Notch-Mediated Expansion of Human Cord Blood Progenitor Cells Results in Rapid Myeloid Reconstitution in Vivo Following Myeloablative Cord Blood Transplantation

Delayed hematopoietic recovery following cord blood transplantation (CBT) is thought to result from inadequate numbers of progenitor cells in the graft and is associated with increased early transplant related morbidity and mortality. Using an engineered form of the Notch ligand, Delta1, we have previously reported on novel ex vivo expansion methods for generating greatly increased numbers of human CD34 progenitor cells that repopulate immunodeficient mice with markedly enhanced rate and magnitude. We now report results of the initial 6 patients enrolled in a phase I study evaluating the safety and potential efficacy of cord blood (CB) progenitors cultured in the presence Delta1 and recombinant cytokines, with the goal of generating increased numbers of short term repopulating cells capable of providing rapid myeloid engraftment. These patients (AML, n=5; bi-phenotypic leukemia, n=1), were treated with a myeloablative preparative regimen consists of cytoxan 120mg/kg, fludarabine 75mg/m² and 1320 cGy TBI, followed one day later by infusion of one non-cultured CB unit and then a second unit that has been CD34 enriched and cultured for 16 days as previously described. The median age and weight of the patients enrolled is 28 years (range 11 to 43) and 61.5 kilograms (range 26 to 76). CB units were selected on the basis of cell dose and a requirement of matching at least 4 of 6 loci with the patient (intermediate resolution for HLA-A and B, and high resolution for HLA-DRB1). The non-cultured unit in all patients was 4/6 matched to the patient. The unit used for expansion was 5/6 matched to the patient in two cases, and 4/6 matched in the other four. After culture, there was an average CD34 fold increase of 160 (range 41 to 382) with an average total nucleated cell (TNC) fold increase of 660 (range 146 to 1496). Average infused TNC/kg x10⁷ was 2.9 (range 1.9–5.8) and 4.6 (range 0.6–9.1) for the non-cultured and cultured cells respectively, and infused CD34 cells/kg (x10⁵) was 2.2 (range 1.1–3.4) and 53.4 (range 9.3–133) respectively. No T cells were generated during culture and no toxicities directly attributable to the cultured product, including infusional or increased acute GVHD, have been observed. All patients have engrafted. Relatively rapid engraftment was observed in 5 out of 6 patients treated to date, with a median time to engraftment of 14 days (range 7 to 34), as compared to 25 days (range 16 to 48) in patients (n=17) undergoing an identical transplant regimen at this center, but with 2 non-cultured CB units. The relative contribution of the expanded and non-cultured grafts over time was determined by a DNA-based assay for short tandem repeat loci on peripheral blood sorted cell fractions to include CD3⁺, CD33⁺, CD56⁺, CD14⁺ and CD19⁺ cells, beginning day +7 post transplant. In the 5 patients with early myeloid engraftment (≤20 days), engrafted myeloid cells present at day 7 were derived almost entirely from the expanded unit. In three of these five, ANC >500 was observed at days 7, 9 and 16 and was mainly derived from the expanded unit, whereas in the other 2 patients who achieved ANC>500 at day 13 and 20, myeloid engraftment at day 14 was derived from the non-cultured cells. Persistent contribution to engraftment from the expanded cells has been noted in two patients, one through 280 days post transplant but no longer present at one year, and in a more recently treated patient who is currently 75 days post transplant, the expanded cells continue to dominate in CD33, CD14 and CD56, but not CD3, sorted cell fractions. Furthermore, time to platelet engraftment (>20k) has averaged 30 days (range 19–53). Average follow-up time is 277 days (range 70–632). One patient died on day 462 from complications of VZV myelitis; all other patients are alive and in remission. Overall, accelerated myeloid engraftment has been observed in 5/6 patients treated to date as a direct result of Notch-mediated ex vivo expansion of one of two CB units prior to transplantation with ultimate engraftment from the T replete non-cultured unit. These results further suggest that improvement in early myeloid reconstitution may result from provision of short term repopulating cells and/or of cells able to facilitate engraftment of the non-cultured unit. These studies continue with the goal of achieving consistent, rapid engraftment in recipients of hematopoietic cell transplants to decrease morbidity and mortality in the early post-transplant setting.