The CD19 antigen, expressed on most B-cell acute lymphoblastic leukemias (B-ALL), can be targeted with chimeric antigen receptor–armed T cells (CART-19), but relapses with epitope loss occur in 10% to 20% of pediatric responders. We detected hemizygous deletions spanning the CD19 locus and de novo frameshift and missense mutations in exon 2 of CD19 in some relapse samples. However, we also discovered alternatively spliced CD19 mRNA species, including one lacking exon 2. Pull-down/siRNA experiments identified SRSF3 as a splicing factor involved in exon 2 retention, and its levels were lower in relapsed B-ALL. Using genome editing, we demonstrated that exon 2 skipping bypasses exon 2 mutations in B-ALL cells and allows expression of the N-terminally truncated CD19 variant, which fails to trigger killing by CART-19 but partly rescues defects associated with CD19 loss. Thus, this mechanism of resistance is based on a combination of deleterious mutations and ensuing selection for alternatively spliced RNA isoforms. Significance CART-19 yield 70% response rates in patients with B-ALL, but also produce escape variants. We discovered that the underlying mechanism is the selection for preexisting alternatively spliced CD19 isoforms with the compromised CART-19 epitope. This mechanism suggests a possibility of targeting alternative CD19 ectodomains, which could improve survival of patients with B-cell neoplasms.

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Adoptive immunotherapy using chimeric antigen receptor (CAR) expressing T cells targeting the CD19 B lineage receptor has demonstrated marked success in relapsed pre-B-cell acute lymphoblastic leukaemia (ALL). Persisting CAR-T cells generate sustained pressure against CD19 that may drive unique mechanisms of resistance. Pre-B ALL originates from a committed pre-B cell or an earlier progenitor, with potential to reprogram into other hematopoietic lineages. Here we report changes in lineage markers including myeloid conversion in patients following CD19 CAR therapy. Using murine ALL models we study the long-term effects of CD19 CAR-T cells and demonstrate partial or complete lineage switch as a consistent mechanism of CAR resistance depending on the underlying genetic oncogenic driver. Deletion of Pax5 or Ebf1 recapitulates lineage reprogramming occurring during CD19 CAR pressure. Our findings establish lineage switch as a mechanism of CAR resistance exposing inherent plasticity in genetic subtypes of pre-B-cell ALL.

Some solid tumors have reduced posttranscriptional RNA editing by adenosine deaminase acting on RNA (ADAR) enzymes, but the functional significance of this alteration has been unclear. Here, we found the primary RNA-editing enzyme ADAR1 is frequently reduced in metastatic melanomas. In situ analysis of melanoma samples using progression tissue microarrays indicated a substantial downregulation of ADAR1 during the metastatic transition. Further, ADAR1 knockdown altered cell morphology, promoted in vitro proliferation, and markedly enhanced the tumorigenicity in vivo. A comparative whole genome expression microarray analysis revealed that ADAR1 controls the expression of more than 100 microRNAs (miRNAs) that regulate many genes associated with the observed phenotypes. Importantly, we discovered that ADAR1 fundamentally regulates miRNA processing in an RNA binding-dependent, yet RNA editing-independent manner by regulating Dicer expression at the translational level via let-7. In addition, ADAR1 formed a complex with DGCR8 that was mutually exclusive with the DGCR8-Drosha complex that processes pri-miRNAs in the nucleus. We found that cancer cells silence ADAR1 by overexpressing miR-17 and miR-432, which both directly target the ADAR1 transcript. We further demonstrated that the genes encoding miR-17 and miR-432 are frequently amplified in melanoma and that aberrant hypomethylation of the imprinted DLK1-DIO3 region in chromosome 14 can also drive miR-432 overexpression.

Glioblastomas (GBM), the most common and aggressive malignant astrocytic tumors, contain a small subpopulation of cancer stem cells (GSCs) that are implicated in therapeutic resistance and tumor recurrence. Here, we study the expression and function of miR-137, a putative suppressor miRNA, in GBM and GSCs. We found that the expression of miR-137 was significantly lower in GBM and GSCs compared to normal brains and neural stem cells (NSCs) and that the miR-137 promoter was hypermethylated in the GBM specimens. The expression of miR-137 was increased in differentiated NSCs and GSCs and overexpression of miR-137 promoted the neural differentiation of both cell types. Moreover, pre-miR-137 significantly decreased the self-renewal of GSCs and the stem cell markers Oct4, Nanog, Sox2 and Shh. We identified RTVP-1 as a novel target of miR-137 in GSCs; transfection of the cells with miR-137 decreased the expression of RTVP-1 and the luciferase activity of RTVP-1 3'-UTR reporter plasmid. Furthermore, overexpression of RTVP-1 plasmid lacking its 3'-UTR abrogated the inhibitory effect of miR-137 on the self-renewal of GSCs. Silencing of RTVP-1 decreased the self-renewal of GSCs and the expression of CXCR4 and overexpression of CXCR4 abrogated the inhibitory effect of RTVP-1 silencing on GSC self-renewal. These results demonstrate that miR-137 is downregulated in GBM probably due to promoter hypermethylation. miR-137 inhibits GSC self-renewal and promotes their differentiation by targeting RTVP-1 which downregulates CXCR4. Thus, miR-137 and RTVP-1 are attractive therapeutic targets for the eradication of GSCs and for the treatment of GBM.

