Alpha4 integrins regulate the proliferation/differentiation balance of multilineage hematopoietic progenitors in vivo

Heme oxygenase-1 deletion affects stress erythropoiesis

Background

Homeostatic erythropoiesis leads to the formation of mature red blood cells under non-stress conditions, and the production of new erythrocytes occurs as the need arises. In response to environmental stimuli, such as bone marrow transplantation, myelosuppression, or anemia, erythroid progenitors proliferate rapidly in a process referred to as stress erythropoiesis. We have previously demonstrated that heme oxygenase-1 (HO-1) deficiency leads to disrupted stress hematopoiesis. Here, we describe the specific effects of HO-1 deficiency on stress erythropoiesis.

Methodology/Principal Findings

We used a transplant model to induce stress conditions. In irradiated recipients that received hmox^+/− or hmox^+/+ bone marrow cells, we evaluated (i) the erythrocyte parameters in the peripheral blood; (ii) the staining intensity of CD71-, Ter119-, and CD49d-specific surface markers during erythroblast differentiation; (iii) the patterns of histological iron staining; and (iv) the number of Mac-1⁺-cells expressing TNF-α. In the spleens of mice that received hmox^+/− cells, we show (i) decreases in the proerythroblast, basophilic, and polychromatophilic erythroblast populations; (ii) increases in the insoluble iron levels and decreases in the soluble iron levels; (iii) increased numbers of Mac-1⁺-cells expressing TNF-α; and (iv) decreased levels of CD49d expression in the basophilic and polychromatophilic erythroblast populations.

Conclusions/Significance

As reflected by effects on secreted and cell surface proteins, HO-1 deletion likely affects stress erythropoiesis through the retention of erythroblasts in the erythroblastic islands of the spleen. Thus, HO-1 may serve as a therapeutic target for controlling erythropoiesis, and the dysregulation of HO-1 may be a predisposing condition for hematologic diseases.

Embryonic origin of the adult hematopoietic system: advances and questions

Definitive hematopoietic stem cells (HSCs) lie at the foundation of the adult hematopoietic system and provide an organism throughout its life with all blood cell types. Several tissues demonstrate hematopoietic activity at early stages of embryonic development, but which tissue is the primary source of these important cells and what are the early embryonic ancestors of definitive HSCs? Here, we review recent advances in the field of HSC research that have shed light on such questions, while setting them into a historical context, and discuss key issues currently circulating in this field.

Combinatorial and distinct roles of α₅ and α₄ integrins in stress erythropoiesis in mice

To delineate the role of specific members of β₁ integrins in stress erythropoiesis in the adult, we compared the response to phenylhydrazine stress in 3 genetically deficient models. The survival of β₁-conditionally deficient mice after phenylhydrazine is severely compromised because of their inability to mount a successful life saving splenic erythroid response, a phenotype reproduced in β₁(Δ/Δ) reconstituted animals. The response of bone marrow to phenylhydrazine-induced stress was, unlike that of spleen, appropriate in terms of progenitor cell expansion and mobilization to peripheral blood although late differentiation defects qualitatively similar to those in spleen were present in bone marrow. In contrast to β₁-deficient mice, α₄(Δ/Δ) mice showed only a kinetic delay in recovery and similar to β₁(Δ/Δ), terminal maturation defects in both bone marrow and spleen, which were not present in VCAM-1(Δ/Δ) mice. Convergence of information from these comparative studies lends new insight to the distinct in vivo roles of α₄ and α₅ integrins in erythroid stress, suggesting that the presence of mainly α₅β₁ integrin in all hematopoietic progenitor cells interacting with splenic microenvironmental ligands/cells is instrumental for their survival and accumulation during hemolytic stress, whereas presence of α₄ or of both α₅ and α₄, is important for completion of terminal maturation steps.

Physicochemical control of adult stem cell differentiation: shedding light on potential molecular mechanisms

Realization of the exciting potential for stem-cell-based biomedical and therapeutic applications, including tissue engineering, requires an understanding of the cell-cell and cell-environment interactions. To this end, recent efforts have been focused on the manipulation of adult stem cell differentiation using inductive soluble factors, designing suitable mechanical environments, and applying noninvasive physical forces. Although each of these different approaches has been successfully applied to regulate stem cell differentiation, it would be of great interest and importance to integrate and optimally combine a few or all of the physicochemical differentiation cues to induce synergistic stem cell differentiation. Furthermore, elucidation of molecular mechanisms that mediate the effects of multiple differentiation cues will enable the researcher to better manipulate stem cell behavior and response.

Essential roles of VLA-4 in the hematopoietic system

Integrins are one of the major families of adhesion molecules and make various kinds of biological effects by mediating cell-cell and cell-matrix interactions. Among integrins, VLA-4 is expressed on many types of hematopoietic cells including stem/progenitor cells and it is considered as a critical regulator of adult hematopoiesis. Recent studies revealed that VLA-4 is not necessarily required for the development or maintenance of adult hematopoietic cells. On the other hand, it was proved that VLA-4 is essential for homeostasis of distribution of hematopoietic stem/progenitor cells (HSPCs) and mature lymphocytes in the body. The dynamic regulation of VLA-4 function is mediated by its conformational change, which is strictly linked to the interaction between alpha and beta cytoplasmic domains. The study using knockin mice showed that GFFKR sequence, a well-preserved motif in the alpha cytoplasmic domain of VLA-4, is critical for binding of alpha and beta cytoplasmic domains as well as regulation of hematopoietic cell distribution. Small molecules targeting this cytoplasmic interaction or ligand-VLA-4 interaction may become good candidates of new drugs for mobilization of hematopoietic stem cells. Several studies have suggested the impact of VLA-4 on chemotherapy sensitivity and prognosis in hematological malignancies, which awaits further investigations.

gp96, an endoplasmic reticulum master chaperone for integrins and Toll-like receptors, selectively regulates early T and B lymphopoiesis

Integrins contribute to lymphopoiesis, whereas Toll-like receptors (TLRs) facilitate the myeloid replenishment during inflammation. The combined role of TLRs and integrin on hematopoiesis remains unclear. gp96 (grp94, HSP90b1) is an endoplasmic reticulum master chaperone for multiple TLRs. We report herein that gp96 is also essential for expression of 14 hematopoietic system-specific integrins. Genetic deletion of gp96 thus enables us to determine the collective roles of gp96, integrins, and TLRs in hematopoiesis. We found that gp96-null hematopoietic stem cells could support long-term myelopoiesis. B- and T-cell development, however, was severely compromised with transitional block from pro-B to pre-B cells and the inability of thymocytes to develop beyond the CD4(-)CD8(-) stage. These defects were cell-intrinsic and could be recapitulated on bone marrow stromal cell culture. Furthermore, defective lymphopoiesis correlated strongly with failure of hematopoietic progenitors to form close contact with stromal cell niche and was not the result of the defect in the assembly of antigen receptor or interleukin-7 signaling. These findings define gp96 as the only known molecular chaperone to specifically regulate T- and B-cell development.

