Here, we report that cytokine-induced (granulocyte colony-stimulating factor and IL-8) hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) mobilization is completely inhibited after low-dose (0.5 Gy) total-body irradiation (TBI). Because neutrophil granular proteases are regulatory mediators in cytokineinduced HSC͞HPC mobilization, we considered a possible role for protease inhibitors in the induction of HSC͞HPC mobilization. Bone marrow (BM) extracellular extracts that were obtained from murine femurs after 0.5 Gy of TBI contained an inhibitor of elastase. Also, after low-dose TBI, both Serpina1 mRNA and protein concentrations were increased in BM extracts, compared with extracts that were obtained from controls. The inhibitory activity in BM extracts of irradiated mice was reversed by addition of an Ab directed against Serpina1. To further study a possible in vivo role of Serpina1 in HSC͞HPC mobilization, we administered Serpina1 before IL-8 injection. This administration resulted in an almost complete inhibition of HSC͞HPC mobilization, whereas heat-inactivated Serpina1 had no effect. These results indicate that low-dose TBI inhibits cytokine-induced HSC͞HPC mobilization and induces Serpina1 in the BM. Because exogenous administration of Serpina1 inhibits mobilization, we propose that radiation-induced Serpina1 is responsible for the inhibition of HSC͞HPC mobilization. Also, we hypothesize that cytokine-induced HSC͞HPC mobilization is determined by a critical balance between serine proteases and serine protease inhibitors.protease ͉ protease inhibitors ͉ bone marrow ͉ adhesion molecules

IntroductionTo reduce the risk of infections in neutropenic patients a combination of protective isolation and gut decontamination by oral administration of antibiotics is often used. [1][2][3][4] Since it has been shown that selective elimination of the major potentially pathogenic microorganisms from the digestive tract is as effective or even superior to total decontamination in preventing infections in neutropenic patients, 5-8 selective gut decontamination 9 or partial antibiotic decontamination 10 mainly directed against aerobic Gram-negative rods and fungi is usually applied. The various antibiotics used for selective decontamination can be divided into 2 main groups, those that are absorbed 11-13 after oral administration (eg, cotrimoxazole, quinolones) and those that are not absorbed 2-4 after oral administration (eg, polymyxin, colistin, aminoglycosides).Mobilized hematopoietic progenitor cells (HPCs) are increasingly used for hematopoietic recovery instead of bone marrow to allow rapid hematopoietic recovery after autologous and allogeneic transplantation. [14][15][16][17][18] An increasing number of different growth factors and chemokines such as granulocyte colony-stimulating factor (G-CSF), 19,20 granulocyte-macrophage colony-stimulating factor (GM-CSF), 19,21 stem cell factor (SCF), 22 flt3 ligand (FL), 23 interleukin-1 (IL-1), 24 26 and thrombopoietin 27 when either administered alone or in combination 28 are capable of mobilizing peripheral blood progenitor cells. In the clinical setting, the growth factor G-CSF has become the standard for autologous as well as allogeneic peripheral blood stem cell transplantation. [29][30][31] Although patients receiving selective gut decontamination or systemic antibiotic treatment while concurrently being treated with G-CSF are able to mobilize peripheral blood progenitor cells, little is known of the effect of antibiotics on the efficacy of mobilization.Endotoxins are ubiquitously present and constitute an important component of the normal intestinal flora and are known to induce HPC mobilization. [32][33][34] We therefore hypothesized that low levels of endotoxins passing through the intestinal mucosa into the blood may contribute to the levels of circulating HPCs. Here, we used a mouse model to study the effects of selective antibiotic decontamination on cytokine-induced stem cell mobilization. We found that the ability to mobilize in response to IL-8, G-CSF, or FL was significantly reduced in animals receiving endotoxin binding antibiotics or antibiotics that modulate the anaerobic flora. The mobilizing capacity was partially restored after adding endotoxins to the drinking water of decontaminated mice. These observations indicate that endotoxins are involved as cofactors in cytokineinduced hematopoietic stem cell mobilization. Supported by a grant from the Fund for Scientific Research FWO-Vlaanderen, the "Geconcerteerde Onderzoeks Acties" (GAO), and by the Dutch Cancer Society (NKB-RUL 99-2029).Reprints: Willem E. Fibbe, Department of Hematology, Lei...