IL-18 receptor signaling regulates tumor-reactive CD8+ T-cell exhaustion via activation of the IL-2/STAT5/mTOR pathway in a pancreatic cancer model.

Intratumoral cytotoxic CD8+ T cells (CTLs) enter a dysfunctional state characterized by expression of coinhibitory receptors, loss of effector function, and changes in the transcriptional landscape. Even though several regulators of T-cell exhaustion have been identified, the molecular mechanisms inducing T-cell exhaustion remain unclear. Here, we show that IL-18 receptor (IL-18R) signaling induces CD8+ T-cell exhaustion in a murine pancreatic cancer model. Adoptive transfer of Il18r-/- OT-1 CD8+ CTLs resulted in enhanced rejection of subcutaneous tumors expressing OVA as a model antigen (PancOVA), compared to wild-type (WT) OT-1 CTLs. Transferred intratumoral IL18R-deficient CTLs expressed higher levels of effector cytokines TNF and IFN-γ and had reduced expression of coinhibitory receptors (PD-1, TIM-3, 2B4, LAG-3) and the transcription factors Eomes and TOX. Lower expression of coinhibitory receptors and TOX on IL-18R-deficient vs -sufficient CD8+ T cells were confirmed in an orthotopic KPC model. IL-18R-induced T-cell exhaustion was regulated by IL-2/STAT5 and AKT/mTOR pathways, as demonstrated in an in vitro exhaustion assay. Concordantly, mice deficient in NLRP3, the molecular complex activating IL-18, had decreased expression of coinhibitory receptors on intratumoral T cells and similar changes in signaling pathways at the transcriptome level. Thus, molecular pathways promoting T-cell exhaustion indicate an involvement of an NLRP3 expressing tumor microenvironment, which mediates IL-18 release. TCGA analysis of patients with pancreatic carcinoma showed an association between NLRP3-mediated IL-18 signaling and shorter survival. These findings indicate NLRP3-mediated IL-18R signaling as a regulator of intratumoral T-cell exhaustion and a possible target for immunotherapy.