J.F.M. Pruijt

Neutrophils are indispensable for hematopoietic stem cell mobilization induced by interleukin-8 in mice

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 that in vivo activated 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 the in vivo activation of circulating PMNs.

Biology of IL‐8‐Induced Stem Cell Mobilization

Abstract: The CXC chemokine interleukin‐8 (IL‐8) has profound hematopoietic activities following systemic administration. It induces the rapid mobilization of cells with lymphomyeloid repopulating ability in mice and of hematopoietic progenitor cells in monkeys. In this paper, evidence is presented that stem cell mobilization in mice requires the functional expression on the β2‐integrin leukocyte function‐associated antigen‐1 (LFA‐1). In monkeys, systemic injection of IL‐8 is followed by a significant increase in the circulating levels of the matrix metallo proteinase gelatinase‐B (MMP‐9). Based on these findings, the hypothesis is discussed that mature neutrophils serve as intermediate cells in IL‐8‐induced stem cell mobilization by the release of proteinases.

Leukemia Inhibitory Factor Induces In Vivo Expansion of Bone Marrow Progenitor Cells that Accelerate Hematopoietic Reconstitution but Do Not Enhance Radioprotection in Lethally Irradiated Mice

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine with distinct hematopoietic activities. In vivo treatment of mice with recombinant murine LIF induces thrombocytosis and increases the number of hematopoietic progenitor cells (HPCs) in spleen and bone marrow (BM). In this study, we applied LIF to expand HPCs in vivo prior to syngeneic BM transplantation. BALB/c donor mice were treated with recombinant human LIF at a dose of 2.5 μg/day s.c. for seven days. This resulted in a 1.6‐fold increment in platelet counts from 941 to 1,470 × 109/l (mean, n = 20). Mean spleen weight increased from 120 mg to 160 mg (n = 5). The total numbers of HPCs in the spleen as well as in the BM, as assessed in a CFU‐GM (colony forming unit‐granulocyte‐macrophage) assay, were significantly higher in LIF‐treated donors than in saline‐treated controls (30.1 ± 14.5 versus 7.4 ± 5.3 × 103 per spleen; mean ± SD, n = 22, p < 0.001 and 74.4 ± 17.1 versus 55.3 ± 16.1 × 103 per femur, p < 0.001). Recipient mice were lethally (8.5 Gy) irradiated and transplanted with 3 × 105 BM cells derived from LIF‐ or saline‐treated donors. Hematopoietic reconstitution was monitored by tail bleeding at three‐day intervals. Platelet and WBC nadir counts in control animals were reached at day 9 (31 ± 25 × 109/l for platelets and 0.40 ± 0.10 × 109/l for WBC; mean ± SD, n = 29 per treatment group); in animals transplanted with LIF‐treated BM cells, these counts were 44 ± 25 × 109/l for platelets, p < 0.05 and 0.60 ± 0.38 × 109/l for WBC, p < 0.01. In addition, platelet reconstitution was faster in recipients of LIF‐treated BM cells (226 ± 118 versus 126 ± 62 × 109/l at day 12 and 633 ± 174 versus 434 ± 180 × 109/l at day 15, p < 0.001). Similarly, the reconstitution of WBC was also significantly enhanced. The radioprotection rate of lethally irradiated recipients with increasing cell doses of BM cells derived from LIF‐treated donors was higher at all cell doses tested then of control animals, but did not reach statistical significance. These results show that in vivo treatment with LIF expands the number of committed progenitor cells and BM repopulating cells that accelerate short‐term hematopoietic reconstitution without increasing radioprotection. Our data do not support a major role for LIF as a single factor inducing expansion of hematopoietic stem cells in vivo.

Biology of Stem Cell Mobilization Induced by Cytokines

Mobilized autologous or allogeneic hematopoietic progenitor cells are currently the preferential source for transplantation in the treatment of a variety of malignant and non-malignant diseases. In spite of the widespread clinical use of these peripheral blood stem cells the mechanisms underlying mobilization are still largely unknown.