CCL5-deficiency enhances intratumoral infiltration of CD8+ T cells in colorectal cancer

Colorectal cancer (CRC) is the third most common solid tumor in the world and shows resistance to several immunotherapies, particularly immune checkpoint blockade which has therapeutic effects on many other types of cancer. Cytotoxic CD8(+) T cell has been considered as one of the main populations of effector immune cells in antitumor immunity; however, the absence of CD8(+) T cells in the central tumor area has become a major obstacle for solid tumor immunotherapy, particularly for CRC. Thus, novel therapeutic strategies that could promote CD8(+) T cells to accumulate in the central tumor area are urgently needed. Here, we demonstrated that CCL5-deficiency delayed tumor growth and metastasis via facilitating CD8(+) T cells to accumulate into tumor site in CRC mouse models. Furthermore, CCL5-deficiency could upregulate PD-1 and PD-L1 expression and reduce the resistance to anti-PD-1 antibody therapy in CRC mouse model. Mechanically, the results of RNA-sequencing, in vitro coculture system and hypoxia measurements demonstrated that knockdown of CCL5 could result in the metabolic disorders in CD11b(hi)F4/80(low) TAMs and suppress the expression of S100a9 to promote the migration of CD8(+) T cells in the tumor microenvironment. These findings were verified by the data of clinical samples from CRC patients, suggesting that CCL5 may provide a potential therapeutic target for the combined PD-1-immunotherapy of CRC.