Impact of Donor-Derived CD34+ Infused Cell Dose on Outcomes of Patients Undergoing Allo-HCT Following Reduced Intensity Regimen for Myelofibrosis:a Study from the Chronic Malignancies Working Party of the EBMT
Authors: Czerw Tomasz , Malpassuti Vittoria , Iacobelli Simona , Koster Linda , Kröger Nicolaus , Robin Marie , Maertens Johan , Chevallier Patrice , Mielke Stephan , Poire Xavier , Snowden John A , Petersen Eefke J. , Malladi Ram , Blaise Didier , Reményi Péter , Gandemer Virginie , Cammenga Jörg , Rubio Marie Thérèse , Maury Sébastien , Deconinck Eric , Finnegan Damian , Hayden Patrick , Hernandez Boluda Juan Carlos , McLornan Donal P. , Yakoub-Agha Ibrahim 
Published: November 2019

Introduction:
Cellular composition of grafts is believed to be one of the significant determinants of outcomes of reduced-intensity conditioning (RIC) allogenic hematopoietic cell transplantation (allo-HCT). However, inconsistent results of the influence of CD34+ cells dose on the incidence of graft-versus-host disease (GVHD), disease control and survival have been reported in studies published thus far. Allo-HCT remains the only potentially curative treatment modality for eligible patients with myelofibrosis (MF). As patients with this diagnosis were underrepresented or not included in the above mentioned analyses, this EBMT registry study aimed to determine impact of CD34+ cell counts on allo-HCT results in this population.
Methods:
Six hundred and fifty seven patients with primary or secondary MF transplanted with use of peripheral blood (PB) as a source of stem cells after RIC regimen (Fludarabine/Melphalan, N=536 (82%); Fludarabine/Busulphan, N=121 (18%)) between 2000 and 2016 were included. Stem cell donor type was HLA-identical sibling (MSD; N=249, 38%) or unrelated (UD; N=408, 62%). Use of ex vivo T-cell depletion and post-transplant cyclophosphamide to prevent GVHD were the exclusion criteria. Median patient age was 58 (range, 22-76) years. In-vivo T-cell depletion was used in 526 (80%) of cases. Median transplanted CD34+ dose was 6.6 (2-29) x 10^6 per kg of recipient body weight. Median follow-up was 46 (2-194) months. Patient and transplant characteristics are summarized in Table 1.
Results:
We did not observe any impact of CD34+ cells dose on outcome variables in the whole study group. However, in an analysis adjusted for donor type, among patients transplanted from a MSD, the 2yr overall survival (OS) probability was 77% for those transplanted with higher doses of CD34+ cells (defined as >7.0 x 10^6/kg) compared to 60% for lower CD34+ counts (below 7.0 x 10^6/kg), P=0.007 (Figure 1). The corresponding probabilities of 2yr non-relapse mortality (NRM) were 16% and 29% (P=0.04), respectively (Figure 2). In multivariate analysis, for patients transplanted from a MSD, higher CD34+ dose was an independent predictor for better survival (HR 0.56; P=0.01) and lower risk of NRM (HR 0.54; P=0.02). We did not find any association between CD34+ counts with probability of engraftment, the incidence of acute (aGVHD) and chronic (cGVHD) nor disease relapse. The effect of cell dose was not seen in the UD cohort.
Factors associated independently with an unfavorable outcome in the whole study group were: older patient age and transplantation from CMV seronegative donors to seropositive recipients for OS and NRM and use of an UD and female donor for NRM. Allo-HCT from UD was also an independent predictor for higher incidence of aGVHD and lower probability of engraftment. Use of in vivo T-cell depletion was a protective factor for aGVHD. It was in turn associated with higher risk of relapse.
Conclusions:
Our analysis suggests a potential survival benefit for MF patients treated with RIC PB-allo-HCT from MSD with higher doses of CD34+ cells (threshold > 7.0 x 10^6/kg). We suggest that this factor should be taken into consideration when planning and performing hematopoietic cell apheresis.

Disclosures
Kröger: Celgene: Honoraria, Research Funding; DKMS: Research Funding; JAZZ: Honoraria; Medac: Honoraria; Neovii: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Riemser: Research Funding; Sanofi-Aventis: Research Funding. Robin:Novartis Neovii: Research Funding. Chevallier:Daiichi Sankyo: Honoraria; Jazz Pharmaceuticals: Honoraria; Incyte: Consultancy, Honoraria. Mielke:Bellicum: Consultancy, Honoraria, Other: Travel (via institution); EBMT/EHA: Other: Travel support; Miltenyi: Consultancy, Honoraria, Other: Travel and speakers fee (via institution), Speakers Bureau; IACH: Other: Travel support; Jazz Pharma: Honoraria, Other: Travel support, Speakers Bureau; ISCT: Other: Travel support; GILEAD: Consultancy, Honoraria, Other: travel (via institution), Speakers Bureau; Celgene: Honoraria, Other: Travel support (via institution), Speakers Bureau; DGHO: Other: Travel support; Kiadis Pharma: Consultancy, Honoraria, Other: Travel support (via institution), Speakers Bureau. Snowden:Janssen: Honoraria; Mallinckrodt: Honoraria; Kiadis: Membership on an entity's Board of Directors or advisory committees; IDMC: Honoraria; Jazz: Honoraria; Gilead: Honoraria. Blaise:Sanofi: Honoraria; Pierre Fabre medicaments: Honoraria; Molmed: Consultancy, Honoraria; Jazz Pharmaceuticals: Honoraria. Hernandez Boluda:Incyte: Other: Travel expenses paid. McLornan:Jazz Pharmaceuticals: Honoraria, Speakers Bureau; Novartis: Honoraria.