Neutrophil mobilization and clearance in the bone marrow

Growth factors and cytokines in acute lung injury

Cytokines and growth factors play an integral role in the maintenance of immune homeostasis, the generation of protective immunity, and lung reparative processes. However, the dysregulated expression of cytokines and growth factors in response to infectious or noxious insults can initiate and perpetuate deleterious lung inflammation and fibroproliferation. In this article, we will comprehensively review the contribution of individual cytokines and growth factors and cytokine networks to key pathophysiological events in human and experimental acute lung injury (ALI), including inflammatory cell recruitment and activation, alveolar epithelial injury and repair, angiogenesis, and matrix deposition and remodeling. The application of cytokines/growth factors as prognostic indicators and therapeutic targets in human ALI is explored.

Telomeres shorten at equivalent rates in somatic tissues of adults

Telomere shortening in somatic tissues largely reflects stem cell replication. Previous human studies of telomere attrition were predominantly conducted on leukocytes. However, findings in leukocytes cannot be generalized to other tissues. Here we measure telomere length in leukocytes, skeletal muscle, skin and subcutaneous fat of 87 adults (aged 19–77 years). Telomeres are longest in muscle and shortest in leukocytes, yet are strongly correlated between tissues. Notably, the rates of telomere shortening are similar in the four tissues. We infer from these findings that differences in telomere length between proliferative (blood and skin) and minimally proliferative tissues (muscle and fat) are established during early life, and that in adulthood, stem cells of the four tissues replicate at a similar rate.

Telomere shortening as a result of cell proliferation has been implicated in human ageing. Here, Daniali and colleagues show that telomere length and the rate of age-dependent shortening vary between adults but are similar within tissues of the same individual.

Local inflammatory reaction induced by Scolopendra viridicornis centipede venom in mice

Centipede envenomation is generally mild, and human victims usually manifest burning pain, erythema and edema. Despite the abundance and ubiquity of these animals, centipede venom has been poorly characterized in literature. For this reason, the aim of this work was to investigate local inflammatory features induced by Scolopendra viridicornis centipede envenomation in mice, evaluating edema formation, leukocyte infiltration, production of inflammatory mediators, and also performing histological analysis. The highest edematogenic activity induced by the venom, determined by plethysmometry, was noticed 0.5 h after injection in mice footpad. At 24 h, edema was still detected in animals that received 15 and 60 μg of venom, and at 48 h, only in animals injected with 60 μg of venom. In relation to leukocyte count, S. viridicornis venom induced cell recruitment, mainly neutrophils and monocytes/macrophages, in all doses and time periods analyzed in comparison with PBS-injected mice. An increase in lymphocytes was detected especially between 1 and 24 h at 60 μg dose. Besides, eosinophil recruitment was observed mainly for 15 and 60 μg doses in early time periods. Edema formation and cell recruitment were also confirmed by histological analysis. Moreover, S. viridicornis venom stimulated the release of IL-6, MCP-1, KC, and IL-1β. Conversely, S. viridicornis venom did not induce the release of detectable levels of TNF-α. We demonstrated that the edematogenic activity induced by S. viridicornis venom was of rapid onset, and the venom stimulated secretion of pro-inflammatory mediators which contribute to the inflammatory reaction induced by S. viridicornis venom in an experimental model.

Blood neutrophil counts in HIV-infected patients with pulmonary tuberculosis: association with sputum mycobacterial load

Background

Increasing evidence suggests that neutrophils play a role in the host response to Mycobacterium tuberculosis. We determined whether neutrophil counts in peripheral blood are associated with tuberculosis (TB) and with mycobacterial load in sputum in HIV-infected patients.

Methodology/Principal Findings

Adults enrolling in an antiretroviral treatment (ART) clinic in a Cape Town township were screened for TB regardless of symptoms. Paired sputum samples were examined using liquid culture, fluorescence microscopy, and the Xpert MTB/RIF assay. Absolute neutrophil counts (ANC) were measured in blood samples. Of 602 HIV-infected patients screened, 523 produced one or more sputum samples and had complete results available for analysis. Among these 523 patients, the median CD4 count was 169×10⁹/L (IQR, 96–232) and median ANC was 2.6×10⁹/L (IQR, 1.9–3.6). Culture-positive pulmonary tuberculosis was diagnosed in 89 patients. Patients with TB had a median ANC of 3.4×10⁹/L (IQR, 2.4–5.1) compared to 2.5×10⁹/L (IQR, 1.8–3.4) among those who were culture negative (p<0.0001). In multivariable analyses, having pulmonary TB was associated with an adjusted risk ratio (aRR) of 2.6 (95%CI, 1.5–4.5) for having an ANC level that exceeded the median value (ANC ≥2.6×10⁹/L; p = 0.0006) and an aRR of 6.8 (95%CI, 2.3–20.4) for having neutrophilia defined by a neutrophil count exceeding the upper limit of the normal range (ANC >7.5×10⁹/L; p = 0.0005). Patients were then classified into four mutually exclusive groups with increasing sputum mycobacterial load as defined by the results of culture, Xpert MTB/RIF and sputum smear microscopy. Multivariable analyses demonstrated that increasing sputum mycobacterial load was positively associated with blood ANC ≥2.6×10⁹/L and with neutrophilia.

Conclusions/Significance

Increased blood neutrophil counts were independently associated with pulmonary TB and sputum mycobacterial burden in this HIV-infected patient group. This observation supports the growing body of literature regarding the potential role for neutrophils in the host response to TB.

Kinetics of neutrophils in mice exposed to radiation and/or granulocyte colony-stimulating factor treatment

Astronauts have the potential to develop the hematopoietic syndrome as a result of exposure to radiation from a solar particle event (SPE) during exploration class missions. This syndrome is characterized by a reduction in the number of circulating blood cells (cytopenias). In the present study the effects of SPE-like proton and γ radiation on the kinetics of circulating neutrophils were evaluated during a one-month time period using mice as a model system. The results revealed that exposure to a 2 Gy dose of either SPE-like proton or γ radiation significantly decreased the number of circulating neutrophils, with two nadirs observed on day 4 and day 16 postirradiation. Low circulating neutrophil count (neutropenia) is particularly important because it can increase the risk of astronauts developing infections, which can compromise the success of the mission. Thus, two granulocyte colony-stimulating factors (G-CSFs), filgrastim and pegfilgrastim were evaluated as countermeasures for this endpoint. Both forms of G-CSF significantly increased neutrophil counts in irradiated mice, however, the effect of pegfilgrastim was more potent and lasted longer than filgrastim. Using the expression of CD11b, CD18 and the production of reactive oxygen species (ROS) as markers of neutrophil activation, it was determined that the neutrophils in the irradiated mice treated with pegfilgrastim were physiologically active. Thus, these results suggest that pegfilgrastim could be a potential countermeasure for the reduced number of circulating neutrophils in irradiated animals.

