[Tumor of the parapharyngeal space]

Increased extracellular ubiquitin in surgical wound fluid provides a chemotactic signal for myeloid dendritic cells

PURPOSE

Myeloid dendritic cells (MDC) decline significantly after multiple traumas which might be due to an increased migration into injured regions. Ubiquitin is released from dying cells and is increased in serum after trauma. Ubiquitin can bind to the chemokine receptor CXCR4. Thus, we hypothesized that elevated ubiquitin provides a chemotactic signal for MDC to injured regions.

METHODS

Surgical wound fluid (SWF) and serum from patients with mono-trauma (n = 20) were used to simulate the humoral situation in injured tissue. MDC were identified by flow cytometry. Chemotaxis was measured using transwell migration assays. Ubiquitin and CXCL12 (natural CXCR4 ligand) were determined by ELISA.

RESULTS

MDC express CXCR4 and fluorescence-labeled ubiquitin binds to MDC. Ubiquitin exerts a dose-dependent chemotactic effect (fourfold at 100 ng/mL, p < 0.05). Ubiquitin concentration was sixfold higher in SWF (p < 0.05), whereas CXCL12 was increased in serum. MDC migration towards SWF was significantly reduced (- 40%, p < 0.05), if ubiquitin was neutralized by specific antibodies.

CONCLUSIONS

Ubiquitin is increased in SWF and exerts a significant chemotactic effect on MDC. This mechanism might play a role in attraction of immune cells to injured regions and might contribute to the decline of circulating MDC in multiple traumas.

The tetraspanin CD82 regulates bone marrow homing and engraftment of hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cell (HSPC) transplantation represents a treatment option for patients with malignant and nonmalignant hematological diseases. Initial steps in transplantation involve the bone marrow homing and engraftment of peripheral blood–injected HSPCs. In recent work, we identified the tetraspanin CD82 as a potential regulator of HSPC homing to the bone marrow, although its mechanism remains unclear. In the present study, using a CD82 knockout (CD82KO) mouse model, we determined that CD82 modulates HSPC bone marrow maintenance, homing, and engraftment. Bone marrow characterization identified a significant decrease in the number of long-term hematopoietic stem cells in the CD82KO mice, which we linked to cell cycle activation and reduced stem cell quiescence. Additionally, we demonstrate that CD82 deficiency disrupts bone marrow homing and engraftment, with in vitro analysis identifying further defects in migration and cell spreading. Moreover, we find that the CD82KO HSPC homing defect is due at least in part to the hyperactivation of Rac1, as Rac1 inhibition rescues homing capacity. Together, these data provide evidence that CD82 is an important regulator of HSPC bone marrow maintenance, homing, and engraftment and suggest exploiting the CD82 scaffold as a therapeutic target for improved efficacy of stem cell transplants.

Mobilized Peripheral Blood versus Cord Blood: Insight into the Distinct Role of Proinflammatory Cytokines on Survival, Clonogenic Ability, and Migration of CD34+ Cells

Inflammation may play a role in cancer. However, the contribution of cytokine-mediated crosstalk between normal hemopoietic stem/progenitor cells (HSPCs) and their (inflammatory) microenvironment is largely elusive. Here we compared survival, phenotype, and function of neonatal (umbilical cord blood (CB)) and adult (normal G-CSF-mobilized peripheral blood (mPB)) CD34⁺ cells after in vitro exposure to combined crucial inflammatory factors such as interleukin- (IL-) 1β, IL-6, tumor necrosis factor- (TNF-) α, or tissue inhibitor of metalloproteinases-1 (TIMP-1). To mimic bone marrow (BM) niche, coculture experiments with normal BM stromal cells (BMSCs) were also performed. We found that combined inflammatory cytokines increased only the in vitro survival of CB-derived CD34⁺ cells by reducing apoptosis. Conversely, selected combinations of inflammatory cytokines (IL-1β + TNF-α, IL-6 + TNF-α, and IL-1β + TNF-α + TIMP-1) mainly enhanced the in vitro CXCR4-driven migration of mPB-derived CD34⁺ cells. TNF-α, alone or in combination, upregulated CD44 and CD13 expression in both sources. Finally, BMSCs alone increased survival/migration of CB- and mPB-derived CD34⁺ cells at the same extent of the combined inflammatory cytokines; importantly, their copresence did not show additive/synergistic effect. Taken together, these data indicate that combined proinflammatory stimuli promote distinct in vitro functional activation of neonatal or adult normal HSPCs.

