Moderating Regulatory T Cell Brakes in CAR T Cell Immunotherapy of Solid Tumors Through the CD4-CD28 Co-stimulation Axis 2260886

Abstract Introduction CAR-T cell immunotherapies are at the translational frontline for solid tumors. However, loss of T cell function over time, and the immunosuppressive tumor microenvironment (TME) limit the efficacy of cell therapies in solid tumors. Regulatory T cells (Treg) in the TME are key immunosuppressive mediators that derail therapeutic efficacy. However, in human clinical trials, severe systemic autoimmune toxicity remains a primary roadblock in clinical advancement of Treg-targeting immunotherapies for solid tumors. Here, we elucidate the mechanisms of Treg depletion mediated toxicity and identify key therapeutic interventions for translating balanced safety and efficacy of CAR T cell immunotherapy for solid tumors. Methods Towards developing strategies that remove Treg-mediated brakes on tumor-reactive T cells, while concurrently preventing autoimmune toxicity, we used immunocompetent murine models of CAR T cell therapy of syngeneic solid tumors ranging from “hot” (melanoma) to “cold” (neuroblastoma). Foxp3-DTR transgenic mice were engaged to achieve targeted Treg ablation using low dose diphtheria toxin administration. Results Our studies show that ablation of Tregs enhances tumor control by overcoming the brakes on anti-tumor effector cytotoxic T lymphocyte (CTL) program. Mechanistic studies establish that CD28-CD80/86 costimulation axis plays a dominant role in Treg-depletion toxicity. Paradoxically, blocking of CD4 T cell activation via CD28 causes a complete rescue of autoimmune toxicity, without compromising anti-tumor therapeutic efficacy and development of systemic memory CAR T cells in the peripheral lymphoid and non-lymphoid tissues for lifelong protection from tumor relapse. Furthermore, we show that Tregs exert the autoimmune toxicity almost entirely via the inhibitory receptor CTLA-4 expressed on Treg cells. Conclusion Our data lay the preclinical groundwork for translating Treg-ablation strategies during CAR T cell immunotherapy for safe and durable remission of solid tumors. Funding Source This study was funded by the National Institutes of Health (National Cancer Institute, and National Institute of Allergy Immunology and Infectious Diseases) and Pediatric Cancer Research Foundation. Topic Categories Tumor Immunology: Cellular Responses and Tumor Microevironment (TIME)