Make Your Cells Engraft Better

For almost any cell therapy, a key step is engraftment. Successful engraftment is a culmination of many steps. The cells need to stay alive after entering the body, home to their appropriate niche, and maintain their expected lifetime. This process cannot be taken for granted, a cell that has been engineered ex vivo faces many challenges upon re-entry.

• They are often stressed and display markers that may lead to their deletion by endogenous immune cells
• They cannot compete for resources against the more ‘fit’ endogenous cells and may die from deprivation
• They cannot home to the appropriate location. How cells localize to their target niche is a complex process and often in cell therapy the cells that have been injected back into the patients blood are ill equipped to travel to their desired target area
• If it is an allogeneic cell therapy, often the greatest challenge of all is a rejection reaction by the patient’s immune system. The immune system is very proficient at identifying and destroying any foreign cells (hence why cancer isn't contagious for example)  so an allogeneic cell therapy tends to have many challenges when seeking to evade these rejection mechanisms

A prominent example of the challenges of engraftment is the transplantation of genetically engineered hematopoietic stem cells. The recent approval of Casgevy was a major milestone for the field however it also cast a spotlight on the major challenges associated with engraftment.
To achieve therapeutic benefit, not all HSCs in the patient need to be replaced by the CRISPR engineered ones. By some estimates, around 20% chimerism is sufficient. However, as part of the current treatment paradigm, the patient’s must undergo a harsh ablation of endogenous HSCs to facilitate the engraftment of Casgevy. The toxicities, including risks of death and infertility, associated with this process have led to many patients steering away from this approach despite its curative potential. This harsh pre-conditioning regimen is believed to be necessary because otherwise the CRISPR engineered HSCs would not be able to effectively engraft. The transplanted HSCs are fragile due to their ex vivo processing during production and would be attempting to enter a bone marrow niche that is otherwise crowded with the patient’s normal HSCs. Pre-conditioning solves this by ablating the normal HSCs inhabiting this niche so that the more delicate engineered HSCs have an improved chance of setting up shop.

An alternative would be to forgo preconditioning and instead super-charge the engineered HSCs for engraftment. For example, by delivery of survival factors, such as BCL-2, one could significantly improve the resilience of the CRISPR-engineered HSCs before injection. Moreover, temporary over-expression of chemokine receptors, such as CXCR4, could improve the homing ability of these HSCs to the bone marrow niche. More sophisticated strategies coupling this approach with CXCR4 and/or BCL-2 inhibitors while using mutant forms of the proteins in the engineered HSCs could be used to exert selective pressure on the endogenous HSCs simultaneous to providing a selective advantage to the engineered ones. It is important to note that some of these pathways, like BCL-2, would be too dangerous to permanently modify because they could be oncogenic. Therefore transient approaches, such as RNA-based expression, could prove critical to the feasibility and safety of such strategies.

All-in-all leveraging such powerful mechanisms through a transient modification could drastically improve the patient journey and yield substantially higher adoption while simultaneously lowering patient risk and hospital costs associated with the toxic conditioning regimens.

CRISPR edited HSCs for beta thalassemia and sickle cell disease are ultimately one of many examples of cell therapies that could potentially benefit from improved engraftment capabilities. Cell fitness when entering the patient’s body can be a crucial factor in optimizing for efficacy vs. toxicity. If these principles of non-genetic cell engineering can be robustly applied across other cell therapy strategies, it can mark a profound improvement in patient outcomes while simultaneously reducing the burden of administering care!