PSGL-1 function in immunity and steady state homeostasis

The substantial importance of P-selectin glycoprotein ligand 1 (PSGL-1) in leukocyte trafficking has continued to emerge beyond its initial identification as a selectin ligand. PSGL-1 seemed to be a relatively simple molecule with an extracellular mucin domain extended as a flexible rod, teleologically consistent with its primary role in tethering leukocytes to endothelial selectins. The rolling interaction between leukocyte and endothelium mediated by this selectin-PSGL-1 interaction requires branched O-glycan extensions on specific PSGL-1 amino acid residues. In some cells, such as neutrophils, the glycosyltransferases involved in formation of the O-glycans are constitutively expressed, while in other cells, such as T cells, they are expressed only after appropriate activation. Thus, PSGL-1 supports leukocyte recruitment in both innate and adaptive arms of the immune response. A complex array of amino acids within the selectins engage multiple sugar residues of the branched O-glycans on PSGL-1 and provide the molecular interactions responsible for the velcro-like catch bonds that support leukocyte rolling. Such binding of PSGL-1 can also induce signaling events that influence cell phenotype and function. Scrutiny of PSGL-1 has revealed a better understanding of how it performs as a selectin ligand and yielded unexpected insights that extend its scope from supporting leukocyte rolling in inflammatory settings to homeostasis including stem cell homing to the thymus and mature T-cell homing to secondary lymphoid organs. PSGL-1 has been found to bind homeostatic chemokines CCL19 and CCL21 and to support the chemotactic response to these chemokines. Surprisingly, the O-glycan modifications of PSGL-1 that support rolling mediated by selectins in inflammatory conditions interfere with PSGL-1 binding to homeostatic chemokines and thereby limit responsiveness to the chemotactic cues used in steady state T-cell traffic. The multi-level influence of PSGL-1 on cell traffic in both inflammatory and steady state settings is therefore substantially determined by the orchestrated addition of O-glycans. However, central as specific O-glycosylation is to PSGL-1 function, in vivo regulation of PSGL-1 glycosylation in T cells remains poorly understood. It is our purpose herein to review what is known, and not known, of PSGL-1 glycosylation and to update understanding of PSGL-1 functional scope.

Regulation of Lymphoid and Myeloid Leukemic Cell Survival: Role of Stromal Cell Adhesion Molecules

Several laboratories have documented the necessity for direct contact of lymphoid and myeloid leukemic cells with bone marrow stromal cells for optimal survival. Subsequent studies have identified various stromal cell adhesion molecules and soluble factors that facilitate survival through leukemic cell anti-apoptotic signal transduction pathways. This report provides an overview of enhanced leukemic cell survival through adhesive interactions with bone marrow expressed molecules. In addition, we describe the establishment of cloned murine stromal cell lines engineered to constitutively express human VCAM-1 protein on their surface. These stromal cell lines will be useful in studies aimed at better understanding the specific contribution of VCAM-1: VLA-4 signaling in maintenance of residual leukemic disease.

Evidence for a regulatory role of alpha 4-integrins in the maturation of eosinophils generated from the bone marrow in the presence of dexamethasone

BACKGROUND

Although eosinophils co-express multiple integrin receptors, the contributions of integrins to eosinophil development have not been explored. We previously described extensive aggregation and cytological immaturity in eosinophils developing in bone-marrow (BM) cultures exposed to dexamethasone. Here we examined the relationship of alpha 4 integrins with these effects of dexamethasone.

OBJECTIVES

We evaluated: (a) the effects of exposure to dexamethasone in BM culture on eosinophil expression of alpha 4 integrin receptors and ligands; (b) the contribution of alpha 4 integrins to eosinophil aggregation and maturation.

METHODS

Cultures were established with IL-5 (alone or with dexamethasone) for up to 7 days, and eosinophil production, alpha 4 integrin receptor/ligand expression, aggregation and morphology were evaluated before and after targeting alpha 4 integrin-dependent adhesions. Because prostaglandin E2 (PGE2) modifies the effects of dexamethasone on eosinophilopoiesis, PGE2 effects on alpha 4 integrin expression and function were also evaluated.

RESULTS

Dexamethasone increased the yield of eosinophils up to day 7. The frequency of eosinophils expressing alpha 4, beta1 and beta 7 integrin receptors at day 7 was also increased by dexamethasone. Eosinophils also expressed the alpha 4 beta 1 ligand, VCAM-1. Dexamethasone increased the expression of alpha 4 integrin and VCAM-1 in aggregates containing eosinophils as early as day 3. PGE2, added up to day 3, modified the effects of dexamethasone to suppress the expression of alpha 4 integrin, decrease aggregation and promote cytological maturation of eosinophils recovered at day 7. Dissociation of immature eosinophils from clusters present at day 3 by reagents targeting alpha 4 or beta1 integrins or VCAM-1 also induced cytological maturation. The concordant effects of targeting alpha 4 integrins with drugs and antibodies support a relationship between alpha 4-mediated aggregation and maturational arrest.

CONCLUSIONS

These observations support a novel role for alpha 4 integrin receptors and ligands in eosinophilopoiesis. In addition, increased alpha 4 expression following glucocorticoid exposure may contribute to the retention and accumulation of eosinophils in haemopoietic tissue.

