Downregulated Smad3 signaling impairs the maturation of MO-MDSC in colorectal cancer

Myeloid-derived suppressor cells (MDSC) are a heterogeneous group of immunosuppressive cells that expand in tumors due to impaired differentiation into macrophages (MΦ), dendritic cells (DC), or granulocytes. Despite extensive research, the mechanisms governing their differentiation remain incompletely understood. Transforming growth factor β1 (Tgfβ1), abundant in the tumor microenvironment (TME), regulates cellular differentiation via the classical Smad2/3 pathway, yet paradoxically fails to drive MDSC maturation. In this study, we demonstrate that Smad3 is significantly downregulated in MDSC from colorectal cancer (CRC) patients and tumor-bearing mouse models, impairing monocytic lineage maturation. Myeloid-specific Smad3 overexpression promotes the differentiation of MO-MDSC into mature MΦ and DC, enhances MHC-II expression, and reduces immunosuppressive molecules, thereby attenuating tumor progression. Mechanistically, Tgfβ1 induces MO-MDSC expansion via the non-classical PI3K/AKT pathway, which is counteracted by Smad3 overexpression. Furthermore, Mettl3-mediated m⁶A modification destabilizes Smad3 mRNA at the 3'UTR, linking RNA epigenetics to MDSC dysfunction. Clinically, plasma Tgfβ1 levels correlate with the MO-MDSC/PMN-MDSC ratio across cancer types, highlighting its biomarker potential. Our findings unveil Smad3 as a critical regulator of MDSC fate and propose novel therapeutic targets for tumor immunotherapy.