LFA-1 is sufficient in mediating neutrophil emigration in Mac-1-deficient mice

Leukocyte engagement of platelet glycoprotein Ibalpha via the integrin Mac-1 is critical for the biological response to vascular injury

BACKGROUND

Leukocyte-platelet interactions are critical in the initiation and progression of atherosclerosis as well as restenosis. Although the leukocyte integrin Mac-1 (alphaMbeta2, CD11b/CD18) has been implicated in the firm adhesion and transmigration of leukocytes at sites of platelet deposition, the precise alphaMbeta2 counterligand responsible for mediating adhesion-strengthening interactions between neutrophils and platelets in vivo has not previously been identified.

METHODS AND RESULTS

Our previous studies have established the P201-K217 sequence in the alphaMI domain as the binding site for platelet glycoprotein (GP) Ibalpha. Here we report that antibody targeting of alphaM(P201-K217) reduced alphaMbeta2-dependent adhesion to GP Ibalpha but not other alphaMbeta2 ligands, including fibrinogen, intercellular adhesion molecule-1, and junctional adhesion molecule-3. Anti-alphaM(P201-K217) inhibited the firm adhesion of both human and murine leukocytes to adherent platelets under laminar flow conditions. In a mouse femoral artery wire injury model, antibody targeting of alphaM(P201-K217) reduced leukocyte accumulation after injury that was accompanied by inhibition of cellular proliferation and neointimal thickening.

CONCLUSIONS

This study demonstrates that GP Ibalpha is a physiologically relevant ligand for alphaMbeta2 and that integrin engagement of GP Ibalpha is critical to leukocyte function and the biological response to vascular injury. These observations establish a molecular target for selectively disrupting leukocyte-platelet complexes that promote inflammation in thrombosis and restenosis.

Differential roles for beta2 integrins in experimental autoimmune bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune disease associated with autoantibodies directed against the hemidesmosomal antigens anti-BP230 and anti-B180. Neonatal mice injected with rabbit anti-mouse BP180 (mBP10) IgG develop a BP-like disease. Complement, immune complexes, mast cells, and neutrophils play a key role in subepidermal blistering in this animal model. In this study we investigated the role of beta2 integrins in experimental BP. Wild-type (WT) mice pretreated with neutralizing antibody against CD11a (LFA-1), CD11b (Mac-1), CD11a plus CD11b, or CD18 alone failed to develop BP when injected with pathogenic anti-mBP180 IgG. This was associated with a significant reduction in neutrophil accumulation in neutralizing antibody-treated mice. Mac-1-deficient (Mac-1 knockout [KO]) mice were resistant to experimental BP despite normal complement deposition and mast cell and neutrophil degranulation. Neutrophil infiltration in Mac-1 KO mice was severely impaired at 24 hours. However, more neutrophils accumulated in the skin of Mac-1 KO mice compared with WT mice at early time points (2-4 hours), which was associated with an increase in their survival as determined by apoptosis markers. These data suggest that beta2 integrins play differential roles in experimental BP: LFA-1 is required for neutrophil recruitment, while Mac-1 mediates late neutrophil accumulation and apoptosis of infiltrating neutrophils.

Wound-healing defect of CD18(-/-) mice due to a decrease in TGF-beta1 and myofibroblast differentiation

We studied the mechanisms underlying the severely impaired wound healing associated with human leukocyte-adhesion deficiency syndrome-1 (LAD1) using a murine disease model. In CD18(-/-) mice, healing of full-thickness wounds was severely delayed during granulation-tissue contraction, a phase where myofibroblasts play a major role. Interestingly, expression levels of myofibroblast markers alpha-smooth muscle actin and ED-A fibronectin were substantially reduced in wounds of CD18(-/-) mice, suggesting an impaired myofibroblast differentiation. TGF-beta signalling was clearly involved since TGF-beta1 and TGF-beta receptor type-II protein levels were decreased, while TGF-beta(1) injections into wound margins fully re-established wound closure. Since, in CD18(-/-) mice, defective migration leads to a severe reduction of neutrophils in wounds, infiltrating macrophages might not phagocytose apoptotic CD18(-/-) neutrophils. Macrophages would thus be lacking their main stimulus to secrete TGF-beta1. Indeed, in neutrophil-macrophage cocultures, lack of CD18 on either cell type leads to dramatically reduced TGF-beta1 release by macrophages due to defective adhesion to, and subsequent impaired phagocytic clearance of, neutrophils. Our data demonstrates that the paracrine secretion of growth factors is essential for cellular differentiation in wound healing.

Experimental approaches to lymphocyte migration in dermatology in vitro and in vivo

Lymphocyte trafficking through the dermal compartment is part of the physiological surveillance process of the adaptive immune system. On the other hand, persistent or recurrent lymphocyte infiltrates are hallmarks of both types of chronic inflammatory skin diseases, Th1-type such as psoriasis or Th2/allergic-type like atopic dermatitis. A better understanding of the mechanisms underlying lymphocyte movements is one of the key prerequisites for developing more effective therapies. In this review, we introduce a range of simple-to-sophisticated experimental in vitro and in vivo approaches to analyze lymphocyte migration. These methods start from static in vitro adhesion and chemotaxis assays, include dynamic endothelial flow chamber, intravital dual photon, and transcutaneous live-video microscopy, and finally encompass specific genetically deficient or engineered animal models. Discussing pros and cons of these assay systems hopefully generates both state-of-the-art knowledge about the factors involved in most common chronic skin diseases as well as an improved understanding of the limitations and chances of new biologic pharmaceuticals that are currently introduced into clinical practice.

A specific role of integrin Mac-1 in accelerated macrophage efflux to the lymphatics

In response to injury, monocytes migrate to the site of inflammation, where they differentiate into macrophages and participate in various biologic processes. However, their fate during the resolution of acute inflammation is not fully understood. Here, we show that inflammatory macrophages do not die locally by apoptosis; rather, they migrate across the peritoneal mesothelium to the lymphatics, through which they further migrate to the lymph nodes and to the blood circulation. Macrophage efflux is enhanced considerably on cell activation, and such accelerated macrophage migration is dependent specifically on integrin Mac-1, and can be blocked by addition of its antagonist. Thus, genetic inactivation of Mac-1 in mice inhibits the accelerated macrophage efflux from the inflammatory site to the lymphatics, but it does not compromise the accumulation of blood monocytes into the inflammatory site. Together, our study demonstrates that Mac-1 is involved specifically in the efflux of activated macrophages to the lymphatics, suggesting that Mac-1 may play an important role in the removal of local inflammatory macrophages and in their subsequent migration to the lymph nodes, a process that is critical to the development of the adaptive immunity.

Manifestations of inflammatory arthritis are critically dependent on LFA-1

Leukocyte infiltration of synovial fluid and tissues is the hallmark of inflammatory arthritis. Selectins and beta2 integrins have been implicated in the multistep process of leukocyte adhesion to vascular endothelium. However, previous work has revealed disparate requirements for leukocyte recruitments to specific anatomic locales. Moreover, the mechanisms regulating recruitment of leukocytes to the joint in inflammatory arthritis models are not fully understood. We hypothesized that beta2 integrins, expressed on leukocytes, might play a pathogenic role in synovial inflammation. Using mice deficient in all beta2 integrins (CD18 null mice), we demonstrate that expression of these heterodimeric adhesion molecules is critical for arthritis induction in the K/B x N serum transfer model. Using null-allele mice and blocking mAbs, we demonstrate specifically that CD11a/CD18 (LFA-1) is absolutely required for the development of arthritis in this model. Blocking mAbs further revealed an ongoing requirement for LFA-1 I-domain adhesive function in disease perpetuation. These findings suggest that the LFA-1 I-domain forms an attractive target for treatment of human inflammatory arthritis.

