Inducible MyD88/CD40 Allows Rimiducid-Dependent Activation to Control Proliferation and Survival of Chimeric Antigen Receptor-Modified T Cells

Introduction: Adoptive transfer of T cells, genetically engineered to express chimeric antigen receptors (CARs) containing costimulatory domains, such as CD28 or 4-1BB, has yielded impressive clinical results in some blood cancers, but severe toxicities have been observed due to unchecked T cell activation. In contrast, CAR-T cells have demonstrated limited clinical efficacy, associated with poor engraftment, survival and proliferation of adoptively transferred cells when used to target a variety of solid tumors. Thus, technologies that can regulate T cell activation and proliferation in vivo should both mitigate toxicities and maximize anti-tumor efficacy, expanding their clinical utility to a wider range of indications. Here, we describe a novel T cell costimulation switch, inducible MyD88/CD40 (iMC), activated by a small molecule chemical inducer of dimerization, rimiducid, to enhance survival and drive T cell proliferation.

Methods: T cells were activated with anti-CD3/28 and transduced with a retrovirus encoding tandem rimiducid-binding domains (FKBP12v36),cloned in-frame with MyD88 and CD40 signaling elements, and first generation CARs (CAR.ζ) targeting CD19 or PSCA (SFG-iMC-2A-CD19.ζ or SFG-iMC-2A-PSCA.ζ, respectively). iMC activation was measured by treating T cells with and without rimiducid and measuring cytokine production by ELISA and T cell activation markers by flow cytometry. Coactivation through iMC and CAR was tested in coculture assays with or without rimiducid using various tumor cells (CD19⁺, Raji and Daudi lymphoma; PSCA⁺, Capan-1 and HPAC pancreatic adenocarcinoma). Efficacy of iMC-modified CAR-T cells were assessed using an immune-deficient NSG mouse tumor model. For CD19-targeted CARs, 1x10⁵ Raji tumor cells were injected i.v. followed on day 7 by a single i.v. injection at various doses of iMC-CD19.ζ-modified T cells. For PSCA-targeted CARs, 2x10⁶ HPAC tumor cells were injected s.c. followed by iMC-PSCA.ζ-modified T cells on day 10. In both models, iMC was activated in vivo by weekly i.p. injections of rimiducid (5 mg/kg). In some experiments, iMC-CAR-modified T cells were engrafted into tumor-free mice. Tumor burden and CAR-T cell expansion in vivo was assessed using luciferase bioluminescent imaging and flow cytometry.

Results: T cells transduced with either iMC-CD19.ζ or iMC-PSCA.ζ produce cytokines (e.g., IFN-γ and IL-6) in response to rimiducid; however, the key growth and survival cytokine, IL-2, was only produced when both iMC and CAR were activated simultaneously by rimiducid and tumor antigen, respectively. CD19⁺ Raji tumor-bearing mice treated with iMC-CD19.ζ-modified T cells with or without rimiducid administration increased survival compared to non-transduced T cells (p = 0.01). However, rimiducid treatment induced a 7.3-fold CAR-T cell expansion compared to mice infused with iMC-CD19.ζ, but untreated with dimer drug (p = 0.02). Additionally, treatment of NSG mice bearing large (>200 mm³) HPAC tumors with a single dose iMC-PSCA.ζ, resulted in complete elimination in 10/10 mice (100%) of tumors both with and without rimiducid treatment compared to mice receiving non-transduced T cells (p = 0.0003). Rimiducid administration again dramatically increased CAR-T cell levels, resulting in a 23-fold expansion of iMC-PSCA.ζ-modified T cells compared to mice not receiving rimiducid (p = 0.02), justifying ongoing experiments using larger tumors at baseline with fewer T cells. In addition, in tumor-free mice, rimiducid prolonged iMC-PSCA.ζ-modified T cell engraftment and survival for 28 days compared to those mice not treated with dimerizer (p = 0.03). Importantly, following rimiducid withdrawal, CAR-T cell numbers declined, consistent with the requirement of MC-mediated costimulation in combination with CAR activation.

Summary: Inducible MyD88/CD40 represents a novel activation switch that can be used to provide a controllable costimulatory signal to T cells transduced with a first generation CAR. The separation of the cytolytic signal 1 (CD3ζ) domain from a potent, regulatable, signal 2 costimulation (iMC) in the novel platform, called "GoCAR-T", allows the expansion of T cells only in response to both rimiducid and tumor antigen, and their decrease in number by withdrawal of rimiducid-induced iMC costimulation. The "GoCAR-T" platform may allow the development of a new generation of more effective CAR-T cell therapies.