Neutrophil roles in left ventricular remodeling following myocardial infarction

Polymorphonuclear granulocytes (PMNs; neutrophils) serve as key effector cells in the innate immune system and provide the first line of defense against invading microorganisms. In addition to producing inflammatory cytokines and chemokines and undergoing a respiratory burst that stimulates the release of reactive oxygen species, PMNs also degranulate to release components that kill pathogens. Recently, neutrophil extracellular traps have been shown to be an alternative way to trap microorganisms and contain infection. PMN-derived granule components are also involved in multiple non-infectious inflammatory processes, including the response to myocardial infarction (MI). In this review, we will discuss the biological characteristics, recruitment, activation, and removal of PMNs, as well as the roles of PMN-derived granule proteins in inflammation and innate immunity, focusing on the MI setting when applicable. We also discuss future perspectives that will direct research in PMN biology.

The receptor for complement component C3a mediates protection from intestinal ischemia-reperfusion injuries by inhibiting neutrophil mobilization

C3a is a key complement activation fragment, yet its neutrophil-expressed receptor (C3aR) still has no clearly defined role. In this study, we used a neutrophil-dependent mouse model of intestinal ischemia-reperfusion (IR) injury to explore the role of C3aR in acute tissue injuries. C3aR deficiency worsened intestinal injury, which corresponded with increased numbers of tissue-infiltrating neutrophils. Circulating neutrophils were significantly increased in C3aR(-/-) mice after intestinal ischemia, and C3aR(-/-) mice also mobilized more circulating neutrophils after granulocyte colony-stimulating factor infusion compared with WT mice, indicating a specific role for C3aR in constraining neutrophil mobilization in response to intestinal injury. In support of this role, C3aR(-/-) mice reconstituted with WT bone marrow reversed IR pathology back to WT levels. Complement C5a receptor (C5aR) antagonism in C3aR(-/-) mice also rectified the worsened pathology after intestinal IR injury but had no effect on circulating neutrophils, highlighting the opposing roles of C3a and C5a in disease pathogenesis. Finally, we found that using a potent C3a agonist to activate C3aR in vivo reduced neutrophil mobilization and ameliorated intestinal IR pathology in WT, but not C3aR(-/-), mice. This study identifies a role for C3aR in regulating neutrophil mobilization after acute intestinal injury and highlights C3aR agonism as a potential treatment option for acute, neutrophil-driven pathologies.

Another look at the life of a neutrophil

Neutrophils are considered as the privates of the innate immune system. They are born in the bone marrow, migrate to the tissues where they kill putative intruders. After their job they are quickly removed from the battlefield by macrophages. This view of a predetermined pathway fitted nicely in their short lifespan of 5 h. However, recent studies indicated that their lifespan was in the order of several days. Recently, it became clear that neutrophils have functions beyond killing of pathogens. The reported half-life of 5 h is hardly compatible with those functions. Moreover, the organism actively invests in rescuing primed neutrophils from clearance by the body. It appears that their half-life is highly dependent on the method used to measure their life span. Here, we discuss the literature and show that neutrophils compartmentalize which could explain partially the differences reported for their lifespan. Moreover, the methodology to label neutrophils ex-vivo could have similar deteriorating effects on their lifespan as found for transfused red blood cells.

Vascular repair and regeneration as a therapeutic target for pulmonary arterial hypertension

The last decade has seen substantial changes in our understanding of the pathobiology of pulmonary arterial hypertension (PAH), a severe and devastating disease without curative treatment. It is now accepted that injury to the endothelial cells of the pulmonary arteries is central for the subsequent development of lumen-obliterative lung vascular lesions. A variety of circulating and lung-resident progenitor and stem cells likely contribute to vascular integrity, and evidence for the presence of cells expressing stem and progenitor cell markers is found inside and in the immediate vicinity of pulmonary vascular lesions in PAH. The currently available vasodilator therapies mainly target enhanced vasoconstriction in the lung circulation and help to maintain or improve right ventricular function, but do not treat pulmonary vascular remodeling, the underlying cause of the disease. Vascular gene therapy and cell therapy with progenitor and stem cells is a progressing field in the context of the development of novel treatment options for PAH, but the majority of the studies are currently performed at the level of preclinical studies in animal models. The current review provides an overview of the current knowledge on cell- and gene therapy-based approaches for vascular repair and regeneration in PAH.

Localized bacterial infection induces systemic activation of neutrophils through Cxcr2 signaling in zebrafish

Neutrophils are the first line of defense against tissue damage and are rapidly mobilized to sites of bacterial infection. However, the signals that regulate neutrophil recruitment are not well defined. Here, using photolabel-enabled fate mapping in zebrafish larvae, we show that localized otic infection with Pseudomonas aeruginosa induces systemic activation and mobilization of neutrophils from the CHT through Cxcr2 signaling. We have cloned the zebrafish Cxcr1 and Cxcr2 receptors and show that Cxcr2 functions as a Cxcl8 receptor in live zebrafish. With the use of morpholino-mediated depletion, we show that infection-induced neutrophil mobilization from the CHT is mediated by Cxcr2 but not Cxcr1. By contrast, Cxcr2 depletion does not affect neutrophil recruitment to the chemoattractant LTB4. Taken together, our findings identify Cxcl8-Cxcr2 signaling as an infection-induced long-range cue that mediates neutrophil motility and mobilization from hematopoietic tissues, positioning Cxcr2 as a critical pathway that mediates infection-induced systemic activation of neutrophils.

Immunological cells and functions in Gaucher disease

The macrophage (MΦ) has been the focus of causality, research, and therapy of Gaucher disease, but recent evidence casts doubt its solitary role in the disease pathogenesis. The excess of glucosylceramide (GC) in such cells accounts for some of the disease manifestations. Evidence of increased expression of C-C and C-X-C chemokines (i.e., CCL2,CXCL1, CXCL8) in Gaucher disease could be critical for monocyte transformation to inflammatory subsets of macrophages and dendritic cells (DC) as well as neutrophil (PMNs) recruitment to visceral organs. These immune responses could be essential for activation of T- and B-cell subsets, and the induction of numerous cytokines and chemokines that participate in the initiation and propagation of the molecular pathogenesis of Gaucher disease. The association of Gaucher disease with a variety of cellular and humoral immune responses is reviewed here to provide a potential foundation for expanding the complex pathophysiology of Gaucher disease.