Fibrin(ogen)-alpha M beta 2 interactions regulate leukocyte function and innate immunity in vivo

In addition to its well-characterized role in hemostasis, fibrin(ogen) has been proposed to be a central regulator of the inflammatory response. Multiple in vitro studies have demonstrated that this hemostatic factor can alter leukocyte function, including cell adhesion, migration, cytokine and chemokine expression, degranulation, and other specialized processes. One important link between fibrin(ogen) and leukocyte biology appears to be the integrin receptor alpha(M)beta(2)/Mac-1, which binds to immobilized fibrin(ogen) and regulates leukocyte activities. Although it is well established that fibrin(ogen) is a ligand for alpha(M)beta(2), the precise molecular determinants that govern this interaction are only now becoming clear. A novel line of mice expressing a mutant form of fibrinogen (Fib gamma(390-396A)) has revealed that gamma chain residues 390-396 are important for the high-affinity engagement of fibrinogen by alpha(M)beta(2) and leukocyte function in vivo. Fibrinogen gamma(390-396A) failed to support alpha(M)beta(2)-mediated adhesion of primary neutrophils, monocytes, and macrophages, and mice expressing this fibrinogen variant were found to exhibit a major defect in the host inflammatory response following acute challenges. Most notably, Fib gamma(390-396A) mice display a profound impediment in Staphylococcus aureus elimination by leukocytes following intraperitoneal inoculation. These findings have positively established the physiological importance of fibrin(ogen) as a ligand for alpha(M)beta(2) and illustrate that the fibrin(ogen) gamma chain residues 390-396 constitute a critical feature of the alpha(M)beta(2) binding motif. Finally, the Fib gamma(390-396A) mice represent a valuable system for better defining the contribution of fibrin(ogen) to the inflammatory response in the absence of any confounding alteration in clotting function.

ANCA induces β2integrin and CXC chemokine-dependent neutrophil-endothelial cell interactions that mimic those of highly cytokine-activated endothelium

Antineutrophil cytoplasm antibodies (ANCA) activate neutrophils to undergo a series of coordinated interactions, leading to transendothelial migration, eventually culminating in vascular destruction. The molecular events underlying neutrophil recruitment in ANCA-associated vasculitis need to be defined to enable effective therapeutic manipulation. A flow-based adhesion assay was used to investigate the role of beta2 integrins (CD11a/CD18 and CD11b/CD18) and chemokine receptors [CXC chemokine receptor (CXCR)1 and CXCR2] in neutrophil migration through the endothelium. Two endothelial models were used: a highly activated model stimulated with 100 U/ml tumor necrosis factor alpha (TNF-alpha) and a minimally activated model stimulated with 2 U/ml TNF-alpha and in which ANCA was present as a secondary neutrophil stimulus. CD11a/CD18, CD11b/CD18, and CXCR2 contributed to adhesion and transendothelial migration in both models. However, when the endothelium was minimally activated with TNF-alpha, CD11b/CD18 played an important role in stabilizing adhesion induced by ANCA immunoglobulin G (IgG). Analysis of beta2 integrins and chemokine receptors demonstrated that ANCA IgG had no effect on expression levels at the neutrophil surface but enabled an active conformational change of CD11b/CD18. Similar molecular mechanisms control neutrophil adhesion and migration through highly or minimally TNF-alpha-activated endothelium. However, the direct ANCA-mediated neutrophil stimulation is needed to drive migration through minimally activated endothelium, and CD11b/CD18 is recruited for greater stability of adhesion during this process and can undergo an activatory, conformational change in response to ANCA IgG.

The lectin-like domain of complement receptor 3 protects endothelial barrier function from activated neutrophils

The adhesion of neutrophils to endothelial cells is a central event leading to diapedesis and involves the binding of the I-domain of beta(2) integrins (CD11/CD18) to endothelial ICAMs. In addition to the I-domain, the beta(2) integrin complement receptor 3 (CR3) (CD11b/CD18) contains a lectin-like domain (LLD) that can alter leukocyte functions such as chemotaxis and cytotoxicity. The present study demonstrates that, in contrast to the CR3 I-domain, Ab blockade of the CR3 LLD has no role in mediating neutrophil-induced loss of endothelial barrier function. However, activation of CR3 with the LLD agonist beta-glucan protects the barrier function of endothelial cells in the presence of activated neutrophils and reduces transendothelial migration without affecting adhesion of the neutrophils to the endothelium. The LLD site-specific mAb VIM12 obviates beta-glucan protection while activation of the LLD by VIM12 cross-linking mimics the beta-glucan response by both preserving endothelial barrier function and reducing neutrophil transendothelial migration. beta-glucan has no direct effect on endothelial cell function in the absence of activated neutrophils. These findings demonstrate that signaling through the CR3 LLD prevents neutrophil-induced loss of endothelial barrier function and reduces diapedesis. This suggests that the LLD may be a suitable target for oligosaccharide-based anti-inflammatory therapeutics.

A novel method to analyze leukocyte rolling behavior in vivo

Leukocyte endothelial cell interaction is a fundamentally important process in many disease states. Current methods to analyze such interactions include the parallel-plate flow chamber and intravital microscopy. Here, we present an improvement of the traditional intravital microscopy that allows leukocyte-endothelial cell interaction to be studied from the time the leukocyte makes its initial contact with the endothelium until it adheres to or detaches from the endothelium. The leukocyte is tracked throughout the venular tree with the aid of a motorized stage and the rolling and adhesive behavior is measured off-line. Because this method can involve human error, methods to automate the tracking procedure have been developed. This novel tracking method allows for a more detailed examination of leukocyte-endothelial cell interactions.

Reduction of the multiple organ injury and dysfunction caused by endotoxemia in 5-lipoxygenase knockout mice and by the 5-lipoxygenase inhibitor zileuton

The role of 5-lipoxygenase (5-LOX) in the pathophysiology of the organ injury/dysfunction caused by endotoxin is not known. Here, we investigate the effects of treatment with 5-LOX inhibitor zileuton in rats and targeted disruption of the 5-LOX gene in mice (5-LOX(-/-)) on multiple organ injury/dysfunction caused by severe endotoxemia. We also investigate the expression of beta2-integrins CD11a/CD18 and CD11b/CD18 on rat leukocytes by flow cytometry. Zileuton [3 mg/kg intravenously (i.v.)] or vehicle (10% dimethyl sulfoxide) was administered to rats 15 min prior to lipopolysaccharide (LPS; Escherichia coli, 6 mg/kg i.v.) or vehicle (saline). 5-LOX(-/-) mice and wild-type littermate controls were treated with LPS (E. coli, 20 mg/kg intraperitoneally) or vehicle (saline). Endotoxemia for 6 h in rats or 16 h in mice resulted in liver injury/dysfunction (increase in the serum levels of aspartate aminotransferase, alanine aminotransferase, gamma-glutamyl transferase, alkaline phosphatase, bilirubin), renal dysfunction (creatinine), and pancreatic injury (lipase, amylase). Absence of functional 5-LOX (zileuton treatment or targeted disruption of the 5-LOX gene) reduced the multiple organ injury/dysfunction caused by endotoxemia. Polymorphonuclear leukocyte infiltration (myeloperoxidase activity) in the lung and ileum as well as pulmonary injury (histology) were markedly reduced in 5-LOX(-/-) mice. Zileuton also reduced the LPS-induced expression of CD11b/CD18 on rat leukocytes. We propose that endogenous 5-LOX metabolites enhance the degree of multiple organ injury/dysfunction caused by severe endotoxemia by promoting the expression of the adhesion molecule CD11b/CD18 and that inhibitors of 5-LOX may be useful in the therapy of the organ injury/dysfunction associated with endotoxic shock.

