Mettl3-catalyzed m 6 A methylation determines CD8 + T cell differentiation fate in tumor

Abstract The heterogeneity in patient responses to immune checkpoint blockade (ICB) is dictated by the relative abundance of exhausted CD8⁺ T cell (Tex) subsets with distinct therapeutic responsiveness. Progenitor exhausted (pTex) cells remain sensitive to ICB, whereas terminally exhausted (tTex) cells are refractory; however, the molecular cues that bias differentiation toward these divergent fates remain poorly defined. Here, we identify the RNA methyltransferase Mettl3 as a central regulator of Tex fate. Across murine tumor models, human T cells, and adoptive transfer systems, Mettl3 expression is selectively enriched in tTex cells and inversely correlated with TCF1⁺ pTex populations. Mechanistically, Mettl3 drives terminal exhaustion by stabilizing DNMT3B transcripts via m⁶A modification, enforcing CpG methylation and chromatin compaction at memory-associated loci. Inhibition of the Mettl3-Dnmt3b axis reprograms chromatin accessibility toward memory-like states, thereby preserving progenitor potential and effector function. Consequently, T cells lacking Mettl3-Dnmt3b activity persist longer, mount robust recall responses, and achieve superior tumor control with enhanced responsiveness to PD-1 blockade. These findings establish the Mettl3-m⁶A-Dnmt3b axis as a molecular rheostat of CD8⁺ T cell fate, coupling epitranscriptomic regulation to epigenetic remodeling, and reveal a tractable pathway to improve the durability of cancer immunotherapy.