Coxsackievirus B3 infects the bone marrow and diminishes the restorative capacity of erythroid and lymphoid progenitors

Coxsackievirus B3 (CVB3) is known to infect stem cells in the neonatal central nervous system. Here, we evaluated the effects of CVB3 infection on the major source and repository of stem cells, the bone marrow (BM). Viral genome was detectable in BM within 24 h of infection, and productive infection of BM cells was evident, peaking at 48 h postinfection (p.i.), when ∼1 to 2% of BM cells produced infectious virus particles. Beginning at 2 to 3 days p.i., a dramatic and persistent loss of immature erythroid cells, B and T lymphocytes, and neutrophils was observed in BM and, by day 3 to 4 p.i., the femoral BM stroma was largely destroyed. Analysis of peripheral blood revealed a modest neutrophilia, a loss of reticulocytes, and a massive lymphopenia. The abundance of multipotent progenitor cells (Lin(-)/c-kit(+)/Flt3(+)) in BM declined ∼10-fold during CVB3 infection and, consistent with a deficiency of primitive hematopoietic progenitors, serum levels of the hematopoietic growth factor Flt3 ligand were dramatically elevated. Therefore, we analyzed the regenerative capacity of BM from CVB3-infected mice. Granulocyte/macrophage progenitors displayed a relatively normal proliferative ability, consistent with the fact that the peripheral blood level of neutrophils-which are very short-lived cells-remained high throughout infection. However, erythroid and lymphoid hematopoietic progenitors in BM from CVB3-infected mice showed a markedly reduced colony-forming capacity, consonant with the observed loss of both lymphocytes and immature erythroid cells/reticulocytes from the BM and peripheral blood. In summary, CVB3 infects the BM and exerts differential effects on the various hematopoietic progenitor populations.

Hepatocyte-specific S100a8 and S100a9 transgene expression in mice causes Cxcl1 induction and systemic neutrophil enrichment

Background

Calprotectin consists of the Ca²⁺-binding proteins S100a8 and S100a9 that are induced in epithelial cells in response to tissue damage and infection. Both proteins are also secreted by activated innate immune cells and numerous studies demonstrate their crucial role in pathological conditions of acute and chronic inflammation.

Results

Here, we established a conditional mouse model with simultaneous S100a8 and S100a9 transgene expression in hepatocytes (TgS100a8a9^hep) under the control of doxycycline to unravel the role of epithelial-derived Calprotectin on tissue homeostasis and inflammation. TgS100a8a9^hep mice displayed a significant enrichment of neutrophils in peripheral blood and tissues with high blood content. Interestingly, Cxcl1 transcription was significantly induced in the liver of TgS100a8a9^hep mice and primary hepatocytes derived thereof as compared to Control mice, accompanied by an increase of Cxcl1 serum levels. However, expression of other chemokines with a known function in neutrophil mobilization from the bone marrow, e.g. Csf3 and Cxcl2, was not altered. Doxycycline treatment of TgS100a8a9^hep mice reduced Cxcl1 expression in the liver and resulted in normal numbers of neutrophils.

Conclusion

In summary, our data demonstrate for the first time that hepatocyte-specific S100a8 and S100a9 expression induces a systemic mobilization of neutrophils by a specific activation of Cxcl1 transcription in the liver.

Design of a novel cyclotide-based CXCR4 antagonist with anti-human immunodeficiency virus (HIV)-1 activity

Herein, we report for the first time the design and synthesis of a novel cyclotide able to efficiently inhibit HIV-1 viral replication by selectively targeting cytokine receptor CXCR4. This was accomplished by grafting a series of topologically modified CVX15 based peptides onto the loop 6 of cyclotide MCoTI-I. The most active compound produced in this study was a potent CXCR4 antagonist (EC50≈20 nM) and an efficient HIV-1 cell-entry blocker (EC50≈2 nM). This cyclotide also showed high stability in human serum, thereby providing a promising lead compound for the design of a novel type of peptide-based anticancer and anti-HIV-1 therapeutics.

Current insights into neutrophil homeostasis

Neutrophil granulocytes represent the first immunologic barrier against invading pathogens, and neutropenia predisposes to infection. However, neutrophils may also cause significant collateral inflammatory damage. Therefore, neutrophil numbers are tightly regulated by an incompletely understood homeostatic feedback loop adjusting the marrow's supply to peripheral needs. Granulocyte colony-stimulating factor (G-CSF) is accepted to be the major determinant of neutrophil production, and G-CSF levels have, soon after its discovery, been described to be inversely correlated with neutrophil counts. A neutrophil sensor, or "neutrostat," has, therefore, been postulated. The prevailing feedback hypothesis was established in adhesion molecule-deficient mice; it includes macrophages and Th17 cells, which determine G-CSF levels in response to the number of peripherally transmigrated, apoptosing neutrophils. Recent work has deepened our understanding of homeostatic regulation of neutrophil granulopoiesis, but there are still inconsistent findings and unresolved questions when it comes to a plausible hypothesis, similar to the feedback control models of red cell or platelet homeostasis.

G-CSF-activated STAT3 enhances production of the chemokine MIP-2 in bone marrow neutrophils

Neutrophil mobilization from the bone marrow is a critical aspect of the innate immune response, enabling a rapid deployment of phagocytes to infected or inflamed tissue. The cytokine G-CSF, which is induced rapidly during infection, elicits a swift and potent mobilizing response, yet its mechanisms of action remain poorly understood. Here, we studied the role of G-CSF and its principal signal transducer STAT3 in regulating expression of the neutrophil chemoattractant MIP-2. Our studies revealed Gr-1(hi) mature neutrophils as major sources of Cxcl2 (MIP-2) mRNA in bone marrow and G-CSF-responsive MIP-2 protein production. Induction of Cxcl2 was regulated directly by G-CSF-activated STAT3 via interaction at a STAT consensus element in the Cxcl2 promoter. G-CSF coordinately stimulated the association of STAT3, induction of the transcriptionally active H3K4me3 modification, and recruitment of RNA Pol II at the Cxcl2 proximal promoter, as well as the promoter region of Il8rb, encoding the MIP-2 receptor. These results suggest that the G-CSF-STAT3 pathway directly regulates transcriptional events that induce neutrophil mobilization.

The function of neutrophils in sepsis

PURPOSE OF REVIEW

Neutrophils are an essential arm of the innate immune response. In patients with sepsis, reprogramming of neutrophil occurs, manifest by impaired recruitment of neutrophils to sites of infection, abnormal accumulation of neutrophils to remote sites, and dysregulation of neutrophil effector responses. This review examines the mechanisms underlying dysregulated neutrophil trafficking and function during sepsis.

RECENT FINDINGS

Mechanisms governing neutrophil function in sepsis are complex. Bacterial products, cytokines/chemokines, leukotrienes, and immunomodulatory hormones can modulate neutrophil migratory responses during sepsis via induction of cytoskeletal changes, disruption of polymorphonuclear leukocyte (PMN)-endothelial cell interactions, and alterations in G-protein-coupled receptor expression or signaling. Impaired chemotactic responses and alterations in neutrophil function can occur as a result of dysregulated PMN G-protein-coupled receptor and Toll-like receptor expression and/or signaling. As sepsis evolves, neutrophil gene expression is altered, leading to suppression of proinflammatory and immunomodulatory genes, as well as decreased production of reactive oxygen species. Neutrophil extracellular traps are produced to contain and kill invading pathogens, but can paradoxically promote further tissue damage.

SUMMARY

Neutrophil migration is a coordinated process that is altered at multiple stages during sepsis. In combination with impaired neutrophil function, these alterations culminate in defective innate immunity in septic patients. Defining the mechanisms involved and strategies to interrupt these deleterious responses requires further investigation.

