Expression and T cell Regulatory Action of the PD-1 Immune Checkpoint in the Ovary and Fallopian Tube

The P rogrammed Cell D eath Protein-1 (PD-1/PDCD-1/CD279) checkpoint has powerful immunomodulatory action, including in the context of cancer. PD-1 receptor activation by its ligands (PD-L1/2) is associated with downregulated immune response, and tumor cells can avoid surveillance via PD-1 and/or ligand expression. While receptor expression is largely limited to lymphoid, myeloid, and tumor cells, we show that membrane bound and soluble variants of PD-1 and ligands are also expressed by permanent constituent cell types of the human ovary and fallopian tube, including granulosa cells and oocytes. PD-1 and soluble ligands were highly enriched in exosome fractions in human follicular fluid at bioactive levels that can control T cell PD-1 activation. PD-1 checkpoint signaling may be involved in physiological ovarian functions including follicle, and ultimately, germline and embryo immune-privilege.