Activation of TLR9 signaling in fibroblastic reticular cells enhances anti-tumor immunity in peritoneal tumor via suppressing peritoneal resident macrophage retention.

Abstract

Abstract Immune checkpoint blockade (ICB) has been shown to improve the prognosis of patients with advanced malignancy; however, most patients do not respond to ICB therapy. Peritoneal resident macrophages (PRMs) are critical for peritoneal immunity in health and disease. A recent study has demonstrated that PRMs dampen the therapeutic efficacy of ICB via impairing anti-tumor CD8+ T cell immunity. Here we show that activation of TLR9, an endosomal DNA sensor, in fibroblastic reticular cells (FRCs) negatively regulates PRM retention. Mechanistically, TLR9 in FRCs suppresses retinol metabolism and thus suppresses GATA6 expression in PRMs. Notably, combined treatment of ODN1585, a TLR9 agonist, and anti-PD1 substantially improves the efficacy of anti-PD-1 immune checkpoint blockade therapies in MC-38 and CT26 colorectal carcinoma cell lines-induced peritoneal tumor in mice, evidenced by decreased tumor burden and mortality rate. The improvement is associated with an increase in the percent of CD8+ T cells and a decrease in the percent of CD39+ exhausted CD8+ T cells. Furthermore, global knockout of TLR9 or specific knockout of TLR9 in FRC diminished the beneficial effect of ODN1585 and anti-PD-1 combined treatment on tumor burden and survival. These data demonstrate the potential utility of the FRC-TLR9 signaling in modulating PRM retention to improve the therapeutic efficacy of ICB in peritoneal tumors. Supported by grants from NIH / startup fund