Repopulating hematopoietic stem cells from steady-state blood before and after ex vivo culture are enriched in the CD34+CD133+CXCR4low fraction

The feasibility of ex vivo expansion allows us to consider the steady-state peripheral blood as an alternative source of hematopoietic stem progenitor cells for transplantation when growth factor-induced cell mobilization is contraindicated or inapplicable. Ex vivo expansion dramatically enhances the in vivo reconstituting cell population from steady-state blood. In order to investigate phenotype and the expression of homing molecules, the expression of CD34, CD133, CD90, CD45RA, CD26 and CD9 was determined on sorted CD34⁺ cells according to CXCR4 (“neg”, “low” “bright”) and CD133 expression before and after ex vivo expansion. Hematopoietic stem cell activity was determined in vivo on the basis of hematopoietic repopulation of primary and secondary recipients - NSG immuno-deficient mice. In vivo reconstituting cells in the steady-state blood CD34⁺ cell fraction before expansion belong to the CD133⁺ population and are CXCR4^low or, to a lesser extent, CXCR4^neg, while after ex vivo expansion they are contained only in the CD133⁺CXCR4^low cells. The failure of the CXCR4^bright population to engraft is probably due to the exclusive expression of CD26 by these cells. The limiting-dilution analysis showed that both repopulating cell number and individual proliferative capacity were enhanced by ex vivo expansion. Thus, steady-state peripheral blood cells exhibit a different phenotype compared to mobilized and cord blood cells, as well as to those issued from the bone marrow. These data represent the first phenotypic characterization of steady-state blood cells exhibiting short- and long-term hematopoietic reconstituting potential, which can be expanded ex vivo, a sine qua non for their subsequent use for transplantation.

Stem cells as delivery vehicles for regenerative medicine-challenges and perspectives

The use of stem cells as carriers for therapeutic agents is an appealing modality for targeting tissues or organs of interest. Combined delivery of cells together with various information molecules as therapeutic agents has the potential to enhance, modulate or even initiate local or systemic repair processes, increasing stem cell efficiency for regenerative medicine applications. Stem-cell-mediated delivery of genes, proteins or small molecules takes advantage of the innate capability of stem cells to migrate and home to injury sites. As the native migratory properties are affected by in vitro expansion, the existent methods for enhancing stem cell targeting capabilities (modified culture methods, genetic modification, cell surface engineering) are described. The role of various nanoparticles in equipping stem cells with therapeutic small molecules is revised together with their class-specific advantages and shortcomings. Modalities to circumvent common challenges when designing a stem-cell-mediated targeted delivery system are described as well as future prospects in using this approach for regenerative medicine applications.

Novel evidence that extracellular nucleotides and purinergic signaling induce innate immunity-mediated mobilization of hematopoietic stem/progenitor cells

Pharmacological mobilization of hematopoietic stem progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB) is a result of mobilizing agent-induced “sterile inflammation” in the BM microenvironment due to complement cascade (ComC) activation. Here we provide evidence that ATP, as an extracellular nucleotide secreted in a pannexin-1-dependent manner from BM cells, triggers activation of the ComC and initiates the mobilization process. This process is augmented in a P2X7 receptor-dependent manner, and P2X7-KO mice are poor mobilizers. Furthermore, after its release into the extracellular space, ATP is processed by ectonucleotidases: CD39 converts ATP to AMP, and CD73 converts AMP to adenosine. We observed that CD73-deficient mice mobilize more HSPCs than do wild-type mice due to a decrease in adenosine concentration in the extracellular space, indicating a negative role for adenosine in the mobilization process. This finding has been confirmed by injecting mice with adenosine along with pro-mobilizing agents. In sum, we demonstrate for the first time that purinergic signaling involving ATP and its metabolite adenosine regulate the mobilization of HSPCs. Although ATP triggers and promotes this process, adenosine has an inhibitory effect. Thus, administration of ATP together with G-CSF or AMD3100 or inhibition of CD73 by small molecule antagonists may provide the basis for more efficient mobilization strategies.

