NSUN2‐Mediated m 5 C Modification of TGFB1 in Trophoblasts Remodels Macrophage Function to Prevent URSA

ABSTRACT Unexplained Recurrent Spontaneous Abortion (URSA) is a prevalent early pregnancy disorder with substantial implications for reproductive health, yet its pathogenesis remains poorly understood. Here, we uncover a previously unrecognized role of NOP2/Sun RNA methyltransferase 2 (NSUN2), an RNA 5‐methylcytosine (m 5 C) methyltransferase, in safeguarding pregnancy by remodeling trophoblast mediated macrophage function. We demonstrate that NSUN2 expression and global m 5 C modification levels are significantly diminished in villous tissues from URSA patients. Trophoblast‐specific ablation of Nsun2 in mice resulted in embryonic absorption and decreased placental and fetal weight in mice underscoring its functional necessity in pregnancy maintenance. Mechanistically, loss of NSUN2 in trophoblasts impairs m 5 C deposition on TGFB1 mRNA, reducing the recruitment of the m 5 C reader YBX1, which in turn destabilizes the transcript and diminishes TGF‐β1 secretion, thereby suppressing M2 macrophage polarization and skewing the balance toward a pro‐inflammatory M1 phenotype. Notably, restoration of NSUN2 via adenovirus‐mediated gene delivery effectively rescues TGF‐β1 expression, rectifies macrophage imbalance, and alleviates embryo resorption in URSA mouse models. Collectively, our findings establish NSUN2 as an essential epigenetic regulator governing maternal‐fetal immune tolerance through m 5 C‐dependent stabilization of TGFB1 mRNA, and identify the NSUN2 /YBX1/TGFB1 axis as a potential diagnostic biomarker and therapeutic target for URSA.