Equine pregnancy-specific glycoprotein CEACAM49 secreted by endometrial cup cells activates TGFB

Abstract

In early equine pregnancy a highly invasive trophoblast cell subpopulation, the chorionic girdle cells, invade the endometrium and form endometrial cups (EC). These cells express classical MHC molecules, thereby stimulating a humoral and cellular immune response, resulting in a massive accumulation of maternal CD4+ and CD8+ T cells around the EC. Nevertheless, no immediate destruction of endometrial cups by maternal lymphoid cells occurs, presumably due to immune tolerance. Although the environment of EC is rich in TGFB and in FOXP3+, CD4+ T cells, the mechanisms leading to tolerance have not been elucidated. Recently, we discovered that equine trophoblast cells secrete pregnancy-specific glycoproteins (PSGs). Since human and murine PSGs activate latent TGFB, we hypothesized that equine PSGs may have a similar activity. We performed plasmon surface resonance experiments to show that equine PSG CEACAM49 can directly bind to the latency-associated peptide (LAP) of both TGFB1 and TGFB2. We then found that the binding of CEACAM49 leads to the activation of TGFB1 as determined by both ELISA and cell-based assays. Furthermore, the activation of TGFB is a unique function of PSGs within the human CEA family, because CEACAM1, 3, 5, 6, 8 do not activate this cytokine. This finding further strengthens the classification of CEACAM49 as an equine PSG. Based on our results, we hypothesize that activation of latent TGFB in the EC environment by equine PSGs secreted by invasive trophoblast cells, could contribute to the generation of regulatory T cells (Tregs) to maintain immune tolerance.

The transcriptome landscapes of allantochorion and vitelline-chorion in equine day 30 conceptus

“…Therefore, we assumed that the AP-1 transcription factor complex may also play important roles in equine CG differentiation and invasion. Additionally, the transforming growth factor-beta (TGFB) family plays a role in controlling the trophoblast cells invasion ( Lash et al, 2005 ), and the TGFB is activated in equine endometrial cups cells to maintain immune tolerance ( Kammerer et al, 2020 ). In our result, we also found that the TGFB1 expression is significantly higher in binuclear CG cells.…”

Pregnancy-specific glycoproteins: evolution, expression, functions and disease associations

“…Reproduction (2022) 163 R11-R23 classification of the equine proteins as PSGs because the inhibition of platelet -fibrinogen interactions (Aleksic et al 2016) -and activation of TGF-β (Kammerer et al 2020), which are conserved functions of human and mouse PSGs, are not exhibited by non-placentaexpressed CEACAMs in any species.…”

“…Notwithstanding the rapid evolution of the PSG loci, the encoded proteins share many identical functions in all species examined, including in human, mouse and horse (Moore & Dveksler 2014, Aleksic et al 2016, Kammerer et al 2020. Therefore, the expansion and rapid evolution of PSG families are currently better explained by selection for increased gene product dosage than for diversification of functions.…”

“…First, PSGs are undergoing active evolution, with recent gene family and protein domain expansions particularly evident in rodents, primates, equids and bats (McLellan et al 2005, Zebhauser et al 2005, Kammerer & Zimmermann 2010, Aleksic et al 2016, Kammerer et al 2017, Zimmermann & Kammerer 2021. Secondly, notwithstanding divergent protein domain structures between rodents, primates and equids, there appears to be considerable conservation of protein functions (Shanley et al 2013, Warren et al 2018, Kammerer et al 2020. This suggests that PSGs have evolved to solve a significant challenge in the evolution of hemochorial placentation, which we have previously interpreted as a response to parent-offspring conflict (Moore 2012).…”

Evolution of Placental Hormones: Implications for Animal Models

“…It is thought that regulatory T-cells play a role in tolerance of the invasive trophoblast ( 145 ). Since at least one of the equine PSGs can activate TGFβ1, they may contribute to differentiation of regulatory T-cells ( 146 ) as shown for human and mouse ( 130 ). PSG expression has not been examined beyond 36 days ( 142 ) and it would be interesting to know if the putative protection is withdrawn at a later stage when the cups become necrotic and eventually are sloughed off.…”