A novel perspective on stem cell homing and mobilization: review on bioactive lipids as potent chemoattractants and cationic peptides as underappreciated modulators of responsiveness to SDF-1 gradients

Abstract

Hematopoietic stem progenitor cells (HSPCs) respond robustly to α-chemokine stromal-derived factor-1 (SDF-1) gradients, and blockage of CXCR4, a seven-transmembrane-spanning G(αI)-protein-coupled SDF-1 receptor, mobilizes HSPCs into peripheral blood. Although the SDF-1-CXCR4 axis has an unquestionably important role in the retention of HSPCs in bone marrow (BM), new evidence shows that, in addition to SDF-1, the migration of HSPCs is directed by gradients of the bioactive lipids sphingosine-1 phosphate and ceramide-1 phosphate. Furthermore, the SDF-1 gradient may be positively primed/modulated by cationic peptides (C3a anaphylatoxin and cathelicidin) and, as previously demonstrated, HSPCs respond robustly even to very low SDF-1 gradients in the presence of priming factors. In this review, we discuss the role of bioactive lipids in stem cell trafficking and the consequences of HSPC priming by cationic peptides. Together, these phenomena support a picture in which the SDF-1-CXCR4 axis modulates homing, BM retention and mobilization of HSPCs in a more complex way than previously envisioned.

Figure 1. S1P receptors on BM HSPCs are downregulated by S1P exposure

BM-derived stem/progenitor cells were identified by their expression of the CD34 marker and the lack of expression of the committed lineage (Lin) markers. At baseline, Lin⁻CD34⁺ cells isolated from the BM express S1P receptors, with the highest percentage of cells expressing S1P receptor-1. After incubation with S1P at 0.25 μM for 2 hours, the expression of S1P receptors is reduced dramatically due to the internalization of the occupied receptors (*P < 0.05).

Figure 2. Differences in responsiveness of BM-, mPB-, and UCB-derived HSPCs to SDF-1, S1P, and C1P gradients

Mononuclear cells isolated from UCB (Panel A), murine mPB (Panel B), and murine BM (Panel C) were evaluated for the chemotactic activity of CFU-GM to SDF-1, S1P, and C1P gradients. The physiological concentration of SDF-1, as measured in plasma by ELISA, is indicated by red arrows. In contrast to their BM-derived counterparts, CFU-GM from UCB and mPB respond poorly to bioactive lipids. This is explained by downregulation of S1P receptors in the presence of elevated levels of S1P in mPB and UCB plasma. The data shown represent the combined results from three independent experiments carried out in triplicate per group (n=9).

Figure 3. The concept of a chemotactic tug-of-war gradient between BM and PB explains mobilization and homing of HSPCs

It has been postulated that an SDF-1 gradient between BM and PB regulates trafficking of HSPCs (homing vs. mobilization). Under steady-state conditions, this gradient should be in balance. New evidence indicates that, rather than changes in the SDF-1 gradient across the BM–PB barrier, upregulation of the S1P and C1P concentrations on either side of the barrier play an important role in homing or mobilization of HSPCs. While considering chemotactic gradients of S1P and C1P one has to remember that both bioactive lipids in order to reveal chemotactic potential i) have to be present in biological fluids as free “unbound” molecules and ii) and their gradient is also influenced by degrading enzymes. Not shown in this figure is that, in addition to upregulation of bioactive lipids during mobilization or homing, some priming molecules related to CC activation (e.g., C3a), as well as granulocyte-derived cationic peptides (e.g., cathelicidin/LL-37 and β2-defensin) may also sensitize the responsiveness of HSPCs to low SDF-1 levels and thus powerfully modulate trafficking of HSPCs.

Figure 4. Potential strategies to improve homing of HSPCs by targeting the S1P–S1P receptors axes

S1P-mediated homing of HSPCs could be improved by upregulating the S1P level in BM of the transplant recipients with DOP or THI, by blockade of S1P2 on transplanted donor cells (with JTE-013), or by inhibiting S1P-degrading enzymes (i.e., LPP and SPP receptors with, e.g., XY-14 analogues) on donor HSPCs. All these strategies await experimental confirmation.

Figure 5. A priming effect increases the responsiveness of HSPCs to low SDF-1 gradients

Left panel: The overall scheme of chemotactic assays performed in the Transwell system to evaluate the HSPC priming phenomenon. Right panel: In the presence of a priming agent (e.g., C3a or cathelicidin), HSPCs respond more robustly to low doses of SDF-1. This phenomenon is currently being tested in the clinic, where UCB are exposed ex vivo to a priming agent (C3a cationic peptide) before transplantation.