Toll-like receptors (TLR) 7 and 8 are pattern recognition receptors expressed in immune cells, such as dendritic cells (DC) and macrophages, that respond to viral and bacterial infections. TLR7/8 activation triggers a pro-inflammatory immune cascade that leads to T cell and NK cell activation. Hence, synthetic imidazoquinoline-structured TLR7/8 agonists were developed and demonstrated as potent immunotherapy candidates for cancer and as infectious disease vaccine adjuvants. However, whether antagonizing TLR7/8 can induce the opposite effect, which is to produce anti-inflammatory cytokines and induce immunosuppressive cellular phenotypes, is a gap in our knowledge. In this study, we investigated the immunosuppressive efficacy of a novel TLR7/8 antagonist (termed "621") using cellular and animal models of inflammation. The potent TLR7/8 agonist 558 was employed as a control group to contrast the underlying immune mechanisms induced by TLR7/8 antagonist 621. Using mouse DC assays, we found that 621 was a potent inducer of the anti-inflammatory cytokine IL-10 without triggering pro-inflammatory TNF production. When administered systemically, 621-treated mice showed an increased serum IL-10 and decreased serum TNF. 621-treated mice also showed increased frequencies of regulatory T cells (Treg) and M2 macrophages when challenged with immunostimulants such as TLR4 agonist lipopolysaccharide (LPS) or the canonical TLR7/8 agonist resiquimod (RESQ). Further, 621 therapy mitigated the DSS-colitis model by reducing colon pro-inflammatory cytokines and increasing splenic Tregs. Combined, our data suggest that 621 can facilitate robust anti-inflammatory and immunosuppressive immune responses and therefore can be applied as a novel therapy for inflammatory diseases.
