Beta1 integrin is not essential for hematopoiesis but is necessary for the T cell-dependent IgM antibody response

Integrin β1 regulates marginal zone B cell differentiation and PI3K signaling

Marginal zone (MZ) B cells represent innate-like B cells that mediate a fast immune response. The adhesion of MZ B cells to the marginal sinus of the spleen is governed by integrins. Here, we address the question of whether β1-integrin has additional functions by analyzing Itgb1fl/flCD21Cre mice in which the β1-integrin gene is deleted in mature B cells. We find that integrin β1-deficient mice have a defect in the differentiation of MZ B cells and plasma cells. We show that integrin β1-deficient transitional B cells, representing the precursors of MZ B cells, have enhanced B cell receptor (BCR) signaling, altered PI3K and Ras/ERK pathways, and an enhanced interaction of integrin-linked kinase (ILK) with the adaptor protein Grb2. Moreover, the MZ B cell defect of integrin β1-deficient mice could, at least in part, be restored by a pharmacological inhibition of the PI3K pathway. Thus, β1-integrin has an unexpected function in the differentiation and function of MZ B cells.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

Mechanistic insights expatiating the biological role and regulatory implications of estrogen and HER2 in breast cancer metastasis

Breast cancer (BCa) has become the leading cause of death in women worldwide. Irrespective of advancement in cancer treatments, e.g., surgery, radiation, chemotherapy, hormonal therapy, immunotherapy, and targeted therapy, recurrence leading to metastasis poses the greatest threat in BCa management. BCa receptors estrogen (ER), progesterone (PR), and human epidermal growth factor receptor-2 (HER2) hold significant reputations as prognostic and predictive biomarkers in therapeutic decision-making. Under normal physiological conditions, these receptors modulate critical biological functions, e.g., cell migration, proliferation, and apoptosis events, etc. However, aberrant expression causes deviations, triggering signaling course to adapt permanent switching "ON" mode. The later events induce rapid and unrestrained proliferation leading to cancer. As conventional ways of cancer management ultimately lead to resistance; therefore, recently targeted therapies have been extensively studied to conquer resistance. Targeting various small molecules in downstream signaling has become an area of interest in scientific society. The severity of cancer converts many folds soon after it takes on a migratory approach that eventually commences metastasis. Cancer migration comprises protrusion of cytoplasm at the leading edge of the migration forward-facing, establishing adhesions with the basic cell-matrix, disassembly of the adhesions at the back end of the cell, and actin-myosin fiber contractions to pull the bulk of the cytoplasm forward. On the other hand, metastatic progression comprises a cascade of events, including invasion, migration, and establishment of tumor microenvironment. The progression of BCa from early stage to metastatic development causes remarkable heterogeneity. Interference at any explicit level could hamper the process, and it has thus become an area of interest for scientists. Metastasis is the ultimate cause of spreading tumor cells to invade distant organs. Recently small molecule inhibitors of protein tyrosine kinases, which can cross the blood-brain barrier, have become a center point of research for investigators in developing novel treatment strategies against BCa management.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Fibronectin and Its Receptors in Hematopoiesis

Fibronectin is a ubiquitous extracellular matrix protein that is produced by many cell types in the bone marrow and distributed throughout it. Cells of the stem cell niche produce the various isoforms of this protein. Fibronectin not only provides the cells a scaffold to bind to, but it also modulates their behavior by binding to receptors on the adjacent hematopoietic stem cells and stromal cells. These receptors, which include integrins such as α4β1, α9β1, α4β7, α5β1, αvβ3, Toll-like receptor-4 (TLR-4), and CD44, are found on the hematopoietic stem cell. Because the knockout of fibronectin is lethal during embryonal development and because fibronectin is produced by almost all cell types in mammals, the study of its role in hematopoiesis is difficult. Nevertheless, strong and direct evidence exists for its stimulation of myelopoiesis and thrombopoiesis using in vivo models. Other reviewed effects can be deduced from the study of fibronectin receptors, which showed their activation modifies the behavior of hematopoietic stem cells. Erythropoiesis was only stimulated under hemolytic stress, and mostly late stages of lymphocytic differentiation were modulated. Because fibronectin is ubiquitously expressed, these interactions in health and disease need to be taken into account whenever any molecule is evaluated in hematopoiesis.

Cytohesin 1 regulates homing and engraftment of human hematopoietic stem and progenitor cells

Adhesion is a key component of hematopoietic stem cell regulation mediating homing and retention to the niche in the bone marrow. Here, using an RNA interference screen, we identify cytohesin 1 (CYTH1) as a critical mediator of adhesive properties in primary human cord blood-derived hematopoietic stem and progenitor cells (HSPCs). Knockdown of CYTH1 disrupted adhesion of HSPCs to primary human mesenchymal stroma cells. Attachment to fibronectin and ICAM1, 2 integrin ligands, was severely impaired, and CYTH1-deficient cells showed a reduced integrin β1 activation response, suggesting that CYTH1 mediates integrin-dependent functions. Transplantation of CYTH1-knockdown cells to immunodeficient mice resulted in significantly lower long-term engraftment levels, associated with a reduced capacity of the transplanted cells to home to the bone marrow. Intravital microscopy showed that CYTH1 deficiency profoundly affects HSPC mobility and localization within the marrow space and thereby impairs proper lodgment into the niche. Thus, CYTH1 is a novel major regulator of adhesion and engraftment in human HSPCs through mechanisms that, at least in part, involve the activation of integrins.

Role of β1 integrins and bacterial adhesins for Yop injection into leukocytes in Yersinia enterocolitica systemic mouse infection

Injection of Yersinia outer proteins (Yops) into host cells by a type III secretion system is an important immune evasion mechanism of Yersinia enterocolitica (Ye). In this process Ye invasin (Inv) binds directly while Yersinia adhesin A (YadA) binds indirectly via extracellular matrix (ECM) proteins to β1 integrins on host cells. Although leukocytes turned out to be an important target of Yop injection by Ye, it was unclear which Ye adhesins and which leukocyte receptors are required for Yop injection. To explain this, we investigated the role of YadA, Inv and β1 integrins for Yop injection into leukocytes and their impact on the course of systemic Ye infection in mice. Ex vivo infection experiments revealed that adhesion of Ye via Inv or YadA is sufficient to promote Yop injection into leukocytes as revealed by a β-lactamase reporter assay. Serum factors inhibit YadA- but not Inv-mediated Yop injection into B and T cells, shifting YadA-mediated Yop injection in the direction of neutrophils and other myeloid cells. Systemic Ye mouse infection experiments demonstrated that YadA is essential for Ye virulence and Yop injection into leukocytes, while Inv is dispensable for virulence and plays only a transient and minor role for Yop injection in the early phase of infection. Ye infection of mice with β1 integrin-depleted leukocytes demonstrated that β1 integrins are dispensable for YadA-mediated Yop injection into leukocytes, but contribute to Inv-mediated Yop injection. Despite reduced Yop injection into leukocytes, β1 integrin-deficient mice exhibited an increased susceptibility for Ye infection, suggesting an important role of β1 integrins in immune defense against Ye. This study demonstrates that Yop injection into leukocytes by Ye is largely mediated by YadA exploiting, as yet unknown, leukocyte receptors.

