Trophoblast m5C Regulation Shapes Maternal–Fetal Immune Tolerance

Unexplained recurrent spontaneous abortion (URSA) is a common early-pregnancy disorder that seriously affects reproductive health, yet its pathogenesis remains poorly understood. Recently, researchers published a study entitled “NSUN2-Mediated m5C Modification of TGFB1 in Trophoblasts Remodels Macrophage Function to Prevent URSA.” The study revealed for the first time that NOP2/Sun RNA methyltransferase 2 (NSUN2), an RNA 5-methylcytosine (m5C) methyltransferase, plays a previously unrecognized role in protecting pregnancy by enabling trophoblasts to regulate macrophage function.

The researchers demonstrated that both NSUN2 expression and global m5C modification levels were significantly reduced in the chorionic villous tissues of patients with URSA. Trophoblast-specific deletion of Nsun2 in mice resulted in embryo resorption and reduced placental and fetal weights, confirming that NSUN2 is essential for maintaining a healthy pregnancy.

Mechanistically, loss of NSUN2 in trophoblasts impaired m5C modification of TGFB1 mRNA and reduced the recruitment of the m5C reader protein YBX1. This destabilized TGFB1 mRNA and decreased TGF-β1 secretion, thereby suppressing macrophage polarization toward the anti-inflammatory M2 phenotype and shifting the macrophage balance toward the proinflammatory M1 phenotype.

Restoring NSUN2 expression through adenovirus-mediated gene delivery effectively increased TGF-β1 levels, corrected the macrophage imbalance, and alleviated embryo resorption in URSA mouse models. Collectively, these findings establish NSUN2 as an important epitranscriptomic regulator of maternal–fetal immune tolerance. NSUN2 exerts its protective effect by stabilizing TGFB1 mRNA through m5C modification, while the NSUN2/YBX1/TGFB1 axis represents a potential diagnostic biomarker and therapeutic target for URSA.

Figure 1. NSUN2 treatment alleviates embryo loss of URSA mice. (Zhu X, et al., 2026)

Maternal–Fetal Immune Tolerance and URSA

Recurrent spontaneous abortion (RSA) remains a major challenge in reproductive medicine. Despite extensive clinical research, the underlying cause remains unknown in approximately 50% of cases, which are classified as unexplained recurrent spontaneous abortion.

In addition to directly impairing reproductive health, URSA imposes a substantial psychological and emotional burden on patients. It also presents major diagnostic and therapeutic challenges for clinicians. Clarifying the molecular mechanisms underlying URSA is therefore essential for developing effective preventive and therapeutic strategies.

From an immunological perspective, successful pregnancy is a classic example of allogeneic tolerance. The semi-allogeneic fetus carries paternal antigens but must remain protected from maternal immune surveillance throughout pregnancy. The maternal–fetal interface (MFI) regulates the local immune environment, while the placenta functions not only as a physical barrier but also as an important immunoregulatory organ.

Villous trophoblasts are key immune-regulatory cells at the maternal–fetal interface. Through direct cell-to-cell interactions and paracrine signaling, they regulate the differentiation and activity of decidual immune cell populations, ultimately establishing maternal–fetal immune tolerance. Disruption of this precisely coordinated interaction between trophoblasts and immune cells can disturb immune homeostasis and contribute to pregnancy complications such as URSA. However, the molecular mechanisms through which trophoblasts regulate immune cells have not been fully elucidated.

Macrophage Polarization at the Maternal–Fetal Interface

Macrophages are among the most abundant immune cell populations at the maternal–fetal interface. They exhibit considerable phenotypic plasticity and play dual roles in pregnancy maintenance.

Most decidual macrophages display an anti-inflammatory M2 phenotype. These cells contribute to tissue remodeling, angiogenesis, and the establishment of immune tolerance. In contrast, a shift toward the proinflammatory M1 phenotype is associated with URSA and other pregnancy-related disorders.

Growing evidence indicates that trophoblasts actively regulate macrophage polarization through paracrine signaling. Nevertheless, the upstream molecular factors responsible for this regulatory effect have remained largely unknown.

The RNA modification 5-methylcytosine is a widespread post-transcriptional modification that regulates gene expression by influencing mRNA stability and translational efficiency. Members of the NOP2/Sun domain family are the principal mRNA m5C methyltransferases in mammals. Using S-adenosyl-L-methionine as the methyl donor, these enzymes catalyze the formation of m5C modifications.

Recent studies have shown that NSUN2, ALYREF, and YBX1 can function cooperatively to regulate the stability of target mRNAs in an m5C-dependent manner. Beyond its physiological functions, NSUN2-mediated m5C modification is also involved in pathological processes such as cancer and inflammatory diseases.

Emerging evidence further suggests that m5C modification plays an important role in reproductive physiology and gynecological disorders. Before this study, however, it remained unknown whether NSUN2 contributed to maternal–fetal immune tolerance, particularly by stabilizing trophoblast-derived functional molecules involved in macrophage regulation.

The NSUN2/YBX1/TGFB1 Axis Maintains Pregnancy

The study found that NSUN2 expression and global m5C modification levels were markedly reduced in chorionic villous tissues from patients with URSA. Integrated transcriptomic, epitranscriptomic, and functional analyses demonstrated that NSUN2 stabilizes transforming growth factor beta 1 (TGFB1) mRNA through m5C modification.

By maintaining TGFB1 mRNA stability, NSUN2 sustains TGF-β1 expression and secretion from trophoblasts. TGF-β1 subsequently promotes macrophage polarization toward the M2 phenotype, helping to preserve immune tolerance at the maternal–fetal interface.

When NSUN2 is lost from trophoblasts, m5C deposition on TGFB1 mRNA is impaired. Reduced m5C modification weakens the binding of YBX1, destabilizes the transcript, and decreases TGF-β1 production. As a result, M2 macrophage polarization is suppressed, while the macrophage population shifts toward the proinflammatory M1 phenotype. This imbalance disrupts immune homeostasis at the maternal–fetal interface and increases the risk of pregnancy failure.

Using trophoblast-specific Nsun2-knockout mice and URSA animal models, the researchers confirmed that this regulatory pathway is indispensable for pregnancy maintenance. Trophoblast-specific Nsun2 deletion caused embryo resorption and reduced placental and fetal growth. Conversely, adenovirus-mediated restoration of NSUN2 expression increased TGF-β1 levels, restored macrophage balance, and reduced embryo resorption.

These findings identify the NSUN2/YBX1/TGFB1 signaling axis as a crucial molecular pathway linking trophoblast epitranscriptomic regulation to maternal–fetal immune tolerance. The study also demonstrates the therapeutic potential of restoring NSUN2 activity and provides a promising foundation for developing new diagnostic and treatment strategies for URSA.