Loss of LFA-1, but not Mac-1, protects MRL/MpJ-Fas(lpr) mice from autoimmune disease

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by immune complex-mediated tissue injury. Many different adhesion molecules are thought to participate in the development of SLE; however, few studies have directly examined the contributions of these proteins. Here we demonstrate that LFA-1 plays an essential role in the development of lupus in MRL/MpJ-Fas(lpr) mice. Mice deficient in LFA-1, but not Mac-1, showed significantly increased survival, decreased anti-DNA autoantibody formation, and reduced glomerulonephritis. The phenotype of the LFA-1-deficient mice was similar to that observed in beta(2) integrin-deficient (CD18-null) MRL/MpJ-Fas(lpr) mice, suggesting a lack of redundancy among the beta(2) integrin family members and other adhesion molecules. These studies identify LFA-1 as a key contributor in the pathogenesis of autoimmune disease in this model, and further suggest that therapeutic strategies targeting this adhesion molecule may be beneficial for the treatment of SLE.

Contributions of LFA-1 and Mac-1 to brain injury and microvascular dysfunction induced by transient middle cerebral artery occlusion

Although the beta2-integrins have been implicated in the pathogenesis of cerebral ischemia-reperfusion (I/R) injury, the relative contributions of the alpha-subunits to the pathogenesis of ischemic stroke remains unclear. The objective of this study was to determine whether and how genetic deficiency of either lymphocyte function-associated antigen-1 (LFA-1) or macrophage-1 (Mac-1) alters the blood cell-endothelial cell interactions, tissue injury, and organ dysfunction in the mouse brain exposed to focal I/R. Middle cerebral artery occlusion was induced for 1 h (followed by either 4 or 24 h of reperfusion) in wild-type mice and in mice with null mutations for either LFA-1 or Mac-1. Neurological deficit and infarct volume were monitored for 24 h after reperfusion. Platelet- and leukocyte-vessel wall adhesive interactions were monitored in cortical venules by intravital microscopy. Mice with null mutations for LFA-1 or Mac-1 exhibited significant reductions in infarct volume. This was associated with a significant improvement in the I/R-induced neurological deficit. Leukocyte adhesion in cerebral venules did not differ between wild-type and mutant mice at 4 h after reperfusion. However, after 24 h of reperfusion, leukocyte adhesion was reduced in both LFA-1- and Mac-1-deficient mice compared with their wild-type counterparts. Platelet adhesion was also reduced at both 4 and 24 h after reperfusion in the LFA-1- and Mac-1-deficient mice. These findings indicate that both alpha-subunits of the beta2-integrins contribute to the brain injury and blood cell-vessel wall interactions that are associated with transient focal cerebral ischemia.

Integrin engagement regulates monocyte differentiation through the forkhead transcription factor Foxp1

The precise signals responsible for differentiation of blood-borne monocytes into tissue macrophages are incompletely defined. "Outside-in" signaling by integrins has been implicated in modulation of gene expression that affects cellular differentiation. Herein, using differential display PCR, we have cloned an 85-kDa forkhead transcription factor (termed Mac-1-regulated forkhead [MFH] and found subsequently to be identical to Foxp1) that is downregulated in beta(2)-integrin Mac-1-clustered compared with Mac-1-nonclustered monocytic THP-1 cells. MFH/Foxp1 is expressed in untreated HL60 cells, and its expression was markedly reduced during phorbol ester-induced monocyte differentiation, but not retinoic acid-induced granulocyte differentiation. Overexpression of MFH/Foxp1 markedly attenuated phorbol ester-induced expression of c-fms, which encodes the M-CSF receptor and is obligatory for macrophage differentiation. This was accompanied by decreased CD11b expression, cell adhesiveness, and phagocytosis. Using electromobility shift and reporter assays, we have established that MFH/Foxp1 binds to previously uncharacterized sites within the c-fms promoter and functions as a transcriptional repressor. Deficiency of Mac-1 is associated with altered regulation of MFH/Foxp1 and monocyte maturation in vivo. Taken together, these observations suggest that Mac-1 engagement orchestrates monocyte-differentiation signals by regulating the expression of the forkhead transcription repressor MFH/Foxp1. This represents a new pathway for integrin-dependent modulation of gene expression and control of cellular differentiation.

Vav GEFs are required for beta2 integrin-dependent functions of neutrophils

Integrin regulation of neutrophils is essential for appropriate adhesion and transmigration into tissues. Vav proteins are Rho family guanine nucleotide exchange factors that become tyrosine phosphorylated in response to adhesion. Using Vav1/Vav3-deficient neutrophils (Vav1/3^ko), we show that Vav proteins are required for multiple β₂ integrin-dependent functions, including sustained adhesion, spreading, and complement-mediated phagocytosis. These defects are not attributable to a lack of initial β₂ activation as Vav1/3^ko neutrophils undergo chemoattractant-induced arrest on intercellular adhesion molecule-1 under flow. Accordingly, in vivo, Vav1/3^ko leukocytes arrest on venular endothelium yet are unable to sustain adherence. Thus, Vav proteins are specifically required for stable adhesion. β₂-induced activation of Cdc42, Rac1, and RhoA is defective in Vav1/3^ko neutrophils, and phosphorylation of Pyk2, paxillin, and Akt is also significantly reduced. In contrast, Vav proteins are largely dispensable for G protein-coupled receptor–induced signaling events and chemotaxis. Thus, Vav proteins play an essential role coupling β₂ to Rho GTPases and regulating multiple integrin-induced events important in leukocyte adhesion and phagocytosis.

Leukocyte engagement of fibrin(ogen) via the integrin receptor alphaMbeta2/Mac-1 is critical for host inflammatory response in vivo

The leukocyte integrin alpha(M)beta(2)/Mac-1 appears to support the inflammatory response through multiple ligands, but local engagement of fibrin(ogen) may be particularly important for leukocyte function. To define the biological significance of fibrin(ogen)-alpha(M)beta(2) interaction in vivo, gene-targeted mice were generated in which the alpha(M)beta(2)-binding motif within the fibrinogen gamma chain (N(390)RLSIGE(396)) was converted to a series of alanine residues. Mice carrying the Fibgamma(390-396A) allele maintained normal levels of fibrinogen, retained normal clotting function, supported platelet aggregation, and never developed spontaneous hemorrhagic events. However, the mutant fibrinogen failed to support alpha(M)beta(2)-mediated adhesion of primary neutrophils, macrophages, and alpha(M)beta(2)-expressing cell lines. The elimination of the alpha(M)beta(2)-binding motif on fibrin(ogen) severely compromised the inflammatory response in vivo as evidenced by a dramatic impediment in leukocyte clearance of Staphylococcus aureus inoculated into the peritoneal cavity. This defect in bacterial clearance was due not to diminished leukocyte trafficking but rather to a failure to fully implement antimicrobial functions. These studies definitively demonstrate that fibrin(ogen) is a physiologically relevant ligand for alpha(M)beta(2), integrin engagement of fibrin(ogen) is critical to leukocyte function and innate immunity in vivo, and the biological importance of fibrinogen in regulating the inflammatory response can be appreciated outside of any alteration in clotting function.