Mammalian sterile 20 kinase 1 and 2 are important regulators of hematopoietic stem cells in stress condition

The mammalian Hippo signaling pathway has been implicated in the self-renewal and differentiation of stem and progenitor cells. MST1 and MST2 (MST1/2) are core serine-threonine kinases in the Hippo signaling pathway, one of which, MST1, has been extensively investigated for its role in T cell and myeloid cell function. These studies have identified MST1 as a promising therapeutic target in immunological disease. However, the roles of MST1/2 in hematopoietic stem cell (HSC) function in vivo are not fully understood. Here, we report that mice with a conditional deletion of Mst1/2 exhibit impaired hematopoietic stem and progenitor cell (HSPC) function under stress condition. Furthermore, Mst1/2 deletion markedly altered mature cell output. Therefore, MST1/2 are indispensable for maintenance as well as function of stem and progenitor cells under steady state conditions and with transplantation stress.

Recent Progress in Stem Cell Modification for Cardiac Regeneration

During the past decades, stem cell-based therapy has acquired a promising role in regenerative medicine. The application of novel cell therapeutics for the treatment of cardiovascular diseases could potentially achieve the ambitious aim of effective cardiac regeneration. Despite the highly positive results from preclinical studies, data from phase I/II clinical trials are inconsistent and the improvement of cardiac remodeling and heart performance was found to be quite limited. The major issues which cardiac stem cell therapy is facing include inefficient cell delivery to the site of injury, accompanied by low cell retention and weak effectiveness of remaining stem cells in tissue regeneration. According to preclinical and clinical studies, various stem cells (adult stem cells, embryonic stem cells, and induced pluripotent stem cells) represent the most promising cell types so far. Beside the selection of the appropriate cell type, researchers have developed several strategies to produce “second-generation” stem cell products with improved regenerative capacity. Genetic and nongenetic modifications, chemical and physical preconditioning, and the application of biomaterials were found to significantly enhance the regenerative capacity of transplanted stem cells. In this review, we will give an overview of the recent developments in stem cell engineering with the goal to facilitate stem cell delivery and to promote their cardiac regenerative activity.

Stem cells and their potential clinical applications in psychiatric disorders

The robustness of stem cells is one of the major factors that directly impacts life quality and life span. Evidence has accumulated that changes in the stem cell compartment affect human mental health and serve as an indicator of psychiatric problems. It is well known that stem cells continuously replace differentiated cells and tissues that are used up during life, although this replacement occurs at a different pace in the various organs. However, the participation of local neural stem cells in regeneration of the central nervous system is controversial. It is known that low numbers of stem cells circulate continuously in peripheral blood (PB) and lymph and undergo a circadian rhythm in their PB level, with the peak occurring early in the morning and the nadir at night, and recent evidence suggests that the number and pattern of circulating stem cells in PB changes in psychotic disorders. On the other hand, progress in the creation of induced pluripotent stem cells (iPSCs) from patient somatic cells provides valuable tools with which to study changes in gene expression in psychotic patients. We will discuss the various potential sources of stem cells that are currently employed in regenerative medicine and the mechanisms that explain some of their beneficial effects as well as the emerging problems with stem cell therapies. However, the main question remains: Will it be possible in the future to modulate the stem cell compartment to reverse psychiatric problems?

Ceramide and S1P Signaling in Embryonic Stem Cell Differentiation

Bioactive sphingolipids are important regulators for stem cell survival and differentiation. Most recently, we have coined the term "morphogenetic lipids" for sphingolipids that regulate stem cells during embryonic and postnatal development. The sphingolipid ceramide and its derivative, sphingosine-1-phosphate (S1P), can act synergistically as well as antagonistically on embryonic stem (ES) cell differentiation. We show here simple as well as state-of-the-art methods to analyze sphingolipids in differentiating ES cells and discuss new protocols to use ceramide and S1P analogs for the guided differentiation of mouse ES cells toward neuronal and glial lineage.

