels of apoptosis of CD8+ T cells and infiltration of T-regulatory cells (T _reg). In mouse cells, RNA interference (RNAi)-mediated knockdown of CCL5 delayed tumor growth in immunocompetent syngeneic hosts but had no effect on tumor growth in immunodeficient hosts. Reduced tumor growth was correlated with a reduction in Treg infiltration and CD8 ⁺ T-cell apoptosis in tumors. Notably,we found that CCL5 enhanced the cytotoxicity of T _reg against CD8 ⁺ T cells. We also found tumor growth to be diminished in mice lacking CCR5, aCCL5 receptor, where a similar decrease in both T _reg cell infiltration and CD8 ⁺ T-cell apoptosis was noted. TGF-β signaling blockade diminished apoptosis of CD8 ⁺ T cells, implicating TGF-β as an effector of CCL5 action. In support of this concept, CCL5 failed to enhance the production of TGF-β by CCR5-deficient T _reg or to enhance their cytotoxic effects against CD8 ⁺ T cells. CCR5 signaling blockade also diminished the in vivo suppressive capacity of T _reg in inhibiting the antitumor responses of CD8 ⁺ T cells, in the sameway as CCL5 signaling blockade. Together, our findings establish that CCL5/CCR5 signaling recruits T _reg to tumors and enhances their ability to kill antitumor CD8 ⁺ T cells, thereby defining a novel mechanism of immune escape in colorectal cancer.