CD4+ T cells represent a critical component of the adaptive immune system and are essential for defense against viral, fungal, and bacterial infections (Martynova et al., 2022). Upon exposure to exogenous pathogens or endogenous self-antigens, CD4+ T cells become activated and subsequently differentiate into diverse effector Th cell subsets (Ruterbusch et al., 2020, Wang et al., 2022). Among them, Th1 cells are central orchestrating immune responses, including protecting hosts against intracellular bacteria and viruses, promoting B cell antibody production, enhancing CD8+ T cell proliferation and function, and modulating the activities of dendritic cells and macrophages (Yin et al., 2021, Goto et al., 2024, Wang et al., 2025). The differentiation and expansion of Th1 cells are tightly regulated by the activation of several key transcription factors (Sun et al., 2023). Specifically, following TCR engagement and exposure to cytokine IL-12, the transcription factors STAT4 and T-bet (encoded by Tbx21) are activated and upregulated, thereby driving Th1 cell differentiation (Liu et al., 2024). T-bet can directly bind to the Ifng gene promoter, enhancing IFN-γ expression, which in turn promotes the expression of the IL-12 receptor β 2-chain (Il12rb2) and T-bet, thereby initiating basal Th1 differentiation. Additionally, IL-2 signaling through JAK1 and JAK3 activates STAT5A and STAT5B, and induces the expression of Il12rb2 and T-bet, further supporting Th1 cell development (Liao et al., 2011, Liao et al., 2013). Despite these advances, the number of transcription factors that are well established in the Th1 differentiation program remains limited. Moreover, our understanding of the molecular network that supports Th1 cell differentiation is still incomplete, highlighting the need for further research to elucidate the regulatory mechanisms governing this critical immune response.

N6-methyladenosine (m6A) represents the most abundant and prevalent RNA modification, particularly on mRNA, in eukaryotic cells. This modification is dynamically and reversibly regulated by two distinct classes of enzymes: methyltransferases ("writers") and demethylases ("erasers"). The m6A methyltransferase complex comprises several key components, including methyltransferase 3 (METTL3), METTL14, Wilms Tumor Associated Protein (WTAP), and vir-like m6A methyltransferase associated (VIRMA). In contrast, the demethylases include the α-ketoglutarate-dependent dioxygenase ABH5 (ALKBH5) and the Fat Mass and Obesity-Associated Protein (FTO) (Jiang et al., 2021). m6A modifications significantly influence the fate and downstream functions of mRNA, including decay, splicing, subcellular localization, and translation. These functions are mediated by m6A-binding proteins, commonly referred to as "readers" (Shi et al., 2019). To date, three major groups of proteins have been identified as m6A readers: (i) the YT521-B homology (YTH)-RNA binding domain family, which includes YTH domain family 1–3 (YTHDF1–3) and YTH domain containing 1–2 (YTHDC1–2); (ii) members of the heterogeneous nuclear ribonucleoprotein (HNRNP) family, such as HNRNPC, HNRNPG, and HNRNPA2B1; and (iii) insulin-like growth factor 2 mRNA-binding proteins 1–3 (IGF2BP1–3). Among these readers, YTHDF2 has been shown to mediate mRNA decay, while YTHDF1 and YTHDF3 promote translation. In contrast, IGF2BPs primarily enhance mRNA stability (Qin et al., 2020, Wang et al., 2015).

Extensive research has highlighted the critical regulatory roles of m6A modifications in T cell homeostasis and functionality (Chao et al., 2021). For instance, the m6A writer METTL3 is essential for maintaining T cell homeostasis through regulation of the IL-7/STAT5/SOCS signaling pathway (Li et al., 2017). Moreover, m6A modifications directly influence T cell differentiation. The absence of METTL3 in T cells results in impaired effector differentiation and memory formation of CD8+ T cells, as well as disrupted differentiation of Th1, Th17, regulatory T (Treg), and follicular helper T (Tfh) cells (Yao et al., 2021, Tong et al., 2018, Guo et al., 2024). Additionally, m6A modifications can indirectly modulate T cell immune responses by affecting antigen-presenting cells (APCs), such as dendritic cells (DCs) and macrophages (Han et al., 2019a, Dong et al., 2021).

FTO, the first identified m6A demethylase, primarily functions to remove m6A modifications through its oxidative demethylase activity (Zhao et al., 2014). It has been implicated in a variety of pathological conditions, including obesity, diabetes, heart failure, neurological disorders, and tumorigenesis (Li et al., 2022). However, the role of FTO in modulating immune functions remains incompletely understood. The direct regulatory effects of FTO on CD4+ T cell differentiation and functionality have yet to be fully elucidated. In the present study, we employed a transgenic mouse model with T cell-specific deletion of FTO to elucidate its role in CD4+ T cell differentiation and effector function during pathogenic infection. Our results demonstrate that FTO is essential for supporting the differentiation of CD4+ T cells into Th1 cells and mediating protective immune responses during acute bacterial infection. Specifically, FTO ablation resulted in a pronounced decrease in IFN-γ secretion and T-bet expression in CD4+ T cells upon activation both in vivo and in vitro. Collectively, our findings highlight the critical role of the RNA epigenetic regulator FTO in mediating m6A modification, which in turn governs CD4+ T cell differentiation and effector responses during acute bacterial infection.