Administration of signalling molecules dictates stem cell homing for in situ regeneration

Ex vivo-expanded stem cells have long been a cornerstone of biotherapeutics and have attracted increasing attention for treating intractable diseases and improving tissue regeneration. However, using exogenous cellular materials to develop restorative treatments for large numbers of patients has become a major concern for both economic and safety reasons. Advances in cell biological research over the past two decades have expanded the potential for using endogenous stem cells during wound healing processes, and in particular, recent insight into stem cell movement and homing has prompted regenerative research and therapy based on recruiting endogenous cells. Inspired by the natural healing process, artificial administration of specific chemokines as signals systemically or at the injury site, typically using biomaterials as vehicles, is a state-of-the-art strategy that potentiates stem cell homing and recreates an anti-inflammatory and immunomodulatory microenvironment to enhance in situ tissue regeneration. However, pharmacologically coaxing endogenous stem cells to act as therapeutics in the field of biomedicine remains in the early stages; its efficacy is limited by the lack of innovative methodologies for chemokine presentation and release. This review describes how to direct the homing of endogenous stem cells via the administration of specific signals, with a particular emphasis on targeted signalling molecules that regulate this homing process, to enhance in situ tissue regeneration. We also provide an outlook on and critical considerations for future investigations to enhance stem cell recruitment and harness the reparative potential of these recruited cells as a clinically relevant cell therapy.

Inducible Nitric Oxide Synthase (iNOS) Is a Novel Negative Regulator of Hematopoietic Stem/Progenitor Cell Trafficking

Nitric oxide (NO) is a gaseous free radical molecule involved in several biological processes related to inflammation, tissue damage, and infections. Based on reports that NO inhibits migration of granulocytes and monocytes, we became interested in the role of inducible NO synthetase (iNOS) in pharmacological mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB). To address the role of NO in HSPC trafficking, we upregulated or downregulated iNOS expression in hematopoietic cell lines. Next, we performed mobilization studies in iNOS^-/- mice and evaluated engraftment of iNOS^-/- HSPCs in wild type (control) animals. Our results indicate that iNOS is a novel negative regulator of hematopoietic cell migration and prevents egress of HSPCs into PB during mobilization. At the molecular level, downregulation of iNOS resulted in downregulation of heme oxygenase 1 (HO-1), and, conversely, upregulation of iNOS enhanced HO-1 activity. Since HO-1 is a negative regulator of cell migration, the inhibitory effects of iNOS identified by us can be at least partially explained by its enhancing the HO-1 level in BM cells.

Hematopoietic stem cells under pressure

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) and progenitors are tasked with maintaining hematopoietic homeostasis in the face of numerous insults and challenges, including infection, inflammation, and exsanguination. HSCs possess the remarkable ability to reconstitute the entire hematopoietic system of an organism whose own hematopoietic system has been ablated. This ability is exploited routinely in the clinic via HSC transplantation (HSCT). Here, we focus on the physiological and molecular bottlenecks overcome by HSCs during transplantation.

RECENT FINDINGS

During transplantation, HSCs encounter a damaged bone marrow niche, characterized molecularly by increases in oxygen concentrations and an altered cytokine milieu. New mechanisms and pathways have been recently implicated during HSCT, including transplanted HSC-dependent secretion of conditioning molecules that facilitate engraftment and pathways that protect HSCs from perturbed organelle homeostasis.

SUMMARY

Better understanding the molecular processes HSCs employ to withstand the stress of transplant will illuminate novel targets for further improving conditioning regimens and engraftment during HSCT.

High Level P2X7-Mediated Signaling Impairs Function of Hematopoietic Stem/Progenitor Cells

Nucleotides, which bind to P2 receptors, have emerged as a family of mediators in intercellular communication. P2X7 is a member of the P2X family ligand-gated ion channels respond to extracellular ATP. High level expression of P2X7 was detected in leukemia samples, especially in relapsed cases. However, the role of P2X7 mediated signaling in hematopoietic stem/progenitor cells (HSPCs) as well as its potential role in leukemogenesis have not been established. In this study, the expression of P2X7 in hematopoietic cells in different lineages and stages was analyzed. Over-expression of P2X7 in HSPCs was carried out by retrovirus infection to study the impact on HSPCs. The results showed that low level expression of P2X7 was detected in HSPCs. Over-expression of P2X7 in HSPCs resulted in decreased colony forming ability in vitro and engraftment potential in vivo. These results suggested that high level purinergic signaling by P2X7 impaired function of HSPCs.

