So, you’re familiar with CRISPR as a gene-editing tool. You might have even seen Dwayne Johnson star in Rampage alongside a mutant albino gorilla fighting a bat-wolf. But you may still find yourself wondering: what has gene editing technology actually done so far to combat human disease?
To date, CRISPR’s biggest impacts in translational medicine have been in monogenic disorders, where a single gene mutation causes disease. Some of these diseases—such as hemoglobinopathies, which affect red blood cells, and lysosomal storage disorders, which impair a cells’ ability to recycle materials—have seen major advances thanks to emerging cell and gene therapies. These treatments mainly rely on autologous cell transplantation, in which a patient’s own stem cells are harvested, edited, and reinfused. While this approach has shown some early success with a new therapy approved last year for sickle cell disease, significant challenges remain. One of the biggest hurdles is ensuring that enough of the edited cells survive and engraft. If the engraftment level falls below 20%, the transplant is no longer effective, and the patient may require another round of therapy.
A new study in Blood Advances from the Kiem lab at Fred Hutch tackles this challenge by using a strategy that protects and enriches transplanted cells to sustain engraftment. Hematopoietic stem and progenitor cells (HSPCs) can be identified by several surface proteins, including CD33. The research team used two different gene-editing approaches—CRISPR/Cas9 and Adenine Base Editors (ABE)—to remove CD33 from HSPCs. The idea was that by eliminating CD33 expression, they could selectively protect transplanted HSPCs from destruction while using a CD33-targeted CAR T cell to eliminate unedited cells, thereby allowing only the modified cells to expand.
Led by graduate student Nick Petty and Hans-Peter Kiem, the study tested whether deleting CD33 in HSPCs could protect them from CD33-targeted CAR T cells, called CAR33. Using a nonhuman primate model, the researchers were able to track immune responses over time in a way that would be difficult in mice or even clinical trials. Petty emphasized the value of this approach: “One thing I find so impactful about the nonhuman primate model is the access to longitudinal samples it allows. For CAR33, we were able to monitor blood values (complete blood counts, cytokine levels, etc.) daily, providing a clear picture of the dynamics of CAR33 expansion and response in vivo.”
The first step was to confirm that CAR33 cells could effectively kill CD33-positive cells while sparing CD33-negative ones in cell culture. They also verified that the CD33-ablated HSPCs could still differentiate into normal immune cell types. Then, it was time for the real test.
T cells were isolated from two Rhesus macaques, expanded in the lab, and engineered into CAR33 cells using simian immunodeficiency virus (SIV) before being reinfused into the test subjects. Both animals had already received CD33-negative HSPC transplants, and as expected, the CAR33 cells successfully eliminated CD33-positive cells in peripheral blood. However, to Petty and Kiem’s surprise, the effects were transient—CD33-positive cells rebounded about three weeks after treatment.
Determined to uncover why, the researchers discovered that Rhesus macaque HSPCs naturally express low levels of CD33, making them harder to target over time. This unexpected finding suggests that expression levels of CD33 in human patients could also impact the efficacy of similar CAR T-based enrichment strategies.
Another unexpected finding emerged from differences between the two test subjects. While one animal developed cytokine release syndrome (CRS), or “cytokine storm,” the other did not. This variation is commonly seen in patients receiving CAR T cell therapy, and Petty noted that the NHP model could help identify predictive factors for CRS: “I think one particularly interesting result that came of this work was the differential response to CAR treatment, where one animal had clinical and physiologic signs of cytokine storm while the other did not. This is a known phenomenon in patients treated with CAR T cells, and seeing that stochastic response recapitulated in the NHP model could allow for further tests to determine predictive factors dictating whether a patient will or won’t develop cytokine storm.”
Despite these hurdles, the study demonstrated that CD33-deleted HSPCs could successfully engraft and resist CAR33-mediated depletion, highlighting the potential of gene editing to enhance stem cell therapies. Importantly, the ability to monitor immune responses in a nonhuman primate model provided valuable insights into CAR T cell dynamics, engraftment efficiency, and cytokine responses—key factors for optimizing future therapies. As Petty noted, the study’s findings lay the groundwork for refining this approach, bringing the field one step closer to more effective and durable cell-based treatments for patients.
The highlighted research was supported by the NIH National Cancer Institute (NCI), National Heart, Lung, and Blood Institute (NHLBI), and the National Institute of Diabetes and Digestive Diseases (NIDDK).
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Keith Jerome, Roland Walter, and Hans-Peter Kiem contributed to this work.
Petty NE, Radtke S, Kanestrom G, Fields E, Humbert O, Fiorenza S, Llewellyn MJ, Laszlo GS, Thomas J, Burger Z, Swing K, Zhu H, Jerome KR, Turtle CJ, Walter RB, Kiem HP. 2025. Protection of CD33-modified hematopoietic stem cell progeny from CD33-directed CAR T cells in nonhuman primates. Blood Advances. DOI: 10.1182/bloodadvances.2024015016.