Multicolor Flowcytometry Analysis of Hematopoietic Stem and Progenitor Cells Subsets Among Basal and Mobilized Peripheral CD34+ Cells

BACKGROUND: In the recent years, numerous studies based on multicolor flowcytometry have analyzed the different subpopulations of bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) (Manz MG et al, PNAS 2002; Majeti R et al, Cell Stem Cell 2007): the common myeloid progenitors (CMPs: Lin^-CD34⁺CD38⁺CD45RA^-CD123⁺), the granulocyte-macrophage progenitors (GMPs: Lin^-CD34⁺CD38⁺CD45RA⁺CD123⁺) and the megakaryocyte-erythroid progenitors (MEPs: Lin^-CD34⁺CD38⁺CD45RA^-CD123^-) constitute the progenitor compartment, while the hematopoietic stem cells (HSCs: Lin^-CD34⁺CD38^- CD45RA^-CD90⁺), the multipotent progenitors (MPPs: Lin^-CD34⁺CD38^- CD45RA^-CD90^-) and the lymphoid-myeloid multipotent progenitors (LMPPs: Lin^-CD34⁺CD38^- CD45RA⁺CD90^-) represent the more immature HSPCs. In animal models, the progenitor compartment includes short-term repopulating cells, leading to the hematological recovery in the first 5 weeks after transplantation, whereas the stem cell compartment comprehends the long-term repopulation cells, responsible for the long-term hematological recovery. However, very little is known about the different subpopulations of HSPCs among peripheral blood (PB) CD34+ in basal state and after mobilization for harvest and transplantation.

Our study was conducted to analyze PB CD34+ cells from healthy volunteers and from hematological patients during CD34+ cells mobilization. Our main aim was to understand if the proportions of different HSPCs among PB CD34+ cells were similar to those found in BM and whether the mobilizing regimens employed in chemo treated patients differently affected CD34+ cells subfractions in PB.

METHODS: multicolor flowcytometry was used to analyze CD34+ cells from 4 BM samples and 9 PB samples from healthy volunteers and 32 PB samples from hematological patients prior CD34+ cells harvesting.

RESULTS: Percentages of CD34+ cells subpopulations were different in basal PB compared to the BM: indeed, CMPs, GMPs and MEPs constituted respectively 27.6% ± 9.5, 23.8% ± 7.2 and 27.6% ± 16.2 of BM CD34+ cells and 47.8% ± 9.5, 10.3% ± 6.9 and 16.1% ± 7.6 of the total PB CD34+ cells. HSCs constituted 2.1% of BM and 1.5% of PB CD34+ cells. The differences between BM and circulating CMPs and GMPs were significant (p<0.005 and p<0.01).

No differences in subpopulations proportions were shown comparing G-CSF mobilized and basal PB CD34+ cells. Interestingly, the 2 patients mobilized with AMD3100 (the inhibitory molecule for CXCR4) showed a higher percentage of GMPs (33.8% and 37.8% versus the average 16.3% ± 9.8 in G-CSF mobilized samples) and a lower fraction of CMPs (29.5% and 41.6% versus the average 58% ± 12 in G-CSF mobilized samples). In order to understand this result, we looked then at the CXCR4 mean fluorescence intensity among the progenitor subsets: GMPs showed significantly higher levels of this molecule compared to CMPs and MEPs.

Regarding the mobilizing chemotherapy regimens, CMPs percentages were higher (61.1% versus 49.1%, p: 0.038) and GMPs’ were significantly lower (11.1% versus 27.6%, p<0.0001) in cyclophosphamide treated patients, compared to patients mobilized with other chemotherapy regimens. The percentage of HSCs did not significantly differ among bone marrow, unmobilized and mobilized PB CD34+ cells. Therefore, since an average collection of mobilized PB cells contains approximately one log more CD34+ cells than a BM harvest, a similarly higher amount of HSC are infused with mobilized CD34+ cell transplantation.

A linear positive correlation between the number of mobilized CD34+ cells and the number of mobilized CMPs, GMPs, and MEPs was observed indicating that the proportions of different HSPCs did not significantly change among high- and low-mobilizers. There were no correlations between the number of mobilized subpopulations and leucocytes, hemoglobin and platelets levels.

CONCLUSIONS: Our data displayed the heterogeneity of HSPC compartment between PB and BM. Many factors could contribute to this variegated scenario. These mechanisms comprehension can help us to choose the most suitable chemotherapy and cytokine administrations in order to improve clinical outcomes as infections complications, length of aplasia and transfusion requirements during an hematopoietic stem cell transplantation.