In recent years, CAR T therapy has become an established treatment for patients suffering from blood cancers derived from B cells; this includes B cell lymphomas, leukemias and multiple myeloma. However, the therapy has yet to achieve similar success for T cell-derived cancers. A small study published in the New England Journal of Medicine (NEJM) suggests that highly precise gene editing could be used to confront this therapeutic gap. Their method demonstrates proof-of-concept and opens doors for CAR T therapy to potentially treat children with aggressive T cell leukemia.
T Cell Acute Lymphoblastic Leukemia
T cell acute lymphoblastic leukemia (T-ALL) is an aggressive, rapidly forming blood cancer borne from white blood cells called T cells. Healthy T cells normally target abnormal or infected cells and aid other immune cells; with this kind of cancer, the immature T cells overcrowd and weaken the immune system.
People with T cell leukemia often experience persistent infections, bruising, and symptoms such as anemia and bone pain. Treatment usually includes chemotherapy and, if that fails, stem cell transplantation. These measures successfully extend survival, but unfortunately relapse can still occur afterwards. When this happens, previous lines of treatment lose their efficacy.
Chimeric Antigen Receptor T Cell (CAR T) therapy, a recent medical innovation, can achieve durable remission for patients with relapsed B cell-derived cancers. With modification, perhaps CAR T therapy could be used to treat T cell cancer patients, too.
CAR T Therapy Challenges for T Cell Leukemia
The current standard for CAR T therapy involves extracting a patient’s T cells, genetically modifying them to improve their cancer detection, and then returning the strengthened cells to the patient to wipe out their cancer. The cells identify a specific biological tag, or antigen, found on the surface of a lineage of white blood cells; this wipes out cancerous and healthy cells in the process. Although effective for patients with B cell derived cancers, this unabashed killing poses challenges for patients with T cell leukemia.
CAR T therapy relies on T cells. Therefore, a CAR T cell designed to attack T cell antigens would inevitably cause CAR T cell fratricide, a phenomenon where CAR T cells attack each other. This would decrease the efficacy of the treatment. Additionally, the CAR T cells may attack too many healthy T cells and lead to T cell deficiency. This deficiency is hard to supplement and compromises the patient’s ability to fight off viral and fungal infections.
Adapting CAR T Therapy for T Cell Leukemia
Researchers from the University College of London and National Health Services Trust collaborated to confront these unique CAR T therapy challenges. They used healthy donor T cells and bioengineered them to express chimeric receptors against a T cell antigen called CD7. Unlike traditional CAR T therapy, this procedure does not alter the patient’s own immune cells.
Most interestingly, the team used CRISPR-Cas9 gene editing to simultaneously and precisely target three genes in the donated T cells. The genes were base-edited, a process which converts a single base of DNA code into another base (ie. C to T, cytosine changed to thymine). This change is read as a stop codon and turns off the gene.
The first gene change involved deactivating T cell receptor genes such as TRBC1 and TRBC2. Disrupting T cell receptor expression means that host T cells will not be able to find and destroy the CAR T cells. This should prevent the donor CAR T cells from being rejected upon infusion as a tissue transplant would, a phenomenon commonly known as graft-versus-host disease (GVHD).
The second gene change terminated CD7 antigen expression on the T cells. Without CD7 on their cell surface, the CAR T cells should be able to ignore each other.
Finally, a gene called CD52 was silenced to prevent T cell depletion. Prospective CAR T therapy candidates usually undergo preparatory chemotherapy to kill some immune cells and make space for the CAR T cell infusion to proliferate. This gene change helps the CAR T cell evade alemtuzumab, a chemotherapy drug which targets CD52.
Figure 1: Study protocol overview, CAR T therapy for T cell acute lymphoblastic leukemia (T-ALL). The donor T cells were first electroporated with guide RNA to assist with CRISPR editing. Next, the T cells were base-edited to inactivate genes TRBC, CD7 and CD52. Lastly, the cells were transduced to express an anti-CD7 chimeric antigen receptor.
CHIESA, ROBERT, ET AL. “BASE-EDITED CAR7 T CELLS FOR RELAPSED T-CELL ACUTE LYMPHOBLASTIC LEUKEMIA.” NEW ENGLAND JOURNAL OF MEDICINE, 2023, HTTPS://DOI.ORG/10.1056/NEJMOA2300709.
Results Demonstrate Proof-of-Concept
The researchers enrolled three children with relapsed leukemia in their study. Each child underwent a chemotherapy regime of alemtuzumab, fludarabine, and cyclophosphamide before they received a single infusion of ready-made CAR T cells. A follow up was made 28 days after the initial infusion to determine the therapy’s safety and measure its ability to achieve remission.
The CAR T therapy sent two patients into remission—a result consistent with CAR T therapy for B cell derived cancers. The third patient responded to the therapy, but also suffered fatal adverse effects.
The first patient, a 14 year old girl, had complete remission of cancer signs at Day 28. Afterwards, she received a stem cell transplant from her original donor and continued to maintain cancer remission. Similarly, one 15 year old boy successfully went into remission and continued on to receive stem cell transplantation. The last patient, a 13 year old boy, had a starkly different experience. This patient responded to the therapy, but later developed fatal fungal lung disease on top of already pre-existing complications.
Significant Adverse Events
The CAR T therapy elicited significant adverse effects—some align with other CAR T therapies and others may be exacerbated by T cell leukemia challenges.
All patients experienced symptoms of cytokine release syndrome (CRS) within a week of the infusion. This is a common yet potentially fatal effect of CAR T therapy. Neurotoxicity, another known CAR T effect, was also observed.
Patients with T cell leukemia may be at higher risk for infections caused by T cell deficiency. All three children had low white blood cell counts; this likely left them vulnerable to viral (cytomegalovirus) reactivations and fungal infections. Overlapping infections can burden the body severely, as seen with the third patient. Previous studies suggest that this deficiency may be attributed to the CAR T cells’ anti-CD7 activity.
It would be useful to control the CAR T cells before they triggered adverse events. The authors mention a possible solution: integrating suicide genes into the CAR T cells. These genes could be triggered externally, allowing the engineered cells to self-destruct before they become overly dangerous. Toxic genes could also be used to terminate CAR T cells that mutate and turn cancerous. These cells would otherwise grow unchecked by the immune system due to reduced T cell receptor expression.
There may be hope yet for patients with difficult-to-treat T cell leukemia. CAR T therapy, an established treatment for certain B cell derived cancers, could potentially achieve remission—but not without substantial changes. The universal CAR T cells in this study are designed to prevent host rejection, T cell deficiency and CAR T cell fratricide. The infusion achieved remission for two out of three patients, but not without significant risks. It will be compelling to see how this ready-made concept performs with further investigation and a larger study cohort.
For more on developing therapeutics for T cell leukemia, please read the article CAR T Cell-Like Therapy To Treat T Cell Leukemia (T-ALL).