Granulocyte-Derived Cationic Peptides (GDCPs) Present in Leucophoresis Products Enhance Homing of Hematopoietic Stem Cells (HSCs) to SDF-1 Gradient; Potential Implications for Accelerated Recovery of Hematopoiesis After Transplantation of Mobilized Peripheral Blood Stem Cells (PBSC).

Abstract 371

Current strategies to accelerate hematopoietic reconstitution after transplantation, include transplantation of greater numbers of HSC or ex vivo expansion of harvested HSC before transplant. However, the number of HSC availabel for allogeneic or autologous transplantation can be low (e.g., umbilical cord blood, poor mobilizers) and strategies to expand HSC and maintain equivalent engraftment capability ex vivo are limited. We reported that some compounds present in leucopheresis products [(e.g., platelet-derived microparticles (Blood 2001, 98: 3143)] and some complement cascade cleavage fragments, e.g., anaphylatoxin C3a (Blood 2005, 101, 3784), enhance the homing responses of HSC to SDF-1 gradient. We recently noted that small cationic peptides released from activated granulocytes (beta2-defensin and cathelicidin) positively prime responsiveness of murine and human HSC to SDF-1 gradient (Leukemia 2009; in press). Accordingly, both compounds enhanced transwell migration of HSC to low threshold doses of SDF-1. This phenomenon was not receptor-dependent, as agonists of membrane receptors that may bind beta2-defensin (FPRL-1), cathelicidin (CCR6) - FPRL-1 agonist, and MIP-3alpha, respectively, did not show similar priming effects. This could be explained by affected distribution of membrane lipids by cationic peptides. In support of this notion, an inhibitor of cell membrane raft formation (methyl-b-cyclodextran) inhibited the priming effect of both compounds, indicating this effect is dependent on CXCR4 incorporation into lipid rafts. Direct confocal analysis of CXCR4 and lipid raft colocalization in the presence or absence of cationic peptides confirmed these findings. Because leucopheresis products are enriched in activated granulocytes that release beta2-defensin and cathelicidin, we tested whether this may explain why mobilized peripheral blood stem cells (PBSC) engraft faster compared to HSC isolated directly from bone marrow (BM) in a murine BM transplant model. Accordingly, syngeneic BMMNCs were exposed ex vivo to beta2-defensin or cathelicidin for 30 minutes and subsequently transplanted into lethally irradiated recipients. We noted that animals transplanted with BM cells primed by those cationic peptides showed accelerated recovery of platelets and neutrophils by ∼3-5 days compared to unprimed control cells. We envision that small cationic peptides, which primarily possess antimicrobial functions and are harmless to mammalian cells, could be clinically applied to prime human HSC before transplantation. This novel approach would be particularly important in cord blood transplantation, where the number of HSC availabel for transplantation is usually limited. We postulate that this promising strategy warrants further investigations.