Deficiency of CD11b or CD11d Results in Reduced Staphylococcal Enterotoxin-Induced T Cell Response and T Cell Phenotypic Changes

The beta(2) integrin CD11a is involved in T cell-APC interactions, but the roles of CD11b, CD11c, and CD11d in such interactions have not been examined. To evaluate the roles of each CD11/CD18 integrin in T cell-APC interactions, we tested the ability of splenocytes of CD11-knockout (KO) mice to respond to staphylococcal enterotoxins (SEs), a commonly used superantigen. The defect in T cell proliferation with SEA was more severe in splenocytes from mice deficient in CD18, CD11b, or CD11d than in CD11a-deficient splenocytes, with a normal response in CD11c-deficient splenocytes. Mixing experiments showed that the defect of both CD11b-KO and CD11d-KO splenocytes was, unexpectedly, in T cells rather than in APC. Cytometric analysis failed to detect CD11b or CD11d on resting or activated T cells or on thymocytes of wild-type adult mice, nor did Abs directed to these integrins block responses in culture, suggesting that T cells educated in CD11b-KO or CD11d-KO mice were phenotypically altered. Consistent with this hypothesis, T cells from CD11b-KO and CD11d-KO splenocytes exhibited reduced intensity of CD3 and CD28 expression and decreased ratios of CD4/CD8 cells, and CD4(+) T cells were reduced among CD11b-KO and CD11d-KO thymocytes. CD11b and CD11d were coexpressed on a subset of early wild-type fetal thymocytes. We postulate that transient thymocyte expression of both CD11b and CD11d is nonredundantly required for normal thymocyte and T cell development, leading to phenotypic changes in T cells that result in the reduced response to SE stimulation.

Mac-1, but not LFA-1, uses intercellular adhesion molecule-1 to mediate slow leukocyte rolling in TNF-alpha-induced inflammation

We have previously shown that Mac-1 and LFA-1 play a cooperative role in slow leukocyte rolling in inflamed vessels, and that, although both have a role in leukocyte adhesion, the contribution from LFA-1 exceeds that of Mac-1. In this study, we used mice deficient in ICAM-1 (ICAM-1(null)) to study the function of ICAM-1 as an endothelial ligand for Mac-1 and LFA-1. The cremaster muscles of these mice were treated with TNF-alpha and prepared for intravital microscopy. We found that the average rolling velocity in venules was not different in ICAM-1(null) mice (4.7 micro m/s) compared with wild-type mice (5.1 micro m/s). Similarly, leukocyte adhesion efficiency in ICAM-1(null) mice (0.11 +/- 0.01 mm) was similar to that in Mac-1(-/-) (0.12 +/- 0.03 mm) mice but significantly increased compared with that in LFA-1(-/-) (0.08 +/- 0.01 mm) mice and significantly reduced from that in wild type (0.26 +/- 0.04 mm). When both LFA-1 and ICAM-1 were blocked, rolling velocity increased, and adhesion efficiency and arrest decreased. However, blocking both Mac-1 and ICAM-1 had no greater effect than either blockade alone. We conclude that endothelial ICAM-1 is the main ligand responsible for slow leukocyte rolling mediated by Mac-1, but not LFA-1.

Lymphocyte function antigen-1 mediates leukocyte adhesion and subsequent liver damage in endotoxemic mice

1. Sepsis is associated with leukocyte activation and recruitment in the liver. We investigated the role of lymphocyte function antigen-1 (LFA-1) in endotoxin-induced leukocyte-endothelium interactions, microvascular perfusion failure, hepatocellular injury and apoptosis in the liver by use of gene-targeted mice, blocking antibodies and a synthetic inhibitor of LFA-1 (LFA703). For this purpose, mice were challenged with lipopolysaccharide (LPS)+D-galactosamine (Gal), and intravital microscopy of the liver microcirculation was conducted 6 h later. 2. The number of firmly adherent leukocytes in response to LPS/Gal was reduced by 48% in LFA-1-deficient mice. Moreover, endotoxin-induced increases of apoptosis and enzyme markers of hepatocellular injury were decreased by 64 and 69-90%, respectively, in LFA-1-deficient mice. Furthermore, sinusoidal perfusion was improved in endotoxemic mice lacking LFA-1. 3. A similar protective pattern was observed in endotoxemic mice pretreated with an antibody against LFA-1. Thus, immunoneutralization of LFA-1 reduced endotoxin-induced leukocyte adhesion by 55%, liver enzymes by 64-66% and apoptosis by 42%, in addition to the preservation of microvascular perfusion. 4. Administration of a novel statin-derived inhibitor of LFA-1, LFA703, significantly decreased leukocyte adhesion (more than 56%) and the subsequent liver injury in endotoxemic mice. 5. Thus, this study demonstrates a pivotal role of LFA-1 in supporting leukocyte adhesion in the liver. Moreover, interference with LFA-1-mediated leukocyte adhesion protects against endotoxemic liver damage, and may constitute a potential therapeutic strategy in sepsis.

The murine myeloid cell line 32Dcl3 as a model system for studying neutrophil functions

The murine myeloid cell line 32Dcl3 is one of the few cell lines that can terminally differentiate into neutrophils. Granulocyte colony-stimulating factor (G-CSF) drives the differentiation of these cells; therefore, G-CSF receptor signaling for neutrophil proliferation and differentiation has been studied extensively using this cell line as a model. Differentiated 32Dcl3 cells exhibit a striking morphologic similarity to normal neutrophils; however, the degree to which differentiated 32Dcl3 cells are functionally similar to normal neutrophils remains unknown. In this study, we compared the function of differentiated 32Dcl3 cells with mouse neutrophils. Our results demonstrate that a subclone of differentiated 32Dcl3 cells (32Dcl3C) exhibits normal neutrophil functions of phagocytosis, degranulation, adhesion and shape change in response to appropriate stimuli. These observations suggest that this cell line can serve as an effective model system to study similar mature neutrophil functions. However, 32Dcl3C cells fail to produce superoxide in response proper stimuli.

SLP-76 regulates Fcgamma receptor and integrin signaling in neutrophils

While the contribution of intracellular adaptor proteins to lymphocyte activation has been well studied, the function of these molecules in innate immune effector cells such as neutrophils has not been extensively addressed. Here we demonstrate a critical role for the adaptor molecule SH2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76) in FcgammaR and integrin signaling. Stimulation of these receptors induces tyrosine phosphorylation and cytoplasmic relocalization of SLP-76 in freshly isolated murine neutrophils. Neutrophils lacking SLP-76 demonstrate decreased FcgammaR-induced calcium flux and reactive oxygen intermediate (ROI) production in response to immune complex stimulation. More dramatically, SLP-76-/- neutrophils fail to produce ROI, spread, or activate critical downstream regulators in response to integrin ligation. These results provide genetic evidence for a critical role of SLP-76 in the regulation of neutrophil function.