Rescuing Self: Transient Isolation and Autologous Transplantation of Bone Marrow Mitigates Radiation-Induced Hematopoietic Syndrome and Mortality in Mice

The inflamed bone marrow niche shortly after total body irradiation (TBI) is known to contribute to loss of hematopoietic stem cells in terms of their number and function. In this study, autologous bone marrow transfer (AL-BMT) was evaluated as a strategy for mitigating hematopoietic form of the acute radiation syndrome by timing the collection phase (2 h after irradiation) and reinfusion (24 h after irradiation) using mice as a model system. Collection of bone marrow (BM) cells (0.5 × 10⁶ total marrow cells) 2 h after lethal TBI rescued different subclasses of hematopoietic stem and progenitor cells (HSPCs) from the detrimental inflammatory and damaging milieu in vivo. Cryopreservation of collected graft and its reinfusion 24 h after TBI significantly rescued mice from lethal effects of irradiation (65% survival against 0% in TBI group on day 30th) and hematopoietic depression. Transient hypometabolic state (HMS) induced 2 h after TBI effectively preserved the functional status of HSPCs and improved hematopoietic recovery even when BM was collected 8 h after TBI. Homing studies suggested that AL-BMT yielded similar percentages for different subsets of HSPCs when compared to syngeneic bone marrow transfer. The results suggest that the timing of collection, and reinfusion of graft is crucial for the success of AL-BMT.

Early assessment of dosimetric and biological differences of total marrow irradiation versus total body irradiation in rodents

PURPOSE

To develop a murine total marrow irradiation (TMI) model in comparison with the total body irradiation (TBI) model.

MATERIALS AND METHODS

Myeloablative TMI and TBI were administered in mice using a custom jig, and the dosimetric differences between TBI and TMI were evaluated. The early effects of TBI/TMI on bone marrow (BM) and organs were evaluated using histology, FDG-PET, and cytokine production. TMI and TBI with and without cyclophosphamide (Cy) were evaluated for donor cell engraftment and tissue damage early after allogeneic hematopoietic cell transplantation (HCT). Stromal derived factor-1 (SDF-1) expression was evaluated.

RESULTS

TMI resulted in similar dose exposure to bone and 50% reduction in dose to bystander organs. BM histology was similar between the groups. In the non-HCT model, TMI mice had significantly less acute intestinal and lung injury compared to TBI. In the HCT model, recipients of TMI had significantly less acute intestinal injury and spleen GVHD, but increased early donor cell engraftment and BM:organ SDF-1 ratio compared to TBI recipients.

CONCLUSIONS

The expected BM damage was similar in both models, but the damage to other normal tissues was reduced by TMI. However, BM engraftment was improved in the TMI group compared to TBI, which may be due to enhanced production of SDF-1 in BM relative to other organs after TMI.

Induced Cell Turnover: A Novel Therapeutic Modality for In Situ Tissue Regeneration

Induced cell turnover (ICT) is a theoretical intervention in which the targeted ablation of damaged, diseased, and/or nonfunctional cells is coupled with replacement by partially differentiated induced pluripotent stem cells in a gradual and multiphasic manner. Tissue-specific ablation can be achieved using pro-apoptotic small molecule cocktails, peptide mimetics, and/or tissue-tropic adeno-associated virus-delivered suicide genes driven by cell type-specific promoters. Replenishment with new cells can be mediated by systemic administration of cells engineered for homing, robustness, and even enhanced function and disease resistance. Otherwise, the controlled release of cells can be achieved using implanted biodegradable scaffolds, hydrogels, and polymer matrixes. In theory, ICT would enable in situ tissue regeneration without the need for surgical transplantation of organs produced ex vivo, and addresses non-transplantable tissues (such as the vasculature, lymph nodes, and the nervous system). This article outlines several complimentary strategies for overcoming barriers to ICT in an effort to stimulate further research at this promising interface of cell therapy, tissue engineering, and regenerative medicine.

Prostaglandin E2 promotes human CD34+ cells homing through EP2 and EP4 in vitro

Recently, certain studies have demonstrated in vitro that prostaglandin E2 (PGE2) promotes human cluster of differentiation (CD)34⁺ cell homing. However, the sub-type receptors activated by PGE2 are unknown, as the PGE2 receptor EP1-4 subtypes (EP1-4) are expressed on the membrane of human CD34⁺ cells. Based on the above, the present study aimed to screen the receptor subtype activity by PGE2 to promote human CD34⁺ cell homing. It was observed that human CD34⁺ cells expressed the four PGE2 sub-receptors, particularly EP2 and 4. PGE2 increased EP2 and 4 mRNA expression significantly, while EP1 and 3 mRNA exhibited no significant alteration. PGE2, EP2 agonist (EP2A), and EP4A upregulated C-X-C chemokine receptor 4 mRNA and protein expression in human CD34⁺ cells, and promoted stromal cell-derived factor 1α (SDF-1α) expression in bone marrow mesenchymal stem cells (BMMSCs). These phenomena were inhibited by the associated receptor antagonists. PGE2, EP2A, and EP4A facilitated human CD34⁺ cell migration towards SDF-1α and BMMSCs. The results of the present study suggested that PGE2 promoted human CD34⁺ cell homing through EP2 and 4 receptors in vitro.