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.

Selected Stro-1-enriched bone marrow stromal cells display a major suppressive effect on lymphocyte proliferation

Mesenchymal stem cells (MSCs) have an immunosuppressive effect and can inhibit the proliferation of alloreactive T cells in vitro and in vivo. Cotransplantation of MSCs and hematopoietic stem cells (HSCs) from HLA-identical siblings has been shown to reduce the incidence of acute graft-vs.-host disease. MSCs are heterogeneous and data on the inhibitory effects of different MSC subsets are lacking. The antigen Stro1 is a marker for a pure primitive MSC subset. We investigated whether Stro-1-enriched induce a more significant suppressive effect on lymphocytes in a mixed lymphocyte reaction (MLR), and whether this action is related to a specific gene expression profile in Stro-1-enriched compared to other MSCs. We demonstrated that the Stro-1-enriched population elicits a significantly more profound dose-dependent inhibition of lymphocyte proliferation in a MLR than MSCs. One thousand expanded Stro-1-enriched induced an inhibitory effect comparable to that of 10 times as many MSCs. Inhibition by Stro-1-enriched was more significant in contact-dependent cultures than in noncontact-dependant cultures at higher ratio. The Stro-1-enriched inhibitory effect in both culture types was linked to increased gene expression for soluble inhibitory factors such as interleukin-8 (IL-8), leukemia inhibitory factor (LIF), indoleamine oxidase (IDO), human leukocyte antigen-G (HLA-G), and vascular cell adhesion molecule (VCAM1). However, tumor growth factor-beta1 (TGF-beta) and IL-10 were only up-regulated in contact-dependant cultures. These results may support using a purified Stro-1-enriched population to augment the suppressive effect in allogeneic transplantation.

Blockade of alpha4 integrin signaling ameliorates the metabolic consequences of high-fat diet-induced obesity

OBJECTIVE

Many prevalent diseases of advanced societies, such as obesity-induced type 2 diabetes, are linked to indolent mononuclear cell-dependent inflammation. We previously proposed that blockade of alpha4 integrin signaling can inhibit inflammation while limiting mechanism-based toxicities of loss of alpha4 function. Thus, we hypothesized that mice bearing an alpha4(Y991A) mutation, which blocks signaling, would be protected from development of high-fat diet-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Six- to eight-week-old wild-type and alpha4(Y991A) C57Bl/6 male mice were placed on either a high-fat diet that derived 60% calories from lipids or a chow diet. Metabolic testing was performed after 16-22 weeks of diet.

RESULTS

Alpha4(Y991A) mice were protected from development of high-fat diet-induced insulin resistance. This protection was conferred on wild-type mice by alpha4(Y991A) bone marrow transplantation. In the reverse experiment, wild-type bone marrow renders high-fat diet-fed alpha4(Y991A) acceptor animals insulin resistant. Furthermore, fat-fed alpha4(Y991A) mice showed a dramatic reduction of monocyte/macrophages in adipose tissue. This reduction was due to reduced monocyte/macrophage migration rather than reduced monocyte chemoattractant protein-1 production.

CONCLUSIONS

Alpha4 integrins contribute to the development of HFD-induced insulin resistance by mediating the trafficking of monocytes into adipose tissue; hence, blockade of alpha4 integrin signaling can prevent the development of obesity-induced insulin resistance.

Localized alpha4 integrin phosphorylation directs shear stress-induced endothelial cell alignment

Vascular endothelial cells respond to laminar shear stress by aligning in the direction of flow, a process which may contribute to atheroprotection. Here we report that localized alpha4 integrin phosphorylation is a mechanism for establishing the directionality of shear stress-induced alignment in microvascular endothelial cells. Within 5 minutes of exposure to a physiological level of shear stress, endothelial alpha4 integrins became phosphorylated on Ser(988). In wounded monolayers, phosphorylation was enhanced at the downstream edges of cells relative to the source of flow. The shear-induced alpha4 integrin phosphorylation was blocked by inhibitors of cAMP-dependent protein kinase A (PKA), an enzyme involved in the alignment of endothelial cells under prolonged shear. Moreover, shear-induced localized activation of the small GTPase Rac1, which specifies the directionality of endothelial alignment, was similarly blocked by PKA inhibitors. Furthermore, endothelial cells bearing a nonphosphorylatable alpha4(S(988)A) mutation failed to align in response to shear stress, thus establishing alpha4 as a relevant PKA substrate. We thereby show that shear-induced PKA-dependent alpha4 integrin phosphorylation at the downstream edge of endothelial cells promotes localized Rac1 activation, which in turn directs cytoskeletal alignment in response to shear stress.

Distinct changes in adult lymphopoiesis in Rag2-/- mice fully reconstituted by alpha4-deficient adult bone marrow cells

OBJECTIVE

alpha4 Integrins are major players in lymphoid cell trafficking and immune responses. However, their importance in lymphoid reconstitution and function, studied by antibody blockade or in genetic models of chimeric animals with alpha4(KO) embryonic stem (ES) cells, competitive repopulation experiments with fetal liver(KO) cells, or in beta1/beta7 doubly-deficient mice has yielded disparate conclusions.

MATERIALS AND METHODS

To study the role of alpha4 integrin (alpha4beta1, alpha4beta7) during adult life, we transplanted lethally irradiated Rag2(-/-) mice with alpha4(Delta/Delta) or alpha4(f/f) adult bone marrow (BM) cells and evaluated recipients at several points after transplantation.

RESULTS

Lymphomyeloid repopulation (8 months later) was entirely donor-derived in all recipients, and novel insights regarding lymphoid reconstitution and function were revealed. Thymic repopulation was impaired in all alpha4(Delta/Delta) recipients, likely because of homing defects of BM-derived progenitors, although a role of alpha4 integrin in intrathymic expansion/maturation of T cells cannot be excluded; reconstitution of gut lymphoid tissue was also greatly diminished because of homing defects of alpha4(Delta/Delta) cells; impaired immunoglobulin (Ig) M and IgE, but normal IgG responses were seen, suggesting compromised initial B-/T-cell interactions, whereas interferon-gamma production from ovalbumin-stimulated cells was increased, possibly reflecting a bias against Th2 stimulation.