Liposomal Alendronate Inhibits Systemic Innate Immunity and Reduces In-Stent Neointimal Hyperplasia in Rabbits

Background— Innate immunity is of major importance in vascular repair. The present study evaluated whether systemic and transient depletion of monocytes and macrophages with liposome-encapsulated bisphosphonates inhibits experimental in-stent neointimal formation.

Methods and Results— Rabbits fed on a hypercholesterolemic diet underwent bilateral iliac artery balloon denudation and stent deployment. Liposomal alendronate (3 or 6 mg/kg) was given concurrently with stenting. Monocyte counts were reduced by >90% 24 to 48 hours after a single injection of liposomal alendronate, returning to basal levels at 6 days. This treatment significantly reduced intimal area at 28 days, from 3.88±0.93 to 2.08±0.58 and 2.16±0.62 mm 2 . Lumen area was increased from 2.87±0.44 to 3.57±0.65 and 3.45±0.58 mm 2 , and arterial stenosis was reduced from 58±11% to 37±8% and 38±7% in controls, rabbits treated with 3 mg/kg, and rabbits treated with 6 mg/kg, respectively (mean±SD, n=8 rabbits/group, P <0.01 for all 3 parameters). No drug-related adverse effects were observed. Reduction in neointimal formation was associated with reduced arterial macrophage infiltration and proliferation at 6 days and with an equal reduction in intimal macrophage and smooth muscle cell content at 28 days after injury. Conversely, drug regimens ineffective in reducing monocyte levels did not inhibit neointimal formation.

Conclusions— Systemic transient depletion of monocytes and macrophages, by a single liposomal bisphosphonates injection concurrent with injury, reduces in-stent neointimal formation and arterial stenosis in hypercholesterolemic rabbits.

Targeting platelet-leukocyte interactions: identification of the integrin Mac-1 binding site for the platelet counter receptor glycoprotein Ibalpha

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are dependent on the interaction of the leukocyte integrin Mac-1 (alphaMbeta2, CD11b/CD18) and the platelet counter receptor glycoprotein (GP) Ibalpha. Previous studies have established a central role for the I domain, a stretch of approximately 200 amino acids within the alphaM subunit, in the binding of GP Ibalpha. This study was undertaken to establish the molecular basis of GP Ibalpha recognition by alphaMbeta2. The P201-K217 sequence, which spans an exposed loop and amphipathic alpha4 helix in the three-dimensional structure of the alphaMI domain, was identified as the binding site for GP Ibalpha. Mutant cell lines in which the alphaMI domain segments P201-G207 and R208-K217 were switched to the homologous, but non-GP Ibalpha binding, alphaL domain segments failed to support adhesion to GP Ibalpha. Mutation of amino acid residues within P201-K217, H210-A212, T213-I215, and R216-K217 resulted in the loss of the binding function of the recombinant alphaMI domains to GP Ibalpha. Synthetic peptides duplicating the P201-K217, but not scrambled versions, directly bound GP Ibalpha and inhibited alphaMbeta2-dependent adhesion to GP Ibalpha and adherent platelets. Finally, grafting critical amino acids within the P201-K217 sequence onto alphaL, converted alphaLbeta2 into a GP Ibalpha binding integrin. Thus, the P201-K217 sequence within the alphaMI domain is necessary and sufficient for GP Ibalpha binding. These observations provide a molecular target for disrupting leukocyte-platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.

Molecular mechanism of nucleotide-induced primary granule release in human neutrophils: role for the P2Y2 receptor

Nucleotides are released during vascular injury from activated platelets and broken cells, which could stimulate human neutrophils. In this study, we characterized the P2Y receptors and investigated the functional effects of extracellular nucleotides on human neutrophils. Pharmacological characterization using selective agonists and pertussis toxin revealed that human neutrophils express only functional P2Y2 receptors. However, P2Y2 receptor agonists ATP or uridine triphosphate (UTP) caused intracellular Ca2+ increases in isolated human neutrophils with an EC50 of 1 microM but failed to cause release of primary granules from human neutrophils. ATP and UTP were equally potent in causing elastase release from human neutrophils in the presence of exogenous soluble fibrinogen, whereas ADP and UDP were without effect. We investigated whether nucleotides depend on generated arachidonic acid metabolites to cause degranulation. However, phenidone and MK-886, inhibitors of the 5-lipoxygenase pathway, failed to block nucleotide-induced intracellular calcium mobilization and elastase release. ATP and UTP caused activation of p38 MAPK and ERK1/2 in human neutrophils. In addition, the inhibitors of the MAPK pathway, SB-203580 and U-0126, inhibited nucleotide-induced elastase release. We conclude that fibrinogen is required for nucleotide-induced primary granule release from human neutrophils through the P2Y2 receptor without a role for arachidonic acid metabolites. Both ERK1/2 and p38 MAPK play an important role in nucleotide-induced primary granule release from human neutrophils.

Host resistance of CD18 knockout mice against systemic infection with Listeria monocytogenes

Mice with targeted mutations of CD18, the common beta2 subunit of CD11/CD18 integrins, have leukocytosis, impaired transendothelial neutrophil emigration, and reduced host defense to Streptococcus pneumoniae, a gram-positive extracellular bacterium. Previous studies using blocking monoclonal antibodies suggested roles for CD18 and CD11b in hepatic neutrophil recruitment and host innate response to Listeria monocytogenes, a gram-positive intracellular bacterium. We induced systemic listeriosis in CD18 knockout (CD18-ko) and wild-type (WT) mice by tail vein injection with Listeria. By 14 days postinjection (dpi), 8 of 10 WT mice died, compared with 2 of 10 CD18-ko mice (P < 0.01). Quantitative organ culture showed that numbers of Listeria organisms in livers and spleens were similar in both groups at 20 min postinfection. By 3, 5, and 7 dpi, however, numbers of Listeria organisms were significantly lower in livers and spleens of CD18-ko mice than in WT mice. Histopathology showed that following Listeria infection, CD18-ko mice had milder inflammatory and necrotizing lesions in both spleens and livers than did WT mice. Cytokine assays indicated that baseline interleukin-1beta and granulocyte colony-stimulating factor (G-CSF) levels were higher in CD18-ko mice than in WT mice and that CD18-ko splenocytes produced higher levels of interleukin-1beta and G-CSF than WT splenocytes under the same amount of Listeria stimulation. These findings show that CD18 is not an absolute requirement for antilisterial innate immunity or hepatic neutrophil recruitment. We propose that the absence of CD18 in the mice results in the priming of innate immunity, as evidenced by elevated cytokine expression, and neutrophilic leukocytosis, which augments antilisterial defense.

