Abstract
Although blocking programmed cell death protein 1 (PD-1) has emerged as a standard treatment for metastatic colorectal cancer (CRC), a vast majority of CRC patients still respond poorly to anti-PD-1 immunotherapy. In this study, we showed that the levels of indoleamine 2,3-dioxygenase 1 (IDO1) and its catabolite kynurenine (Kyn) were higher in late stages (stages III and IV) than in early stages (stages I and II) of CRC patients. We found that Kyn could induce the expression of immune checkpoints and exhaustion markers in CD8+ tumor-infiltrating T cells. Knockdown of IDO1 expression using small hairpin RNAs (shRNAs) in the MC38 and CT26 colorectal cell lines led downregulation of Kyn expression and activation of CD8+ T cells in MC38- or CT26-bearing mice. Subsequent mechanistic study revealed significantly reduced thymocyte selection-associated HMG box (TOX) mRNA levels in CD8+ tumor-infiltrating T cells isolated from IDO1 knockdown MC38-Scr- and CT26-bearing mice. Kyn-induced CD8+ T cell exhaustion was reversed by knockdown of TOX expression. Finally, the application of the well-known IDO1 inhibitors 1MT or NLG919 substantially improved the therapeutic effect of CRC in vivo and restored CD8+ tumor-infiltrating T cells anti-tumor activity. This improvement was further enhanced by an anti-PD-1 combined therapy. In conclusion, our study revealed a novel mechanism underlying the metabolic factors found in tumor microenvironment which could induce CD8+ T cells exhaustion. Our findings provided a new strategy of restoring the antitumor activity of CD8+ T cells through combined targeting of the IDO1/Kyn and PD-1/PD-L1 pathways in patients with CRC.