Th17-driven CD8⁺ T cells in hUC-MSC and CAR T-cell dual immunotherapy for superior anti-tumor efficacy

Chimeric antigen receptor (CAR) T-cell therapy has emerged as a promising treatment for hematological malignancies; however, its efficacy and safety remain challenging, particularly in the context of high tumor burden. High tumor load and substantial residual lesions significantly impair CAR T-cell function and exacerbate cytokine release syndrome (CRS). Here, we report the development of a novel dual cellular immunotherapy in which human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) are co-administered with CD19 CAR T-cells. We demonstrated that this combination therapy enhances the anti-tumor efficacy of CD19 CAR T-cells under high tumor burden condition. In xenograft models of high tumor burden B-cell lymphoma, the dual cellular immunotherapy improved survival, mitigated myelosuppression, and preserved CAR T-cell expansion. Transcriptomic analysis of CAR T-cells revealed enrichment of the Th17 pathway in CAR T-cells, while single-cell RNA sequencing showed enhanced, particularly that of NK-like cytotoxic T lymphocytes characteristics which are associated with Th17 differentiation. Furthermore, in a CRS model, hUC-MSCs attenuate CRS severity by suppressing macrophage activity. Collectively, hUC-MSCs significantly enhance the anti-tumor capability of CD19 CAR T-cells under high tumor burden conditions by inducing CD8⁺ NK-like cytotoxic T lymphocytes through Th17 differentiation, while concurrently mitigating treatment-related side effects. Our study provides a novel therapeutic strategy to improve clinical outcomes in hematological malignancies.