Targeted cellular therapy for treatment of relapsed or refractory leukemia.

While the mainstay of treatment for high-risk or relapsed, refractory leukemia has historically revolved around allogeneic hematopoietic stem cell transplant (allo-HSCT), targeted immunotherapies have emerged as a promising therapeutic option, especially given the poor prognosis of patients who relapse after allo-HSCT. Novel cellular immunotherapies that harness the cytotoxic abilities of the immune system in a targeted manner (often called "adoptive" cell therapy), have changed the way we treat r/r hematologic malignancies and continue to change the treatment landscape given the rapid evolution of these powerful, yet sophisticated precision therapies that often offer a less toxic alternative to conventional salvage therapies. Importantly, adoptive cell therapy can be allo-HSCT-enabling or a therapeutic option for patients in whom transplantation has failed or is contraindicated. A solid understanding of the core concepts of adoptive cell therapy is necessary for stem cell transplant physicians, nurses and ancillary staff given its proximity to the transplant field as well as its inherent complexities that require specific expertise in compliant manufacturing, clinical application, and risk mitigation. Here we will review use of targeted cellular therapy for the treatment of r/r leukemia, focusing on chimeric antigen receptor T-cells (CAR T-cells) given the remarkable sustained clinical responses leading to commercial approval for several hematologic indications including leukemia, with brief discussion of other promising investigational cellular immunotherapies and special considerations for sustainability and scalability.

Posoleucel, an Allogeneic, Off-the-Shelf Multivirus-Specific T-Cell Therapy, for the Treatment of Refractory Viral Infections in the Post-HCT Setting.

PURPOSE: Viral infections are a major cause of morbidity and mortality following allogeneic hematopoietic cell transplantation (allo-HCT). In the absence of safe and effective antiviral treatments, virus-specific T cells have emerged as a promising therapeutic option. Posoleucel is a multivirus-specific T-cell therapy for off-the-shelf use against six viral infections that commonly occur in allo-HCT recipients: adenovirus, BK virus (BKV), cytomegalovirus, Epstein-Barr virus, human herpes virus-6, and JC virus. PATIENTS AND METHODS: We conducted an open-label, phase II trial to determine the feasibility and safety of posoleucel in allo-HCT recipients infected with one or more of these viruses. Infections were either unresponsive to or patients were unable to tolerate standard antiviral therapies. Fifty-eight adult and pediatric patients were enrolled and treated. RESULTS: Posoleucel was well tolerated, with no cytokine release syndrome or other infusion-related toxicities; two patients (3.4%) developed Grade 2 and one patient (1.7%) Grade 3 GvHD during the trial. The overall response rate 6 weeks after the first posoleucel infusion was 95%, with a median plasma viral load reduction of 97%. Of the 12 patients who had two or more target viral infections identified at study entry, 10 (83%) had a clinical response for all evaluable viruses. Of the 23 patients treated for refractory BKV-associated hemorrhagic cystitis, 74% had resolution of symptoms and macroscopic hematuria by 6 weeks post-infusion. CONCLUSIONS: In this open-label trial, treatment of refractory viral infections/disease in allo-HCT recipients with posoleucel was feasible, safe, and effective.

Feasibility and preclinical efficacy of CD7-unedited CD7 CAR T cells for T cell malignancies.

Chimeric antigen receptor (CAR)-mediated targeting of T lineage antigens for the therapy of blood malignancies is frequently complicated by self-targeting of CAR T cells or their excessive differentiation driven by constant CAR signaling. Expression of CARs targeting CD7, a pan-T cell antigen highly expressed in T cell malignancies and some myeloid leukemias, produces robust fratricide and often requires additional mitigation strategies, such as CD7 gene editing. In this study, we show fratricide of CD7 CAR T cells can be fully prevented using ibrutinib and dasatinib, the pharmacologic inhibitors of key CAR/CD3ζ signaling kinases. Supplementation with ibrutinib and dasatinib rescued the ex vivo expansion of unedited CD7 CAR T cells and allowed regaining full CAR-mediated cytotoxicity in vitro and in vivo on withdrawal of the inhibitors. The unedited CD7 CAR T cells persisted long term and mediated sustained anti-leukemic activity in two mouse xenograft models of human T cell acute lymphoblastic leukemia (T-ALL) by self-selecting for CD7-, fratricide-resistant CD7 CAR T cells that were transcriptionally similar to control CD7-edited CD7 CAR T cells. Finally, we showed feasibility of cGMP manufacturing of unedited autologous CD7 CAR T cells for patients with CD7+ malignancies and initiated a phase I clinical trial (ClinicalTrials.gov: NCT03690011) using this approach. These results indicate pharmacologic inhibition of CAR signaling enables generating functional CD7 CAR T cells without additional engineering.

