A Novel Perspective on Stem Cell Homing – Emerging Interplay Between Bioactive Lipids As Potent Chemoattractants and Cationic Peptides As Underappreciated Modulators of Responsiveness to SDF-1 Gradients

Hematopoietic stem progenitor cells (HSPCs) are retained in bone marrow (BM) niches due to the stromal-derived growth factor-1 (SDF-1)–CXCR4 receptor axis and interactions between Very Late Antigen-4 (VLA-4 or a₄b₁ integrin) and its ligand, Vascular Adhesion Molecule-1 (VCAM-1 or CD106). While HSPCs express CXCR4 and VLA-4, their corresponding ligands, SDF-1 and VCAM-1, are expressed by cells in the BM microenvironment (e.g., osteoblasts and fibroblasts). While a role for the SDF-1–CXCR4 axis in the retention of HSPCs in BM under steady state conditions is undisputed, its role in stem cell homing needs further clarification. For many years, it has been proposed that concentration differences between the chemotactic SDF-1 gradient across the BM–peripheral blood (PB) barrier determines whether cells will home from PB into the BM microenvironment. However, this simple explanation has been challenged by several observations supporting the existence of SDF-1–CXCR4-independent homing mechanisms. For example, i) CXCR4^−/– fetal liver HSPCs can home to BM in an SDF-1-independent manner, ii) homing of murine HSPCs made refractory to SDF-1 by preincubation and co-injection with a CXCR4 receptor antagonist (AMD3100) is normal or only mildly reduced, iii) HSPCs in which CXCR4 expression is reduced by a SDF-1 intrakine strategy remain able to engraft in lethally irradiated recipients, and as we recently reported, iv) myeloablative conditioning for transplantation induces a highly proteolytic microenvironment in BM that leads to proteolytic degradation of SDF-1 (Leukemia 2011, doi: 10.1038/leu.2011.185). Aim of the study. Based on these observations that strongly suggest the involvement of other factors and/or supportive mechanisms in the SDF-1-mediated homing of HSPCs we become interested in a role of bioactive lipids (sphingosine-1 phosphate; S1P and ceramide-1 phosphate; C1P) as well as priming phenomenon of SDF-1-CXCR4 axis by cationic antimicrobial peptides (CAMPs) in HSPCs homing. Results. We present evidence that gradients of bioactive lipids, such as S1P and C1P, which are products of membrane lipid metabolism, are involved in stem cell lodgment into BM. In addition, we present another important mechanism that enhances stem cell response to an low SDF-1 concentration gradient, namely, the “priming effect”. This mechanism is based on a phenomenon in which CAMPs such as complement cascade cleavage fragment C3a and stroma fibroblast- and granulocytes-derived antimicrobial peptide LL-37 significantly increase (up to 25 times) the chemotactic responsiveness of HSPCs to very low “tissue concentration-relevant” SDF-1 gradients (1–3 ng/ml). To support this further murine HSPCs primed ex vivo with CAMPs engrafted by 3–5 days faster in lethally irradiated littermates. At the molecular level, this sensitization of the responsiveness to SDF-1 after exposure to CAMPs depends on the incorporation of the CXCR4 receptor into membrane lipid rafts. This phenomenon is specific for SDF-CXCR4 axis as we did not observe that this phenomenon influences S1P- and C1P-mediated in vitro chemotaxis. Overall, these changes of SDF-1, S1P, and C1P chemotactic gradients and the appearance of priming factors in the BM microenvironment and PB plasma are triggered during conditioning for transplantation by radio-chemotherapy by the induction of a proteolytic microenvironment in BM and the activation of the complement cascade (CC). Conclusions. We provide in vivo evidence that C3a and LL-37, which have primarily antimicrobial functions and are harmless to mammalian cells, could be clinically applied to accelerate engraftment as ex vivo priming agent for transplanted human HSPCs. This novel approach based on a short ex vivo incubation of HSPCs in the graft before transplantation by CAMPs (C3a and LL-37) is of particular important in umbilical cord blood transplantations, where the number of available HSPCs in single unit for transplant is limited. This strategy is currently tested in the clinic.

No relevant conflicts of interest to declare.