Shp1 signalling is required to establish the long-lived bone marrow plasma cell pool

Germline or B-cell-specific loss of Ptpn6 gene encoding the Shp1 protein tyrosine phosphatase leads to skewed B lymphopoiesis and systemic autoimmunity. Here, to study its role in B-cell terminal differentiation, we generated Ptpn6^f/fAicda^Cre/+ mice with Shp1 ablated only in activated B cells. We show that Ptpn6^f/fAicda^Cre/+ mice have normal B-cell development but exhibit defective class-switched primary and recalled antibody response to a T-cell-dependent antigen. Germinal centres are present but do not persist and memory B cells are not formed. Interestingly, Shp1-deficient plasma cells are generated in the spleen but do not contribute to the bone marrow long-lived pool. Plasma cells lacking Shp1 exhibit aberrant α4β1 integrin activation due to dysregulated Src- and PI3-kinase signalling and manifest attenuated migration in vitro and defective bone marrow homing when reconstituted in vivo. Interrupting α4β1–VCAM-1 interaction rectifies this defect. These data suggest that Shp1 signalling is required for the establishment of a life-long protective humoral immunity.

SHP-1 signalling is required for the normal development of B lymphocytes but its role in the terminal differentiation of these cells has not been fully established. Here, the authors show that SHP-1 ablation impairs the establishment of long-lived bone marrow-resident plasma cells due to aberrant integrin activation.

Knockdown and knockout of β1-integrin in hepatocytes impairs liver regeneration through inhibition of growth factor signalling

The liver has a unique regenerative capability, which involves extensive remodelling of cell-cell and cell-matrix contacts. Here we study the role of integrins in mouse liver regeneration using Cre/loxP-mediated gene deletion or intravenous delivery of β1-integrin siRNA formulated into nanoparticles that predominantly target hepatocytes. We show that although short-term loss of β1-integrin has no obvious consequences for normal livers, partial hepatectomy leads to severe liver necrosis and reduced hepatocyte proliferation. Mechanistically, loss of β1-integrin in hepatocytes impairs ligand-induced phosphorylation of the epidermal growth factor and hepatocyte growth factor receptors, thereby attenuating downstream receptor signalling in vitro and in vivo. These results identify a crucial role and novel mechanism of action of β1-integrins in liver regeneration and demonstrate that protein depletion by nanoparticle-based delivery of specific siRNA is a powerful strategy to study gene function in the regenerating liver.

Integrin-mediated interactions between B cells and follicular dendritic cells influence germinal center B cell fitness

Integrin-ligand interactions between germinal center (GC) B cells and Ag-presenting follicular dendritic cells (FDCs) have been suggested to play central roles during GC responses, but their in vivo requirement has not been directly tested. In this study, we show that, whereas integrins αLβ2 and α4β1 are highly expressed and functional on mouse GC B cells, removal of single integrins or their ligands had little effect on B cell participation in the GC response. Combined β2 integrin deficiency and α4 integrin blockade also did not affect the GC response against a particulate Ag. However, the combined integrin deficiency did cause B cells to be outcompeted in splenic GC responses against a soluble protein Ag and in mesenteric lymph node GC responses against gut-derived Ags. Similar findings were made for β2-deficient B cells in mice lacking VCAM1 on FDCs. The reduced fitness of the GC B cells did not appear to be due to decreased Ag acquisition, proliferation rates, or pAKT levels. In summary, our findings provide evidence that αLβ2 and α4β1 play overlapping and context-dependent roles in supporting interactions with FDCs that can augment the fitness of responding GC B cells. We also find that mouse GC B cells upregulate αvβ3 and adhere to vitronectin and milk-fat globule epidermal growth factor VIII protein. Integrin β3-deficient B cells contributed in a slightly exaggerated manner to GC responses, suggesting this integrin has a regulatory function in GC B cells.

Selectins and their ligands are required for homing and engraftment of BCR-ABL1+ leukemic stem cells in the bone marrow niche

We investigated adhesion pathways that contribute to engraftment of breakpoint cluster region-Abelson murine leukemia viral oncogene homolog 1 (BCR-ABL1)-induced chronic myelogenous leukemia (CML)-like myeloproliferative neoplasia in a mouse retroviral transduction/transplantation model. Compared with normal stem/progenitor cells, BCR-ABL1(+) progenitors had similar expression of very late antigen-4 (VLA4), VLA5, leukocyte functional antigen-1, and CXCR4 but lower expression of P-selectin glycoprotein ligand-1 (PSGL-1) and of L-selectin. Whereas vascular cell adhesion molecule-1 and P-selectin were not required, deficiency of E-selectin in the recipient bone marrow endothelium significantly reduced engraftment by BCR-ABL1-expressing stem cells following intravenous injection, with leukemogenesis restored by direct intrafemoral injection. BCR-ABL1-expressing cells deficient for PSGL-1 or the selectin ligand-synthesizing enzymes core-2 β1,6-N-acetylglucosaminyltransferase or fucosyltransferases IV/VII were impaired for engraftment, and destruction of selectin ligands on leukemic progenitors by neuraminidase reduced engraftment. BCR-ABL1-expressing L-selectin-deficient progenitors were also defective in homing and engraftment, with leukemogenesis rescued by coexpression of chimeric E/L-selectin. Antibody to L-selectin decreased the engraftment of BCR-ABL1-transduced stem cells. These results establish that BCR-ABL1(+) leukemic stem cells rely to a greater extent on selectins and their ligands for homing and engraftment than do normal stem cells. Selectin blockade is a novel strategy to exploit differences between normal and leukemic stem cells that may be beneficial in autologous transplantation for CML and perhaps other leukemias.

Tetraspanins CD81 and CD82 facilitate α4β1-mediated adhesion of human erythroblasts to vascular cell adhesion molecule-1

The proliferation and terminal differentiation of erythroid progenitors occurs in human bone marrow within erythroblastic islands, specialised structures consisting of a central macrophage surrounded by developing erythroid cells. Many cell-cell and cell-matrix adhesive interactions maintain and regulate the co-ordinated daily production of reticulocytes. Erythroid cells express only one integrin, α4β1, throughout differentiation, and its interactions with both macrophage Vascular Cell Adhesion Molecule-1 and with extracellular matrix fibronectin are critical for erythropoiesis. We observed that proerythroblasts expressed a broad tetraspanin phenotype, and investigated whether any tetraspanin could modulate integrin function. A specific association between α4β1 and CD81, CD82 and CD151 was demonstrated by confocal microscopy and co-immune precipitation. We observed that antibodies to CD81 and CD82 augmented adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 but not to the fibronectin spliceoforms FnIII_12-IIICS-15 and FnIII_12–15. In contrast, different anti-CD151 antibodies augmented or inhibited adhesion of proerythroblasts to Vascular Cell Adhesion Molecule-1 and the fibronectin spliceoform FnIII_12-IIICS-15 but not to FnIII_12–15. These results strongly suggest that tetraspanins have a functional role in terminal erythropoiesis by modulating interactions of erythroblast α4β1 with both macrophages and extracellular matrix.

