Plenary Paper HEMATOPOIESIS AND STEM CELLS Glypican-3–mediated inhibition of CD26 by TFPI: a novel mechanism in hematopoietic stem cell homing and maintenance

Hematopoietic stem cells (HSCs) are responsible for maintaining all blood cells throughout the lifetime of an individual, and are used clinically to treat various malignant and nonmalignant disorders. However, for some HSC grafts, for instance from umbilical cord blood (UCB), limited numbers of HSCs restrict their application to pediatric patients. Expanding HSCs in vitro or improving their homing efficiency would overcome this hurdle. As the HSC niche regulates HSC function in vivo, it is believed that additional insights in the regulation of HSCs by their niche may identify novel ways to manipulate HSCs and enhance their clinical use. A number of niche factors regulate HSC function by interacting with their respective receptors expressed on HSCs. These molecular interactions also play important roles in homing of the transplanted HSCs, which adhere to the vasculature through integrins and pass through the endothelium following rolling mediated by selectins. Directional migration of hematopoietic stem/progenitor cells (HSPCs) is mediated in large part by interaction of cell-surface– expressed CXCR4 with a gradient of CXCL12 or stroma-derived factor-1a expressed in the bone marrow (BM) niche. Loss of CXCR4 or annexin 2, involved in the presentation of CXCL12 to HSCs, severely reduces the number of HSCs in BM of adult mice. Incubation of murine or human HSPCs with anti-CXCR4 antibodies significantly reduces their homing and engraftment ability, while infusion of CXCR4-selective antagonists induces an increase in circulating HSPCs. CD26, a serine protease, cleaves an N-terminal dipeptide from CXCL12 thereby depleting its chemotactic activity. CD26deficient or CD26 inhibitor–treated mouse BM as well as human UCB-derived HSPCs display enhanced migration toward CXCL12, which is translated in improved engraftment. During a screen of stromal feeders from fetal sites of hematopoiesis, used to mimic the hematopoietic niche, we found that transcripts for Tfpi were .20-fold higher in murine stromal cells that supported long-term repopulating (LTR) HSCs in noncontact cultures. Tissue-factor (TF) pathway inhibitor (TFPI) mediates the coagulation cascade. TFPI is a serine protease inhibitor that contains 3 Kunitz-type domains, 2 of which bind to factor VIIa and Xa. Although there was no evidence for a role of TFPI in hematopoiesis, other molecules involved in coagulation such as uPA and uPAR have been shown to affect HSC homeostasis. Here, we report that TFPI acts as a biological inhibitor of CD26 in murine BM as well as human UCB-derived HSPCs. Decrease in CD26 activity led to better chemotactic activity of HSPCs resulting in enhanced homing and engraftment potential. We further demonstrate that TFPI binds to heparan sulfate proteoglycan Glypican-3 (GPC3), which itself is known to inhibit CD26 activity