Mobilization meets antibody conditioning: A path toward safer engraftment

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The first bone marrow transplant (BMT) was performed in 1957. Since then, over 1.5 million patients worldwide have undergone this procedure,¹ offering many a durable cure for severe hematopoietic disorders. However, fewer than 20% of patients have a suitable immunologically matched donor available. For the remaining patients, viral-mediated gene therapies that genetically modify a patient’s own hematopoietic stem cells (HSCs) first entered the clinic in the 1990s but only achieved US Food and Drug Administration (FDA) approval in 2022. Shortly thereafter, the first precision CRISPR-mediated genome-editing therapy in HSCs achieved FDA approval.² These autologous BMT procedures have shown durable clinical efficacy and may be safer than allogeneic transplants, since using a patient’s own HSCs avoids complications such as immune rejection and graft-versus-host disease. However, major concerns exist surrounding the accessibility, scalability, and safety of this technology.³ As a result, most patients choose not to undergo the procedure, opting instead to manage the symptoms and long-term complications of their disease.⁴ Clearly, this option is more acceptable than facing the toxic myeloablation regimens currently required to achieve sufficient long-term engraftment of donor or corrected patient HSCs.

To overcome this barrier, new conditioning regimens are being developed that selectively target and clear HSCs in the bone marrow, offering a more precise and less systemically toxic alternative to the broad-spectrum myeloablation regimens that remain the current standard of care. These therapies typically deploy naked monoclonal antibodies or antibody-drug conjugates to target an intended epitope, such as CD117, which can deplete HSCs but spare other tissues. Early antibody-based conditioning studies in select disease indications have shown considerable promise in the clinic.⁵ However, this targeted conditioning typically yields lower levels of engraftment than traditional myeloablative regimens and remains limited to diseases like Fanconi anemia, where corrected HSCs enjoy a selective advantage; thus, even partial engraftment may be curative. Yet, for the most prevalent hematopoietic disorders like sickle cell disease and thalassemia, the therapeutic threshold is substantially higher, requiring 25% or more donor chimerism in the marrow,⁶ putting them out of reach for current antibody-only regimens. Mobilization, on the other hand, has historically been deployed at the beginning of allogeneic or autologous BMT procedures to coax HSCs out of the marrow and into circulation for harvest. A recent study demonstrated that mobilizing the recipient immediately before HSC transplantation can enable engraftment without conditioning. This approach transiently opens bone marrow niches, improving the engraftment of genome-modified HSCs in preclinical mouse models.⁷ While this strategy provided a substantial boost over the unmobilized cohort, as with antibody-mediated conditioning, the engraftment rates from this mobilization-only regimen remained modest and well below the corrective threshold for sickle cell disease. Now, the recent publication by Ojeda et al. in this issue of Molecular Therapy⁸ combines mobilization and antibody conditioning to achieve a synergistic effect (Figure 1).

Figure 1.

Schematic depicting bone marrow and peripheral blood in the steady state and following mobilization and/or antibody-mediated conditioning plus HSCT

Ab., antibody; HSCT, HSC transplantation.

In this study, mobilization is used to not only coax HSCs from the bone marrow and enhance accessibility to the niche but also to amplify the effectiveness of antibody-mediated clearance, achieving higher engraftment rates than either strategy alone. The authors proceed to show the broad utility of this approach in three independent mouse models. Importantly, this combined regimen appears to maintain a favorable safety profile, with minimal off-target toxicity compared to traditional myeloablation. While this is a promising proof of concept, future work is needed to define the ideal timing and dosage of mobilizers, antibodies, and HSCs and to determine how best to pair specific mobilization regimens with targeted depletion strategies in a given disease context to maximize long-term engraftment. Even with this current study, engraftment rates remained below the clinical threshold needed for the most prevalent hematopoietic disorders.

So, are low engraftment rates simply an inherent limitation of antibody-based conditioning? Evidence from prior and ongoing clinical trials suggests otherwise, but none have yet matched the engraftment levels of traditional myeloablation, likely due to incomplete clearance of host HSCs and concerns about on-target, off-tissue activity. These experiences highlight a critical question for the field: which HSC markers represent the most effective and durable targets for depletion? CD117 has emerged as the leading candidate, given its central role in stem cell maintenance and feasibility demonstrated in clinical studies, though its expression on mast cells, melanocytes, and germ cells raises safety concerns. More recently, the MPL protein, which is the receptor for thrombopoietin, has been proposed as a more specific marker of long-term repopulating HSCs. Looking ahead, high-throughput functional screens may provide an unbiased means of discovering novel surface targets uniquely expressed on long-term HSCs. In this sense, mobilization plus antibody conditioning is not the endpoint but rather a concept on which additional innovations can build.

Toward this end, there is the opportunity to integrate improved conditioning protocols with other emerging technologies in stem cell and gene therapy. For example, strategies that confer a proliferative advantage to corrected HSCs, such as engineering increased sensitivity to erythroid cytokines,⁹ could be particularly powerful when combined with antibody-mediated conditioning. Strategies such as epitope shielding provide another example of an alternative and possibly synergistic approach with the above strategies, enabling targeted ablation of unmodified HSCs while protecting those that have been modified. This approach may even allow repeated administration of antibodies to improve corrected HSC chimerism over time.¹⁰ These examples highlight that by aligning the conditioning environment with the biology of the incoming graft, it may be possible to tilt the balance further in favor of durable engraftment.

Finally, advances in direct genomic modification of HSCs in vivo are a rapidly emerging space, one that could bypass the time- and cost-intensive process of HSC isolation, conditioning, and transplantation. However, HSCs are currently a difficult cell population to modify at clinically meaningful frequencies in vivo, and current genetic modifications remain limited to small, local changes using Cas9 or base editors. Still, one can imagine that any of the concepts described above could synergize with in vivo delivery, such as (1) mobilization to make HSC populations more accessible to in vivo editing machinery, (2) combining antibody-mediated depletion with in-vivo-delivered epitope shielding edits to exclusively clear unmodified HSCs, and (3) introducing modifications that amplify production of clinically relevant cell types to allow lower rates of in vivo HSC editing to achieve clinically meaningful thresholds.

As the landscape of cell and gene therapies continues to evolve, the concept of pairing mobilization with antibody conditioning has the potential to reduce the toxicity of HSC transplantation. In the long run, its success will depend on demonstrating durable engraftment under reduced-intensity regimens and on practical considerations, such as the cost and delivery of clinical-grade antibodies and mobilizers. At the same time, advances in antibody engineering, mobilization regimens, and in vivo editing are steadily expanding what is possible. Conditioning has long been the Achilles’ heel of HSC therapies, and mobilization plus antibody conditioning represents a creative approach to address this barrier. Just as the first BMT in 1957 grew into a therapy that has transformed more than a million lives, these next-generation approaches may finally bring safe and durable cures to the millions still waiting.

Declaration of interests

M.K.C. has equity in Kamau Therapeutics. M.K.C. holds patents related to genome editing and cell-based therapies for hematopoietic disorders (WO/2023/060059, WO 2023/028469, WO 2023/060059, WO 2023/064798, WO 2021/097350, and WO/2021/022189).