Impact of CD34-positive cell dose on outcome after peripheral blood stem cell allogeneic transplantation prepared with ATG-based reduced intensity conditioning regimen
Authors: Aude Collignon, Boris Calmels, Samia Harbi, Sabine Fürst, Angela Granata, Catherine Faucher, Claude Lemarie, Pierre‐Jean Weiller, Christian Chabannon, Didier Blaise, Raynier Devillier
Journal: American Journal of Hematology (2017)

Although the amount of CD34+ cells in peripheral blood stem cell (PBSC) graft could affect engraftment and outcome after allogeneic hematopoietic stem cell transplantation (Allo-HSCT), the optimal CD34+ cell dose remains unknown.1 In the early 2000s, we reported that high dose CD34+ cells (>8 × 106/kg) was associated with an increased incidence of graft-versus-host disease (GVHD) after Allo-HSCT prepared by myeloablative conditioning (MAC) regimens.2 However, transplantation procedures strongly changed during the past 15 years, especially with the development of reduced intensity conditioning (RIC) regimen to treat patients with advanced age and/or comorbidities.3 In this setting, it was shown that the use of antithymocyte globulins (ATG) as part of RIC regimen is highly effective as GVHD prophylaxis.4 Thus, we wondered if the negative impact of high CD34+ cell doses observed in the setting of MAC (initially reported without ATG) remains true after RIC regimens. Registry-database studies have evaluated the impact of CD34+ cell dose on outcome after RIC regimen. The experience of the CIBMTR showed that outcome was better when higher doses of CD34+ cells (>4 × 106/kg) were infused.5 In contrast, the EBMT series found that a high amount of CD34+ cells (>8.25 × 106/kg) was associated with increased risk of GVHD and poor outcome.6 These discordant results may be due to the heterogeneity of included patients in terms of conditioning regimens, in vivo T-cell depletion, disease risk, and/or GVHD prophylaxis. We therefore investigated the role of CD34+ PBSC graft composition on outcome after Allo-HSCT in a cohort of patients homogeneously prepared with an ATG-based RIC regimen. We retrospectively analyzed consecutive patients who underwent Allo-HSCT from 2005 to 2013, at the Paoli Calmettes Institute. Patients with following criteria were eligible for analysis: (1) hematological malignancies; (2) RIC regimen based on fludarabine (150 mg/m2), busulfan (260 mg/m2 total dose) and rabbit ATG (5 mg/kg total dose); (3) PBSC from matched related (MRD) or 10/10 matched unrelated donor (MUD) as graft source; (4) Cyclosporine A (CSA) alone as GVHD prophylaxis. Details about conditioning regimen, donor stem cell mobilization, cell enumeration methods and statistical analyses are available in the Supporting Information File 1. We analyzed 246 patients, with a median age of 59 years (19-71). Median dose of CD34+ cells was 6.5 × 106/kg (range: 1-14) (Details in Supporting Information File 2). Briefly, 142 patients (58%) had a matched-related donor, 44 patients (18%) had low disease risk index, and 110 patients (46%) had a hematopoietic cell transplantation comorbidity index ≥ 3. All patients engrafted and median follow-up after Allo-HSCT was 36 months (5-99). Based on our previous report in the setting of MAC, we decided to first analyze the CD34+ cell dose as a categorical variable using 8 × 106/kg as cut off.2 We found no difference in the cumulative incidences of both grade III-IV acute GVHD (≤ vs. >8 × 106/kg: 9% vs. 10%, P = .705) and extensive chronic GVHD (≤ vs. > 8 × 106/kg: 24% vs. 22%, P = 0.715; Table 1, Supporting Information File 3—Fig. A,B) according to CD34+ cell dose. Adjusted Cox model confirmed that of CD34+ cells in PBSC graft > 8 × 106/kg was not associated with increased incidence of acute or chronic GVHD, (acute III-IV: Hazard ratio (HR) = 0.96, 95% confidence interval (95 CI) = [0.42-2.23], P = .929; extensive chronic: HR = 0.84 [0.44-1.60], P = .603, Table 1). Thus, in a population of patients who were homogeneously prepared by ATG-based RIC regimen, we found no impact of CD34+ cell dose on the incidence of GVHD, contrasting with the previous results obtained in the setting of MAC.2 This could be explained in part by the use of ATG, which probably plays a major role as GVHD prophylaxis and may counterbalance the negative impact of the high CD34+ cell doses. It is also important to notice that optimal cutoff defining high dose of CD34+ is not well defined and varied across different studies. To explore the possibility that the optimal cut off may be different than 8 × 106/kg when patients receive a ATG-based RIC regimen, we also analyzed the CD34+ cell dose as a continuous variable (Table 1) and as a categorical variable dichotomized using the median of CD34+ cell dose (6.5 × 106/kg) or quartiles. As shown using the cutoff of 8 × 106/kg, none of these methods found an association between high dose of CD34+ cells and the occurrence of GVHD (Supporting Information File 4). Moreover, we found no impact of CD34+ cell dose on non-relapse mortality, relapse, progression-free survival, and overall survival (Table 1 and Supporting Information File 4). In addition, we used the random survival forest method to evaluate trends of association between CD34+ cell dose and GVHD. We showed that both day-100 acute and 3-year chronic GVHD were not strongly influenced by CD34+ cell dose, after either MRD or MUD Allo-HSCT (Supporting Information File—Fig. C,D). The trends of these curves (horizontal and flat) support that there is no ideal cutoff for analyzing the impact of CD34+ cell dose. In our series, we were not able to explore the impact of very low CD34+ cell doses, and acknowledge that a minimal amount of CD34+ is needed for good engraftment.1 Indeed, only three patients had a less than 2 × 106/kg of CD34+ cells and no patients experienced graft failure. In conclusion, we found no significant impact of CD34+ PBSC graft composition on GVHD and outcome after ATG-based RIC Allo-HSCT. In this situation, the optimal CD34+ cell dose remains unknown, contradicting what was observed using MAC regimens. Thus, we suppose that there is no evidence for limiting the maximal amount of PBSC dose in Allo-HSCT prepared by ATG-based RIC regimen. The authors declare that they have no competing interests. AC, SF, SH, AG, BC, CF, CL, PJW, CC, DB and RD edited and approved the manuscript; AC, SF, SH, AG, CF, DB and RD enrolled patients and collected the data; BC, CL and CC were in charge of donor mobilization and collection, and performed cell numeration; AC and RD performed statistical analyses; DB and RD design the study; AC and RD wrote the manuscript. Authors thank the nursing staff under supervision of Laurence Caymaris for providing excellent care for our patients. Additional Supporting Information may be found in the online version of this article. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.