Abstract
Background: Microsatellite-stable colorectal cancer (MSS-CRC) exhibits poor response to immune checkpoint blockade, partly due to CD8+ T cell exhaustion. Interferon-gamma (IFN-γ) signaling in tumor cells can have dual effects, but its role in promoting exhaustion in MSS-CRC is unclear. Methods: We analyzed RNA-seq data from 245 MSS-CRC tumors and 30 normal tissues from The Cancer Genome Atlas (TCGA) and a validation cohort of 112 MSS-CRC samples. Tumor-intrinsic IFN-γ signaling was quantified using a 10-gene signature. CD8+ T cell exhaustion was assessed by expression of PD-1, TIM-3, LAG-3, and TIGIT. Indoleamine 2,3-dioxygenase 1 (IDO1) expression was measured by qPCR and immunohistochemistry. In vitro, MSS-CRC cell lines (SW480, HT-29) were treated with recombinant IFN-γ, and IDO1 knockdown was performed using siRNA. Co-cultures with CD8+ T cells from healthy donors were used to evaluate T cell exhaustion markers. Results: High tumor-intrinsic IFN-γ signaling was associated with increased CD8+ T cell exhaustion markers (all pConclusions: Tumor-intrinsic IFN-γ signaling promotes CD8+ T cell exhaustion in MSS-CRC through IDO1 induction. Targeting IDO1 may enhance immunotherapy efficacy in this setting.