The PD-1 receptor triggers a negative immunoregulatory mechanism that prevents overactivation of immune cells and subsequent inflammatory diseases. Because of its biological significance, PD-1 has been a drug target for modulating immune responses. Immunoenhancing anti–PD-1 blocking antibodies have become a widely used cancer treatment; however, little is known about the required characteristics for anti–PD-1 antibodies to be capable of stimulating immunosuppressive activity. Here, we show that PD-1 agonists exist in the group of anti–PD-1 antibodies recognizing the membrane-proximal extracellular region in sharp contrast to the binding of the membrane-distal region by blocking antibodies. This trend was consistent in an analysis of 81 anti-human PD-1 monoclonal antibodies. Because PD-1 agonist antibodies trigger immunosuppressive signaling by cross-linking PD-1 molecules, Fc engineering to enhance FcγRIIB binding of PD-1 agonist antibodies notably improved human T cell inhibition. A PD-1 agonist antibody suppressed inflammation in murine disease models, indicating its clinical potential for treatment of various inflammatory disorders, including autoimmune diseases.

Blockade of PD-1, an indispensable physiological immunoregulatory mechanism, enhances immune activities and is widely used in the immunotherapy of cancer. This treatment often accompanies inflammatory complication called immune-related adverse events (irAE), most frequently in the skin. To analyze how skin inflammation develops by the blockade of PD-1-dependent immunoregulation, we studied the exacerbation of oxazolone-induced contact hypersensitivity by PD-L1 blockade. The inactivation of PD-1 signaling enhanced swelling of the skin with massive CD8+ T cell infiltration. Among PD-1-expressing cells, T cells were the predominant targets of anti-PD-L1 mAb treatment since PD-L1 blockade did not affect skin inflammation in RAG2-/- mice. PD-L1 blockade during immunization with oxazolone significantly promoted the development of hapten-reactive T cells in the draining lymph nodes. The enhancement of local CD8+ T cell-dominant immune responses by PD-L1 blockade was correlated with the upregulation of CXCL9 and CXCL10. Challenges with a low dose of oxazolone did not demonstrate any significant dermatitis; however, the influence of PD-L1 blockade on T cell immunity was strong enough to cause the emergence of notable dermatitis in this suboptimal dosing, suggesting its relevance to dermal irAE development. In the low-dose setting, the blockade of CXCR3, receptor of CXCL9/10, prevented the induction of T cell-dominant inflammation by anti-PD-L1 mAb. This experimental approach reproduced CD8+ T cell-dominant form of cutaneous inflammation by the blockade of PD-L1 that has been observed in dermal irAE in human patients.

The success of immunoenhancing anti-PD-1 blocking antibodies in cancer treatment indicates the biological significance of PD-1-dependent immunoregulation in the human immune system. Active stimulation of PD-1 is potentially an effective approach in treating various inflammatory disorders; however, agonistic anti-PD-1 antibodies stimulating the immunosuppressive activity are not clearly defined. Here, by assessing the biological activities of anti-human PD-1 mAbs, we have identified functional anti-PD-1 agonists in the antibody group recognizing the membrane-proximal extracellular region (MPER). MPER recognition was characteristic to anti-PD-1 agonist mAbs and distinct from blocking mAbs binding to the membrane-distal region of PD-1. The agonistic activity was mediated by Fc receptor-dependent co-ligation of PD-1 molecules. In vivo anti-inflammatory efficacy of the PD-1 agonist in murine disease models suggests its clinical potential to various inflammatory disorders including autoimmune diseases. Meiji Seika Pharma, Co., Ltd. Foundation for Biomedical Research and Innovation at Kobe