The fourth blade within the beta-propeller is involved specifically in C3bi recognition by integrin alpha M beta 2

Interactions between the complement degradation product C3bi and leukocyte integrin alpha M beta 2 are critical to phagocytosis of opsonized particles in host defense against foreign pathogens and certain malignant cells. Previous studies have mapped critical residues for C3bi binding to the I-domains of the alpha M and the beta2 subunits. However, the role of the alpha M beta-propeller in ligand binding remains less well defined, and the functional residues are still unknown. In the present study, we studied the function of the alpha M beta-propeller in specific ligand recognition by alpha M beta 2 using a number of different approaches, and we report four major findings. 1) Substitution of five individual segments (Asp398-Ala402, Leu412-Leu419, Tyr426-Met434, Phe435-Glu443, and Ser444-Thr451) within the W4 blade of the beta-propeller with their homologous counterparts in integrin alpha2 abrogated C3bi binding, whereas substitution of eight other segments outside this blade had no effect. 2) These five mutants defective in C3bi binding supported strong alpha M beta 2-mediated and cation-dependent cell adhesion to fibrinogen, suggesting that the conformations of these five defective mutants were intact. 3) Polyclonal antibodies recognizing sequences within the W4 blade significantly blocked C3bi binding by wild-type alpha M beta 2. 4) A synthetic peptide corresponding to Gln424-Gly440 within W4 interacted directly with C3bi. In conclusion, our data demonstrate that the W4 blade (residues Asp398 to Thr451) is involved specifically in C3bi but not fibrinogen binding to alpha M beta 2. Altogether, our study supports a model in which three separate domains of alpha M beta 2 (the alpha MI-domain, the alpha M beta-propeller, and the beta 2I-domain) function together and contribute to the formation of the C3bi-binding site.

Modulation of leukocyte populations and immune responses in sheep experimentally infected with Anaplasma (formerly Ehrlichia) phagocytophilum

Anaplasma phagocytophilum infection in sheep is characterized by an immune suppression as indicated by impaired antibody response, reduced lymphocyte response and reduced oxidative burst. The effect of A. phagocytophilum infection on leucocyte populations, especially lymphocytes, was therefore investigated in six sheep experimentally infected with A. phagocytophilum, and compared with leucocyte populations from control animals.To investigate the ability of the infection to interfere with the cellular and humoral responses to specific antigens, the animals were vaccinated with commercial vaccines at the time of experimental infection, and monitored for 56 days. There were reduced percentages of gammadelta T-cells and CD4+ T-cells in peripheral blood of infected animals throughout the study period, and these cell populations showed a down-regulation of CD25 expression; while there was a relative increase in CD8+ T-cells. The reduction in CD25+ gammadelta T-cells involved a subpopulation of WC1+ gammadelta T-cells. During the first 2 weeks of the study there were reduced percentages of B-cells and leukocytes expressing MHC II and CD11b, though this decrease changed to a relative increase later in the study. The relative reductions in leucocyte populations corresponded with the observed leucopenia during the first 3 weeks post-infection, which involved lymphocyte, neutrophil and eosinophil subsets [Vet. Immunol. Immunopathol. 86 (2002) 183]. There was a reduced expression of CD11b and CD14 on granulocytes during the first 2 weeks of the study, which corresponded with the previously reported leucopenia involving neutrophils and eosinophils. Antibody responses to vaccines, lymphocyte in vitro proliferative responses to antigens and mitogens, and in vitro IFN-gamma responses to antigens were reduced up to 4 weeks after infection.

A role for Syk-kinase in the control of the binding cycle of the beta2 integrins (CD11/CD18) in human polymorphonuclear neutrophils

A fine control of beta(2) integrin (CD11/CD18)-mediated firm adhesion of human neutrophils to the endothelial cell monolayer is required to allow ordered emigration. To elucidate the molecular mechanisms that control this process, intracellular protein tyrosine signaling subsequent to beta(2) integrin-mediated ligand binding was studied by immunoprecipitation and Western blotting techniques. The 72-kDa Syk-kinase, which was tyrosine-phosphorylated upon adhesion, was found to coprecipitate with CD18, the beta-subunit of the beta(2) integrins. Moreover, inhibition of Syk-kinase by piceatannol enhanced adhesion and spreading but diminished N-formyl-Met-Leu-Phe-induced chemotactic migration. The enhancement of adhesiveness was associated with integrin clustering, which results in increased integrin avidity. In contrast, piceatannol had no effect on the surface expression or on the affinity of beta(2) integrins. Altogether, this suggests that Syk-kinase controls alternation of beta(2) integrin-mediated ligand binding with integrin detachment.

Neutrophilia in LFA-1-deficient mice confers resistance to listeriosis: possible contribution of granulocyte-colony-stimulating factor and IL-17

LFA-1 (CD11a/CD18) plays a crucial role in various inflammatory responses. In this study, we show that LFA-1(-/-) mice are far more resistant to Listeria monocytogenes infection than LFA-1(+/-) mice. Consistent with this, we found the following: 1) the numbers of granulocytes infiltrating the liver were markedly higher in LFA-1(-/-) mice than in LFA-1(+/-) mice, 2) increased antilisterial resistance in LFA-1(-/-) mice was abrogated by depletion of granulocytes, and 3) the numbers of granulocytes in peripheral blood, and the serum levels of both G-CSF and IL-17 were higher in LFA-1(-/-) mice than in LFA-1(+/-) mice. Neither spontaneous apoptosis nor survival of granulocytes from LFA-1(-/-) mice were affected by physiological concentrations of G-CSF. Our data suggest regulatory effects of LFA-1 on G-CSF and IL-17 secretion, and as a corollary on neutrophilia. Consequently, we conclude that increased resistance of LFA-1(-/-) mice to listeriosis is due to neutrophilia facilitating liver infiltration by granulocytes promptly after L. monocytogenes infection, although it is LFA-1 independent.

Roles of neutrophil beta 2 integrins in kinetics of bacteremia, extravasation, and tick acquisition of Anaplasma phagocytophila in mice

Tick saliva contains anti-inflammatory and immunosuppressive substances that facilitate blood feeding and enhance tick-vectored pathogen transmission, including Anaplasma phagocytophila an etiologic agent of granulocytic ehrlichiosis. As such, inflammation at a tick-feeding site is strikingly different than that typically observed at other sites of inflammation. Up-regulation of CD11b/CD18 occurs in host granulocytes following interaction or infection with A phagocytophila, and the absence of CD11b/CD18 results in early increases in bacteremia. We hypothesized that beta 2 integrin-dependent infection kinetics and leukocyte extravasation are important determinants of neutrophil trafficking to, and pathogen acquisition at, tick-feeding sites. A phagocytophila infection kinetics were evaluated in CD11a/CD18, CD11b/CD18, and CD18 knock-out mice using quantitative polymerase chain reaction (PCR) of blood, ticks, and skin biopsies in conjunction with histopathology. A marked increase in the rate of A phagocytophila infection of neutrophils and pathogen burden in blood followed tick feeding. Infection kinetics were modified by beta 2 integrin expression and systemic neutrophil counts. Significant neutrophil-pathogen trafficking was observed to both suture and tick sites. Despite the prominent role for beta 2 integrins in neutrophil arrest in flowing blood, successful pathogen acquisition by ticks occurred in the absence of beta 2 integrins. Establishment of feeding pools that rely less on leukocyte trafficking and more on small hemorrhages may explain the ready amplification of A phagocytophila DNA from ticks infested on CD11/CD18-deficient mouse strains.

