Acid-driven immune suppression by pHLIP-fused PD-L1 under inflammatory conditions

Abstract Programmed cell death–ligand 1 (PD-L1)/PD-1 axis is crucial for maintenance of immune homeostasis and its impairment partially accounts for the pathogenesis of inflammatory diseases. Hence, augmenting PD-L1/PD-1 signals represents a novel strategy to prevent destructive inflammation and induce immune tolerance. Recently, we developed a new cargo by conjugating the ectodomain of PD-L1 with pHLIP, a low pH-responding and membrane-inserting peptide, and demonstrated its potent immune-suppressive activity under acidic conditions in vitro. Herein, we further showed that PD-L1-pHLIP well responded to relatively high acidic buffer, while it could not inhibit T cell expansion in weakly acidic solutions. Furthermore, in a mouse model of acute intestinal inflammation, PD-L1-pHLIP treatment prolonged survival time and attenuated colitis in mice subjected to 6% dextran sulfate sodium (DSS) instead of 3% DSS. The different efficacy was due to the distinct acidity in the lesions, which facilitated PD-L1-pHLIP accumulation in the niche of 6% DSS-triggered severe inflammation. Mechanistic investigations revealed that PD-L1-pHLIP inhibited the release of proinflammatory cytokines in infiltrating macrophages and other immune cells in a PD-1-dependent and/or –independent fashion. Taken together, this study highlights PD-L1-pHLIP as a novel therapeutic avenue for inflammatory diseases.