A Costimulatory CAR Improves TCR-based Cancer Immunotherapy.

T-cell receptors (TCR) recognize intracellular and extracellular cancer antigens, allowing T cells to target many tumor antigens. To sustain proliferation and persistence, T cells require not only signaling through the TCR (signal 1), but also costimulatory (signal 2) and cytokine (signal 3) signaling. Because most cancer cells lack costimulatory molecules, TCR engagement at the tumor site results in incomplete T-cell activation and transient antitumor effects. To overcome this lack of signal 2, we genetically modified tumor-specific T cells with a costimulatory chimeric antigen receptor (CoCAR). Like classical CARs, CoCARs combine the antigen-binding domain of an antibody with costimulatory endodomains to trigger T-cell proliferation, but CoCARs lack the cytotoxic CD3ζ chain to avoid toxicity to normal tissues. We first tested a CD19-targeting CoCAR in combination with an HLA-A*02:01-restricted, survivin-specific transgenic TCR (sTCR) in serial cocultures with leukemia cells coexpressing the cognate peptide-HLA complex (signal 1) and CD19 (signal 2). The CoCAR enabled sTCR+ T cells to kill tumors over a median of four additional tumor challenges. CoCAR activity depended on CD19 but was maintained in tumors with heterogeneous CD19 expression. In a murine tumor model, sTCR+CoCAR+ T cells improved tumor control and prolonged survival compared with sTCR+ T cells. We further evaluated the CoCAR in Epstein-Barr virus-specific T cells (EBVST). CoCAR-expressing EBVSTs expanded more rapidly than nontransduced EBVSTs and delayed tumor progression in an EBV+ murine lymphoma model. Overall, we demonstrated that the CoCAR can increase the activity of T cells expressing both native and transgenic TCRs and enhance antitumor responses.

Replacing CAR-T cell resistance with persistence by changing a single residue.

Sustained persistence of chimeric antigen receptor T (CAR-T) cells is a key characteristic associated with long-term remission in patients with hematologic malignancies. Attempts to uncover mechanisms that enhance persistence and thus functionality will have a substantial impact in broadening application of CAR-T cell therapy, especially for solid tumors. In this issue of the JCI, Guedan et al. describe a promising strategy to limit T cell exhaustion and improve persistence by changing a single amino acid in the costimulatory domain of CD28. The authors demonstrated that this single amino acid substitution in CD28-based mesothelin CAR-T cells results in improved persistence and functionality in a xenograft model of pancreatic cancer. Furthermore, reciprocal alteration of the same residue in inducible costimulator-containing (ICOS-containing) CAR-T cells resulted in limited antitumor activity and persistence. These findings suggest that simple alterations in the costimulatory domain may enhance CAR-T cell persistence, warranting future evaluation in other CD28-costimulatory CARs in an effort to improve durable antitumor effects.

Chimeric Antigen Receptors for T-Cell Malignancies.

Development of chimeric antigen receptor (CAR)-modified T cells for the treatment of T-lineage leukemia and lymphoma has encountered several unique challenges. The most widely expressed tumor antigen targets for malignant T cells are often also expressed on non-malignant T cells. Transducing T cells with CARs targeted to these shared antigens can therefore promote over-activation or fratricide of CAR T cells, reducing their therapeutic potency. If fratricide is resolved, clinical CAR T cell activity may eliminate normal T-cell subsets and cause temporary immunosuppression. In this review, we summarize the preclinical development of CAR-based therapies for T-cell malignancies and discuss strategies to minimize toxicities associated with on-target fratricide and off-tumor activity.