The interplay between integrins alphaMbeta2 and alpha5beta1 during cell migration to fibronectin

A directed migration of leukocytes through the extracellular matrix requires the regulated engagement of integrin cell adhesion receptors. The integrin alpha(M)beta(2) (CD11b/CD18, Mac-1) is progressively upregulated to high levels on migrating phagocytic leukocytes in response to inflammatory stimuli and is able to bind numerous ligands in the interstitial matrix. The role of alpha(M)beta(2) in migration of leukocytes through the extracellular matrix and its cooperation with other leukocyte integrins during migration are not understood. Using a model system consisting of cells that express different levels of alpha(M)beta(2) and an invariable level of endogenous integrin alpha(5)beta(1), we have explored a situation relevant to migrating neutrophils when alpha(M)beta(2) and alpha(5)beta(1) engage the same ligand, fibronectin. We show that fibronectin is a ligand for alpha(M)beta(2) and that both alpha(M)beta(2) and alpha(5)beta(1) on the alpha(M)beta(2)-expressing cells contribute to adhesion to fibronectin. However, migration of these cells to fibronectin is mediated by alpha(5)beta(1), whereas alpha(M)beta(2) retards migration. The decrease in migration correlates directly with the increased alpha(M)beta(2) density. Ligation of alpha(M)beta(2) with function-blocking antibodies can reverse this effect. The restorative effects of antibodies are caused by the removal of restraint imposed by the excess of alpha(M)beta(2)-fibronectin adhesive bonds. These findings indicate that alpha(M)beta(2) can increase general cell adhesiveness which results in braking of cell migration mediated by integrin alpha(5)beta(1). Because alpha(M)beta(2) binds numerous proteins in the extracellular matrix with a specificity overlapping that of the beta(1) integrins, the results suggest that alpha(M)beta(2) can affect the beta(1) integrin-mediated cell migration.

A molecular basis for integrin alphaMbeta 2 ligand binding promiscuity

The leukocyte integrin alpha(M)beta(2) is a highly promiscuous leukocyte receptor capable of binding a multitude of unrelated ligands. To understand the molecular basis for the broad ligand recognition of alpha(M)beta(2), the inter-integrin chimera was created. In the chimeric integrin, the betad-alpha5 loop-alpha5 helix segment comprised of residues Lys(245)-Arg(261) from the alpha(M)I domain of alpha(M)beta(2) was inserted into the framework of alpha(L)beta(2). The construct was expressed in HEK 293 cells, and the ability of generated cells to adhere to fibrinogen and its derivatives was characterized first. Grafting the alpha(M)(Lys(245)-Arg(261)) sequence converted alpha(L)beta(2) into a fibrinogen-binding protein capable of mediating efficient and specific adhesion similar to that of wild-type alpha(M)beta(2). Verifying a switch in the binding specificity of alpha(L)beta(2), the chimeric receptor became competent to support cell migration to fibrinogen. Mutations at positions Phe(246), Asp(254), and Pro(257) within Lys(245)-Arg(261) of alpha(M)beta(2) produced significant decreases in cell adhesion, illustrating the critical role of these residues in ligand binding. The insertion of alpha(M)(Lys(245)-Arg(261)) imparted to the chimeric integrin the ability to recognize many typical alpha(M)beta(2) protein ligands. Furthermore, cells expressing the chimeric receptor, but not alpha(L)beta(2), were able to stick to uncoated plastic, which represents the hallmark of wild-type alpha(M)beta(2). These results suggest that alpha(M)(Lys(245)-Arg(261)) serves as a consensus binding site for interaction with a variety of distinct molecules and, thus, may define the degenerate recognition properties inherent to alpha(M)beta(2).

Inactivation of the F4/80 glycoprotein in the mouse germ line

Macrophages play a crucial role in the defense against pathogens. Distinct macrophage populations can be defined by the expression of restricted cell surface proteins. Resident tissue macrophages, encompassing Kupffer cells of the liver and red pulp macrophages of the spleen, characteristically express the F4/80 molecule, a cell surface glycoprotein related to the seven transmembrane-spanning family of hormone receptors. In this study, gene targeting was used to simultaneously inactivate the F4/80 molecule in the germ line of the mouse and to produce a mouse line that expresses the Cre recombinase under the direct control of the F4/80 promoter (F4/80-Cre knock-in). F4/80-deficient mice are healthy and fertile. Macrophage populations in tissues can develop in the absence of F4/80 expression. Functional analysis revealed that the generation of T-cell-independent B-cell responses and macrophage antimicrobial defense after infection with Listeria monocytogenes are not impaired in the absence of F4/80. Interestingly, tissues of F4/80-deficient mice could not be labeled with anti-BM8, another macrophage subset-specific marker with hitherto undefined molecular antigenic structure. Recombinant expression of a F4/80 cDNA in heterologous cells confirmed this observation, indicating that the targets recognized by the F4/80 and BM8 monoclonal antibodies are identical.

CD11b and intercellular adhesion molecule-1 are involved in pulmonary neutrophil recruitment in lipopolysaccharide-induced airway disease

To better define the roles of CD11b, CD11a, and one of their endothelial cell receptors, intercellular adhesion molecule-1 (ICAM-1), in the lower respiratory tract inflammatory response to inhaled lipopolysaccharide (LPS), we evaluated the physiologic and biologic response to inhaled LPS in mice receiving anti-CD11b antibody, anti-CD11a antibody, and anti-ICAM-1 antibody. Mice receiving anti-CD11b antibody had a dramatic reduction in pulmonary neutrophil recruitment compared with control mice (18,300 versus 143,000 cells/ml, and neutrophils 16.7% versus 77%), whereas mice receiving anti-CD11a antibody did not demonstrate a reduction in lavage cellularity. Mice receiving anti-ICAM-1 antibody also demonstrated a dose-dependent reduction in inflammatory cell recruitment to the alveolar space. Despite the significant reduction in inflammatory cell infiltrate in mice receiving either CD11b or ICAM-1 antibodies, there was no reduction in the development of airway hyperreactivity. These findings suggest that CD11b and ICAM-1 are important mediators of LPS-induced airway inflammation, but do not appear to be critical to the development of LPS-induced airway hyperreactivity.

The effect of isoflurane on neutrophil selectin and beta(2)-integrin activation in vitro

Isoflurane is reported to reduce ischemia-reperfusion injury. Lower expression of CD11b may be responsible for attenuated postischemic neutrophil adhesion to vascular endothelium. However, neutrophil adhesion to vascular endothelium is a multistep process involving several selectins and beta(2)-integrins. Therefore, we assessed whether isoflurane affects the activation of the selectins P-selectin glycoprotein ligand-1 (PSGL-1) and L-selectin and the beta(2)-integrins CD11a and CD11b. Whole blood was incubated for 60 min with 0.5 or 1 minimum alveolar anesthetic concentration (MAC) isoflurane. After incubation, neutrophils were activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP) or phorbol-12-myristate-13-acetate (PMA). Activation of adhesion molecules was evaluated via flow cytometry, and 1 MAC isoflurane reduced the expression of CD11a in the unstimulated samples. After stimulation with FMLP and PMA, shedding of L-selectin was lower in the presence of isoflurane. Furthermore, 1 MAC isoflurane reduced FMLP-induced activation of CD11a and CD11b compared with unexposed blood samples. These results demonstrate that isoflurane affects the activation of three adhesion molecules involved in the multistep process of neutrophil recruitment. First, isoflurane inhibits the activation of L-selectin, which mediates the neutrophil tethering and rolling on the vascular endothelium. Second, isoflurane attenuates the activation of both beta(2)-integrins-CD11a and CD11b-which mediate firm adhesion and transendothelial migration.

IMPLICATIONS

Adhesion of neutrophils to endothelial cells in reperfusion injury is mediated by different adhesion molecules. This study indicates that the inhibiting effect of isoflurane on neutrophil recruitment may be mediated by a decreased activation of the L-selectin and by attenuation of the activation of the beta(2)-integrins CD11a and CD11b.

