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

Introduction/Rationale: 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.