Human pregnancy-specific glycoproteins function as immunomodulators in vitro by inducing secretion of IL-10 and IL-6 in human monocytes

The lack of rejection of the semiallogeneic fetus by the maternal immune system is brought about in part by the maintenance of an anti-inflammatory immune environment at the maternal-fetal interface. The fetoplacental unit produces an array of cytokines and other regulatory molecules that assist in the implantation, survival and development of the fetus. Pregnancy specific glycoproteins (PSGs) are a family of highly conserved, secreted proteins abundantly produced by the placenta in various species including human, mouse and rat. PSGs are composed of repeated immunoglobulin (Ig) related domains, and are part of the Ig superfamily. Abnormally low levels of PSGs in maternal serum have been correlated with complications of pregnancy including spontaneous abortion. A peptide derived from the N-terminal domain of human PSG11 has been shown to bind cells of the promonocyte lineage, suggesting a role for PSGs in modulation of macrophage function during pregnancy.; We investigated the ability of three recombinant human PSGs (PSG1, PSG6 and PSG11), produced using a baculovirus expression system, to regulate the in vitro production of cytokines by human monocytes. Cytokine secretion by monocytes at 24 hours after treatment was measured by quantitative sandwich ELISA. All three PSGs induced dose-dependent secretion of IL-10 and IL-6, but not secretion of TNFα, IL-1β or IL-12. In order to examine the role of the N-terminal Ig-variable-like domain in PSG function, we produced a fusion protein consisting of only the N-terminal domain of PSG6. The PSG6 N-terminal domain was shown to be sufficient for induction of monocyte secretion of IL-10 and IL-6, demonstrating that this domain mediates the interaction with a putative PSG receptor on monocytes. As shown by RT-PCR, increased IL-10 and IL-6 secretion was accompanied by an increase in mRNA after PSG6 treatment. PSG6 induction of IL-10 and IL-6 secretion was inhibited by the tyrosine kinase inhibitor Herbimycin A, the protein kinase C inhibitor Calphostin C, and the specific protein kinase A inhibitor (Rp)cAMPS, suggesting a possible role for these intracellular signalling molecules in PSG signal transduction in monocytes. Also, the specific phosphodiesterase type IV inhibitor and cAMP elevating agent, rolipram, increased monocyte secretion of IL-10 and IL-6 after treatment with PSG6, indicating that increased production of these cytokines in response to PSGs may be mediated by an increase in cAMP. We also showed that PSGs exhibit cross-species activity in cytokine induction using human PSG treatment of a mouse macrophage cell fine, RAW 264.7, and mouse PSG 18 N-domain protein treatment of human monocytes, indicating that PSG function may be highly conserved between species. Our results are consistent with a role for PSGs in modulation of macrophage inflammatory responses at the maternal-fetal interface where PSGs are in high concentration.