Objective To investigate the role of histone deacetylase 3 (HDAC3) in the differentiation and function of peripheral CD4⁺ T cells.Methods CD4cre enzyme mediated HDAC3 heterozygous gene deletion mice (Hdac3^fl/flCD4^cre+/-) and wild-type normal control (Hdac3^fl/fl, WT) mice were used. The effects of HDAC3 deletion on the proportion and number of peripheral CD4⁺ and CD8⁺ T cells were detected by flow cytometry. The effects of HDAC3 deletion on the expression of IFN-γ, IL-4 and IL-17A in CD4+ T cells and Tfh cells were detected under the in vitro PMA and Ionomycin stimulation. The effects of HDAC3 deletion on the expression of IFN-γ, IL-4 and IL-17 in serum were detected by ELISA. The naive CD4⁺ T cells of Hdac3^fl/flCD4^cre+/- and WT mice were sorted and cultured in Th1 and Th2 differentiation conditions respectively. The effects of HDAC3 deletion on the expression of Th1, Th2 and Th17 related cytokines and their specific transcription factors were detected by intracellular staining. The effects of HDAC3 deletion on the expression of genes related to CD4⁺ T cell differentiation subsets were detected by gene expression microarray. The mice treated with streptozotocin (STZ) were used to construct type 1 diabetes mellitus (T1DM) disease model, and the effects of HDAC3 deletion on the pathogenesis of T1DM were detected.Results Compared with WT mice, the proportion and number of peripheral CD4⁺ and CD8⁺ T cells in Hdac3^fl/flCD4^cre+/- mice decreased significantly. The expression of IFN-γ in CD4⁺ T cells and serum of Hdac3^fl/flCD4^cre+/- mice decreased significantly, while the expression of IL-4 and IL-17A increased significantly, and the proportion of Tfh cells also increased significantly. HDAC3 deletion inhibited the differentiation of CD4⁺ T cells into Th1 cells, but promoted their differentiation intoTh2 cells. Microarray analysis showed that the deletion of HDAC3 resulted in the decrease of gene expression in Th1 cell lineage, while the increase of gene expression in Th2, Th17 and Tfh cell lineage. Under the condition of STZ induction, HDAC3 deletion inhibited the development of T1DM and the differentiation of CD4⁺ T cells into Th1.Conclusion HDAC3 promotes the differentiation of peripheral CD4⁺ T cells into Th1 cells and aggravates the occurrence of T1DM.