Common Myeloid Progenitor Content in Allogeneic Graft Products Correlates with Time to Platelet Engraftment.
Authors: Arber Caroline , Halter Joerg , Stern Martin , Alicia Rovò Dominik Heim , Gratwohl Alois , Tichelli Andre 
Published: October 2019

Abstract
Abstract 3215
Poster Board III-152
In clinical hematopoietic cell transplantation (HCT), CD34 is used as a surrogate marker to determine the content of hematopoietic stem and progenitor cells in the graft. Increasing numbers of CD34+ cells in the graft were associated with significantly accelerated neutrophil and platelet engraftment after myeloablative allogeneic HCT with granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood mononuclear cells (G-PBMC). However, the CD34+ population is very heterogeneous and contains several subpopulations, such as hematopoietic stem cells (HSC), common myeloid progenitors (CMP), granulocyte- macrophage progenitors (GMP), megakaryocyte erythroid progenitors (MEP), common lymphoid progenitors (CLP), T/ NK cell progenitors, and pro B cells. So far, these populations have not been quantified and functionally correlated with time to engraftment in patients after allogeneic HCT. This study was designed to (1) assess the distribution of the CD34+ subpopulations in G-PBMC, bone marrow (BM) and cord blood (CB) products and (2) to correlate the content of CD34+ subpopulations with time to engraftment in patients after myeloblative allogeneic HCT. The following populations within the Lin-CD34+ fraction were quantified by flow cytometry: CD38- HSC, CD38+IL3RaloCD45RA- CMP, CD38+IL3RaloCD45RA+ GMP, CD38+IL3Ra-CD45RA- MEP, CD10+CD19- CLP, CD10+CD19+ Pro B cells and CD10-CD7+CD45RA+ T/ NK cell progenitors. First, the relative distribution of these populations in % was determined in G-PBMC, BM and CB. In G-PBMC the percentage of CMP was highest whereas HSC, GMP and T/ NK progenitor percentage was lowest, when compared to BM or CB (Table 1). MEP and CLP percentages were not significantly different. Second, the functional correlation of absolute progenitor counts per kilogram (kg) recipient body weight with regards to neutrophil and platelet engraftment was analyzed in 24 patients with myeloablative allogeneic HCT with G-PBMC (n=21) or BM (n=3) from matched related (n=15) or unrelated donors (n=9). The underlying diseases were acute leukemia in 14, myelodysplastic syndrome in 2, chronic myeloid leukemia in 3, lymphoproliferative disease in 3, severe aplastic anemia in 1 or non-malignant disease in 1. Median age was 51 years (range 12- 65). Conditioning regimens were cyclophosphamide (Cy)/ busulfan in 15, Cy/ total body irradiation (TBI) in 3, BEAM/ fludarabine/ TBI in 3, other in 3. A Cox model was used to determine the correlation between progenitor counts and time to engraftment of neutrophils >0.5×10e9/L, platelets >20×10e9/L (Plt20) or >50×10e9/L (Plt50). In this cohort, only graft source had a significant impact on neutrophil recovery (p=0.01). However, platelet engraftment also correlated with the total CD34+ cell count/ kg (Plt20 p=0.018, Plt50 p=0.020). This correlation was attributed to the content of CMP/ kg contained in the graft (Plt20 p=0.006, Plt50 p=0.005). In conclusion: (1) G-PBMC, BM and CB not only differ in absolute CD34+ cell numbers but also in the relative distribution of CD34+ subpopulations showing that clinical hematopoietic grafts are more complex than simple CD34 counts. (2) In this patient cohort, accelerated platelet engraftment after myeloablative allogeneic HCT is correlated with the content of CMPs. These findings underline that there is a need for better understanding of hematopoietic graft compositions as this could have further implications for graft manipulation in the future.
Table 1: Distribution of Lin-CD34+ populations in % in G-PBMC, BM, or CB. Comparisons by Mann Whitney test, NS= not significant. Populations in % of Lin-CD34+ cells G-PBMC N=30 BM N=6 CB N=7 p value, (G-PBMC vs BM) p value, (G-PBMC vs CB) p value, (BMvsCB) HSC Median 1.76 4.15 6.57 0.003 <0.001 NS 95% CI (low/high) 1.54/2.96 2.51/6.47 4.51/14.65 CMP Median 73.4 36.6 3.19 <0.001 <0.001 0.001 95% CI (low/high) 66.12/76.35 22.71/42.88 1.06/10.73 GMP Median 15.80 21.55 72.40 0.027 <0.001 0.001 95% CI (low/high) 13.52/22.76 17.73/27.27 61.89/77.02 MEP Median 0.21 0.91 0.12 NS NS NS 95% CI (low/high) 0.20/0.47 -0.15/2.45 -1.17/4.14 CLP Median 0.76 0.42 1.09 NS NS NS 95% CI (low/high) 0.58/1.05 0.11/1.07 0.33/3.43 Pro B Median 0.49 28.25 3.19 <0.001 <0.001 0.001 95% CI (low/high) 0.37/0.73 15.51/42.09 2.13/5.01 T NK progenitor Median 5.10 7.23 16.90 NS <0.001 0.001 95% CI (low/high) 4.66/7.20 5.08/9.31 12.35/21.22

Disclosures
No relevant conflicts of interest to declare.