CONCLUSION

These data complement previous observations by defending the role of alpha4 integrin in thymic and gut lymphoid tissue homing, and by strengthening evidence of attenuated B-cell responses in alpha4-deficient mice.

Treating multiple sclerosis with monoclonal antibodies

Therapeutic monoclonal antibodies (mAbs) are potent new tools for a molecular targeted approach to modify the course of multiple sclerosis (MS). Besides natalizumab, which was approved in 2006, three other mAbs (alemtuzumab, rituximab and daclizumab) were successfully tested in Phase II MS trials. In this review, introductory notes on the development and systematic nomenclature of therapeutic mAbs in general, set the stage for a detailed discussion of the four mAbs mentioned. We summarize non-MS indications, expression and function of target antigens, scientific rationales for MS therapy, putative modes of action and pharmacological aspects. Particularly, we provide a critical discussion of clinical MS trials, including protocols and interim analyses of trials currently underway. The natalizumab section pays special attention to the clinical handling of safety issues and the diagnostic use of neutralizing antibodies. We finally develop a scenario for how each of the four mAbs might evolve into the market of MS therapeutics within the coming years.

Human bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) have elicited a great clinical interest, particularly in the areas of regenerative medicine and induction of tolerance in allogeneic transplantation. Previous reports demonstrated the feasibility of transplanting MSCs, which generates new prospects in cellular therapy. Recently, injection of MSCs induced remission of steroid-resistant acute graft-versus-host disease (GVHD). This review summarizes the knowledge and possible future clinical uses of MSCs.

Segregation of megakaryocytic or erythroid cells from a megakaryocytic leukemia cell line (JAS-R) by adhesion during culture

Adhesion is one of the important biologic characteristics of leukemic cells. We previously reported a new megakaryocytic-erythroid cell line, JAS-R. In this study, JAS-R cells were segregated into two types by the differences of attachment to culture dishes. One type (designated as JAS-RAD cells) adhered to the substratum of the culture dishes, while the other (JAS-REN cells) grew as a single-cell suspension. Adhesion of JAS-RAD was inhibited by treatment with RGDS oligopeptide. Flow cytometric analysis revealed that JAS-RAD cells had high expression of CD41a and CD61 versus low CD235a expression, and JAS-REN showed low expression of CD41a, and CD61, and high CD235a. The two phenotypes were reciprocally exchangeable by selecting adherent or suspended cells from each type of culture. Microarray analysis and RT-PCR revealed that JAS-RAD cells expressed four major alpha-granule genes and JAS-REN cells expressed beta-globin. Interestingly, erythropoietin was only secreted by JAS-RAD cells. With regard to transcription factors, it was shown that GFI1, FLI1 and RUNX1 were strongly expressed in JAS-RAD cells while GATA1, FOG1 and NFE2 were equally expressed by both types. These findings indicate that adhesion via integrins is related to the phenotypic shift of JAS-R cells between megakaryocytic and erythroid lineages.

Identification and molecular characterization of multiple phenotypes in integrin knockout mice

Each of the 24 known integrin subunits has now been inactivated in mice, and a growing number of conditional null lines are becoming available. Lines of mice expressing null mutations in integrin subunit genes have taught us a great deal about the remarkably diverse functions that integrins perform in vivo in mammals. Thorough evaluation of the phenotypes manifested by these lines has also revealed a number of previously unexpected integrin ligands and signaling partners. In this article, we review approaches that can contribute to optimal use of this valuable resource.

Effects of mannosylated glycopolymers on specific interaction to bone marrow hematopoietic and progenitor cells derived from murine species

Poly[N-pvinylbenzyl-O-D-galactopyranosyl-(1-4)-D-glucoamide], poly[N-pvinylbenzyl-O-D-glucopyranosyl-(1-4)-D-glucoamide], and poly[N-p-vinylbenzyl-O-mannopyranosyl-(1-4)-D-gluconamide] (referred to as PVLA, PVMA, and PV-Man) are polystyrene derivatives that contain galactose, glucose, and mannose moieties, which interact with hematopoietic cells (HCs). To clarify the specific interaction between the glucopolymers and hematopoietic cells, glycopolymers labeled with fluorescent isothiocyanate (FITC) were used to follow the specific interaction, which was visualized by confocal laser microscopy. We found that PV-Man binds strongly to HCs, probably because of a specific interaction mediated by specific receptors present on the cell membrane, while some cytotoxicity when was observed when PV-Man interacted with the cell membrane. The fluorescence intensity between PV-Man and HCs was up to four-fold (0.14 +/- 0.04) that of PVMA and PVLA with hematopoietic HCs (0.033 +/- 0.01). Moreover, cellular fluorescence increased significantly with increasing incubation time and increasing polymer concentration. Using hematopoietic lineage-specific antibodies, cells were stained and analyzed by flow cytometry to confirm which HCs showed specific binding with glycopolymers, especially hematopoietic stem cells and progenitor cells (c-kit+), B-lymphocyte progenitor cells (B220+), monocyte cells (CD11b+), and erythrocytes (Ter119+).

Primitive and committed human hematopoietic progenitor cells interact with primary murine neural cells and are induced to undergo self-renewing cell divisions

OBJECTIVE

Studies in animal models have indicated that hematopoietic progenitor cells (HPC) migrate and home to the central nervous system and might acquire neural features under specific circumstances. The interaction between HPC and the neural environment and the functional effect on hematopoiesis have not yet been defined.

METHODS

CD34(+)133(+) cells from mobilized peripheral blood were cocultured with primary murine neurons or astrocytes. Chemotaxis and adhesive interactions were studied by applying beta(1)- and beta(2)-integrin function-blocking anibodies. The impact of neural feeder layers on integrin expression of HPC and the presence of appropriate adhesion ligands on neural cells were determined by immunostaining and flow cytometry. The hematopoietic long-term fate was monitored by time-lapse microscopy of individual cell-division history followed by long-term culture-initiating cell (LTC-IC) and colony-forming cell (CFC) assays. Neural differentiation was assessed by immunostaining against specific neuronal and glial antigens.

