Tumor-associated macrophages expressing the transcription factor IRF8 promote T cell exhaustion in cancer

Tumor progression is associated with overstimulation of cytotoxic T lymphocytes (CTLs), resulting in a dysfunctional state of exhaustion. However, the identity of antigen-presenting cells that drive CTL exhaustion is poorly defined. Here we show that tumor-associated macrophages (TAMs) expressing the transcription factor interferon regulatory factor-8 (IRF8) promote CTL exhaustion in a murine model of breast cancer. While CTL priming in tumor-draining lymph nodes was enabled by IRF8-dependent type 1 dendritic cells, CTL exhaustion in tumor required TAM expression of IRF8 that supported the cancer cell antigen-presenting function of TAMs. Macrophage-specific ablation of IRF8 reversed exhaustion of cancer cell antigen-reactive CTLs, and suppressed tumor growth. Analysis of the immune-infiltrated human renal cell carcinoma tumors revealed abundant TAMs that expressed IRF8 and were enriched for an IRF8 gene expression signature. Furthermore, the TAM-IRF8 signature co-segregated with CTL exhaustion signatures across multiple cancer types. Thus, CTL exhaustion is promoted by TAMs via IRF8. Overall design: Bulk RNAseq on antigen-presenting cells from mouse PyMT breast tumor, tumor-associated macrophages expressing or not the transcription factor IRF8, and scRNA-seq on CD8 T cells in PyMT breast tumor from IRF8FLPyMT or MafBiCreIRF8FLPyMT mice.