Chemokine receptor CXCR2: physiology regulator and neuroinflammation controller?

The innate immune system is a crucial component of inflammatory reactions, while the central nervous system (CNS) is the most vulnerable site of the body to inflammatory tissue injury. Neuroinflammatory brain pathologies are disorders in which the CNS is threatened by its own immune system. Chemokine receptor CXCR2 and its ligands have been implicated in several neuroinflammatory brain pathologies, as well as in neutrophil recruitment and in the developmental positioning of neural cells. This review focuses on the basics of CXCR2, its regulating role in bone marrow neutrophil recruitment, oligodendrocyte progenitor cell positioning and neural repair mechanisms, as well as its diverse roles in neuroinflammatory brain pathologies.

Regulation of neutrophil trafficking from the bone marrow

Neutrophils are an essential component of the innate immune response and a major contributor to inflammation. Consequently, neutrophil homeostasis in the blood is highly regulated. Neutrophil number in the blood is determined by the balance between neutrophil production in the bone marrow and release from the bone marrow to blood with neutrophil clearance from the circulation. This review will focus on mechanisms regulating neutrophil release from the bone marrow. In particular, recent data demonstrating a central role for the chemokines CXCL12 and CXCL2 in regulating neutrophil egress from the bone marrow will be discussed.

Proper desensitization of CXCR4 is required for lymphocyte development and peripheral compartmentalization in mice

Desensitization controls G protein-dependent signaling of chemokine receptors. We investigate the physiologic implication of this process for CXCR4 in a mouse model harboring a heterozygous mutation of the Cxcr4 gene, which engenders a desensitization-resistant receptor. Such anomaly is linked to the warts, hypogammaglobulinemia, infections, myelokathexis (WHIM) syndrome, a human rare combined immunodeficiency. Cxcr4(+/mutant(1013)) mice display leukocytes with enhanced responses to Cxcl12 and exhibit leukopenia as reported in patients. Treatment with CXCL12/CXCR4 antagonists transiently reverses blood anomalies, further demonstrating the causal role of the mutant receptor in the leukopenia. Strikingly, neutropenia occurs in a context of normal bone marrow architecture and granulocyte lineage maturation, indicating a minor role for Cxcr4-dependent signaling in those processes. In contrast, Cxcr4(+/1013) mice show defective thymopoiesis and B-cell development, accounting for circulating lymphopenia. Concomitantly, mature T and B cells are abnormally compartmentalized in the periphery, with a reduction of primary follicles in the spleen and their absence in lymph nodes mirrored by an unfurling of the T-cell zone. These mice provide a model to decipher the role of CXCR4 desensitization in the homeostasis of B and T cells and to investigate which manifestations of patients with WHIM syndrome may be overcome by dampening the gain of CXCR4 function.

Granulocyte colony stimulating factor prevents kidney infarction and attenuates renovascular hypertension

BACKGROUND

G-CSF is a critical regulator of hematopoietic cell proliferation, differentiation and survival. It has been reported that G-CSF attenuates renal injury during acute ischemia-reperfusion. In this study we evaluated the effects of G-CSF on the renal and cardiovascular systems of 2K1C hypertensive mice.

METHODS

Male C57BL/6 mice were subjected to left renal artery clipping (2K1C) or sham operation and were then administered G-CSF (100 μg/kg/day) or vehicle for 14 days.

RESULTS

Arterial pressure was higher in 2K1C + vehicle animals than in 2K1C + G-CSF (150±5 vs. 129±2 mmHg, p<0.01, n=8). Plasma angiotensin I, II and 1-7 concentrations were significantly increased in 2K1C + Vehicle when compared to the normotensive Sham group. G-CSF prevented the increase of these vasoactive peptides. The clipped kidney/contralateral kidney weight ratio showed a less atrophy of the ischemic kidney in the treated group (0.50±0.02 vs. 0.66±0.01, p<0.05). The infarction area in the clipped kidney was completely prevented in 7 out of 8 2K1C + G-CSF mice. Administration of G-CSF protected the clipped kidney from apoptosis.

CONCLUSION

Our data indicate that G-CSF prevents kidney infarction and markedly attenuates the increases in plasma angiotensin levels and hypertension in 2K1C mice, reinforcing the protective effect of G-CSF on kidney ischemia.

Cxcr2 and Cxcl5 regulate the IL-17/G-CSF axis and neutrophil homeostasis in mice

Neutrophils are essential for maintaining innate immune surveillance under normal conditions, but also represent a major contributor to tissue damage during inflammation. Neutrophil homeostasis is therefore tightly regulated. Cxcr2 plays a critical role in neutrophil homeostasis, as Cxcr2(-/-) mice demonstrate mild neutrophilia and severe neutrophil hyperplasia in the bone marrow. The mechanisms underlying these phenotypes, however, are unclear. We report here that Cxcr2 on murine neutrophils inhibits the IL-17A/G-CSF axis that regulates neutrophil homeostasis. Furthermore, enterocyte-derived Cxcl5 in the gut regulates IL-17/G-CSF levels and contributes to Cxcr2-dependent neutrophil homeostasis. Conversely, G-CSF was required for Cxcl5-dependent regulation of neutrophil homeostasis, and inhibition of IL-17A reduced plasma G-CSF concentrations and marrow neutrophil numbers in both Cxcl5(-/-) and Cxcr2(-/-) mice. Cxcr2(-/-) mice constitutively expressed IL-17A and showed increased numbers of IL-17A-producing cells in the lung, terminal ileum, and spleen. Most IL-17-producing splenocytes were responsive to IL-1β plus IL-23 in vitro. Depletion of commensal microbes by antibiotic treatment in Cxcr2(-/-) mice markedly decreased IL-17A and G-CSF expression, neutrophilia, and marrow myeloid hyperplasia. These data suggest a critical role for Cxcr2, Cxcl5, and commensal bacteria in regulation of the IL-17/G-CSF axis and neutrophil homeostasis at mucosal sites and have implications for the development of treatments for pathologies resulting from either excessive or ineffective neutrophil responses.

Genetics of leukocyte telomere length and its role in atherosclerosis

Humans display a large inter-individual variation in leukocyte telomere length (LTL), which is influenced by heredity, sex, race/ethnicity, paternal age at conception and environmental exposures. LTL dynamics (birth LTL and its age-dependent attrition thereafter) mirror telomere dynamics in hematopoietic stem cells (HSCs). LTL at birth is evidently a major determinant of LTL throughout the human lifespan, such that individuals endowed with short (or long) LTL at birth probably have short (or long) LTL later in life. Therefore, the associations of short LTL with atherosclerosis and with diminished survival in the elderly may relate to short birth LTL, accelerated age-dependent LTL attrition, or both. The mechanisms underlying these associations are still not well understood, but they stem in part from genetic factors in control of telomere maintenance and the rate of HSC replication.

