Induction and activation of latent TGF-β1 is carried out by two distinct domains of Pregnancy-Specific Glycoprotein 1.

Pregnancy-specific glycoproteins (PSGs) are a family of immunoglobulin (Ig)-like proteins secreted by specialized placental cells. PSG1 structure is comprised of a single Ig-variable like N-terminal domain (N domain) and three Ig-constant-like domains termed A1, A2 and B2. Members of the human and murine PSG family have been shown to induce anti-inflammatory cytokines from monocytes and macrophages, and to stimulate angiogenesis. We recently showed that recombinant forms of PSG1 (PSG1-Fc and PSG1-His) and PSG1 purified from the serum of pregnant women were associated with the immunoregulatory cytokine TGF-β1 and activated latent TGF-β1. Here, we sought to examine the requirement of specific PSG1 domains in the activation of latent TGF-β1. Plasmon surface resonance studies showed that PSG1 directly binds to the small latent complex (SLC) and to the latency-associated peptide (LAP) of TGF-β1 and that this binding is mediated through the B2-domain. Furthermore, the B2-domain alone was sufficient for activating the SLC. In separate experiments we found that the PSG1- mediated induction of TGF-β1 secretion in macrophages was dependent on the N-domain. Mutagenesis revealed that 4 amino acids (LYHY) of the CC' loop of the N-domain were required for induction of latent TGF-β1 secretion. Together our results show that two distinct domains of PSG1 are involved in the regulation of TGF-β1 and provide a mechanistic framework for how PSGs modulate the immunoregulatory environment at the maternal-fetal interface for successful pregnancy outcome.