Exclusive antagonism of the α4 β7 integrin by vedolizumab confirms the gut-selectivity of this pathway in primates

BACKGROUND

Biological therapies that antagonize specific molecules have demonstrated efficacy in inflammatory bowel diseases, but infections resulting from systemic immunosuppression underscore the need for safer therapies. The objective of this investigation was to determine if antagonism of the α(4) β(7) integrin would exclusively yield gut-selective antiinflammatory activity in primates.

METHODS

A series of experiments were conducted to investigate potential intra- and extraintestinal effects in healthy nonhuman primates dosed repeatedly with the α(4) β(7) -exclusive antagonist vedolizumab (former versions: MLN0002, MLN02, LDP-02) for 4, 13, and 26 weeks.

RESULTS

No adverse clinical effects of vedolizumab were observed in healthy cynomolgus monkeys up to the highest doses tested (100 mg/kg). Histomorphologic analyses indicated a reduction in the frequency of leukocytes in gastrointestinal tissue, but not other organs. A significant (P < 0.05) decrease in the frequency of β 7+ lymphocytes in gastrointestinal tissues corresponded to a significant (P < 0.05) increase in α(4) β 7+ memory helper T lymphocytes in peripheral blood. This elevation was specific to α(4) β 7+ memory helper T lymphocytes; levels of other leukocyte subsets remained unaffected. Systemic opportunistic infections were not observed, and vedolizumab did not inhibit adaptive or innate immune responses systemically.

CONCLUSIONS

These data demonstrate that blocking the α(4) β(7) integrin exclusively yields gut-selective antiinflammatory activity in primates.

Mobilization of hematopoietic stem and progenitor cells using inhibitors of CXCR4 and VLA-4

Successful hematopoietic stem cell transplant requires the infusion of a sufficient number of hematopoietic stem/progenitor cells (HSPCs) that are capable of homing to the bone marrow cavity and regenerating durable trilineage hematopoiesis in a timely manner. Stem cells harvested from peripheral blood are the most commonly used graft source in HSCT. Although granulocyte colony-stimulating factor (G-CSF) is the most frequently used agent for stem cell mobilization, the use of G-CSF alone results in suboptimal stem cell yields in a significant proportion of patients. Both the chemokine receptor CXCR4 and the integrin α(4)β(1) (very late antigen 4 (VLA-4)) have important roles in the homing and retention of HSPCs within the bone marrow microenvironment. Preclinical and/or clinical studies have shown that targeted disruption of the interaction of CXCR4 or VLA-4 with their ligands results in the rapid and reversible mobilization of hematopoietic stem cells into the peripheral circulation and is synergistic when combined with G-CSF. In this review, we discuss the development of small-molecule CXCR4 and VLA-4 inhibitors and how they may improve the utility and convenience of peripheral blood stem cell transplantation.

Epratuzumab targeting of CD22 affects adhesion molecule expression and migration of B-cells in systemic lupus erythematosus

INTRODUCTION

Epratuzumab, a humanized anti-CD22 monoclonal antibody, is under investigation as a therapeutic antibody in non-Hodgkin's lymphoma and systemic lupus erythematosus (SLE), but its mechanism of action on B-cells remains elusive. Treatment of SLE patients with epratuzumab leads to a reduction of circulating CD27(negative) B-cells, although epratuzumab is weakly cytotoxic to B-cells in vitro. Therefore, potential effects of epratuzumab on adhesion molecule expression and the migration of B-cells have been evaluated.

METHODS

Epratuzumab binding specificity and the surface expression of adhesion molecules (CD62L, β7 integrin and β1 integrin) after culture with epratuzumab was studied on B-cell subsets of SLE patients by flow cytometry. In addition, in vitro transwell migration assays were performed to analyze the effects of epratuzumab on migration towards different chemokines such as CXCL12, CXCL13 or to CXCR3 ligands, and to assess the functional consequences of altered adhesion molecule expression.

RESULTS

Epratuzumab binding was considerably higher on B-cells relative to other cell types assessed. No binding of epratuzumab was observed on T-cells, while weak non-specific binding of epratuzumab on monocytes was noted. On B-cells, binding of epratuzumab was particularly enhanced on CD27(negative) B-cells compared to CD27(positive) B-cells, primarily related to a higher expression of CD22 on CD27(negative) B-cells. Moreover, epratuzumab binding led to a decrease in the cell surface expression of CD62L and β7 integrin, while the expression of β1 integrin was enhanced. The effects on the pattern of adhesion molecule expression observed with epratuzumab were principally confined to a fraction of the CD27(negative) B-cell subpopulation and were associated with enhanced spontaneous migration of B-cells. Furthermore, epratuzumab also enhanced the migration of CD27(negative) B-cells towards the chemokine CXCL12.

CONCLUSIONS

The current data suggest that epratuzumab has effects on the expression of the adhesion molecules CD62L, β7 integrin and β1 integrin as well as on migration towards CXCL12, primarily of CD27(negative) B-cells. Therefore, induced changes in migration appear to be part of the mechanism of action of epratuzumab and are consistent with the observation that CD27(negative) B-cells were found to be preferentially reduced in the peripheral blood under treatment.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

Thrombin-cleaved osteopontin regulates hemopoietic stem and progenitor cell functions through interactions with alpha9beta1 and alpha4beta1 integrins

Osteopontin (OPN), a multifunctional acidic glycoprotein, expressed by osteoblasts within the endosteal region of the bone marrow (BM) suppresses the proliferation of hemopoietic stem and progenitor cells and also regulates their lodgment within the BM after transplantation. Herein we demonstrate that OPN cleavage fragments are the most abundant forms of this protein within the BM. Studies aimed to determine how hemopoietic stem cells (HSCs) interact with OPN revealed for the first time that murine and human HSCs express alpha(9)beta(1) integrin. The N-terminal thrombin cleavage fragment of OPN through its binding to the alpha(9)beta(1) and alpha(4)beta(1) integrins plays a key role in the attraction, retention, regulation, and release of hemopoietic stem and progenitor cells to, in, and from their BM niche. Thrombin-cleaved OPN (trOPN) acts as a chemoattractant for stem and progenitor cells, mediating their migration in a manner that involves interaction with alpha(9)beta(1) and alpha(4)beta(1) integrins. In addition, in the absence of OPN, there is an increased number of white blood cells and, specifically, stem and progenitor cells in the peripheral circulation.

CD98hc facilitates B cell proliferation and adaptive humoral immunity

The proliferation of antigen-specific lymphocytes and resulting clonal expansion are essential for adaptive immunity. We report here that B cell-specific deletion of the heavy chain of CD98 (CD98hc) resulted in lower antibody responses due to total suppression of B cell proliferation and subsequent plasma cell formation. Deletion of CD98hc did not impair early B cell activation but did inhibit later activation of the mitogen-activated protein kinase Erk1/2 and downregulation of the cell cycle inhibitor p27. Reconstitution of CD98hc-deficient B cells with CD98hc mutants showed that the integrin-binding domain of CD98hc was required for B cell proliferation but that the amino acid-transport function of CD98hc was dispensable for this. Thus, CD98hc supports integrin-dependent rapid proliferation of B cells. We propose that the advantage of adaptive immunity favored the appearance of CD98hc in vertebrates.