Immunostimulation of sugar cane extract on neutrophils to Salmonella typhimurium infection in mice

The aim of this study was to evaluate the immunomodulatory effects of sugar cane extract (SCE) on the biological activities of neutrophils in mice. Six-week-old BALB/c mice were fed 1250 mg/kg of SCE once. The generation, migration and biological functions of neutrophils and the survival rates of the mice in response to Salmonella typhimurium infection were evaluated. The results show that the numbers of both bone marrow cells and neutrophils were significantly increased in response to SCE administration (p < 0.05) compared with controls. The migration, phagocytosis and H₂O₂ generation of neutrophils were all significantly enhanced in SCE-treated mice (p < 0.05). After challenge with S. typhimurium (lethal dose, 50% (LD₅₀), SCE-treated mice had a 19.2% higher survival rate and milder hepatic lesions than the controls. Additionally, fewer invasive bacteria were recovered from the spleens of SCE-treated mice. In conclusion, our results suggest that SCE has a positive regulatory effect on the biological function of mouse neutrophils that may increase host resistance against bacterial infections.

Heparin-induced leukocytosis requires 6-O-sulfation and is caused by blockade of selectin- and CXCL12 protein-mediated leukocyte trafficking in mice

Leukocytosis refers to an increase in leukocyte count above the normal range in the blood and is a common laboratory finding in patients. In many cases, the mechanisms underlying leukocytosis are not known. In this study, we examined the effects, the structural determinants, and the underlying mechanisms of heparin-induced leukocytosis, a side effect occurring in 0.44% of patients receiving heparin. We observed that heparin induced both lymphocytosis and neutrophilia, and the effects required heparin to be 6-O-sulfated but did not require its anticoagulant activity. Cell mobilization studies revealed that the lymphocytosis was attributable to a combination of blockage of lymphocyte homing and the release of thymocytes from the thymus, whereas the neutrophilia was caused primarily by neutrophil release from the bone marrow and demargination in the vasculature. Mechanistic studies revealed that heparin inhibits L- and P-selectin, as well as the chemokine CXCL12, leading to leukocytosis. Heparin is known to require 6-O-sulfate to inhibit L- and P-selectin function, and in this study we observed that 6-O-sulfate is required for its interaction with CXCL12. We conclude that heparin-induced leukocytosis requires glucosamine 6-O-sulfation and is caused by blockade of L-selectin-, P-selectin-, and CXCL12-mediated leukocyte trafficking.

Coordinate regulation of neutrophil homeostasis by liver X receptors in mice

The most abundant immune cell type is the neutrophil, a key first responder after pathogen invasion. Neutrophil numbers in the periphery are tightly regulated to prevent opportunistic infections and aberrant inflammation. In healthy individuals, more than 1 × 10⁹ neutrophils per kilogram body weight are released from the bone marrow every 24 hours. To maintain homeostatic levels, an equivalent number of senescent cells must be cleared from circulation. Recent studies indicate that clearance of senescent neutrophils by resident tissue macrophages and DCs helps to set homeostatic levels of neutrophils via effects on the IL-23/IL-17/G-CSF cytokine axis, which stimulates neutrophil production in the bone marrow. However, the molecular events in phagocytes underlying this feedback loop have remained indeterminate. Liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate both lipid metabolic and inflammatory gene expression. Here, we demonstrate that LXRs contribute to the control of neutrophil homeostasis. Using gain- and loss-of-function models, we found that LXR signaling regulated the efficient clearance of senescent neutrophils by peripheral tissue APCs in a Mer-dependent manner. Furthermore, activation of LXR by engulfed neutrophils directly repressed the IL-23/IL-17/G-CSF granulopoietic cytokine cascade. These results provide mechanistic insight into the molecular events orchestrating neutrophil homeostasis and advance our understanding of LXRs as integrators of phagocyte function, lipid metabolism, and cytokine gene expression.

The neutrophil in vascular inflammation

Here we focus on how neutrophils have a key regulatory role in vascular inflammation. Recent studies using advanced imaging techniques have yielded new insights into the mechanisms by which neutrophils contribute to defense against bacterial infections and also against sterile injury. In these settings, neutrophils are recruited by various mechanisms depending on the situation. We also describe how these processes may be disrupted in systemic infections, with a particular emphasis on mouse models of sepsis. Neutrophils are often immobilized in the lungs and liver during systemic infections, and this immobilization may be a mechanism through which bacteria can evade the innate immune response or allow neutrophils to form neutrophil extracellular traps that trap and kill bacteria in blood. The platelet is also an important player in sepsis, and we describe how it collaborates with neutrophils in the formation of neutrophil extracellular traps.

Dual roles for Rac2 in neutrophil motility and active retention in zebrafish hematopoietic tissue

Neutrophil homeostasis is essential for host defense. Here we identify dual roles for Rac2 during neutrophil homeostasis using a zebrafish model of primary immune deficiency induced by the human inhibitory Rac2D57N mutation in neutrophils. Noninvasive live imaging of Rac2 morphants or Rac2D57N zebrafish larvae demonstrates an essential role for Rac2 in regulating 3D motility and the polarization of F-actin dynamics and PI(3)K signaling in vivo. Tracking of photolabeled Rac2-deficient neutrophils from hematopoietic tissue also shows increased mobilization into the circulation, indicating that neutrophil mobilization does not require traditionally defined cell motility. Moreover, excessive neutrophil retention in hematopoietic tissue resulting from a constitutively active CXCR4 mutation in zebrafish warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is partially rescued by the inhibitory Rac2 mutation. These findings reveal that Rac2 signaling is necessary for both neutrophil 3D motility and CXCR4-mediated neutrophil retention in hematopoietic tissue, thereby limiting neutrophil mobilization, a critical first step in the innate immune response.

Downregulation of CXCL12 signaling and altered hematopoietic stem and progenitor cell trafficking in a murine model of acute Anaplasma phagocytophilum infection

Infection with a variety of bacterial pathogens results in hematopoietic stem and progenitor cell (HSPC) mobilization. The mechanism and kinetics of HSPC mobilization during infection are largely unknown. Previously, we found altered HSPC activity in bone marrow, spleen and blood during infection with Anaplasma phagocytophilum, the agent of granulocytic anaplasmosis. We hypothesized that altered CXCL12/CXCR4 signaling, a central pathway for HSPC homing to, and retention within, the bone marrow, plays a role in infection-induced alterations in HSPC number and trafficking. Mice were infected with A. phagocytophilum. Lineage-cKit+ HSPCs were enumerated and proliferation determined. CXCL12 and CXCR4 mRNA were quantified along with CXCL12 protein, and CXCR4 surface, intracellular and total protein expression in HSPCs was determined. Increased bone marrow proliferation of HSPCs began at 2 d post-infection followed by HSPC mobilization and splenic homing. Proliferation of resident HSPCs contributed to increased splenic HSPC numbers. Bone marrow CXCL12 mRNA and protein levels were decreased at 4-8 d post-infection concurrent with HSPC mobilization. CXCR4 protein parameters were decreased in bone marrow HSPCs throughout 2-6 d post-infection. Reduction of CXCL12/CXCR4 signaling simultaneously occurs with HSPC mobilization from bone marrow. Findings suggest that deranged CXCL12/CXCR4 signaling plays a causal role in HSPC mobilization during acute A. phagocytophilum infection.