RESULTS

The 23.0% +/- 4.9% of HPC showed stromal cell-derived factor-1-induced migration toward neural cells, and 20.2% +/- 1.6% displayed firm beta(1)-integrin-mediated adhesion to astrocytes. The latter expressed appropriate adhesion ligands, stabilized beta(1)-integrin expression, and increased beta(2)-integrin expression of HPC. Neural differentiation of HPC could not be identified but astrocytes were able to induce limited self-renewing cell divisions of HPC and thus maintain 25.8% +/- 3.4% of the initial LTC-IC and 80.7% +/- 1.9% of the initial CFC.

CONCLUSION

Human HPC are able to interact with neural cells and interaction maintains, albeit to a limited extent, the self-renewal capability of HPC.

Alpha4beta1 integrin and erythropoietin mediate temporally distinct steps in erythropoiesis: integrins in red cell development

Erythropoietin (Epo) is essential for the terminal proliferation and differentiation of erythroid progenitor cells. Fibronectin is an important part of the erythroid niche, but its precise role in erythropoiesis is unknown. By culturing fetal liver erythroid progenitors, we show that fibronectin and Epo regulate erythroid proliferation in temporally distinct steps: an early Epo-dependent phase is followed by a fibronectin-dependent phase. In each phase, Epo and fibronectin promote expansion by preventing apoptosis partly through bcl-xL. We show that α₄, α₅, and β₁ are the principal integrins expressed on erythroid progenitors; their down-regulation during erythropoiesis parallels the loss of cell adhesion to fibronectin. Culturing erythroid progenitors on recombinant fibronectin fragments revealed that only substrates that engage α₄β₁-integrin support normal proliferation. Collectively, these data suggest a two-phase model for growth factor and extracellular matrix regulation of erythropoiesis, with an early Epo-dependent, integrin-independent phase followed by an Epo-independent, α₄β₁-integrin–dependent phase.

Distinct roles of integrins alpha6 and alpha4 in homing of fetal liver hematopoietic stem and progenitor cells

Homing of hematopoietic stem cells (HSCs) into the bone marrow (BM) is a prerequisite for establishment of hematopoiesis during development and following transplantation. However, the molecular interactions that control homing of HSCs, in particular, of fetal HSCs, are not well understood. Herein, we studied the role of the alpha6 and alpha4 integrin receptors for homing and engraftment of fetal liver (FL) HSCs and hematopoietic progenitor cells (HPCs) to adult BM by using integrin alpha6 gene-deleted mice and function-blocking antibodies. Both integrins were ubiquitously expressed in FL Lin(-)Sca-1(+)Kit(+) (LSK) cells. Deletion of integrin alpha6 receptor or inhibition by a function-blocking antibody inhibited FL LSK cell adhesion to its extracellular ligands, laminins-411 and -511 in vitro, and significantly reduced homing of HPCs to BM. In contrast, the anti-integrin alpha6 antibody did not inhibit BM homing of HSCs. In agreement with this, integrin alpha6 gene-deleted FL HSCs did not display any homing or engraftment defect compared with wild-type littermates. In contrast, inhibition of integrin alpha4 receptor by a function-blocking antibody virtually abrogated homing of both FL HSCs and HPCs to BM, indicating distinct functions for integrin alpha6 and alpha4 receptors during homing of fetal HSCs and HPCs.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Erythropoietin modulation of podocalyxin and a proposed erythroblast niche

Epo's erythropoietic capacity is ascribed largely to its antiapoptotic actions. In part via gene profiling of bone marrow erythroblasts, Epo is now shown to selectively down-modulate the adhesion/migration factors chemokine receptor-4 (Cxcr4) and integrin alpha-4 (Itga4) and to up-modulate growth differentiation factor-3 (Gdf3), oncostatin-M (OncoM), and podocalyxin like-1 (PODXL). For PODXL, Epo dose-dependent expression of this CD34-related sialomucin was discovered in Kit(+)CD71(high) proerythroblasts and was sustained at subsequent Kit(-)CD71(high) and Ter119(+) stages. In vivo, Epo markedly induced PODXL expression in these progenitors and in marrow-resident reticulocytes. This was further associated with a rapid release of PODXL(+) reticulocytes to blood. As studied in erythroblasts expressing minimal Epo receptor (EpoR) alleles, efficient PODXL induction proved dependence on an EpoR-PY343 Stat5 binding site. Moreover, in mice expressing an EpoR-HM F343 allele, compromised Epo-induced PODXL expression correlated with abnormal anucleated red cell representation in marrow. By modulating this select set of cell-surface adhesion molecules and chemokines, Epo is proposed to mobilize erythroblasts from a hypothesized stromal niche and possibly promote reticulocyte egress to blood.

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).

Endothelin receptors and endothelin-1 in developing rat teeth

INTRODUCTION

The endothelins are a family of small peptides with multiple roles in a variety of tissues. Signaling is mediated through two receptor subtypes, the endothelin A receptor (ET(A)) specific for Et-1 and the non-specific endothelin B receptor (ET(B)).

OBJECTIVE

Our goal was to determine the location of immunoreactivity (IR) for ET(A) and ET(B) in developing and mature rat teeth as indicators of endothelin (Et) regulatory sites and to compare this to the Et-1 (ligand)-IR expression patterns.

DESIGN

We used immunohistochemistry to study developing and mature rat molars and continuously developing incisors.

RESULTS

We demonstrate ET(A), ET(B), and Et-1 expression patterns in teeth, for the first time. ET(A) was found in developing molar root pulp, pulpal vasculature, and preodontoblasts, and then persisted in odontoblasts or cellular cementocytes at the root apices of mature teeth. ET(B) was found at the molar (Hertwig's) root sheath during root formation and in molar ameloblasts, nerve fibers and odontoblasts of immature and mature teeth. In incisors, ET(B)-IR was associated with ameloblasts and the stem cell niche of the cervical loop while ET(A) was located in the substratum layer. Et-1 was found throughout the dental and periodontal tissues with higher concentrations associated with odontoblasts, nerves and incisor layers that expressed ET(B).