Cannabinoid receptor 2 mediates the retention of immature B cells in bone marrow sinusoids

Immature B cells developing in the bone marrow are found in the parenchyma and sinusoids. The mechanisms that control the positioning of B cells in the sinusoids are not understood. Here we show that the integrin alpha(4)beta(1) (VLA-4) and its ligand VCAM-1 were required, whereas the chemokine receptor CXCR4 was dispensable, for sinusoidal retention of B cells. Instead, cannabinoid receptor 2 (CB2), a Galpha(i) protein-coupled receptor upregulated in immature B cells, was required for sinusoidal retention. Using two-photon microscopy, we found immature B cells entering and crawling in sinusoids; these immature B cells were displaced by CB2 antagonism. Moreover, CB2-deficient mice had a lower frequency of immunoglobulin lambda-chain-positive B cells in the peripheral blood and spleen. Our findings identify unique requirements for the retention of B cells in the bone marrow sinusoidal niche and suggest involvement of CB2 in the generation of the B cell repertoire.

Beta1 integrins differentially control extravasation of inflammatory cell subsets into the CNS during autoimmunity

Inhibiting the alpha(4) subunit of the integrin heterodimers alpha(4)beta(1) and alpha(4)beta(7) with the monoclonal antibody natalizumab is an effective treatment for multiple sclerosis (MS). However, the pharmacological action of natalizumab is not understood conclusively. Previous studies suggested that natalizumab inhibits activation, proliferation, or extravasation of inflammatory cells. To specify which mechanisms, cell types, and alpha(4) heterodimers are affected by the antibody treatment, we studied MS-like experimental autoimmune encephalomyelitis (EAE) in mice lacking the beta(1)-integrin gene either in all hematopoietic cells or selectively in T lymphocytes. Our results show that T cells critically rely on beta(1) integrins to accumulate in the central nervous system (CNS) during EAE, whereas CNS infiltration of beta(1)-deficient myeloid cells remains unaffected, suggesting that T cells are the main target of anti-alpha(4)-antibody blockade. We demonstrate that beta(1)-integrin expression on encephalitogenic T cells is critical for EAE development, and we therefore exclude alpha(4)beta(7) as a target integrin of the antibody treatment. T cells lacking beta(1) integrin are unable to firmly adhere to CNS endothelium in vivo, whereas their priming and expansion remain unaffected. Collectively, these results suggest that the primary action of natalizumab is interference with T cell extravasation via inhibition of alpha(4)beta(1) integrins.

Homing of mouse spermatogonial stem cells to germline niche depends on beta1-integrin

Spermatogonial stem cells (SSCs) provide the foundation for spermatogenesis. In a manner comparable to hematopoietic stem cell transplantation, SSCs colonize the niche of recipient testes and reinitiate spermatogenesis following microinjection into the seminiferous tubules. However, little is known about the homing mechanism of SSCs. Here we examined the role of adhesion molecules in SSC homing. SSCs isolated from mice carrying loxP-tagged beta1-integrin alleles were ablated for beta1-integrin expression by in vitro adenoviral cre transduction. The beta1-integrin mutant SSCs showed significantly reduced ability to recolonize recipient testes in vivo and to attach to laminin molecules in vitro. In contrast, genetic ablation of E-cadherin did not impair homing, and E-cadherin mutant SSCs completed normal spermatogenesis. In addition, the deletion of beta1-integrin on Sertoli cells reduced SSC homing. These results identify beta1-integrin as an essential adhesion receptor for SSC homing and its association with laminin is critical in multiple steps of SSC homing.

Loss of integrin alpha9beta1 results in defects in proliferation, causing poor re-epithelialization during cutaneous wound healing

The wound microenvironment comprises constituents, such as the extracellular matrix (ECM), that regulate with temporal and spatial precision, the migratory, proliferative, and contractility of wound cells. Prompt closure of the wound is an early and critical phase of healing and beta1 integrins are important in this process. We previously reported a marked increase in integrin alpha9beta1 expression in epidermal keratinocytes in cutaneous and corneal wounds. However, the functional role of keratinocyte alpha9beta1 during re-epithelialization is unknown and analysis has been precluded by the lethal phenotype of integrin alpha9beta1 knockout mice. We now report that in conditional integrin alpha9 knockout (K14-alpha9 null) mice, normal proliferation occurs in epidermal keratinocytes and corneal basal cells. Normal epidermal keratinocyte morphology is also retained. However, corneal basal cell morphology and epithelial thickness are altered, suggesting that loss of integrin alpha9beta1 results in abnormal corneal differentiation. In cutaneous wounds, the number of proliferating epidermal keratinocytes is significantly reduced in K14-alpha9 null mice compared with alpha9(fl/-) mice, but not in Cre (control) mice. The decreased keratinocyte proliferation observed in K14-alpha9 null mice negatively impacts healing, resulting in a thinner migrating epithelium, demonstrating that alpha9beta1 is crucial for efficient and proper re-epithelialization during cutaneous wound healing.

Myocardial ischaemia-reperfusion injury in haematopoietic cell-restricted beta1 integrin knockout mice

Evidence indicates that the intercellular adhesion molecule-1 and its counter-receptor beta2 integrin are cardioprotective proteins during myocardial ischaemia-reperfusion, but no data are available concerning the role of blood cell beta1 integrins in this process. We studied the effects of temporary myocardial ischaemia and reperfusion in blood cell-restricted beta1 integrin knockout mice (beta1(-/-)). The left descending coronary artery in conditional beta1(-/-) integrin (beta1(-/-)), beta1 integrin +/+ (beta1(+/+)) and beta1 integrin -/- bone marrow chimeric (beta1(-/-) BM) mice was ligated for 30 min, followed by reperfusion of either 3 h or 3 weeks. Plasma levels of troponin T were evaluated as an index of cardiac cellular damage. The histological evaluation of tissue damage was performed with Haematoxylin and Eosin stained sections. Cell infiltrations in the ischaemic area were investigated by immunofluorescence studies. It was found that plasma troponin T was at a similar level in beta1(-/-), beta1(+/+) and beta1(-/-) BM mice treated with 30 min ischaemia and 3 h reperfusion. Histological analysis showed that ischaemia-reperfusion resulted in marked myocardial injury in all groups of animals, but the damage score of the hearts was not significantly different between beta1(-/-), beta1(+/+) and beta1(-/-) BM mice after 3 h of reperfusion following 30 min of ischaemia (2.8 +/- 0.5, 2.6 +/- 0.5 and 2.8 +/- 0.6, respectively, n.s.). Furthermore, no difference in scar sizes in ischaemia-injured hearts was found 3 weeks after ischaemia. Semi-quantification of cells demonstrated that, compared with beta1(+/+) mice, the number of infiltrating neutrophils was significantly reduced in beta1(-/-) and beta1(-/-) BM mice, whereas MAC-1(CD11b/CD18)-positive cells in the ischaemic regions were similar in myocardial tissues of all groups. We conclude that absence of beta1 integrin expression in haematopoietic cells results in reduced neutrophil infiltration in the ischaemic regions, but does not influence myocardial damage of ischaemic hearts.

