Galectin-1 (gal-1) is expressed at the feto-maternal interface and plays a role in regulating the maternal immune response against placental alloantigens, contributing to pregnancy maintenance. Both decidua and placenta contribute to gal-1 expression and may be important for the maternal immune regulation. The expression of gal-1 within the placenta is considered relevant to cell-adhesion and invasion of trophoblasts, but the role of gal-1 in the immune evasion machinery exhibited by trophoblast cells remains to be elucidated. In this study, we analyzed gal-1 expression in preimplantation human embryos and first-trimester decidua-placenta specimens and serum gal-1 levels to investigate the physiological role played by this lectin during pregnancy. The effect on human leukocyte antigen G (HLA-G) expression in response to stimulation or silencing of gal-1 was also determined in the human invasive, proliferative extravillous cytotrophoblast 65 (HIPEC65) cell line. Compared with normal pregnant women, circulating gal-1 levels were significantly decreased in patients who subsequently suffered a miscarriage. Human embryos undergoing preimplantation development expressed gal-1 on the trophectoderm and inner cell mass. Furthermore, our in vitro experiments showed that exogenous gal-1 positively regulated the membrane-bound HLA-G isoforms (HLA-G1 and G2) in HIPEC65 cells, whereas endogenous gal-1 also induced expression of the soluble isoforms (HLA-G5 and -G6). Our results suggest that gal-1 plays a key role in pregnancy maternal immune regulation by modulating HLA-G expression on trophoblast cells. Circulating gal-1 levels could serve as a predictive factor for pregnancy success in early human gestation.

Preeclampsia (PE) is a pregnancy-specific disorder characterized by sudden onset of hypertension and proteinuria in the second half of pregnancy (>20 wk). PE is strongly associated with abnormal placentation and an excessive maternal inflammatory response. Galectin-1 (Gal-1), a member of a family of carbohydrate-binding proteins, has been shown to modulate several processes associated with placentation and to promote maternal tolerance toward fetal antigens. Here, we show that Gal-1 exhibits proangiogenic functions during early stages of pregnancy, promoting decidual vascular expansion through VEGF receptor 2 signaling. Blocking Gal-1-mediated angiogenesis or lectin, galactoside-binding, soluble, 1 deficiency results in a spontaneous PE-like syndrome in mice, mainly by deregulating processes associated with good placentation and maternal spiral artery remodeling. Consistent with these findings, we observed a down-regulation of Gal-1 in patients suffering from early onset PE. Collectively, these results strengthen the notion that Gal-1 is required for healthy gestation and highlight Gal-1 as a valuable biomarker for early PE diagnosis.

Transforming growth factor beta (TGF-βs) are secreted from cells as latent complexes and the activity of TGF-βs is controlled predominantly through activation of these complexes. Tolerance to the fetal allograft is essential for pregnancy success; TGF-β1 and -β2 play important roles in regulating these processes. Pregnancy-specific β-glycoproteins (PSGs) are present in the maternal circulation at high concentration throughout pregnancy and have been proposed to have anti-inflammatory functions. We found that recombinant and native PSG1 activate TGF-β1 and TGF-β2 in vitro. Consistent with these findings, administration of PSG1 protected mice from DSS-induced colitis, reduced the secretion of pro-inflammatory cytokines and increased the number of T regulatory cells. The PSG1-mediated protection was greatly inhibited by the co-administration of neutralizing anti-TGF-β Ab. Our results indicate that proteins secreted by the placenta directly contribute to the generation of active TGF-β and identify PSG1 as one of the few known biological activators of TGF-β2.

The involvement of gal-1 in the regulation of different processes during the establishment, development and maintenance of pregnancy could be described as unique. Gal-1 has emerged as an important lectin with major functions in pregnancy.

Galectins are a phylogenetically conserved family of soluble β-galactoside binding proteins, consisting of 15 different types, each with a specific function. Galectins contribute to placentation by regulating trophoblast development, migration, and invasion during early pregnancy. In addition, galectins are critical players regulating maternal immune tolerance to the embedded embryo. Recently, the role of galectins in angiogenesis during decidualization and in placenta formation has gained attention. Altered expression of galectins is associated with abnormal pregnancies and infertility. This review focuses on the role of galectins in pregnancy-associated processes and discusses the relevance of galectin-glycan interactions as potential therapeutic targets in pregnancy disorders.