Systemic Administration of Pleiotrophin Induces Hematopoietic Stem Cell Regeneration In Vivo.

Abstract 1498

Poster Board I-521

Significant progress has been made toward delineating the intrinsic and extrinsic signaling pathways that regulate hematopoietic stem cell (HSC) self-renewal. However, much less is known regarding the process of HSC regeneration or the extrinsic signals that regulate hematopoietic reconstitution following stress or injury. Elucidation of the microenvironmental signals which promote HSC regeneration in vivo would have important implications for the treatment of patients undergoing radiation therapy, chemotherapy and stem cell transplantation. We recently reported that pleiotrophin, a soluble heparin-binding growth factor, induced a 10-fold expansion of murine long-term repopulating HSCs in short term culture (Himburg et al. Blood (ASH Annual Meeting Abstracts), Nov 2008; 112: 78). Based on this observation, we hypothesized that PTN might also be a regenerative growth factor for HSCs. Here we tested the effect of systemic administration of PTN to non-irradiated and irradiated C57Bl6 mice to determine if PTN could promote HSC regeneration in vivo. C57Bl6 mice were irradiated with 700 cGy total body irradiation (TBI) followed by intraperitoneal administration of 2 μg PTN or saline x 7 days, followed by analysis of BM stem and progenitor cell content. Saline-treated mice demonstrated significant reductions in total BM cells, BM c-kit+sca-1+lin- (KSL) cells, colony forming cells (CFCs) and long term culture-initiating cells (LTC-ICs) compared to non-irradiated control mice. In contrast, PTN-treated mice demonstrated a 2.3-fold increase in total BM cells (p=0.03), a 5.6-fold increase in BM KSL stem/progenitor cells (p=0.04), a 2.9-fold increase in BM CFCs (p=0.004) and an 11-fold increase in LTC-ICs (p=0.03) compared to saline-treated mice. Moreover, competitive repopulating transplantation assays demonstrated that BM from PTN-treated, irradiated mice contained 5-fold increased competitive repopulating units (CRUs) compared to saline-treated, irradiated mice (p=0.04). Taken together, these data demonstrate that the administration of PTN induces BM HSC and progenitor cell regeneration in vivo following injury. Comparable increases in total BM cells, BM KSL cells and BM CFCs were also observed in PTN-treated mice compared to saline-treated controls following 300 cGy TBI, demonstrating that PTN is a potent growth factor for hematopoietic stem/progenitor cells in vivo at less than ablative doses of TBI. In order to determine whether PTN acted directly on BM HSCs to induce their proliferation and expansion in vivo, we exposed mice to BrDU in their drinking water x 7 days and compared the response to saline treatment versus PTN treatment. PTN-treated mice demonstrated a significant increase in BrDU+ BM KSL cells compared to saline-treated controls (p=0.04) and cell cycle analysis confirmed a significant increase in BM KSL cells in S phase in the PTN-treatment group compared to saline-treated controls (p=0.04). These data indicate that PTN serves as a soluble growth factor for BM HSCs and induces their proliferation and expansion in vivo while preserving their repopulating capacity. These results suggest that PTN has therapeutic potential as a novel growth factor to accelerate hematopoietic reconstitution in patients undergoing myelosuppressive radiotherapy or chemotherapy.