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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 cytokine-induced HSC/HPC mobilization, we considered a possible role for protease inhibitors in the induction of HSC/HPC 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 possiblein vivorole of Serpina1 in HSC/HPC mobilization, we administered Serpina1 before IL-8 injection. This administration resulted in an almost complete inhibition of HSC/HPC mobilization, whereas heat-inactivated Serpina1 had no effect. These results indicate that low-dose TBI inhibits cytokine-induced HSC/HPC 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 HSC/HPC mobilization. Also, we hypothesize that cytokine-induced HSC/HPC mobilization is determined by a critical balance between serine proteases and serine protease inhibitors.

The CXC chemokine interleukin-8 (IL-8/CXCL8) induces rapid mobilization of hematopoietic progenitor cells (HPCs). Previously we showed that mobilization could be prevented completely in mice by pretreatment with neutralizing antibodies against the β2-integrin LFA-1 (CD11a). In addition, murine HPCs do not express LFA-1, indicating that mobilization requires a population of accessory cells. Here we show that polymorphonuclear cells (PMNs) serve as key regulators in IL-8-induced HPC mobilization. The role of PMNs was studied in mice rendered neutropenic by administration of a single injection of antineutrophil antibodies. Absolute neutropenia was observed up to 3–5 days with a rebound neutrophilia at day 7. The IL-8-induced mobilizing capacity was reduced significantly during the neutropenic phase, reappeared with recurrence of the PMNs, and was increased proportionally during the neutrophilic phase. In neutropenic mice, the IL-8-induced mobilizing capacity was restored by the infusion of purified PMNs but not by infusion of mononuclear cells. Circulating metalloproteinase gelatinase B (MMP-9) levels were detectable only in neutropenic animals treated with PMNs in combination with IL-8, showing thatin vivoactivated PMNs are required for the restoration of mobilization. However, IL-8-induced mobilization was not affected in MMP-9-deficient mice, indicating that MMP-9 is not indispensable for mobilization. These data demonstrate that IL-8-induced mobilization of HPCs requires thein vivoactivation of circulating PMNs.