Chimeric antigen receptor (CAR) T cell therapy relies on the ex vivo manipulation of patient T cells to create potent, cancer-targeting therapies, shown to be capable of inducing remission in patients with acute lymphoblastic leukemia and large B cell lymphoma. However, current CAR T cell engineering methods use viral delivery vectors, which induce permanent CAR expression and could lead to severe adverse effects. Messenger RNA (mRNA) has been explored as a promising strategy for inducing transient CAR expression in T cells to mitigate the adverse effects associated with viral vectors, but it most commonly requires electroporation for T cell mRNA delivery, which can be cytotoxic. Here, ionizable lipid nanoparticles (LNPs) were designed for ex vivo mRNA delivery to human T cells. A library of 24 ionizable lipids was synthesized, formulated into LNPs, and screened for luciferase mRNA delivery to Jurkat cells, revealing seven formulations capable of enhanced mRNA delivery over lipofectamine. The top-performing LNP formulation, C14–4, was selected for CAR mRNA delivery to primary human T cells. This platform induced CAR expression at levels equivalent to electroporation, with substantially reduced cytotoxicity. CAR T cells engineered via C14–4 LNP treatment were then compared to electroporated CAR T cells in a coculture assay with Nalm-6 acute lymphoblastic leukemia cells, and both CAR T cell engineering methods elicited potent cancer-killing activity. These results demonstrate the ability of LNPs to deliver mRNA to primary human T cells to induce functional protein expression, and indicate the potential of LNPs to enhance mRNA-based CAR T cell engineering methods.

Improved outcomes for patients with cancer hinge on the development of new targeted therapies with acceptable short-term and long-term toxicity. Progress in basic, preclinical, and clinical arenas spanning cellular immunology, synthetic biology, and cell-processing technologies has paved the way for clinical applications of chimeric antigen receptor– based therapies. This new form of targeted immunotherapy merges the exquisite targeting specificity of monoclonal antibodies with the potent cytotoxicity and long-term persistence provided by cytotoxic T cells. Although this field is still in its infancy, clinical trials have already shown clinically significant antitumor activity in neuroblastoma, chronic lymphocytic leukemia, and B cell lymphoma, and trials targeting a variety of other adult and pediatric malignancies are under way. Ongoing work is focused on identifying optimal tumor targets and on elucidating and manipulating both cell- and host-associated factors to support expansion and persistence of the genetically engineered cells in vivo. The potential to target essentially any tumor-associated cell-surface antigen for which a monoclonal antibody can be made opens up an entirely new arena for targeted therapy of cancer.

Engineered T cell therapies have begun to demonstrate impressive clinical responses in patients with B cell malignancies. Despite this efficacy, many patients are unable to receive T cell therapy because of failure of in vitro expansion, a necessary component of cell manufacture and a predictor of in vivo activity. To evaluate the biology underlying these functional differences, we investigated T cell expansion potential and memory phenotype during chemotherapy in pediatric patients with acute lymphoblastic leukemia (ALL) and non-Hodgkin lymphoma (NHL). We found that patients with T cell populations enriched for early lineage cells expanded better in vitro and that patients with ALL had higher numbers of these cells with a corresponding enhancement in expansion as compared to cells from patients with NHL. We further demonstrated that early lineage cells were selectively depleted by cyclophosphamide and cytarabine chemotherapy and that culture with interleukin-7 (IL-7) and IL-15 enriched select early lineage cells and rescued T cell expansion capability. Thus, early lineage cells are essential to T cell fitness for expansion, and enrichment of this population either by timing of T cell collection or culture method can increase the number of patients eligible to receive highly active engineered cellular therapies.

Primary resistance to CD19-directed chimeric antigen receptor T-cell therapy (CART19) occurs in 10% to 20% of patients with acute lymphoblastic leukemia (ALL); however, the mechanisms of this resistance remain elusive. Using a genome-wide loss-offunction screen, we identifi ed that impaired death receptor signaling in ALL led to rapidly progressive disease despite CART19 treatment. This was mediated by an inherent resistance to T-cell cytotoxicity that permitted antigen persistence and was subsequently magnifi ed by the induction of CAR T-cell functional impairment. These fi ndings were validated using samples from two CAR T-cell clinical trials in ALL, where we found that reduced expression of death receptor genes was associated with worse overall survival and reduced T-cell fi tness. Our fi ndings suggest that inherent dysregulation of death receptor signaling in ALL directly leads to CAR T-cell failure by impairing T-cell cytotoxicity and promoting progressive CAR T-cell dysfunction. SIGNIFICANCE:Resistance to CART19 is a signifi cant barrier to effi cacy in the treatment of B-cell malignancies. This work demonstrates that impaired death receptor signaling in tumor cells causes failed CART19 cytotoxicity and drives CART19 dysfunction, identifying a novel mechanism of antigenindependent resistance to CAR therapy.

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