Reprogramming intratumoral T_reg cells by morpholino-mediated splicing of FOXP3 for cancer immunotherapy

Regulatory T cells (T_reg cells) represent a primary barrier to the development of effective antitumor immunity. Here, we report that reprogramming T_reg cells by shifting the expression of FOXP3 from its full-length isoform (FOXP3^FL) to a short isoform with exon 2 skipped (FOXP3^dE2) promotes CD8 T cell-mediated antitumor immunity. FOXP3^dE2 mRNA expression in triple-negative breast cancer tissue positively correlated with overall patient survival. Mice expressing only the FOXP3^dE2 isoform were resistant to the development of multiple types of tumors. Tumor-infiltrating T_reg cells expressing the FOXP3^dE2 isoform exhibited lower immunosuppressive activity and promoted CD8 T cell activation. In addition, we designed a morpholino oligo to induce FOXP3 exon 2 skipping, which similarly enhanced antitumor activity in mouse tumor models and the killing capacity of autologous tumor-infiltrating T cells against patient-derived tumor organoids. Our results suggest that promoting FOXP3^dE2 expression reprograms T_reg cells to T helper-like cells, thereby enhancing antitumor immunity.