Targeting FOXP3 Isoforms disrupts Tregs suppression in Colorectal Cancer 2259576

Abstract Introduction MSI-H colorectal cancer (CRC) displays a highly immunogenic TME. FOXP3+ Tregs are key mediators of immunosuppression within the TME. Notably, human FOXP3 exists as two major isoforms: the full-length (FOXP3FL) and an exon 2-skipped (FOXP3dE2), but the impact of FOXP3 isoform switching in CRC remains unexplored. We hypothesized that reprogramming tumor-infiltrating Tregs to express the FOXP3dE2 isoform would attenuate their immunosuppressive activity and enhance anti-tumor immunity. Methods We utilized a novel mouse model selectively expressing only the FOXP3ΔE2 isoform, allowing isoform-specific functional analyses without inducing global Treg depletion or autoimmunity. This model was employed to investigate the role of the FOXP3ΔE2 isoform in MC38-derived CRC tumors through both in vitro supression assay and in vivo tumor development studies. Results FOXP3dE2 mice showed increased Ki67+ Tregs, indicating higher proliferation. In vitro suppression assay revealed ∼80% reduced suppression of CD4 T-cell proliferation by tumor-infiltrating dE2 Tregs as compared to WT Tregs. In vivo, FOXP3dE2 mice exhibited enhanced anti-tumor immunity, with reduced tumor volumes and weights in the CRC mouse model. Further analysis revealed that FOXP3dE2 tumors exhibit significantly enhanced cytotoxic CD8 T cells with an increased proportion of CD8 T cells expressing IFN-γ and granzyme B. Conclusion These findings highlight the potential of targeting Treg isoforms to promote anti-tumor responses and offer a new approach for MSI-H CRC immunotherapy, potentially overcoming current ICB limitations while avoiding systemic autoimmunity. Funding Source n/a Topic Categories Tumor Immunology: Cellular Responses and Tumor Microevironment (TIME)