Non-genotoxic Restoration of the Hematolymphoid System in Fanconi Anemia

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for patients with many different blood and immune diseases; however, current treatment regimens contain non-specific chemotherapy and/or irradiation conditioning, which carry both short-term and long-term toxicities. The use of such agents may be particularly harmful for patients with Fanconi anemia (FA), who have genetic mutations resulting in deficiencies in DNA repair, leading to increased sensitivity to genotoxic agents. mAb-based conditioning has been proposed as an alternative conditioning strategy for HSCT that minimizes these toxicities by eliminating collateral tissue damage. Given the high need for improved treatments for FA patients, we aimed to evaluate the efficacy of different αCD117 mAb agents and immunosuppression on hematopoietic stem cell (HSC) depletion and explored their ability to safely establish therapeutic donor hematopoiesis post-HSCT in FA disease models. We evaluated the effects of different concentrations of αCD117 mAbs in vitro and in vivo on HSC growth and depletion. To further assess the efficacy of mAb-based conditioning, Fancd2^-/- animals were treated with αCD117 mAb and combination agents with αCD47 mAb and antibody-drug-conjugates (ADCs) for syngeneic HSCT. Immunosuppression αCD4 mAb was added to all in vivo experiments due to a slightly mismatched background between the donor grafts and recipients. Immunosuppressant cocktails were also given to Fancd2^-/- animals to evaluate the efficacy of mAb-based conditioning in the haploidentical setting. Statistical analyses were done using the unpaired t-test. We found that antagonistic αCD117 mAbs alone do not deplete host HSCs or enhance HSCT effectively in FA mouse models; however, the potency of αCD117 mAbs can be safely augmented through combination with αCD47 mAbs and with ADCs, both of which lead to profound HSC depletion and establishment of long-term donor engraftment post-syngeneic HSCT. This is the first time these approaches have been tested in parallel in any disease setting, with the greatest donor engraftment observed after CD117-ADC conditioning. Interestingly, our data also suggest that HSC-targeted conditioning is not necessary in HSCT for FA, as high donor HSC engraftment was observed with mAb-based immune suppression alone with immunologically matched and mismatched haploidentical grafts. These results demonstrate the safety and efficacy of several different non-genotoxic mAb-based conditioning strategies in the FA setting. In addition, they show that if sufficient immunosuppression is given to obtain initial donor HSC engraftment, turnover of a majority of the hematolymphoid system can result, likely owing to the survival advantage of wild-type HSCs over FA HSCs. Such non-toxic all-mAb-based conditioning strategies could be transformative for FA patients and those with other hematolymphoid diseases.