Objectives: Research indicates that low doses of interleukin-2 (IL-2) can effectively mitigate RA symptoms by promoting Treg cells, while high doses may enhance immune responses. Consequently, this study employed mutated IL-2 to minimize its impact on CD8 ⁺ T and NK cell activation while preserving its influence on Treg cells. Methods: We constructed IL-2 mutants by overlap PCR and assessed its impact on the proliferation and functionality of Treg cells by flow cytometry and PCR. Furthermore, the synergistic effects of mutated IL-2 and MSC on collagen-induced arthritis (CIA) in mice were evaluated through the infusion of lentiviral-transfected mesenchymal stromal cell (MSC) for CIA treatment and through pathological section staining to assess inflammatory joint injury, cartilage destruction, and osteoclast infiltration. Results: Mutant IL-2 demonstrated targeted enhancement of both the proportion and proliferative activity of Treg cells with a diminished capacity to stimulate the proliferation of CD8 ⁺ T cells and NK cells relative to wild-type IL-2. Moreover, MSC-mutant IL-2 significantly augmented the proportion of Treg cells compared to either MSC or mutant IL-2 in isolation. Treatment with MSC-mutant IL-2 infusion in CIA mice ameliorated arthritis symptoms and reduced inflammatory infiltration and cartilage damage in their joints. Conclusion: Mutant IL-2 enhances Treg function and proliferation while exerting reduced effects on CD8 ⁺ and NK cell activation. MSC expressing mutant IL-2 demonstrates therapeutic benefits in CIA by increasing the proportion of Treg cells and reducing the proportion of CD8 ⁺ T cells.