Ulcerative colitis and autoimmunity induced by loss of myeloid alphav integrins

The gastrointestinal tract is constantly challenged by foreign antigens and commensal bacteria but nonetheless is able to maintain a state of immunological quiescence. Recent advances have highlighted the importance of active suppression by regulatory lymphocytes and immunosuppressive cytokines in controlling mucosal immunity. Failures of these mechanisms contribute to the development of inflammatory bowel disease, but how these regulatory networks are established remains unclear. Here, we demonstrate key roles for alphav integrins in regulation of mucosal immunity. We report that deletion of alphav in the immune system causes severe colitis, autoimmunity, and cancer. Mice lacking immune cell alphav have fewer regulatory T (Treg) cells in the colon and corresponding increases in activated T cells and T cell cytokine production, leading to colitis. Using conditional gene targeting, we demonstrate that this is specifically attributable to loss of alphav from myeloid cells. Furthermore, we show that gut-associated macrophages and dendritic cells fail both to remove apoptotic cells efficiently and to induce Treg cells. Our results identify a vital role for myeloid alphav integrins in generating mucosal Treg cells and emphasize the importance of antigen-presenting cells in establishing immune tolerance.

VCAM-1 ablation in nonhematopoietic cells in MxCre+ VCAM-1f/f mice is variable and dictates their phenotype

OBJECTIVE

The goal of the present study was to assess the extent of vascular cell adhesion molecule-1 (VCAM-1) gene deletion in hematopoietic vs nonhematopoietic cells in the bone marrow (BM) of MxCre(+)VCAM-1(f/f) mice and its impact on the phenotypic features of these mice.

METHODS

VCAM-1 ablation was evaluated at the genomic level by polymerase chain reaction (PCR), at the mRNA level by real-time PCR, and at the protein level by fluorescein-activated cell sorting and immunohistochemistry. The homing or mobilization of colony-forming unit cultures was assessed by standard assays.

RESULTS

A previously accepted interferon-induction scheme yielded efficient VCAM-1 ablation in hematopoietic cells but variable ablation in BM fibroblasts and endothelial cells. The level of ablation in the latter populations correlated with alterations in the hematopoietic phenotype.

CONCLUSIONS

Poly(I:C)-induced MxCre-mediated gene ablation is highly efficient in hematopoietic cells but variable and partial in nonhematopoietic cells in BM. Ablation of VCAM-1 in hematopoietic cells does not contribute to their mobilization, nor does it impair their homing. The latter is dependent on VCAM-1 ablation in nonhematopoietic cells of BM.

Natural history and early diagnosis of LAD-1/variant syndrome

The syndrome of leukocyte adhesion deficiency (LAD) combined with a severe Glanzmann-type bleeding disorder has been recognized as a separate disease entity. The variability in clinical and cell biological terms has remained largely unclear. We present data on 9 cases from 7 unrelated families, with 3 patients being actively followed for more than 12 years. The disease entity, designated LAD-1/variant syndrome, presents early in life and consists of nonpussing infections from bacterial and fungal origin, as well as a severe bleeding tendency. This is compatible with 2 major blood cell types contributing to the clinical symptoms (ie, granulocytes and platelets). In granulocytes of the patients, we found adhesion and chemotaxis defects, as well as a defect in NADPH oxidase activity triggered by unopsonized zymosan. This last test can be used as a screening test for the syndrome. Many proteins and genes involved in adhesion and signaling, including small GTPases such as Rap1 and Rap2 as well as the major Rap activity-regulating molecules, were normally present. Moreover, Rap1 activation was intact in patients' blood cells. Defining the primary defect awaits genetic linkage analysis, which may be greatly helped by a more precise understanding and awareness of the disease combined with the early identification of affected patients.

Bruton's tyrosine kinase and phospholipase Cgamma2 mediate chemokine-controlled B cell migration and homing

Control of integrin-mediated adhesion and migration by chemokines plays a critical role in B cell development, differentiation, and function; however, the underlying signaling mechanisms are poorly defined. Here we show that the chemokine SDF-1 induced activation of Bruton's tyrosine kinase (Btk) and that integrin-mediated adhesion and migration in response to SDF-1 or CXCL13, as well as in vivo homing to lymphoid organs, was impaired in Btk-deficient (pre-)B cells. Furthermore, SDF-1 induced tyrosine phosphorylation of Phospholipase Cgamma2 (PLCgamma2), which, unlike activation of the migration regulatory GTPases Rac or Rap1, was mediated by Btk. PLCgamma2-deficient B cells also exhibited impaired SDF-1-controlled migration. These results reveal that Btk and PLCgamma2 mediate chemokine-controlled migration, thereby providing insights into the control of B cell homeostasis, trafficking, and function, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulinemia (XLA).

Human mesenchymal stromal cells regulate initial self-renewing divisions of hematopoietic progenitor cells by a beta1-integrin-dependent mechanism

In previous reports, we have demonstrated that only direct cell-cell contact with stromal cells, such as the murine stromal cell line AFT024, was able to alter the cell division kinetics and self-renewing capacity of hematopoietic progenitor cells (HPC). Because beta(1)-integrins were shown to be crucial for the interaction of HPC with the bone marrow microenvironment, we have studied the role of beta(1)-integrins in the regulation of self-renewing cell divisions. For this purpose, we used primary human mesenchymal stromal (MS) cells as in vitro surrogate niche and monitored the division history and subsequent functional fate of individually plated CD34(+)133(+) cells in the absence or presence of an anti-beta(1)-integrin blocking antibody by time-lapse microscopy and subsequent long-term culture-initiating cell (LTC-IC) assays. beta(1)-Integrin-mediated contact with MS cells significantly increased the proportion of asymmetrically dividing cells and led to a substantial increase of LTC-IC. Provided that beta(1)-integrin-mediated contact was available within the first 72 hours, human MS cells were able to recruit HPC into cell cycle and accelerate their division kinetics without loss of stem cell function. Activation of beta(1)-integrins by ligands alone (e.g., fibronectin and vascular cell adhesion molecule-1) was not sufficient to alter the cell division symmetry and promote self-renewal of HPC, thus indicating an indirect effect. These results have provided evidence that primary human MS cells are able to induce self-renewing divisions of HPC by a beta(1)-integrin-dependent mechanism.

Adult murine hematopoiesis can proceed without β1 and β7 integrins

The function of α4β1 and α4β7 integrins in hematopoiesis is controversial. While some experimental evidence suggests a crucial role for these integrins in retention and expansion of progenitor cells and lymphopoiesis, others report a less important role in hematopoiesis. Using mice with a deletion of the β1 and the β7 integrin genes restricted to the hematopoietic system we show here that α4β1 and α4β7 integrins are not essential for differentiation of lymphocytes or myelocytes. However, β1β7 mutant mice displayed a transient increase of colony-forming unit (CFU-C) progenitors in the bone marrow and, after phenylhydrazine-induced anemia, a decreased number of splenic erythroid colony-forming units in culture (CFUe's). Array gene expression analysis of CD4+CD8+ double-positive (DP) and CD4–CD8– double-negative (DN) thymocytes and CD19+ and CD4+ splenocytes did not provide any evidence for a compensatory mechanism explaining the mild phenotype. These data show that α4β1 and α4β7 are not required for blood cell differentiation, although in their absence alterations in numbers and distribution of progenitor cells were observed.