CONCLUSION

The patterns of ET(A) and ET(B) in teeth differ from each other and from those of adjacent tissues suggesting multiple tooth-specific functions for endothelin during development and mature dental function.

Altered membrane dynamics of quantum dot-conjugated integrins during osteogenic differentiation of human bone marrow derived progenitor cells

Functionalized quantum dots offer several advantages for tracking the motion of individual molecules on the cell surface, including selective binding, precise optical identification of cell surface molecules, and detailed examination of the molecular motion without photobleaching. We have used quantum dots conjugated with integrin antibodies and performed studies to quantitatively demonstrate changes in the integrin dynamics during osteogenic differentiation of human bone marrow derived progenitor cells (BMPCs). Consistent with the unusually strong BMPC adhesion previously observed, integrins on the surface of undifferentiated BMPC were found in clusters and the lateral diffusion was slow (e.g., approximately 10(-11) cm2/s). At times as early as those after a 3-day incubation in the osteogenic differentiation media, the integrin diffusion coefficients increased by an order of magnitude, and the integrin dynamics became indistinguishable from that measured on the surface of terminally differentiated human osteoblasts. Furthermore, microfilaments in BMPCs consisted of atypically thick bundles of stress fibers that were responsible for restricting the integrin lateral mobility. Studies using laser optical tweezers showed that, unlike fully differentiated osteoblasts, the BMPC cytoskeleton is weakly associated with its cell membrane. Based on these findings, it appears likely that the altered integrin dynamics is correlated with BMPC differentiation and that the integrin lateral mobility is restricted by direct links to microfilaments.

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.

alpha4-Integrin(+) endothelium derived from primate embryonic stem cells generates primitive and definitive hematopoietic cells

The mechanism of commencement of hematopoiesis in blood islands of the yolk sac and the aorta-gonad-mesonephros (AGM) region during primate embryogenesis remains elusive. In this study, we demonstrated that VE-cadherin(+)CD45(-) endothelial cells derived from nonhuman primate embryonic stem cells are able to generate primitive and definitive hematopoietic cells sequentially, as revealed by immunostaining of floating erythrocytes and colony-forming assay in cultures. Single bipotential progenitors for hematopoietic and endothelial lineages are included in this endothelial cell population. Furthermore, hemogenic activity of these endothelial cells is observed exclusively in the alpha4-integrin(+) subpopulation; bipotential progenitors are 4-fold enriched in this subpopulation. The kinetics of this hemogenic subpopulation is similar to that of hemogenic endothelial cells previously reported in the yolk sac and the AGM region in vivo in that they emerge for only a limited time. We suggest that VE-cadherin(+)CD45(-)alpha4-integrin(+) endothelial cells are involved in primitive and definitive hematopoiesis during primate embryogenesis, though VE-cadherin(-)CD45(-)alpha4-integrin(+) cells are the primary sources for primitive hematopoiesis.

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 Rap GTPases mediate CXCL13- and sphingosine1-phosphate-induced chemotaxis, adhesion, and Pyk2 tyrosine phosphorylation in B lymphocytes

The localization of B cells to lymphoid organs where they can become activated and differentiate into antibody-secreting plasma cells is controlled by multiple chemoattractants that promote cell migration and integrin-mediated adhesion. CXCL13 and sphingosine 1-phosphate (S1P) are two important chemoattractants that control the trafficking of B cells. CXCL13 directs B lymphocytes to lymphoid follicles where they receive survival signals and, if activated, undergo a germinal center response. In contrast, S1P allows B cells and plasma cells to exit lymphoid organs and re-enter the circulation. The Rap1 GTPase is a key regulator of cell adhesion and cell migration in a number of systems. We now show that Rap activation is required for CXCL13 and S1P to induce B cell migration as well as adhesion to ICAM-1 and VCAM-1. We also show that Pyk2, a tyrosine kinase involved in cytoskeleton rearrangements and B cell migration, is a downstream target of both CXCL13 and S1P signaling and that Rap activation is important for CXCL13 and S1P to stimulate tyrosine phosphorylation of Pyk2, a modification that increases Pyk2 kinase activity. This suggests that the ability of CXCL13 and S1P to direct the trafficking and localization of B cells in vivo may be dependent on Rap activation.

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

Primitive erythroblasts (EryPs) are the first hematopoietic cell type to form during mammalian embryogenesis and emerge within the blood islands of the yolk sac. Large, nucleated EryPs begin to circulate around midgestation, when connections between yolk sac and embryonic vasculature mature. Two to 3 days later, small cells of the definitive erythroid lineage (EryD) begin to differentiate within the fetal liver and rapidly outnumber EryPs in the circulation. The development and maturation of EryPs remain poorly defined. Our analysis of embryonic blood at different stages reveals a stepwise developmental progression within the EryP lineage from E9.5 to E12.5. Thereafter, EryDs are also present in the bloodstream, and the 2 lineages are not easily distinguished. We have generated a transgenic mouse line in which the human epsilon-globin gene promoter drives expression of green fluorescent protein exclusively within the EryP lineage. Here, we have used this line to characterize changes in cell morphology and surface-marker expression as EryPs mature and to track EryP numbers and enucleation throughout gestation. This study identifies previously unrecognized synchronous developmental stages leading to the maturation of EryPs in the mouse embryo. Unexpectedly, we find that EryPs are a stable cell population that persists through the end of gestation.

New approaches to blockade of alpha4-integrins, proven therapeutic targets in chronic inflammation

The recruitment of leukocytes into tissue is a pivotal step in inflammation. alpha4-Integrins are adhesion receptors on circulating leukocytes that mediate attachment to the endothelium and facilitate their migration into the inflamed tissue. This multistep process is mediated by the interaction of alpha4-integrins with their counter receptors VCAM-1 and MadCAM-1 that are expressed on endothelial cells. alpha4-Integrins act as both adhesive and signaling receptors. Paxillin, a signaling adaptor molecule, binds directly to the alpha4 cytoplasmic tail and its binding is important for cell migration. Blocking the adhesive functions of alpha4-integrins has been shown to be an effective therapeutic approach in the treatment of autoimmune diseases, but also carries the risk of defects in development, hematopoiesis and immune surveillance. Interfering with alpha4 signaling by inhibiting the alpha4-paxillin interaction decreases alpha4-mediated cell migration and adhesion to VCAM-1 and MadCAM under shear flow. These in vitro effects are accompanied by a selective impairment of leukocyte migration into inflammatory sites when the alpha4-paxillin interaction is blocked in vivo. Thus, blockade of alpha4-integrin signaling may offer a novel strategy for interfering with the functions of these receptors in pathological events while sparing important physiological functions.

