Differential Responsiveness of HSPCs Harvested From Bone Marrow (BM), Mobilized Peripheral Blood (mPB) and Umbilical Cord Blood (UCB) to the BM Homing Factors Stromal-Derived Factor-1 (SDF-1), Sphingosine-1 Phosphate (S1P) and Ceramide-1 Phosphate (C1P) Is Related to the Desensitization of mPB and UCB HSPCs by S1P and C1P Present in mPB and UCB Plasma

Hematopoietic stem progenitor cells (HSPCs) are retained in bone marrow (BM) niches in stromal-derived growth factor-1 (SDF-1)–CXCR4 receptor axis-dependent manner. While a role for the SDF-1–CXCR4 axis in the retention of HSPCs in BM under steady state conditions is undisputed, recent evidence confirms that due to induction of proteolytic microenvironment in BM after myeloablative radio-chemotherapy, SDF-1 level in BM decreases (Leukemia 2011, doi: 10.1038/leu.2011.185) which supports the potential role of other non-SDF-1-mediated homing mechanisms. The cumulative evidence indicates that gradients of bioactive lipids, such as sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P), are important homing factors. To support this further we demonstrated recently that S1P and C1P are upregulated in BM after conditioning for transplantation (Leukemia 2011, doi: 10.1038/leu.2011.185). Hypothesis. Based on the fact that SDF-1, S1P and C1P are present at relatively high concentrations in umbilical cord blood (UCB) and mobilized peripheral blood (mPB) plasma, they may desensitize responsiveness of HSPCs to BM homing gradients of SDF-1, S1P and C1P. Experimental approach. Based on this we i) measured concentration of SDF-1, S1P and C1P in BM aspirates, mPB and UCB, ii) evaluated chemotactic responsiveness of BM-, UCB- and mPB-derived HSPCs to homing gradients of SDF-1, S1P and C1P and iii) investigated molecular mechanisms potentially involved in this phenomenon by examining the surface expression of CXCR4 and S1P receptors 1–5 (S1PR1-5) at baseline and following exposure to appropriate ligands. Finally, we modulated the expression of S1PR1-5 on HSPCs and their responsiveness to chemotactic gradients by removal of S1PR1-5 ligands from the culture medium. Results. The highest expression of S1PR1-5 was detected on the surface of BM-derived CD34⁺/Lin⁻ cells as compared to mPB and UCB counterparts. The downregulation of S1PR1-5 on mPB- and UCB-derived BM CD34⁺Lin⁻ cells correlated with elevated level of circulating S1P in UCB and mPB plasma. Next, we found that BM-derived HSPCs responded more robustly to S1P and C1P as compared to SDF-1 when these chemoattractants were employed at physiologically relevant concentrations. Addition of S1P to the BM-derived HSPCs or incubation of BM HSPCs in S1P and C1P rich UCB or mPB plasma lead to downregulation of S1PR1-5 and desensitized their responsiveness to S1P and C1P, but not SDF-1 gradient. The expression of S1PR1-5 and responsiveness to S1P gradient by UCB- and mPB-derived HSPCs, however, could be re-established after incubating HSPCs in S1P-free medium. At the same time, since SDF-1 concentration in mPB and UCB is very low, the responsiveness of mPB- and UCB-derived HSPCs to SDF-1 gradient was not affected. Conclusions. We demonstrate, for the first time, significant differences in responsiveness of HSPCs from different sources to homing gradients of bioactive lipids. The relatively high concentration of S1P and C1P in mPB and UCB plasma may potentially desensitize responsiveness of HSPCs to BM homing gradients of bioactive lipids and interfere with their homing. These observations will have substantial clinical implications in HSPCs' transplantation.

No relevant conflicts of interest to declare.