Leveraging Stem Cell Homing for Therapeutic Regeneration

Resident stem cell pools in many tissues/organs are responsible not only for tissue maintenance during physiologic turnover but also for the process of wound repair following injury. With inspiration from stem cell trafficking within the body under physiologic and pathologic conditions, recent advances have been made toward inducing stem cell mobilization and directing patients' own cells to sites of interest for treating a broad spectrum of diseases. An evolving body of work corroborates that delivering guidance cues can mobilize stem cells from the bone marrow and drive these cells toward a specific region. In addition, the transplantation of cell-friendly biomaterials incorporating certain biomolecules has led to the regeneration of lost/damaged tissue without the need for delivering cellular materials manipulated ex vivo. Recently, cell homing has resulted in remarkable biological discoveries in the laboratory as well as great curative successes in preclinical scenarios. Here, we review the biological evidence underlying in vivo cell mobilization and homing with the aim of leveraging endogenous reparative cells for therapeutic applications. Considering both the promise and the obstacles of this approach, we discuss how matrix components of the in vivo milieu can be modified to promote the native regenerative process and inspire future tissue-engineering design.

LCN2 overexpression in bone enhances the hematopoietic compartment via modulation of the bone marrow microenvironment

Lipocalin-2 (LCN2) is a member of the lipocalin family whose expression is modulated in several conditions, including cell differentiation, innate immunity, stress, and cancer. Although it is known that it is expressed in bone, its function in this tissue remains poorly studied. To this end, we took advantage of transgenic mice lines that expressed LCN2 driven by a bone specific type I collagen (LCN2-Tg). In the bone marrow (BM) of LCN2-Tg mice we observed an increased number of phenotypically long-term hematopoietic stem cells (LT-HSC) that also displayed a higher proliferation rate compared to wild-type controls (Wt). Furthermore, hematopoietic progenitor cells, obtained from LCN2-Tg BM showed an increased clonogenic capacity compared to those obtained from LCN2-Tg spleen, a higher concentration of serum erythropoietin and a higher number of mature erythrocytes in the peripheral blood of old LCN2-Tg animals compared to aged-matched wt. The findings of a combined increase in the BM of the LCN2-Tg mice of SDF-1, SCF, and TIMP-1 levels along with the reduction of both MMP-9 activity and cathepsin K concentration may explain the observed effects on the HSC compartment. This study shows that LCN2 overexpression in bones modifies the BM microenvironment via modulation of the expression of key secreted factors and cytokines, which in turn regulate the HSC niche behavior enhancing both HSC homing in young mice and erythrocytes production in older mice.

Poor Mobilization in T-Cell-Deficient Nude Mice Is Explained by Defective Activation of Granulocytes and Monocytes

It has been reported that both SCID mice and SCID patients poorly mobilize hematopoietic stem/progenitor cells (HSPCs) in response to granulocyte colony-stimulating factor (G-CSF). This defect has been proposed to result from a lack of naturally occurring IgM immunoglobulins to trigger activation of the complement cascade (ComC) and release of C5 cleavage fragments crucial in the mobilization process. However, SCID individuals also have T-cell deficiency, and T cells have been shown to modulate trafficking of HSPCs. To learn more about the role of T lymphocytes, we performed mobilization studies in T-lymphocyte-deficient nude mice and found that these mice respond poorly to G-CSF and zymosan but are normal mobilizers in response to AMD3100. Since nude mice have normal levels of IgM immunoglobulins in peripheral blood and may activate the ComC, we focused on the potential involvement of Gr1+ granulocytes and monocytes, which show defective maturation in these animals. Using a nude mouse mobilization model, we found further support for the proposition that proper function of Gr1+ cells is crucial for optimal mobilization of HSPCs.