〔Abstract〕 Objective To investigate the therapeutic effects of a small interfering RNA(siRNA) vector targeting peptidyl arginine deaminase 4(PAD4) and splenocytes infected with a PAD4-siRNA virus, on collagen-induced arthritis(CIA) mice, and to elucidate the underlying mechanisms. Methods The experiment mice were divided into four groups: control group, model group, therapy group 1 and therapy group 2, with 7 mice in each group. Control group mice were not treated. Initially, collagen-induced arthritis(CIA) mice model were established using bovine type II collagen. Model group mice were injected by PBS buffer Therapy group 1 mice were injected of PAD4-siRNA virus solution into the tail vein of the CIA mice, while therapy group 2 mice were injected of splenocytes infected with PAD4-siRNA virus via the same route. These injections were carried out once a week for a total of eight weeks. Subsequently, the alterations in T follicular helper(Tfh), T follicular regulatory(Tfr), T helper 1(Th1), and CD4+IL-10+T cells in the splenocytes of the mice were analyzed. Additionally, the pathological changes in the articular cartilage of the mice joints were detected. Results Comparison with control group, mice of model group exhibited a significant increase in the proportions of Tfh and Th1 cells in the spleen(P<0.05), while the proportions of Tfr and CD4+IL-10+T cells remained unchanged. Comparison with model group, therapy group 1 and therapy group 2 demonstrated a significant decrease in the proportions of Tfh and Th1 cells(P<0.05), with no changes were observed in the proportions of Tfr and CD4+IL-10+T cells. Additionally, the articular surface in the mice of control group was smooth, whereas model group showed signs of inflammatory cell infiltration, rough articular surface, and cartilage destruction. Following treatment with PAD4-siRNA, the infiltration of inflammatory cells and cartilage destruction in the hind paws of CIA mice in therapy group 1 were reduced. However, no reduction was observed in the infiltration of inflammatory cells and cartilage destruction in the front paws of CIA mice. In contrast, therapy group 2 exhibited a reduction in the infiltration of inflammatory cells and cartilage destruction in both the front and hind paws of CIA mice. Conclusion Gene silencing of PAD4 expression can decrease the proportion of Tfh and Th1 cells, leading to an amelioration of pathological changes in joints and cartilage of hind paws. Furthermore, the therapeutic efficacy is observed in the front paws of CIA mice, and PAD4-siRNA plays a role on CIA mice by regulating T cells subpopulation of splenocytes.