Abstract 556

Bone and bone marrow are dynamic tissues with specialized niches that support numerous bone derived stem and progenitor cells with hematopoietic, mesenchymal and endothelial potentials. These niches are comprised of osteoblasts, stromal cells and an extracellular matrix of collagens, fibronectin and proteoglycans. Osteoblasts (OB) lining the endosteum are a significant regulatory component of hematopoiesis and numerous studies have demonstrated that stem cells reside in close proximity to OB. Prostaglandin E2 (PGE2) is an abundant lipid mediator that exerts numerous activities including inhibition of myelopoiesis and regulation of hematopoietic stem cell self-renewal, and is also a positive regulator of OB number and activity. We investigated the therapeutic potential of inhibiting PGE2 biosynthesis via NSAID administration to expand progenitor cells and disrupt the bone marrow niche in order to facilitate stem and progenitor cell mobilization. Mice were treated with NSAID for 1–4 days, and femurs analyzed for OB and stem and progenitor content. Endosteal OB lining both bony trabeculae and cortical bone in the diaphyseal and epiphyseal regions of the femur demonstrated a time dependent “flattening”, as determined by width measurement of 50 random OB per femur (2.82 to 1.45 microns after 4 days, P<0.001; n=4-5 femurs/group). Flushed bones were further subjected to sequential collagenase digestions and assessed for CD45- CD31- TER119- Sca-1- osteopontin (OPN)+ OB and for the Sca-1+ c-kit+ Lin- (SKL) and CD150+ CD48- SKL (SLAM SKL) hematopoietic cell populations. NSAID treatment resulted in a significant reduction in OPN+ OB (24±3%, P<0.05), and an increase in endosteal SKL and SLAM SKL (1.7±0.1 and 1.9±0.1 fold, respectively; P<0.05). This niche disruption directly correlated with an increase in hematopoietic stem and progenitor cell (HSPC) mobilization in mice and in peripheral CD34+ cells in baboons (2.07±0.12 fold, P<0.01). To determine if NSAIDs are also capable of HSPC mobilization in humans, peripheral blood (PB) of 7 healthy volunteers was analyzed for CD34+ cells by the ISHAGE protocol, and CFU-GM, BFU-E and CFU-GEMM in methylcellulose culture, before and after a 5 day regimen of the NSAID meloxicam. Meloxicam treatment resulted in significant mobilization of CD34+ cells, CFU-GM and CFU-GEMM (2.0±0.5, 3.8±0.9 and 3.4±0.7 fold, respectively; P<0.05) with no change in BFU-E, consistent with the erythropoietic stimulatory effect of PGE2. In addition to hematopoiesis, marrow progenitors are also implicated in neoangiogenesis. Endothelial progenitors are found in mouse bone marrow and blood, and in the CD34+ adult and cord blood populations, and can be mobilized by ischemic injury and various cytokines and growth factors. Utilizing a functional approach to define a rare population of circulating endothelial colony forming cells (ECFC) we previously described (Yoder et al., 2007, Blood), we assessed the level of ECFC in volunteers before and after meloxicam administration. Only 4/7 volunteers had detectable ECFC in 40 mL of blood before treatment, while 7/7 had detectable ECFC post-meloxicam. In addition, NSAID treatment increased ECFC number and reduced culture time to first ECFC detection. These data demonstrate that NSAID administration alters the bone marrow niche and mobilizes both hematopoietic and endothelial progenitors to PB. This provides a safe and effective strategy that can be easily added to existing mobilization regimens to improve efficacy. We also report for the first time the ability of NSAIDs to increase peripheral ECFC; perhaps suggesting an exciting possible mechanism, in addition to inhibition of platelets, for the beneficial effects of NSAIDs like aspirin on the cardiovascular system. It is interesting to note that the highly COX2 selective NSAID valdecoxib was recently withdrawn from the market due to adverse cardiovascular events, and our studies demonstrate that valdecoxib is unable to mobilize progenitors from the bone marrow, while aspirin is highly effective, possibly explaining some of the cardiovascular differences between these two compounds. Future pre-clinical and clinical studies utilizing NSAID administration, with and without other agents, are likely to lead to new strategies for acquisition of bone marrow derived hematopoietic and endothelial progenitors.

Disclosures:

No relevant conflicts of interest to declare.

Author notes

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Asterisk with author names denotes non-ASH members.