[The PAI-1 swing: microenvironment and cancer cell migration]

Cancer is a complex and dynamic process caused by a cellular dysfunction leading to a whole organ or even organism vital perturbation. To better understand this process, we need to study each one of the levels involved, which allows the scale change, and to integrate this knowledge. A matricellular protein, PAI-1, is able to induce in vitro cell behaviour modifications, morphological changes, and to promote cell migration. PAI-1 influences the mesenchymo-amaeboid transition. This matricellular protein should be considered as a potential 'launcher' of the metastatic process acting at the molecular, cellular, tissular levels and, as a consequence, at the organism's level.

Hematopoietic stem cells and their niche

Recent evidence indicates that osteoblasts are crucial components of the particular microenvironments, or niches, for hematopoietic stem cells (HSCs) in adult bone marrow (BM). Stem cells persist in an immature state within the BM. The quiescence of HSCs is controlled dynamically by the signaling of receptors-ligands and cell-adhesion molecules. In this review, the characteristics of HSCs in the niche are discussed. The understanding of the relationship between normal and cancer stem cells and their niches should lead to the development of new strategies directed toward regeneration medicine and cancer therapeutics.

Sustained alterations in biodistribution of stem/progenitor cells in Tie2Cre+ alpha4(f/f) mice are hematopoietic cell autonomous

We have generated Tie2Cre+alpha4(f/f) mice with documented alpha4-integrin ablation in hematopoietic and endothelial cells. A prominent feature in this model is a sustained, significant increase in circulating progenitors at levels higher than the levels seen with Tie2Cre+VCAM-1(f/f) mice. To test whether phenotypic differences are due to contributions by ligands other than VCAM-1 in bone marrow, or to alpha4-deficient endothelial cells or pericytes, we carried out transplantation experiments using these mice as donors or as recipients. Changes in progenitor biodistribution after transplantation were seen only with alpha4-deficient donor cells, suggesting that these cells were necessary and sufficient to reproduce the phenotype with no discernible contribution by alpha4-deficient nonhematopoietic cells. Because several similarities are seen after transplantation between our results and those with CXCR4-/- donor cells, the data suggest that VLA4/VCAM-1 and CXCR4/CXCL12 pathways contribute to a nonredundant, ongoing signaling required for bone marrow retention of progenitor cells during homeostasis.

Beta1 integrins: zip codes and signaling relay for blood cells

At least eight of the twelve known members of the beta1 integrin family are expressed on hematopoietic cells. Among these, the VCAM-1 receptor alpha4beta1 has received most attention as a main factor mediating firm adhesion to the endothelium during blood cell extravasation. Therapeutic trials are ongoing into the use of antibodies and small molecule inhibitors to target this interaction and hence obtain anti-inflammatory effects. However, extravasation is only one possible process that is mediated by beta1 integrins and there is evidence that they also mediate leukocyte retention and positioning in the tissue, lymphocyte activation and possibly migration within the interstitium. Genetic mouse models where integrins are selectively deleted on blood cells have been used to investigate these functions and further studies will be invaluable to critically evaluate therapeutic trials.

A multistep adhesion cascade for lymphoid progenitor cell homing to the thymus

Homing of bone marrow (BM)-derived progenitors to the thymus is essential for T cell development. We have previously reported that two subpopulations of common lymphoid progenitors, CLP-1 and CLP-2, coexist in the BM and give rise to lymphocytes. We demonstrate that CLP-2 migrate to the thymus more efficiently than any other BM-derived progenitors. Short-term adoptive transfer experiments revealed that CLP-2 homing involves P-selectin/P-selectin glycoprotein ligand 1 interactions, pertussis toxin-sensitive chemoattractant signaling by CC chemokine ligand 25 through CC chemokine receptor 9, and binding of the integrins alpha4beta1 and alphaLbeta2 to their respective ligands, vascular cell adhesion molecule 1 and intercellular adhesion molecule 1. Preferential thymus-tropism of CLP-2 correlated with higher chemokine receptor 9 expression than on other BM progenitors. Thus, CLP access to the thymus is controlled by a tissue-specific and subset-selective multistep adhesion cascade.

gamma-Parvin is dispensable for hematopoiesis, leukocyte trafficking, and T-cell-dependent antibody response

Integrins regulate cell behavior through the assembly of multiprotein complexes at the site of cell adhesion. Parvins are components of such a multiprotein complex. They consist of three members (alpha-, beta-, and gamma-parvin), form a functional complex with integrin-linked kinase (ILK) and PINCH, and link integrins to the actin cytoskeleton. Whereas alpha- and beta-parvins are widely expressed, gamma-parvin has been reported to be expressed in hematopoietic organs. In the present study, we report the expression pattern of the parvins in hematopoietic cells and the phenotypic analysis of gamma-parvin-deficient mice. Whereas alpha-parvin is not expressed in hematopoietic cells, beta-parvin is only found in myeloid cells and gamma-parvin is present in both cells of the myeloid and lymphoid lineages, where it binds ILK. Surprisingly, loss of gamma-parvin expression had no effect on blood cell differentiation, proliferation, and survival and no consequence for the T-cell-dependent antibody response and lymphocyte and dendritic cell migration. These data indicate that despite the high expression of gamma-parvin in hematopoietic cells it must play a more subtle role for blood cell homeostasis.

Zinc-finger transcription factor Gfi-1: versatile regulator of lymphocytes, neutrophils and hematopoietic stem cells

PURPOSE OF REVIEW

Gfi-1, originally identified as a transcriptional repressor in lymphoid cells, is now recognized to have essential, independent functions in neutrophil maturation and hematopoietic stem cell biology. Here we review recent studies pertaining to Gfi-1 and its cell context specific functions in hematopoietic development.

RECENT FINDINGS

Progress in mapping the precise timing of requirements for myeloid transcription factors during hematopoietic development reveals a more refined picture of their sequence of action. We contrast Gfi-1's role in neutrophil development with PU.1, C/EBPalpha, and C/EBPepsilon. Gfi-1 has been found to be a major regulator of adult hematopoietic stem cells. It is required for restricting the proliferation of hematopoietic stem cells and maintaining their functional integrity. We discuss its role and compare its function with that of other regulators recently implicated in the biology of hematopoietic stem cells.

SUMMARY

Considerable progress has been made in understanding Gfi-1's context-sensitive roles at defined stages of hematopoietic differentiation.

Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis

T-cell development in the thymus requires periodic importation of hematopoietic progenitors from the bone marrow. Such thymus settling progenitors arise from hematopoietic stem cells (HSCs) that are retained in a specific bone marrow microenvironmental niche. Vacation of this niche is required for HSC proliferation and differentiation into downstream progenitors. In order to reach the thymus, progenitors must then be mobilized from bone marrow to blood. Finally, progenitors in blood must settle in the thymus. Here we review signals and molecular interactions that are likely to play a role in trafficking from the bone marrow to the thymus, focusing on how these interactions may regulate which progenitors physiologically contribute to thymopoiesis.

Lsc regulates marginal-zone B cell migration and adhesion and is required for the IgM T-dependent antibody response

The humoral immune response to protein antigens is composed of a rapid low-affinity IgM antibody response followed by an IgG response exhibiting higher affinity. Here, we demonstrate that Lsc, a protein that regulates G protein-coupled-receptor signaling and RhoA activation, is required by B lymphocytes for the antigen-specific IgM antibody response to a protein antigen. We further show that in lsc(-/-) mice, MZB cells are selectively affected such that naive and in vivo-activated MZB cells migrate toward sphingosine-1-phosphate at increased proportions but release inefficiently from integrin ligands. Consequently, lsc(-/-) MZB cells do not traffick appropriately in an immune response and do not contribute to the TD antibody response. These data demonstrate that Lsc regulates the migration and adhesion of MZB cells, and this regulation appears to be required for these cells to contribute to the antibody response to TD antigens.

The role of Smad signaling in hematopoiesis

The TGF-beta family of ligands, including TGF-beta, bone morphogenetic protein (BMP) and activin, signal through Smad pathways to regulate the fate of hematopoietic progenitor and stem cells during development and postnatally. BMP regulates hematopoietic stem cell (HSC) specification during development, while TGF-beta1, 2 and 3 are not essential for the generation of HSCs. BMP4 can increase proliferation of human hematopoietic progenitors, while TGF-beta acts as a negative regulator of hematopoietic progenitor and stem cells in vitro. In contrast, TGF-beta signaling deficiency in vivo does not affect proliferation of HSCs and does not affect lineage choice either. Therefore, the outcome of Smad signaling is very context dependent in hematopoiesis and regulation of hematopoietic stem and progenitor cells is more complicated in the bone marrow microenvironment in vivo than is seen in liquid cultures ex vivo. Smad signaling regulates hematopoiesis by crosstalk with other regulatory signals and future research will define in more detail how the various pathways interact and how the knowledge obtained can be used to develop advanced cell therapies.

β1 integrins are not required for the maintenance of lymphocytes within intestinal epithelia

beta(1) integrins are thought to play a central role in maintaining lymphocytes within mucosal epithelia via their interactions with extracellular matrix proteins and subepithelial cellular components within and underlying the basement membrane. In the current study type a (CD8alphabetaTCRalphabeta) and type b (CD8alphaalphaTCRgammadelta and CD8alphaalphaTCRalphabeta) intraepithelial lymphocyte (IEL) subsets within the mouse small intestine were found to express functional beta(1) integrin and the beta(1) integrin alpha chain partners alpha(1), alpha(2), and alpha(4). Using inducible beta(1) integrin-knockout bone marrow-chimeric mice we demonstrate that IEL expression of alpha(1) and alpha(2) but not alpha(4) is dependent on expression of the beta(1) chain. Importantly, deletion of the beta(1) chain in IEL did not alter the number or composition of lymphocytes within the intestinal epithelium. Thus, while IEL express functional beta(1) integrins, these are not required to maintain lymphocytes within intestinal epithelia. This result is discussed in the light of conventional views of intestinal lymphocyte homing and localization.

Fc gammaRIIIB stimulation promotes beta1 integrin activation in human neutrophils

The molecular stimuli involved in receptor-induced integrin activation are still poorly defined. We have investigated the role of receptors for the Fc portion of immunoglobulin G molecules (Fc gammaR) on activation of integrins in human neutrophils. Cross-linking of Fc gammaRIIA induced an increase in surface expression of beta2 integrins but had no effect on beta1 integrins. In contrast, cross-linking of Fc gammaRIIIB not only increased beta2 integrins on the cell surface but also induced beta1 integrin activation, as indicated by an increase in binding to fibronectin and the appearance of an activation epitope detected by the monoclonal antibody 15/7. The Fc gammaRIIIB-induced increase of beta2 integrins required Src-family tyrosine kinases, Syk kinase, and phosphatidylinositol-3 kinase (PI-3K), as the corresponding, specific inhibitors, PP2, Piceatannol, and LY294002, completely blocked it. Contrary to this, Fc gammaRIIIB-induced beta1 integrin activation was not blocked by PP2 or LY294002. It was, however, enhanced by Piceatannol. After Fc gammaRIIIB cross-linking, colocalization of Fc gammaRIIIB and active beta1 integrins was detected on the neutrophil membrane. These data show, for the first time, that cross-linking of Fc gammaRIIIB induces an inside-out signaling pathway that leads to beta1 integrin activation. This activation is independent of Src-family kinases, and PI-3K and may be induced in part by the interaction of Fc gammaRIIIB with beta1 integrins.

Prespecification and plasticity: shifting mechanisms of cell migration

Cell migration is a universal process involving different morphologies and mechanisms in different cell types and tissue environments. Prespecified cell-type-specific patterns of cell migration can be classified into single cell migration (amoeboid, mesenchymal) and collective migration modes (cell sheets, strands, tubes, clusters). These intrinsic molecular programs are associated with a characteristic structure of the actin cytoskeleton, as well as the cell-type-specific use of integrins, matrix-degrading enzymes (matrix metalloproteinases and serine proteases), cell-cell adhesion molecules (cadherins and activated leukocyte adhesion molecule), and signaling towards the cytoskeleton (carried out by RHO GTPases). In response to the gain or loss of these key molecular determinants, significant adaptation reactions can modify the cell's shape, pattern, and migration mechanism; examples of this include the epithelial-mesenchymal transition, mesenchymal-amoeboid transition and collective-amoeboid transition.

Tel/Etv6 is an essential and selective regulator of adult hematopoietic stem cell survival

Hematopoietic stem cells (HSCs) sustain blood formation throughout life. Pathways regulating maintenance of adult HSCs are largely unknown. Here we report that the Ets-related transcription factor Tel/Etv6, the product of a locus frequently involved in translocations in leukemia, is a selective regulator of HSC survival. Following inactivation of Tel/Etv6, HSCs are lost in the adult bone marrow but their progeny are unaffected and transiently sustain blood formation. Accordingly, absence of Tel/Etv6 after lineage commitment is ostensibly without consequence except for unexpected impairment of maturation of megakaryocytes. Thus, we establish Tel/Etv6 as a selective and essential regulator of postembryonic HSCs.

Integrin engagement-induced inhibition of human myelopoiesis is mediated by proline-rich tyrosine kinase 2 gene products

OBJECTIVE

Hematopoietic progenitor proliferation and differentiation are inhibited by integrin engagement of fibronectin (FN). Focal adhesion kinases have been shown to mediate intracellular signaling from integrins, and we recently demonstrated that gene expression and pre-mRNA splicing of the focal adhesion kinase, PYK2, is abnormal in CD34(+) cells from chronic myelogenous leukemia (CML) patients. Here we investigated whether PYK2 gene products mediate integrin signaling in hematopoietic stem and progenitor cells.