Cell Adhesion Regulates CDC25A Expression and Proliferation in Acute Myeloid Leukemia

The effects of cell adhesion on leukemia cell proliferation remain poorly documented and somehow controversial. In this work, we investigated the effect of adhesion to fibronectin on the proliferation of acute myeloid leukemia (AML) cell lines (U937 and KG1a) and CD34+ normal or leukemic primary cells. We observed an increased rate of proliferation of AML cells when adhered to fibronectin, concomitant with accelerated S-phase entry and accumulation of CDC25A. Conversely, normal CD34+ cell proliferation was decreased by adhesion to fibronectin with a concomitant drop in CDC25A expression. Importantly, we showed that both small interfering RNA (siRNA)-mediated CDC25A down-regulation and a recently developed CDC25 pharmacologic inhibitor impaired this adhesion-dependent proliferation, establishing a functional link between CDC25A accumulation and adhesion-dependent proliferation in leukemic cells. CDC25A accumulation was found only slightly dependent on transcriptional regulation and essentially due to modifications of the proteasomal degradation of the protein as shown using proteasome inhibitors and reverse transcription-PCR. Interestingly, CDC25A regulation was Chk1 dependent in these cells as suggested by siRNA-mediated down-regulation of this protein. Finally, we identified activation of the phosphatidylinositol 3-kinase/Akt pathway as an adhesion-dependent regulation mechanism of CDC25A protein expression. Altogether, our data show that in leukemic cells adhesion to fibronectin increases CDC25A expression through proteasome- and Chk1-dependent mechanisms, resulting in enhanced proliferation. They also suggest that these adhesion-dependent proliferation properties of hematopoietic cells may be modified during leukemogenesis.

The differentiation program of embryonic definitive hematopoietic stem cells is largely α4 integrin independent

Previous analyses of the roles of α4 integrins in hematopoiesis by other groups have led to conflicting evidence. α4 integrin mutant cells developing in [α4 integrin–/–: wt] chimeric mice are not capable of completing lymphomyeloid differentiation, whereas conditional inactivation of α4 integrin in adult mice has only subtle effects. We show here that circumventing the fetal stage of hematopoietic stem cell (HSC) development by transplantation of embryonic α4 integrin–/– cells into the adult microenvironment results in robust and stable long-term generation of α4 integrin–/– lymphoid and myeloid cells, although colonization of Peyer patches and the peritoneal cavity is significantly impaired. We argue here that collectively, our data and the data from other groups suggest a specific requirement for α4 integrin during the fetal/neonatal stages of HSC development that is essential for normal execution of the lymphomyeloid differentiation program.

Clustering of Pre-B Cell Integrins Induces Galectin-1-Dependent Pre-B Cell Receptor Relocalization and Activation

Interactions between B cell progenitors and bone marrow stromal cells are essential for normal B cell differentiation. We have previously shown that an immune developmental synapse is formed between human pre-B and stromal cells in vitro, leading to the initiation of signal transduction from the pre-BCR. This process relies on the direct interaction between the pre-BCR and the stromal cell-derived galectin-1 (GAL1) and is dependent on GAL1 anchoring to cell surface glycosylated counterreceptors, present on stromal and pre-B cells. In this study, we identify alpha(4)beta(1) (VLA-4), alpha(5)beta(1) (VLA-5), and alpha(4)beta(7) integrins as major GAL1-glycosylated counterreceptors involved in synapse formation. Pre-B cell integrins and their stromal cell ligands (ADAM15/fibronectin), together with the pre-BCR and GAL1, form a homogeneous lattice at the contact area between pre-B and stromal cells. Moreover, integrin and pre-BCR relocalizations into the synapse are synchronized and require actin polymerization. Finally, cross-linking of pre-B cell integrins in the presence of GAL1 is sufficient for driving pre-BCR recruitment into the synapse, leading to the initiation of pre-BCR signaling. These results suggest that during pre-B/stromal cell synapse formation, relocalization of pre-B cell integrins mediated by their stromal cell ligands drives pre-BCR clustering and activation, in a GAL1-dependent manner.

Blocking the alpha 4 integrin-paxillin interaction selectively impairs mononuclear leukocyte recruitment to an inflammatory site

Antagonists to alpha4 integrin show promise for several autoimmune and inflammatory diseases but may exhibit mechanism-based toxicities. We tested the capacity of blockade of alpha4 integrin signaling to perturb functions involved in inflammation, while limiting potential adverse effects. We generated and characterized mice bearing a Y991A mutation in alpha4 integrin [alpha4(Y991A) mice], which blocks paxillin binding and inhibits alpha4 integrin signals that support leukocyte migration. In contrast to the embryonic-lethal phenotype of alpha4 integrin-null mice, mice bearing the alpha4(Y991A) mutation were viable and fertile; however, they exhibited defective recruitment of mononuclear leukocytes into thioglycollate-induced peritonitis. Alpha4 integrins are essential for definitive hematopoiesis; however, the alpha4(Y991A) mice had intact lymphohematopoiesis and, with the exception of reduced Peyer's patches, normal architecture and cellularity of secondary lymphoid tissues. We conclude that interference with alpha4 integrin signaling can selectively impair mononuclear leukocyte recruitment to sites of inflammation while sparing vital functions of alpha4 integrins in development and hematopoiesis.

