Stem Cell-Derived CAR T Cells Open Door To Universal Donor Cell Lines

What if an effective cancer treatment became cheaper and easier to produce? One study published in the journal Nature Biomedical Engineering builds on this vision to improve medical innovation CAR T therapy by using stem cells and extensive gene editing. The results demonstrate that the experimental CAR T cells—derived from stem cell donors instead of patients—can be used to effectively treat mice models of cancer. Looking forward, clinical translation of this model could lower the cost, time and labor barriers which make CAR T therapy prohibitive for most.

The High Cost of CAR T Therapy

Patients who undergo Chimeric Antigen Receptor T cell (CAR T) therapy receive a treatment that is personalized to their cancer. The process involves extracting a patient’s immune cells, altering them in the lab to express a new receptor, and reinfusing them to fight their difficult-to-treat cancer. The procedure can achieve a long-sought reduction in symptoms or even remission, but this success comes at a high price—up to $475,000 per infusion, depending on the product. This price takes into account all of the time and labor required to genetically modify each patient’s cells for the treatment.

A ready-made CAR T product would theoretically be easier to produce at a large scale; this would bypass the need to create a custom batch of CAR T cells for each patient. An example effort to create an “off-the-shelf” version of CAR T therapy uses healthy donor T cells as the basis for the therapy. Here, we describe an alternative method that relies on stem cell-grown immune cells—not patient cells or donor T cells—to establish its universal source.

Testing a Solution: Stem Cell-Derived CAR T Cells

In their study, researchers Wang et al. sought to grow a universal source of cytotoxic T cells to use in cell therapies such as CAR T. These cells would be allogeneic, or not derived from the patient. The team turned to induced pluripotent stem cells in particular to accomplish this task.

Induced pluripotent stem cells (iPSC) are adult cells that have been reprogrammed to revert into an immature state. Similar to embryonic stem cells, induced pluripotent cells can thus develop into different cell types. This unique ability means that, in theory, an unlimited number of antigen-specific T cells could be grown from these cells. Expanding these cells can also create a sea of clones with the same genetic characteristics. This is ideal for testing genetic changes; researchers can make multiple clones and reliably compare the effects caused with each gene edit.

Confronting Host Rejection

The use of stem cells always incurs the risk of host rejection. This phenomenon known as Graft-vs-Host Disease (GvHD) occurs when a patient’s body does not recognize a cell transplant as its own. The immune system may engage immune cells such as cytotoxic T cells, helper T cells, or natural killer cells to eliminate the graft. To overcome this, several genes were knocked out from the stem cell-derived T cells using CRISPR-Cas9 gene editing.

First, gene B2M was knocked out to eliminate the expression of a cell surface molecule called human leukocyte antigen I (HLA-I). This removal prevents host cytotoxic T cells from mounting an unwanted immune response and killing the stem cell T cells.

Second, gene CIITA was inactivated to prevent human leukocyte antigen II (HLA-II) expression. Without this antigen present, helper T cells did not target and eliminate the stem cell T cells as they normally would.

The first modification could lead to an attack from natural killer cells. To counter this, gene PVR was eliminated. This downregulated expression of a ligand that natural killer cells normally recognize and destroy. With this ligand gone, the stem cell-derived T cells could escape scrutiny.

Results: Slowed Tumor Growth

The heavily edited T cells were then retrovirally modified to carry different chimeric antigen receptors and tested on mouse models of cancer. Their performance was compared to stem cell-derived T cells that carried a chimeric receptor, but lacked the extensive gene modifications to prevent host rejection.

In the first test, the T cells carried an anti-CD19 chimeric receptor. CD19 is a common therapeutic target for conventional CAR T therapies against certain lymphomas and leukemias. The mice here carried either leukemia or lymphoma tumor cells before receiving an injection of edited or unedited CAR T cells. The results show that the heavily gene-edited CAR T cell injection slowed tumor growth and prolonged the survival of the mice in both groups. This suggests that the edited CAR T cells were effective against the tumors, while harmless against normal cells.

Next, the T cells were modified to carry an anti-CD20 chimeric receptor. CD20 is mostly used as an experimental target for CAR T therapy, an alternative to antigen CD19. The mice here carried CD-20 expressing leukemia or lymphoma cells, and were given multiple cycles of either edited or unedited CAR T cells. The edited CAR T cells immediately slowed tumor growth, while the unedited alternative took 12 days to show a similar effect. The delay could be attributed to the host cytotoxic cells expanding to oppose the tumor.

After each injection, the edited CAR T cells proved more stable than their unedited counterparts. The authors posit that the body rejected the unedited T cells much more. Overall, the extensively edited CAR T cells survived and inhibited tumor growth in both tests.

Possible Complications

The universal donor T cells here are designed to be invisible to the immune system. Notably, such cells may eventually become a source of new tumors and cancers as a result of mutations that arise from uncontrolled cell growth.

To analyze this risk, the authors performed whole-genome sequencing on the cytotoxic T cell lines and their parent stem cell lines to determine the possibility of permanent mutations occuring in the coding regions of the genome. The analysis yielded a few single nucleotide variations with seven or more mismatched base pairs compared to all three genomic RNA-targeting sequences. Coupled with the fact that mice injected with edited CAR T cells did not develop tumors, it seemed unlikely that unwanted genome mutations would occur.

It may also be necessary to integrate inducible suicide genes into the chimeric receptors to further reduce the possibility of developing tumors. This would allow the cells to be intentionally destroyed if they ever posed a danger to the patient. Examples of inducible toxic genes include molecules such as Fas or Cas9, herpes simplex virus thymidine kinase, and truncated epidermal growth factor receptor.

Future Implications

This study challenges conventional methods of sourcing cytotoxic T cells. CAR T therapy traditionally uses a patient’s own cells, but this method is costly and resource-intensive. In comparison, induced pluripotent stem cells can be expanded and cloned in a way that patient cells and donor T cells cannot. The authors demonstrate that their stem cell-derived cytotoxic T cells can survive allogeneic transplant and suppress tumor growth in mouse models of cancer. This optimistic combination of stem cell technology and CAR T therapy may bring the cancer treatment closer to a more ideal, ready-made form.