Chimeric antigen receptor (CAR)-expressing T cells induce durable remissions in patients with relapsed/refractory B cell malignancies. CARs are synthetic constructs that, when introduced into mature T cells, confer a second, non-major histocompatibility complex-restricted specificity in addition to the endogenous T cell receptor (TCR). The implications of TCR activation on CAR T cell efficacy has not been well defined. Using an immunocompetent, syngeneic murine model of CD19-targeted CAR T cell therapy for pre-B cell acute lymphoblastic leukemia in which the CAR is introduced into T cells with known TCR specificity, we demonstrate loss of CD8 CAR T cell efficacy associated with T cell exhaustion and apoptosis when TCR antigen is present. CD4 CAR T cells demonstrate equivalent cytotoxicity to CD8 CAR T cells and, in contrast, retain in vivo efficacy despite TCR stimulation. Gene expression profiles confirm increased exhaustion and apoptosis of CD8 CAR T cells upon dual receptor stimulation compared to CD4 CAR T cells and indicate inherent differences between CD4 and CD8 CAR T cells in the use of T cell-associated signaling pathways. These results provide insights into important aspects of CAR T cell immune biology and indicate opportunities to rationally design CAR constructs to optimize clinical efficacy.

Axicabtagene ciloleucel (YESCARTA; Kite Pharma, a Gilead Company, Los Angeles CA) and tisagenlecleucel (KYM-RIAH; Novartis Pharmaceuticals Corp., Basel, Switzerland) are two CD19-directed chimeric antigen receptor (CAR) T cell products currently approved by the US Food and Drug Administration; the European Medicines Agency; Health Canada; Ministry of Health, Labor and Welfare (Japan); and Therapeutic Goods Administration (Australia) for treatment of specific subtypes of relapsed/refractory aggressive B cell non-Hodgkin lymphoma (NHL). Although this approval has been transformative in the use of cellular immunotherapy in lymphoma, there are concerns regarding appropriate use of this novel therapy and of short-and long-term toxicities. To address these issues, representatives of the American Society of Transplantation and Cellular Therapy convened to recognize and address key issues surrounding the clinical application of CD19 CAR T cell therapy in B cell lymphomas, in collaboration with worldwide experts. The aim of this article is to provide consensus opinion from experts in the fields of hematopoietic cell transplantation, cellular immunotherapy, and lymphoma regarding key clinical questions pertinent to the use of CD19 CAR T cell products for the treatment of NHL. As the clinical practice using CAR T cells grows worldwide, we anticipate that this guidance will be relevant for hematology/oncology physicians who care for patients with lymphomas.

Autologous T cells engineered to express a chimeric antigen receptor (CAR) against the CD19 antigen are in the frontline of contemporary hemato-oncology therapies, leading to high remission rates in B-cell malignancies. Although effective, major obstacles involve the complex and costly individualized manufacturing process, and CD19 target antigen loss or modulation leading to resistant and relapse following CAR therapy. A potential solution for these limitations is the use of donor-derived γδT cells as a CAR backbone. γδT cells lack allogenecity and are safely used in haploidentical transplants. Moreover, γδT cells are known to mediate natural anti-tumor responses. Here, we describe a 14-day production process initiated from peripheral-blood mononuclear cells, leading to a median 185-fold expansion of γδ T cells with high purity (>98% CD3+ and >99% γδTCR+). CAR transduction efficacy of γδ T cells was equally high when compared to standard CART cells (60.5 ± 13.2 and 65.3 ± 18.3%, respectively). CD19-directed γδCAR-T cells were effective against CD19+ cell lines in vitro and in vivo, showing cytokine production, direct target killing, and clearance of bone marrow leukemic cells in an NSG model. Multiple injections of γδCAR-T cells and priming of mice with zoledronate lead to enhanced tumor reduction in vivo. Unlike standard CD19 CART cells, γδCAR-T cells were able to target CD19 antigen negative leukemia cells, an effect that was enhanced after priming the cells with zoledronate. In conclusion, γδCAR-T cell production is feasible and leads to highly pure and efficient effector cells. γδCAR-T cell may provide a promising platform in the allogeneic setting, and may target leukemic cells also after antigen loss.