Annexin A1 regulates neutrophil clearance by macrophages in the mouse bone marrow

Under homeostatic conditions, a proportion of senescent CXCR4(hi) neutrophils home from the circulation back to the bone marrow, where they are phagocytosed by bone marrow macrophages. In this study, we have identified an unexpected role for the anti-inflammatory molecule annexin A1 (AnxA1) as a critical regulator of this process. We first observed that AnxA1(-/-) mice have significantly increased neutrophil numbers in their bone marrow while having normal levels of GM and G colony-forming units, monocytes, and macrophages. Although AnxA1(-/-) mice have more neutrophils in the bone marrow, a greater proportion of these cells are senescent, as determined by their higher levels of CXCR4 expression and annexin V binding. Consequently, bone marrow neutrophils from AnxA1(-/-) mice exhibit a reduced migratory capacity in vitro. Studies conducted in vitro also show that expression of AnxA1 is required for bone marrow macrophages, but not peritoneal macrophages, to phagocytose apoptotic neutrophils. Moreover, in vivo experiments indicate a defect in clearance of wild-type neutrophils in the bone marrow of AnxA1(-/-) mice. Thus, we conclude that expression of AnxA1 by resident macrophages is a critical determinant for neutrophil clearance in the bone marrow.

Use of neutrophil count in early diagnosis and risk stratification of AMI

BACKGROUND

Neutrophils are rapidly released into the circulation upon acute stress such as trauma or acute myocardial infarction (AMI). We hypothesized that neutrophil count might provide incremental value in the early diagnosis and risk stratification of AMI.

METHODS

We conducted a prospective observational multicenter study to examine the diagnostic accuracy of the combination of neutrophil count and cardiac troponin T from 1125 consecutive patients who presented to the Emergency Department with symptoms suggestive of acute myocardial infarction. The final diagnosis was adjudicated by 2 independent cardiologists.

RESULTS

Neutrophil count was higher in patients with acute myocardial infarction compared with other diagnoses (median 6.7 vs. 5.0×10(9)/L, respectively, P <.001). The accuracy of the neutrophil count for diagnosing acute myocardial infarction, quantified by the area under the receiver operating characteristic curve (AUC) was 0.69, which was significantly lower than that of cardiac troponin T (AUC 0.89, P <.001). The combination of the neutrophil count and cardiac troponin T did not improve the early diagnosis of acute myocardial infarction versus cardiac troponin T alone (P=.79). The prognostic accuracy of neutrophil count for death and AMI was significantly lower than that of cardiac troponin T. However, patients in the highest tertile of neutrophil count had a significantly increased risk of death and AMI at 90 and 360 days compared with patients in the lowest tertile (hazard ratios 2.47 [95% confidence interval, 1.63-3.72] and 2.28 [95% confidence interval, 1.55-3.36], respectively).

CONCLUSION

The neutrophil count does not improve the early diagnosis of AMI in patients presenting with chest pain but identifies patients at increased risk of death.

Exercise and leukocyte interchange among central circulation, lung, spleen, and muscle

Circulating leukocytes increase rapidly with exercise then quickly decrease when the exercise ends. We tested whether exercise acutely led to bidirectional interchange of leukocytes between the circulation and the lung, spleen, and active skeletal muscle. To accomplish this it was necessary to label a large number of immune cells (granulocytes, monocytes, and lymphocytes) in a way that resulted in minimal perturbation of cell function. Rats were injected intravenously with a single bolus of carboxyfluorescein diacetate succinamidyl ester (CFSE) dye which is rapidly and irreversibly taken up by circulating cells. The time course of the disappearance of labeled cells and their reappearance in the circulation following exercise was determined via flow cytometry. The majority of circulating leukocytes were labeled at 4h. post-injection and this proportion slowly declined out to 120 h. At both 24 and 120 h, running resulted in an increase in the proportion of labeled leukocytes in the circulation. Analysis of the skeletal muscle, spleen and lung indicated that labeled leukocytes had accumulated in those tissues and were mobilized to the circulation in response to exercise. This indicates that there is an ongoing exchange of leukocytes between the circulation and tissues and that exercise can stimulate their redistribution. Exchange was slower with muscle than with spleen and lung, but in all cases, influenced by exercise. Exercise bouts redistribute leukocytes between the circulation and the lung, spleen and muscle. The modulatory effects of exercise on the immune system may be regulated in part by the systemic redistribution of immune cells.

Dual protective mechanisms of matrix metalloproteinases 2 and 9 in immune defense against Streptococcus pneumoniae

A localized and effective innate immune response to pathogenic bacterial invasion is central to host survival. Identification of the critical local innate mediators of lung defense against such pathogens is essential for a complete understanding of the mechanism(s) underlying effective host defense. In an acute model of Streptococcus pneumoniae lung infection, deficiency in matrix metalloproteinase (MMP)2 and MMP9 (Mmp2/9(-/-)) conferred a survival disadvantage relative to wild-type mice treated under the same conditions. S. pneumoniae-infected Mmp2/9(-/-) mice recruited more polymorphonuclear leukocytes to the lung but had higher bacterial burdens. Mmp2/9(-/-) mice showed significantly higher levels of IL-17A, IP-10, and RANTES in the lung. Although MMP2-dependent cleavage partially inactivated IL-17A, MMP9 was critical for effective bacterial phagocytosis and reactive oxygen species generation in polymorphonuclear neutrophils. These data demonstrate critical nonredundant and protective roles for MMP2 and MMP9 in the early host immune response against S. pneumoniae infection.

A multiscale model of the bone marrow and hematopoiesis

The bone marrow is necessary for renewal of all hematopoietic cells and critical for maintenance of a wide range of physiologic functions. Multiple human diseases result from bone marrow dysfunction. It is also the site in which liquid tumors, including leukemia and multiple myeloma, develop as well as a frequent site of metastases. Understanding the complex cellular and microenvironmental interactions that govern normal bone marrow function as well as diseases and cancers of the bone marrow would be a valuable medical advance. Our goal is the development of a spatially-explicit in silico model of the bone marrow to understand both its normal function and the evolutionary dynamics that govern the emergence of bone marrow malignancy. Here we introduce a multiscale computational model of the bone marrow that incorporates three distinct spatial scales, cell, hematopoietic subunit, whole marrow. Our results, using parameter estimates from literature, recapitulates normal bone marrow function and suggest an explanation for the fractal-like structure of trabeculae and sinuses in the marrow, which would be an optimization of the hematopoietic function in order to maximize the number of mature blood cells produced daily within the volumetric restrictions of the marrow.

