Allograft T Cell Content Influences Early Engraftment after Reduced-Intensity Stem Cell Transplantation (RIST).
Authors: Dean Robert M. , Fowler Daniel H. , Hardy Nancy M. , Odom Jeanne , Castro Kathleen , Krumlauf Michael , Hakim Frances T. , Sportes Claude , Gress Ronald E. , Bishop Michael R. 
Published: April 2020

Abstract
Allogeneic hematopoietic stem cells (HSC) generally engraft rapidly and completely after myeloablative conditioning. However, with reduced-intensity conditioning (RIC), mixed chimerism and graft failure are more common. Host immune status and HSC number are factors known to affect engraftment after reduced-intensity stem cell transplantation (RIST). In addition, donor T cells within the allograft may also influencethe kinetics of donor engraftment after RIST. To evaluate this, we performed a controlled comparison of engraftment outcomes among 3 groups undergoing RIST, varying by ex vivo T cell depletion (TCD) or in vivo depletion of activated T cells with methotrexate (MTX) to prevent graft-versus-host disease (GVHD). Group I (n = 50) received T cell replete (TCR) peripheral blood stem cells (PBSC) with cyclosporine (CSA) alone for GVHD prophylaxis. Group II (n = 17) received ex vivo TCD PBSC (positive/negative selection with T cell add-back to uniform dose of 1 x 105 CD3+ cells/kg) with CSA alone for GVHD prophylaxis. Group III (n = 31) received TCR PBSC with CSA plus MTX (5 mg/m2 IV x 4 doses) for GVHD prophylaxis. The 3 groups were similarly immunosuppressed from prior therapy before RIST (median absolute lymphocyte counts 330/μL, 260/μL, and 307/μL for Groups I, II, and III, respectively), and received an identical RIC regimen (fludarabine/cyclophosphamide) plus comparable numbers of filgrastim-mobilized PBSC from HLA-matched sibling donors (median 7.9 x 106, 7.6 x 106, and 6.8 x 106 CD34+ cells/kg, respectively; median 3.6 x 108, 1.0 x 105, and 3.2 x 108 CD3+ cells/kg, respectively). Hematopoietic recovery was slowest in Group III, consistent with the myelosuppressive effects of MTX (Table). A greater proportion of patients in Group I achieved complete donor chimerism (≥ 95%) by day +28 than in Groups II or III (P &lt; 0.025), and at day +100, mixed donor chimerism persisted more often in Groups II and III than in Group I patients (P &lt; 0.01). Correspondingly, early (&lt; day +42) occurrence of grade 3–4 acute GVHD, before initiation of planned sequential donor lymphocyte infusions (DLI) in Group II, was more frequent in Group I than in either Groups II or III (p=0.08).
Table: Hematopoietic Recovery, Engraftment, and GVHD Group Days to ANC &gt; 500, median (range) Days to plt &gt; 100, median (range) Donor chimerism ≥ 95% Early acute GVHD, grades 3–4 Day +28 Day +100 I 9 (7–13) 15.5 (12-42) 37/44 (84%) 36/38 (95%) 9/50 (18%) II 9 (7–10) 17.5 (11–40) 8/17 (47%) 9/14 (65%) 0/17 (0%) III 14 (7–21) 21.5 (12–85) 23/31 (74%) 21/31 (68%) 2/31 (6%)
Thus, the deletion of T cells by either ex vivo TCD or in vivo MTX administration measurably alters the kinetics and degree of donor T cell engraftment after RIST. These observations provide evidence that donor T cells are an independent factor affecting engraftment of allogeneic HSC after RIST by compensating for incomplete host immune ablation. These data also support the hypothesis that a graft-versus-host effect plays a significant role in engraftment after RIST. Manipulation of donor T cells through graft engineering techniques may be a useful strategy to enhance engraftment in the setting of RIST.