METHODS

Cord blood CD34(+) cells were retrovirally transduced with vectors encoding Pyk2H, Pyk2, or the dominant negative-acting, kinase-deficient, C-terminal PYK2 fragment, PRNK, and myeloid proliferation and differentiation was assessed using colony-forming cell (CFC), long-term culture-initiating cell (LTC-IC), and liquid culture assays.

RESULTS

CD34(+) cells overexpressing Pyk2H or Pyk2 generated 50% less colony-forming unit granulocyte/macrophage (CFU-GM) than eGFP-transduced controls. Although the number of CFC generated by PRNK-expressing cells was unchanged, LTC-IC were significantly reduced. Culture of CD34(+) cells on FN significantly reduced the generation of mature myeloid cells vs those cultured on BSA-coated wells, and could be overcome by addition of SCF. As is observed when integrins are engaged, overexpression of either Pyk2H or Pyk2 decreased committed myeloid progenitor proliferation and differentiation; however, SCF could not override this inhibition. Finally, as is observed when integrins are not engaged, PRNK-mediated inhibition of endogenous Pyk2H resulted in integrin-nonresponsive proliferation and differentiation of myeloid precursors and accelerated differentiation of primitive hematopoietic progenitors.

CONCLUSION

These studies indicate that PYK2 gene products mediate integrin-induced signals that regulate myelopoiesis.

Deletion of alpha4 integrins from adult hematopoietic cells reveals roles in homeostasis, regeneration, and homing

We have explored the functional implications of inducible alpha4 integrin deletion during adult hematopoiesis by generating a conditional-knockout mouse model, and we show that alpha4 integrin-deficient hematopoietic progenitor cells accumulate in the peripheral blood soon after interferon-induced gene deletion. Although their numbers gradually stabilize at a lower level, progenitor cell influx into the circulation continues at above-normal levels for more than 50 weeks. Concomitantly, a progressive accumulation of progenitors occurs within the spleen. In addition, the regeneration of erythroid and myeloid progenitor cells is delayed during stress hematopoiesis induced by phenylhydrazine or by 5-fluorouracil, suggesting impairment in early progenitor expansion in the absence of alpha4 integrin. Moreover, in transplantation studies, homing of alpha4(-/-) cells to the bone marrow, but not to the spleen, is selectively impaired, and short-term engraftment is critically delayed in the early weeks after transplantation. Thus, conditional deletion of alpha4 integrin in adult mice is accompanied by a novel hematopoietic phenotype during both homeostasis and recovery from stress, a phenotype that is distinct from the ones previously described in alpha4 integrin-null chimeras and beta1 integrin-conditional knockouts.

The B cell antigen receptor controls integrin activity through Btk and PLCgamma2

Integrin-mediated adhesion and B cell antigen receptor (BCR) signaling play a critical role in B cell development and function, including antigen-specific B cell differentiation. Here we show that the BCR controls integrin α4β1 (VLA-4)-mediated adhesion of B cells to vascular cell adhesion molecule-1 and fibronectin. Molecular dissection of the underlying signaling mechanism by a combined biochemical, pharmacological, and genetic approach demonstrates that this BCR-controlled integrin-mediated adhesion requires the (consecutive) activation of Lyn, Syk, phosphatidylinositol 3-kinase, Bruton's tyrosine kinase (Btk), phospholipase C (PLC)γ2, IP3R-mediated Ca²⁺ release, and PKC. In contrast, activation of mitogen-activated protein kinase kinase (MEK) or extracellular signal–regulated kinase (ERK) is not required, and simultaneous activation of MEK, ERK, and PKB is not sufficient either. Furthermore, Btk is also involved in the control of integrin-mediated adhesion of preB cells. The control of integrin α4β1-mediated B cell adhesion by the BCR involves cytoskeletal reorganization and integrin clustering. These results reveal a novel function for the BCR and Btk, i.e., regulation of integrin α4β1 activity, thereby providing new insights into the control of B cell development and differentiation, as well as into the pathogenesis of the immunodeficiency disease X-linked agammaglobulineamia (XLA).

Amoeboid shape change and contact guidance: T-lymphocyte crawling through fibrillar collagen is independent of matrix remodeling by MMPs and other proteases

The passage of leukocytes through basement membranes involves proteolytic degradation of extracellular matrix (ECM) components executed by focalized proteolysis. We have investigated whether the migration of leukocytes through 3-dimensional collagenous tissue scaffolds requires similar ECM breakdown. Human T blasts and SupT1 lymphoma cells expressed mRNA of MMP-9, MT1-MMP, MT4-MMP, cathepsin L, uPA, and uPAR as well as ADAM-9, -10, -11, -15, and -17. Upon long-term migration within 3-dimensional collagen matrices, however, no in situ collagenolysis was obtained by sensitive fluorescein isothiocyanate-collagen fragmentation analysis and confocal fluorescence/backscatter microscopy. Consistent with nonproteolytic migration, T-cell crawling and path generation were not impaired by protease inhibitor cocktail targeting MMPs, serine proteases, cysteine proteases, and cathepsins. Dynamic imaging of cell-ECM interactions showed T-cell migration as an amoeba-like process driven by adaptive morphology, crawling along collagen fibrils (contact guidance) and squeezing through pre-existing matrix gaps by vigorous shape change. The concept of nonproteolytic amoeboid migration was confirmed for multicomponent collagen lattices containing hyaluronan and chondroitin sulfate and for other migrating leukocytes including CD8+ T blasts, monocyte-derived dendritic cells, and U937 monocytic cells. Together, amoeboid shape change and contact guidance provide constitutive protease-independent mechanisms for leukocyte trafficking through interstitial tissues that are insensitive toward pharmacologic protease inhibitors.

TGF-beta signaling-deficient hematopoietic stem cells have normal self-renewal and regenerative ability in vivo despite increased proliferative capacity in vitro

Studies in vitro implicate transforming growth factor beta (TGF-beta) as a key regulator of hematopoiesis with potent inhibitory effects on progenitor and stem cell proliferation. In vivo studies have been hampered by early lethality of knock-out mice for TGF-beta isoforms and the receptors. To directly assess the role of TGF-beta signaling for hematopoiesis and hematopoietic stem cell (HSC) function in vivo, we generated a conditional knock-out model in which a disruption of the TGF-beta type I receptor (T beta RI) gene was induced in adult mice. HSCs from induced mice showed increased proliferation recruitment when cultured as single cells under low stimulatory conditions in vitro, consistent with an inhibitory role of TGF-beta in HSC proliferation. However, induced T beta RI null mice show normal in vivo hematopoiesis with normal numbers and differentiation ability of hematopoietic progenitor cells. Furthermore HSCs from T beta RI null mice exhibit a normal cell cycle distribution and do not differ in their ability long term to repopulate primary and secondary recipient mice following bone marrow transplantation. These findings challenge the classical view that TGF-beta is an essential negative regulator of hematopoietic stem cells under physiologic conditions in vivo.