Nuclear phospholipid scramblase 1 prolongs the mitotic expansion of granulocyte precursors during G-CSF-induced granulopoiesis

PLSCR1-/- mice exhibit normal, steady-state hematologic parameters but impaired emergency granulopoiesis upon in vivo administration of G-CSF. The mechanism by which PLSCR1 contributes to G-CSF-induced neutrophil production is largely unknown. We now report that the expansion of bone marrow myelocytes upon in vivo G-CSF treatment is reduced in PLSCR1-/- mice relative to WT. Using SCF-ER-Hoxb8-immortalized myeloid progenitors to examine the progression of G-CSF-driven granulocytic differentiation in vitro, we found that PLSCR1 prolongs the period of mitotic expansion of proliferative granulocyte precursors, thereby giving rise to increased neutrophil production from their progenitors. This effect of PLSCR1 is blocked by a ΔNLS-PLSCR1, which prevents its nuclear import. By contrast, mutation that prevents the membrane association of PLSCR1 has minimal impact on the role of PLSCR1 in G-CSF-induced granulopoiesis. These data imply that the capacity of PLSCR1 to augment G-CSF-dependent production of mature neutrophils from myeloid progenitors is unrelated to its reported activities at the endofacial surface of the plasma membrane but does require entry of the protein into the nucleus, suggesting that this response is mediated through the observed effects of PLSCR1 on gene transcription.

Rapid immunomagnetic negative enrichment of neutrophil granulocytes from murine bone marrow for functional studies in vitro and in vivo

Polymorphonuclear neutrophils (PMN) mediate early immunity to infection but can also cause host damage if their effector functions are not controlled. Their lack or dysfunction is associated with severe health problems and thus the analysis of PMN physiology is a central issue. One prerequisite for PMN analysis is the availability of purified cells from primary organs. While human PMN are easily isolated from peripheral blood, this approach is less suitable for mice due to limited availability of blood. Instead, bone marrow (BM) is an easily available reservoir of murine PMN, but methods to obtain pure cells from BM are limited. We have developed a novel protocol allowing the isolation of highly pure untouched PMN from murine BM by negative immunomagnetic isolation using a complex antibody cocktail. The protocol is simple and fast (∼1 h), has a high yield (5–10*10⁶ PMN per animal) and provides a purity of cells equivalent to positive selection (>80%). Most importantly, cells obtained by this method are non-activated and remain fully functional in vitro or after adoptive transfer into recipient animals. This method should thus greatly facilitate the study of primary murine PMN in vitro and in vivo.

Bone marrow-targeted liposomal carriers

INTRODUCTION

Bone marrow-targeted drug delivery systems appear to offer a promising strategy for advancing diagnostic, protective and/or therapeutic medicine for the hematopoietic system. Liposome technology can provide a drug delivery system with high bone marrow targeting that is mediated by specific phagocytosis in bone marrow.

AREA COVERED

This review focuses on a bone marrow-specific liposome formulation labeled with technetium-99 m. Interspecies differences in bone marrow distribution of the bone marrow-targeted formulation are emphasized. This review provides a liposome technology to target bone marrow. In addition, the selection of proper species for the investigation of bone marrow targeting is suggested.

EXPERT OPINION

It can be speculated that the bone marrow macrophages have a role in the delivery of lipids to the bone marrow as a source of energy and for membrane biosynthesis or in the delivery of fat-soluble vitamins for hematopoiesis. This homeostatic system offers a potent pathway to deliver drugs selectively into bone marrow tissues from blood. High selectivity of the present bone marrow-targeted liposome formulation for bone marrow suggests the presence of an active and specific mechanism, but specific factors affecting the uptake of the bone marrow mononuclear phagocyte system are still unknown. Further investigation of this mechanism will increase our understanding of factors required for effective transport of agents to the bone marrow, and may provide an efficient system for bone marrow delivery for therapeutic purposes.

Neutrophil survival and c-kit(+)-progenitor proliferation in Staphylococcus aureus-infected skin wounds promote resolution

Polymorphonuclear neutrophils (PMNs) are critical for the formation, maintenance, and resolution of bacterial abscesses. However, the mechanisms that regulate PMN survival and proliferation during the evolution of an abscess are not well defined. Using a mouse model of Staphylococcus aureus abscess formation within a cutaneous wound, combined with real-time imaging of genetically tagged PMNs, we observed that a high bacterial burden elicited a sustained mobilization of PMNs from the bone marrow to the infected wound, where their lifespan was markedly extended. A continuous rise in wound PMN number, which was not accounted for by trafficking from the bone marrow or by prolonged survival, was correlated with the homing of c-kit(+)-progenitor cells from the blood to the wound, where they proliferated and formed mature PMNs. Furthermore, by blocking their recruitment with an antibody to c-kit, which severely limited the proliferation of mature PMNs in the wound and shortened mouse survival, we confirmed that progenitor cells are not only important contributors to PMN expansion in the wound, but are also functionally important for immune protection. We conclude that the abscess environment provides a niche capable of regulating PMN survival and local proliferation of bone marrow-derived c-kit(+)-progenitor cells.

Effect of an extract based on the medicinal mushroom Agaricus blazei Murill on expression of cytokines and calprotectin in patients with ulcerative colitis and Crohn's disease

An immunomodulatory extract (AndoSan™) based on the medicinal mushroom Agaricus blazei Murill (AbM) has shown to reduce blood cytokine levels in healthy volunteers after 12 days' ingestion, pointing to an anti-inflammatory effect. The aim was to study whether AndoSan™ had similar effects on cytokines in patients with ulcerative colitis (UC) and Crohn's disease (CD). Calprotectin, a marker for inflammatory bowel disease (IBD), was also measured. Patients with CD (n = 11) and with UC (n = 10) consumed 60 ml/day of AndoSan™. Patient blood plasma was harvested before and after 6 h LPS (1 ng/ml) stimulation ex vivo. Plasma and faecal calprotectin levels were analysed using ELISA and 17 cytokines [IL-2, IFN-γ, IL-12 (Th1), IL-4, IL-5, IL-13 (Th2), IL-7, IL-17, IL-1β, IL-6, TNF-α, IL-8, MIP-1β, MCP-1, G-CSF, GM-CSF and IL-10] by multiplex assay. After 12 days' ingestion of AndoSan™, baseline plasma cytokine levels in UC was reduced for MCP-1 (40%) and in LPS-stimulated blood for MIP-1β (78%), IL-6 (44%), IL-1β (41%), IL-8 (30%), G-CSF (29%), MCP-1 (18%) and GM-CSF (17%). There were corresponding reductions in CD: IL-2 (100%), IL-17 (55%) and IL-8 (29%) and for IL-1β (35%), MIP-1β (30%), MCP-1 (22%), IL-8 (18%), IL-17 (17%) and G-CSF (14%), respectively. Baseline concentrations for the 17 cytokines in the UC and CD patient groups were largely similar. Faecal calprotectin was reduced in the UC group. Ingestion of an AbM-based medicinal mushroom by patients with IBD resulted in interesting anti-inflammatory effects as demonstrated by declined levels of pathogenic cytokines in blood and calprotectin in faeces.

