The human immune checkpoint molecule, HLA-G2, induces tolerance in monocytes and dendritic cells via upregulation of PD-L1

Human leukocyte antigen (HLA)-G is a non-classical HLA class I immunomodulatory molecule with restricted expression in the placenta, thymus and regulatory T cells. The spliced isoforms of HLA-G include an 2 domain-deleted isoform, HLA-G2, which specifically binds to the immune checkpoint leukocyte immunoglobulin-like receptor B2 (LILRB2), to suppress immune responses in myelomonocytic cells. We previously reported the structural and receptor binding characteristics of recombinant HLA-G2 protein and its immunosuppressive effects on inflammation in mouse models. However, the function and the mechanism of action of HLA-G2 on human immune cells have not been elucidated. In the present study, we demonstrate the immunosuppressive effect of HLA-G2 on human CD14-positive monocytic cells. HLA-G2 induced the production of the immunosuppressive cytokine, IL-10, and stimulated IL-6/STAT3/indoleamine-2,3-dioxygenase signaling by binding to LILRB2. HLA-G2 binding to LILRB2 also down-regulated cell surface expression of HLA-DR and CD86. Unexpectedly, HLA-G2 up-regulated cell surface expression of PD-L1 in both CD14-positive monocytic cells and interferon-induced dendritic cells (IFN-DCs). This observation suggests HLA-G2/LILRB2 signaling promotes PD-L1 expression. Furthermore, HLA-G2 treatment of IFN-DCs suppressed T cell proliferation in mixed lymphocyte reactions. These findings provide novel insights into the modulation of human immune responses of tolerogenic myelomonocytic cells induced by HLA-G2 binding to LILRB2, and suggest that targeting the HLA-G2-LILRB2 interaction could be a novel approach for immune checkpoint therapy. Significance statementDuring pregnancy, HLA-G isoforms are expressed by fetal trophoblasts to suppress maternal immune responses. Among various HLA-G isoforms, the HLA-G2 homodimer has been expected as an immunosuppressive biologic targeting myelomonocytic antigen-presenting cells via leukocyte immunoglobulin-like receptor B2. We previously reported significant immunosuppressive effects of HLA-G2 in autoimmune mouse models. Here, we first demonstrate that HLA-G2 isoform induces tolerogenic phenotypes of human peripheral immune cells by significantly upregulating an immune checkpoint molecule, PD-L1. Monocyte-derived dendritic cells stimulated by HLA-G2 suppressed T cell proliferation in mixed lymphocyte reactions. These results suggest that HLA-G2 can be a novel candidate for immune checkpoint therapy.