Marrow Allografts after Nonmyeloablative Conditioning: Effect of Cell Dose on Rejection and Degree of Donor Chimerism.
Authors: Diaconescu Razvan , Leisenring Wendy , Kahl Christoph , Sorror Mohamed , Zellmer Eustacia , Maris Michael , Little Marie-Terese , Storb Rainer 
Published: April 2020

Abstract
Sustained engraftment after marrow transplantation from major histocompatibility complex matched donors depends on the intensities of both pretransplant conditioning and postgrafting immunosuppression. Here we asked, both in a preclinical canine model and in the clinical setting, whether, additionally, the number of transplanted marrow cells had a role in the success of engraftment after minimally intensive conditioning. Thirty-four dogs received marrow from dog leukocyte antigen (DLA)-matched related donors after 1 Gy total body irradiation (TBI). Postgrafting immunosuppression included cyclosporine with or without added drugs, most often mycophenolate mofetil, for no more than 5 weeks. Median total nucleated cell (TNC) doses administered were 3.6 (range 1.7–7.98) x 108/kg. Donor chimerism reached median (range) peaks of 15 (2–75) % for granulocytes and 9 (2–55) % for mononuclear cells (MNC), respectively. Eventually, 31 dogs rejected their grafts, after a median of 10 (range 3–20) weeks. TNC doses had significant impacts on outcomes, after adjustment for immunosuppressive regimens received (Table 1); each increase by 1x108/kg decreased the hazard ratio (HR) of rejection by 0.7, prolonged time to rejection by 2 weeks, and increased maximal percentages of donor chimerism by 8% in granulocytes and by 5% in MNC. Eighteen patients with hematological malignancies received marrow grafts from 10/10 human leukocyte antigen (HLA)-matched unrelated donors, after conditioning with Fludarabine, 90 mg/m2, and 2 Gy TBI, followed by postgrafting mycophenolate mofetil and cyclosporine. Median (range) TNC doses were 3.3 (1.3–4.6) x 108/kg, CD34 cells were 2.8 (0.9–7.2) x 106/kg, and CD3 cells were 31 (16–62) 106/kg. Donor chimerism reached median peaks of 95 (0–100) % in granulocytes and 43 (0–100) % in T cells. Eight patients rejected their grafts after a median of 1 (range 1–4.5) month. The numbers of CD34 and CD3 cells infused were not significantly associated with the risk of rejection (Table 2). However, TNC doses significantly impacted on engraftment; each increase in TNC dose by 1x108/kg decreased the HR of rejection by a factor of 0.3 and increased percentages of donor T-cell chimerism by 18% (p=0.05); there was no correlation with granulocyte chimerism. This effect was not negated by adjustment for patient age, diagnosis group, and numbers of preceding cycles of chemotherapy. In conclusion, while allogeneic marrow grafts are possible following minimal intensity conditioning in dogs and human patients, eventual graft rejection is a major problem, the risk of which can be minimized by transplanting the largest number of nucleated marrow cells possible.
Impact of an increase in TNC dose by 108 cells/kg on outcomes of allogeneic canine marrow transplantation
Outcome p-value 95% CI Rejection HR = 0.7 0.04 0.5–1.0 Time to rejection Prolonged by 2 weeks 0.02 0.4–4.0 Granulocyte chimerism Increased by 8% 0.006 2.4–13.0 Mononuclear cell chimerism Increased by 5% 0.003 1.9–8.6
Impact of marrow cell doses on rejection after allogeneic human marrow transplantation
Cell populations and doses Hazard Ratio of Rejection p-value 95% CI TNC 108/kg increase 0.3 0.02 0.1–0.8 CD34 106/kg increase 0.6 0.21 0.3–1.3 CD3 0.6 0.44 0.2–2.2