ARTICLE ABSTRACTChemokine 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 (Treg). 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 Treg against CD8+ T cells. We also found tumor growth to be diminished in mice lacking CCR5, a CCL5 receptor, where a similar decrease in both Treg 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 Treg or to enhance their cytotoxic effects against CD8+ T cells. CCR5 signaling blockade also diminished the in vivo suppressive capacity of Treg in inhibiting the antitumor responses of CD8+ T cells, in the same way as CCL5 signaling blockade. Together, our findings establish that CCL5/CCR5 signaling recruits Treg to tumors and enhances their ability to kill antitumor CD8+ T cells, thereby defining a novel mechanism of immune escape in colorectal cancer. Cancer Res; 72(5); 1092–102. ©2012 AACR.