Last year marked the first U.S. Food and Drug Administration approvals of autologous T-cell immunotherapies for the treatment of cancer, including acute lymphocytic leukemia (ALL) and non-Hodgkin lymphoma (NHL). However, several challenges have limited the clinical development of chimeric antigen receptor (CAR) T-cell therapies against T-cell malignancies, including difficulties harvesting non-malignant autologous T cells from affected patients. Allogeneic, off-the-shelf CAR T-cell products would overcome some of these limitations, but the shared expression of target antigens between effector cells and malignant cells can lead to "self-killing" of CAR T cells.
Researchers at the Washington University School of Medicine in St. Louis have developed a new CAR T-cell strategy using the gene-editing technology CRISPR/Cas9 to engineer "fratricide-resistant" CAR T cells. Lead author Matthew L. Cooper, PhD, and colleagues published their results in Leukemia.1 The preclinical data from this mouse model demonstrates the feasibility of an allogeneic strategy to treat T-cell malignancies.
"Cancerous T cells and healthy T cells have exactly the same protein – CD7 – on their surfaces, so if we program T cells to target CD7, they would attack both the cancerous cells and each other, thus undermining this approach," senior author John F. DiPersio, MD, PhD, explained.2 "To prevent this T-cell fratricide, we used CRISPR/Cas9 gene editing to remove CD7 from the CAR T cells, so they no longer carry the target." CD7 was also selected because this antigen could be deleted in T cells without affecting immune function, the investigators noted.
The researchers generated the CD7-CAR product, UCART7, using CRISPR/Cas9 gene editing and T-ALL cells collected from patients being treated at their institution. Then, they tested this approach in mice xenografted with either T-ALL cell lines or primary cells from human T cell ALLs. The xenografted mice were randomized to receive either UCART7 or UCART19, an allogeneic T cell product directed against CD19 (expressed on B-cells, not T cells) that served as a negative control. Mice received CAR T-cell doses at 2Ã—106/kg.
Following transduction of T cells, the investigators observed significantly fewer UCART7 cells than UCART19 cells. UCART7 cells were biased toward a CD4 phenotype, compared with UCART19. Mice receiving UCART7 had significantly prolonged survival and reduced tumor burden, compared with mice receiving UCART19 (p=0.0003).
Also, mice treated with the gene-edited T cells targeted to CD7 survived 65 days, compared with 31 days for those that received CD19 targeted cells (p=0.0003).
The investigators then tested the capacity of UCART7 to kill primary T-ALL in vivo without inducing an alloreactive graft-versus-leukemia effect or xenogeneic graft-versus-host disease (GVHD). T-ALL blasts were absent in peripheral blood of mice receiving UCART7, compared with those receiving UCART19 (p<0.0001). UCART7 recipients also had normal-sized spleens, while those receiving UCART19 exhibited splenomegaly.
"These UCART7 cells efficiently kill human T-ALL cell lines and patient-derived primary T-ALL in vitro and in vivo without resulting in xenogeneic [GVHD]," the researchers wrote of their findings, indicating that CD7 is a candidate for gene editing of CAR T cells. "Should rejection occur, UCART7 would still provide a viable bridge to transplantation, which many feel is the primary benefit of [CAR T-cell therapy]."
"[An] additional benefit of this approach is that a patient could receive this therapy much more quickly after diagnosis," said Dr. Cooper. "We wouldn't need to harvest the patient's own T cells and then modify them, which takes time. We also wouldn't have to find a matched donor. We could collect T cells from any healthy donor and have the gene-edited T cells ready in advance."
The study is limited by its use of mice and will need to be validated in a human model. Studies are underway to assess the viability of scaling UCART7 for clinical trials.
The authors report no financial conflicts.
- Cooper ML, Choi J, Staser K, et al. An "off-the-shelf" fratricide-resistant CAR-T for the treatment of T cell hematologic malignancies. Leukemia. 2018 February 20. [Epub ahead of print]
- Washington University School of Medicine in St. Louis press release, March 5, 2018.