In mice with head and neck squamous cell carcinoma, combination of radiation therapy (RT) and PD-L1 blockade led to a transient T cell response followed by upregulation of TIM-3. Adding anti-TIM-3 slowed down tumor growth, decreased Tregs, and enhanced T cell function and survival, but this effect was not durable due to eventual re-emergence of Tregs. Administration of anti-CD25 at the peak response to RT/anti-PD-L1/anti-TIM-3 led to depletion of Tregs, tumor rejection, and immunological memory. Treg enrichment was also observed in two patients not responding to RT + anti-PD-1 compared to responders.
PURPOSE: Radiation therapy (RT) can transform the immune landscape and render poorly immunogenic tumors sensitive to PD-L1 inhibition. Here we established that the response to combined RT and PD-L1 inhibition is transient and investigated mechanisms of resistance. EXPERIMENTAL DESIGN: Mechanisms of resistance to RT and PD-L1 blockade were investigated in orthotopic murine HNSCC tumors using mass cytometry and whole genome sequencing. Mice were treated with anti-PD-L1 or anti-TIM-3 alone and in combination with and without RT. Tumor growth and survival were assessed. Flow cytometry was used to assess phenotypic and functional changes in intratumoral T cell populations. Depletion of regulatory T cells was performed using anti-CD25 antibody. RESULTS: We show that the immune checkpoint receptor, TIM-3, is upregulated on CD8 T cells and Tregs in tumors treated with RT and PD-L1 blockade. Treatment with anti-TIM-3 concurrently with anti-PD-L1 and RT led to significant tumor growth delay, enhanced T cell cytotoxicity, decreased Tregs and improved survival in orthotopic models of head and neck squamous cell carcinoma. Despite this treatment combination, the response was not durable and analysis of relapsed tumors revealed resurgence of Tregs. Targeted Treg depletion, however, restored anti-tumor immunity in mice treated with RT and dual immune checkpoint blockade and resulted in tumor rejection and induction of immunologic memory. CONCLUSIONS: These data reveal multiple layers of immunoregulation that can promote tumorigenesis, and the therapeutic potential of sequential targeting to overcome tumor resistance mechanisms. We propose that targeted Treg inhibitors may be critical for achieving durable tumor response with combined radiotherapy and immunotherapy.
Author Info: (1) Radiation Oncology, University of Colorado Denver. (2) Otolaryngology and Head and Neck Surgery, University of Colorado Denver, Anschutz Medical Campus. (3) Department of Radia
Author Info: (1) Radiation Oncology, University of Colorado Denver. (2) Otolaryngology and Head and Neck Surgery, University of Colorado Denver, Anschutz Medical Campus. (3) Department of Radiation Oncology, University of Colorado Denver. (4) Radiation Oncology, University of Colorado Denver, Anschutz Medical Campus. (5) Radiation Oncology, University of Colorado Denver. (6) Radiation Oncology, University of Colorado Denver. (7) Radiation Oncology, University of Colorado Denver. (8) Radiation Oncology, University of Colorado Denver. (9) Radiation Oncology, University of Colorado Denver. (10) Radiation Oncology, University of Colorado Denver. (11) Department of Medicine, University of Colorado Anschutz Medical Campuso. (12) Radiation Oncology, University of Colorado Denver. (13) Department of Medicine, University of Colorado Denver. (14) Department of Craniofacial Biology, University of Colorado Anschutz Medical Campus. (15) Anesthesiology, University of Colorado Anschutz Medical Campus. (16) Medicine, University of Colorado Anschutz Medical Campus. (17) Radiation Oncology, University of Colorado Denver sana.karam@ucdenver.edu.