Phagocytosis and digestion of pH-sensitive fluorescent dye (Eos-FP) transfected E. coli in whole blood assays from patients with severe sepsis and septic shock

The function of phagocytic and antigen presenting cells is of crucial importance to sustain immune competence against infectious agents as well as malignancies. We here describe a reproducible procedure for the quantification of phagocytosis by leukocytes in whole blood. For this, a pH-sensitive green-fluorescent protein- (GFP) like dye (Eos-FP) is transfected into infectious microroganisms. After UV-irradiation, the transfected bacteria emit green (≈5160 nm) and red (≈581 nm) fluorescent light at 490 nm excitation. Since the red fluorescent light is sensitive to acidic pH, the phagocytosed bacteria stop emitting red fluorescent light as soon as the phagosomes fuse with lysosomes. The green fluorescence is maintained in the phagolysosome until pathogen degradation is completed. Fluorescence emission can be followed by flow cytometry with filter settings documenting fluorescence 1 (FL 1, FITC) and fluorescence 2 (FL 2, phycoerythrin, PE). Eos-FP transfected bacteria can also be traced within phagocytes using microscopical techniques. A standardized assay has been developed which is suitable for clinical studies by providing clinicians with syringes pre-filled with fixed and appropriately UV-irradiated Eos-FP E. coli (TruCulture™). After adding blood or body fluids to these containers and starting the incubation at 37°C, phagocytosis by granulocytes proceeds over time. Cultures can be terminated at a given time by lysing red blood cells followed by flow cytometry. A pilot study demonstrated that Eos-FP E. coli phagocytosis and digestion was up-regulated in the majority of patients with either severe sepsis or septic shock as compared to healthy donors (p < 0.0001 after o/n incubation). Following treatment with recombinant human granulocyte colony-stimulating factor (rhG-CSF) in selected patients with sepsis, phagolysosome fusion appeared to be accelerated.

G-CSF-mediated thrombopoietin release triggers neutrophil motility and mobilization from bone marrow via induction of Cxcr2 ligands

Emergency mobilization of neutrophil granulocytes (neutrophils) from the bone marrow (BM) is a key event of early cellular immunity. The hematopoietic cytokine granulocyte-colony stimulating factor (G-CSF) stimulates this process, but it is unknown how individual neutrophils respond in situ. We show by intravital 2-photon microscopy that a systemic dose of human clinical-grade G-CSF rapidly induces the motility and entry of neutrophils into blood vessels within the tibial BM of mice. Simultaneously, the neutrophil-attracting chemokine KC (Cxcl1) spikes in the blood. In mice lacking the KC receptor Cxcr2, G-CSF fails to mobilize neutrophils and antibody blockade of Cxcr2 inhibits the mobilization and induction of neutrophil motility in the BM. KC is expressed by megakaryocytes and endothelial cells in situ and is released in vitro by megakaryocytes isolated directly from BM. This production of KC is strongly increased by thrombopoietin (TPO). Systemic G-CSF rapidly induces the increased production of TPO in BM. Accordingly, a single injection of TPO mobilizes neutrophils with kinetics similar to G-CSF, and mice lacking the TPO receptor show impaired neutrophil mobilization after short-term G-CSF administration. Thus, a network of signaling molecules, chemokines, and cells controls neutrophil release from the BM, and their mobilization involves rapidly induced Cxcr2-mediated motility controlled by TPO as a pacemaker.

Measuring whole-body neutrophil redistribution using a dedicated whole-body counter and ultra-low doses of 111Indium

BACKGROUND

There is increasing interest in the 'homing' of neutrophils to bone marrow. The aim of this study was to measure the whole-body redistribution of (111) In using a whole-body counter following the administration of ultra-small activities of (111) In-labelled neutrophils.

METHODS

The detectors of a dedicated whole-body counter were fitted with lead collimators. Whole-body (111) In distribution was recorded at 45 min, 24 h, and 2, 4, 7 and 10 days after administration of (111) In-labelled neutrophils (0·29-0·74 MBq) in eight healthy non-smokers, five healthy smokers, eight patients with inactive bronchiectasis, three with asthma and nine with chronic obstructive pulmonary disease (COPD).

RESULTS

Intravascular 45-min (111) In-labelled neutrophil recovery was not significantly different between groups, ranging from 33 (SD 8%) in healthy smokers to 45 (14%) in healthy non-smokers (P > 0·05). Peaks were identified on the whole body count profile corresponding to the chest, upper abdomen (liver/spleen) and pelvis (bone marrow). (111) In distribution changed between 45 min and 24 h and then remained stable thereafter. Peak chest counts increased ∼ 1·5-fold between 45 min and 24 h, whereas upper abdominal peak counts decreased by ∼ 25% with no significant inter-group differences. The increment in pelvic counts (∼ 2·7-fold) was similar between groups, except COPD patients, in whom it was 2·04 (0·35; P < 0·02 vs. healthy participants).

CONCLUSIONS

Assuming neutrophils are distributed only between blood, liver, spleen and bone marrow, the data suggest that marrow pools 25% and destroys 67% of circulating neutrophils, rising in COPD to 40% and 80%, respectively, possibly as a result of the effects on marrow of chronic hypoxaemia.

Discovery of a CXCR4 agonist pepducin that mobilizes bone marrow hematopoietic cells

The G protein-coupled receptor (GPCR), chemokine CXC-type receptor 4 (CXCR4), and its ligand, CXCL12, mediate the retention of polymorphonuclear neutrophils (PMNs) and hematopoietic stem and progenitor cells (HSPCs) in the bone marrow. Agents that disrupt CXCL12-mediated chemoattraction of CXCR4-expressing cells mobilize PMNs and HSPCs into the peripheral circulation and are therapeutically useful for HSPC collection before autologous bone marrow transplantation (ABMT). Our aim was to develop unique CXCR4-targeted therapeutics using lipopeptide GPCR modulators called pepducins. A pepducin is a synthetic molecule composed of a peptide derived from the amino acid sequence of one of the intracellular (IC) loops of a target GPCR coupled to a lipid tether. We prepared and screened a small CXCR4-targeted pepducin library and identified several pepducins with in vitro agonist activity, including ATI-2341, whose peptide sequence derives from the first IC loop. ATI-2341 induced CXCR4- and G protein-dependent signaling, receptor internalization, and chemotaxis in CXCR4-expressing cells. It also induced dose-dependent peritoneal recruitment of PMNs when administered i.p. to mice. However, when administered systemically by i.v. bolus, ATI-2341 acted as a functional antagonist and dose-dependently mediated release of PMNs from the bone marrow of both mice and cynomolgus monkeys. ATI-2341-mediated release of granulocyte/macrophage progenitor cells from the bone marrow was confirmed by colony-forming assays. We conclude that ATI-2341 is a potent and efficacious mobilizer of bone marrow PMNs and HSPCs and could represent a previously undescribed therapeutic approach for the recruitment of HSPCs before ABMT.