Making Better Chimeric Antigen Receptors for Adoptive T-cell Therapy

Abstract

Chimeric antigen receptors (CAR) are engineered fusion proteins constructed from antigen recognition, signaling, and costimulatory domains that can be expressed in cytotoxic T cells with the purpose of reprograming the T cells to specifically target tumor cells. CAR T-cell therapy uses gene transfer technology to reprogram a patient's own T cells to stably express CARs, thereby combining the specificity of an antibody with the potent cytotoxic and memory functions of a T cell. In early-phase clinical trials, CAR T cells targeting CD19 have resulted in sustained complete responses within a population of otherwise refractory patients with B-cell malignancies and, more specifically, have shown complete response rates of approximately 90% in patients with relapsed or refractory acute lymphoblastic leukemia. Given this clinical efficacy, preclinical development of CAR T-cell therapy for a number of cancer indications has been actively investigated, and the future of the CAR T-cell field is extensive and dynamic. Several approaches to increase the feasibility and safety of CAR T cells are currently being explored, including investigation into the mechanisms regulating the persistence of CAR T cells. In addition, numerous early-phase clinical trials are now investigating CAR T-cell therapy beyond targeting CD19, especially in solid tumors. Trials investigating combinations of CAR T cells with immune checkpoint blockade therapies are now beginning and results are eagerly awaited. This review evaluates several of the ongoing and future directions of CAR T-cell therapy.

Figure 1

Overview of CAR T-cell therapy in the clinic. A patient's T cells are harvested through leukapheresis, followed by T-cell activation on antibody-coated beads serving as artificial dendritic cells. The activated T cells are then genetically reprogrammed ex vivo by transduction with a construct encoding the CAR, and the CAR T cells are further expanded ex vivo. When the CAR T-cell product had been prepared and passed all quality control testing, the patient receives lymphodepleting chemotherapy and CAR T cell infusion. © Novartis Pharmaceuticals.

Figure 2

Strategies to regulate CAR T-cell persistence. A. Activated T cells are transfected with mRNA encoding a CAR, resulting in expression of the CAR on the surface of the T cell until the mRNA is degraded. B. T cells are transduced with a construct containing and CAR as well as iCasp9. Administration of AP1903 causes dimerization of iCasp9 within the CAR T cells and subsequent apoptosis, resulting in specific depletion of the CAR T cells. C. T cells are transduced with a construct containing a CAR and the CD20 protein. Upon administration of the CD20-specific antibody rituximab, the CAR T cells (and all other CD20-expressing cells) are depleted. ADCC, antibody-dependent cellular cytotoxicity; CD20, cluster of differentiation 20; CDC, complement-dependent cytotoxicity; mRNA, messenger RNA.

Figure 3

Factors influencing CAR T-cell activity in the immunosuppressive solid tumor microenvironment. FasL, Fas ligand; IL, interleukin; PD-1, programmed death-1; PD-L1, programmed death-1 ligand-1; Treg, regulatory T cell; TRUCK, T cell redirected for universal cytokine-mediated killing.

Figure 4

Heat map indicating geographic location of ongoing or completed trials testing CAR T cells. From search term “chimeric antigen receptor;” source: http://clinicaltrials.gov, accessed January 8, 2016. Of these 92 trials, 80 have been sponsored by academic institutions, while 12 trials are either sponsored by industry or have an industry partner listed as a collaborator on http://clinicaltrials.gov.