A novel role for fibronectin type I domain in the regulation of human hematopoietic cell adhesiveness through binding to follistatin domains of FLRG and follistatin

FLRG and follistatin belong to the family of follistatin proteins involved in the regulation of various biological effects, such as hematopoiesis, mediated by their binding to activin and BMP, both members of the TGFbeta family. To further characterize the function of FLRG, we searched for other possible functional partners using a yeast two-hybrid screen. We identified human fibronectin as a new partner for both FLRG and follistatin. We also demonstrated that their physical interaction is mediated by type I motifs of fibronectin and follistatin domains. We then analyzed the biological consequences of these protein interactions on the regulation of hematopoiesis. For the first time, we associated a biological effect with the regulation of human hematopoietic cell adhesiveness of both the type I motifs of fibronectin and the follistatin domains of FLRG and follistatin. Indeed, we observed a significant and specific dose-dependent increase of cell adhesion to fibronectin in the presence of FLRG or follistatin, using either a human hematopoietic cell line or primary cells. In particular, we observed a significantly increased adhesion of immature hematopoietic precursors (CFC, LTC-IC). Altogether these results highlight a new mechanism by which FLRG and follistatin regulate human hematopoiesis.

Contribution of alpha6 integrins to hematopoietic stem and progenitor cell homing to bone marrow and collaboration with alpha4 integrins

The laminin receptor integrin alpha6 chain is ubiquitously expressed in human and mouse hematopoietic stem and progenitor cells. We have studied its role for homing of stem and progenitor cells to mouse hematopoietic tissues in vivo. A function-blocking anti-integrin alpha6 antibody significantly reduced progenitor cell homing to bone marrow (BM) of lethally irradiated mice, with a corresponding retention of progenitors in blood. Remarkably, the anti-integrin alpha6 antibody profoundly inhibited BM homing of long-term multilineage engrafting stem cells, studied by competitive repopulation assay and analysis of donor-derived lymphocytes and myeloid cells in blood 16 weeks after transplantation. A similar profound inhibition of long-term stem cell homing was obtained by using a function-blocking antibody against alpha4 integrin, studied in parallel. Furthermore, the anti-integrin alpha6 and alpha4 antibodies synergistically inhibited homing of short-term repopulating stem cells. Intravenous injection of anti-integrin alpha6 antibodies, in contrast to antibodies against alpha4 integrin, did not mobilize progenitors or enhance cytokine-induced mobilization by G-CSF. Our results provide the first evidence for a distinct functional role of integrin alpha6 receptor during hematopoietic stem and progenitor cell homing and collaboration of alpha6 integrin with alpha4 integrin receptors during homing of short-term stem cells.

Lack of alpha4 integrin expression in stem cells restricts competitive function and self-renewal activity

Alpha4 integrin or VLA4 (CD49d/CD29) is a multitask molecule with wide expression within and outside the hematopoietic system. Because targeted ablation of alpha4 integrin leads to embryonic lethality, to study its effects on adult hematopoiesis, we used animals with conditional excision of alpha4 integrin (alpha4Delta/Delta) in hematopoietic cells. In such animals, we previously documented weakened bone marrow retention of progenitor cells during homeostasis and impaired homing and short-term engraftment after transplantation. In the present study we show that long-term repopulating cells lacking alpha4 integrins display a competitive disadvantage in hematopoietic reconstitution compared to normal competitors. Although initial dominance of alpha4+ competitors is due to their better homing and proliferative expansion early after transplantation, a progressive decline in contribution of alpha4Delta/Delta hematopoiesis is compatible with neither normal homing nor normal function of alpha4Delta/Delta hematopoietic stem cells (HSCs) in post-homing hematopoiesis. In the absence of alpha4+ competitor cells, alpha4Delta/Delta HSCs can establish long-term hematopoiesis in primary recipients, however, some resurgence of host hematopoiesis is evident, and it becomes dominant in secondary transplants, so that no survivors with exclusively alpha4Delta/Delta cells are seen in tertiary transplants. Collectively, our data provide compelling evidence that under regenerative stress alpha4 integrin assumes a greater importance than for maintenance of steady-state hematopoiesis.

Therapeutic anti-integrin (alpha4 and alphaL) monoclonal antibodies: two-edged swords?

Anti-alpha4 and anti-alphaL integrin chain monoclonal antibodies have shown a clear-cut beneficial effect in different animal models of autoimmune and inflammatory disorders as well as in human diseases, including multiple sclerosis, inflammatory bowel disease, and psoriasis. It has been widely assumed that this therapeutic effect is mainly consequence of the blockade of leucocyte adhesion to endothelium, inhibiting thus their extravasation and the inflammatory phenomenon. However, it is evident that both alpha4beta1 (very late antigen-4) and alphaLbeta2 (leucocyte function-associated antigen-1) integrins have additional important roles in other immune phenomena, including the formation of the immune synapse and the differentiation of T helper 1 lymphocytes. Therefore, it is very feasible that the long-term administration of blocking agents directed against these integrins to patients with inflammatory/autoimmune conditions may have undesirable or unexpected effects.

Enhanced engraftment of hematopoietic stem/progenitor cells by the transient inhibition of an adaptor protein, Lnk

Hematopoietic stem cells (HSCs) are the key elements responsible for maintaining blood-cell production throughout life and for lymphohematopoietic reconstitution following bone marrow (BM) transplantation. Enhancement of the engrafting potential and expansion capabilities of HSCs as well as hematopoietic progenitor cells (HPCs) has been a long-time desire as a means of reducing the risks and difficulties that accompany BM transplantation. The ability of HSCs/HPCs to reconstitute the hematopoietic system of irradiated hosts is negatively regulated by an intracellular adaptor protein, Lnk. Here we have identified the functional domains of Lnk and developed a dominant-negative (DN) Lnk mutant that inhibits the functions of Lnk endogenously expressed in the HSCs/HPCs and thereby potentiates the HSCs/HPCs for engraftment. Importantly, even transient expression of DN-Lnk in HSCs/HPCs facilitated their engraftment under nonmyeloablative conditions and fully reconstituted the lymphoid compartments of immunodeficient host animals. HPCs expressing DN-Lnk were efficiently trapped by immobilized vascular cell adhesion molecule-1 (VCAM-1) in a transwell migration assay, suggesting involvement of Lnk in the regulation of cell mobility or cellular interaction in microenvironments. Transient inhibition of Lnk or Lnk-mediated pathways could be a potent approach to augment engraftment of HSCs/HPCs without obvious side effects.