LETTER TO THE EDITOR Excellent outcome with a high proportion of mixed chimerism in patients with severe aplastic anemia treated with partially T-cell-depleted peripheral hematopoietic stem cell transplants Bone Marrow Transplantation (2016) 51, 860–862; doi:10.1038/ bmt.2015.356; published online 8 February 2016 The outcome of hematopoietic stem cell transplantation (HSCT) in patients with severe aplastic anemia (SAA) has improved considerably over the last few decades, 1 but graft rejection rates at 10–15%, acute GvHD grade II–IV rates at 11–26% and, parti- cularly, chronic GvHD rates at 30% remain important challenges. 2,3 Optimal transplantation strategies for patients undergoing HSCT from an HLA-matched sibling donor (MSD) are considered to be conditioning with cyclophosphamide (CY) and ATG, cyclosporin-A (CsA) and short-term methotrexate as GvHD prophylaxis and bone marrow (BM), as a stem cell source given lower GvHD risks. 4 However, the advantage of BM over PBSC may be less evident when modulated in vitro T-cell depletion is used. Here we report a series of 12 patients transplanted for SAA at the Bone Marrow Transplant Unit of the Geneva University Hospital with T-cell depleted PBSC followed by a T-cell add-back the next day. We report the incidence of post-transplant infections and an analysis of the hematopoietic chimerism associated with a low incidence of both acute and chronic GvHD, sustained engraftment and excellent long-term survival. We retrospectively evaluated 12 consecutive patients with acquired SAA receiving first transplants from an MSD or HLA-matched unrelated donors after a conditioning regimen consisting of CY and ATG with partially T-cell-depleted PBSC from 1997 to 2015. All patients provided written informed consent. The stem cell source was G-CSF-mobilized peripheral blood of which a part was T-cell depleted with 20 mg of alemtuzumab (CAMPATH-1H (Genzyme Corporation, Cambridge, MA, USA)) in vitro for infusion at day 0, whereas the rest, containing 100 × 10 6 CD3+ cells/kg, was given as a T-cell add-back at day 1. 5 GvHD prophylaxis consisted of CsA, titrated to trough levels between 150 and 200 μg/L and continued for at least 12 months post HSCT in combination with a short course of methotrexate. Patients and transplantation-related parameters, as well as hematological recovery data are presented in Table 1. With a median follow-up of 91 months (range 4–131), all patients engrafted and are alive in sustained CR. No acute GvHD was observed, whereas moderate chronic GvHD occurred only in 1 patient. Low hematopoietic stem cell doses are associated with an increased risk of graft failure. 6 Moreover, in order to ensure engraftment, 'the more the better' seems to be the rule. BM is strongly recommended as the stem cell source for all SAA patients in spite of the fact that it does not always contain an adequate cell dose. On the other hand, PBSC grafts contain considerably more cells, but their use is associated with an increased risk of GvHD and a higher mortality rate. 7,8 Indeed, according to the European Society for Blood and Marrow Transplantation (EBMT) analysis of the outcome of 1448 SAA patients transplanted between 2005 and 2009, the use of PBSC as stem cell source is the strongest negative predictor of survival. 3 Our partially T-cell-depleted PBSC grafts that contain a median CD34+ cell dose of 9.7 × 10 6 /kg and not enough T cells to cause severe GvHD may be an efficient approach. Obviously, we do not pretend that the outcome of our small series calls into question the recommendation for BM donation, but we do think that our method may be considered in cases where transplanting with PBSC is the only available option. The favorable impact of in vivo T-depletion by alemtuzumab- containing regimens in SAA transplants on the incidence and severity of GvHD has been noticed previously. 9–11 However, the increased risk of infections, particularly viral, associated with the use of alemtuzumab is of concern. 11 In our study, we assessed CD4+ T-cell counts and serum concentration of Igs, as well as the incidence and the severity of infections at different time points after transplantation. All patients displayed 4200 CD4+T Table 1. Patients and transplantation characteristics Patients and transplantation characteristics Patients (n = 12) Male/female 7/5 Age Years (range) Patient age at HSCT 32 (15–58) Donor age at HSCT 36 (16–57) Time from dx to HSCT median (months, range) 3 (1–43) Previous IST Patients (n) No previous IST 8 ATG+CsA 2 ATG 2 MMF+ prednisone 1 CsA+ prednisone 1 CD34+ cell count median (range) 9.7 (5.5–40.1) × 10 6 /kg Donor type HLA-identical sibling 11 Matched unrelated 1 Female donor/male recipient 3/12 Engraftment Days (range) Neutrophil engraftment 15 (8–29) Platelet engraftment 18 (8–30) GvHD Patients (n) Acute grade II–IV 0 Chronic 1 Follow-up median (months, range) 91 (4–131) Status at last follow-up Patients (n) CR 12 Graft failure/relapse 0 Death 0 Abbreviations: CsA = cyclosporin-A; dx = diagnosis; HSCT = hematopoietic stem cell transplantation; IST = immunosuppressive treatment; MMF = mycophenolate mofetil. Bone Marrow Transplantation (2016) 51, 860 – 862 © 2016 Macmillan Publishers Limited All rights reserved 0268-3369/16 www.nature.com/bmt