Macrophage depletion by clodronate-containing liposomes reduces neointimal formation after balloon injury in rats and rabbits

BACKGROUND

Inflammation is critical to vascular repair after mechanical injury. Excessive inflammation enhances neointimal formation and restenosis. We examined whether transient systemic inactivation of macrophages at the time of vascular intervention could attenuate the degree of expected restenosis.

METHODS AND RESULTS

Liposomal clodronate (LC) inhibited the growth of cultured macrophages but had no effect on endothelial or smooth muscle cells and suppressed neointimal hyperplasia in hypercholesterolemic rabbits and rats after intravenous administration of LC, with no adverse effects. LC treatment reduced the number of blood monocytes and decreased macrophage infiltration in the injured arteries as well as smooth muscle cell proliferation, interleukin-1beta transcription, and production and matrix metalloproteinase-2 activity.

CONCLUSIONS

Macrophages play a pivotal role in vascular repair after mechanical arterial injury. Systemic inactivation and depletion of monocytes and macrophages by LC reduce neointimal hyperplasia and restenosis.

Dynamic regulation of LFA-1 activation and neutrophil arrest on intercellular adhesion molecule 1 (ICAM-1) in shear flow

Neutrophil recruitment during acute inflammation is triggered by G-protein-linked chemotactic receptors that in turn activate beta(2) integrin (CD18), deemed a critical step in facilitating cell capture and arrest under the shear force of blood flow. A conformational switch in the I domain allosteric site (IDAS) and in CD18 regulates LFA-1 affinity for endothelial ligands including intercellular adhesion molecule 1 (ICAM-1). We examined the dynamics of CD18 activation in terms of the efficiency of neutrophil capture of ICAM-1, and we correlated this with the membrane topography of 327C, an antibody that recognizes the active conformation of CD18 I-like domain. Adhesion increased in direct proportion to chemotactic stimulus rising 7-fold over a log range of interleukin-8 (IL-8). A threshold dose of approximately 75 pm IL-8, corresponding to ligation of only approximately 10-100 receptors, was sufficient to activate approximately 20,000 CD18 and a rapid boost in the capture efficiency on ICAM-1. This was accompanied by a rapid redistribution of active LFA-1, but not Mac-1, into membrane patches, a necessary component for optimum adhesion efficiency. Shear-resistant arrest on a monolayer of ICAM-1 was reversed within minutes of chemotactic stimulation correlating with a shift from high to low affinity CD18 and dispersal of patches of active CD18. Mobility of active CD18 into high avidity patches was dependent on phosphatidylinositol 3-kinase activity and not F-actin polymerization. The data reveal that the number of chemotactic receptors bound and the topography and lifetime of high affinity LFA-1 tightly regulate the efficiency of neutrophil capture on ICAM-1.

Absence of GPIbalpha is responsible for aberrant membrane development during megakaryocyte maturation: ultrastructural study using a transgenic model

OBJECTIVE

The glycoprotein Ib/IX/V complex (GPIb-IX-V) mediates platelet attachment to von Willebrand factor in exposed subendothelium. Molecular defects in the genes for GPIbalpha, GPIbbeta, and GPIX give rise to the Bernard-Soulier syndrome, in which thrombocytopenia and giant platelets suggest that this receptor also is involved in platelet production. To study how giant platelets are produced in vivo, we used a model of GPIbalpha-deficient mice (GPIbalpha(null)) and mice rescued with the human GPIbalpha transgene (GPIbalpha(null;hTg)).

MATERIALS AND METHODS

Using electron microscopy and immunogold labeling, we examined megakaryocytopoiesis in the bone marrow of these mice and developed a method to quantify the membranes of megakaryocytes (MK) and proplatelets by computer analysis.

RESULTS

Abnormal membrane development in the perinuclear zone was found in immature MK of GPIbalpha(null) mice. This led to a poorly developed demarcation membrane system and other ultrastructural changes. As a result, well-organized platelet territories were rarely seen within the cytoplasm of mature MK. Membrane quantification confirmed that MK of GPIbalpha(null) mice had a reduced internal membrane pool. Whereas these MK normally crossed the endothelial barrier, their migration was accompanied by the production of unusually large MK fragments or proplatelets in the vascular sinus with an approximately 50% decrease in internal membrane content compared to wild-type. In the rescued GPIbalpha(null;hTg) model, GPIbalpha was normally localized in MK, and there was a total correction of the ultrastructural defects.

CONCLUSIONS

GPIbalpha is essential for membrane development and distribution in maturing MK. Its absence leads to abnormal partitioning of the membrane systems and abnormal proplatelet production.

Syk is required for integrin signaling in neutrophils

The Syk tyrosine kinase plays a critical role in the signaling machinery of various receptors of the adaptive immune system. Here we show that Syk is also an essential component of integrin signaling in neutrophils. syk(-/-) neutrophils failed to undergo respiratory burst, degranulation, or spreading in response to proinflammatory stimuli while adherent to immobilized integrin ligands or when stimulated by direct crosslinking of integrins. Signaling from the beta(1), beta(2), or beta(3) integrins was defective in syk(-/-) cells. Syk colocalized with CD18 during cell spreading and initiated downstream signaling events leading to actin polymerization. Surprisingly, these defects in integrin-mediated activation did not impair the integrin-dependent in vitro or in vivo migration of syk(-/-) neutrophils or of cells deficient in Src-family kinases. Thus, integrins use different signaling mechanisms to support migration and adherent activation.

Kinetics of CD11b/CD18 up-regulation during infection with the agent of human granulocytic ehrlichiosis in mice

The agent of human granulocytic ehrlichiosis (aoHGE) is a tick-borne, obligate intracellular, granulocytotropic bacterium able to infect numerous host species. Given its unique niche and the leukopenia often noted with infection, we investigated the effect of acute aoHGE infection on neutrophil activation by evaluating surface expression of the beta 2 integrin CD11b/CD18 in a mouse model using FACS analysis. Infection resulted in neutrophil activation with up-regulation of CD11b/CD18 in multiple strains of mice, however, hematologic analysis showed no apparent role for CD11b/CD18 in mediating peripheral leukopenia. Because IFN-gamma is an important cytokine during granulocytic ehrlichiosis and is known to activate leukocytes, we investigated the potential role of IFN-gamma in CD11b/CD18 up-regulation. Neutrophils from IFN-gamma knock-out mice became activated during aoHGE infection, however, the kinetics of activation differed from wild-type mice. In addition, activation correlated directly with the presence of bacteria because neutrophils with large intracytoplasmic morula also expressed higher levels of CD11b/CD18. CD11b/CD18 seemed to be critical to early bacterial clearance and killing in vivo because infection of mice with targeted genetic disruption of CD11b/CD18 resulted in an initial increase in bacterial burden compared with wild-type mice. Similarly, in vitro culture of neutrophils from infected CD11b/CD18 knock-out mice resulted in a marked increase in bacterial proliferation compared with congenic controls. The data support crucial roles of CD11b/CD18 and IFN-gamma-mediated cell activation as mechanisms that limit bacterial replication.

A new assay to monitor the degranulation process in phagocytizing human neutrophils

oxidation, we set up a method for measuring MPO intraphagosomal release in human neutrophils. The method is based on the passive engulfment of DAB together with the phagocytosable particle. Inside the vacuole, this substrate is oxidized by MPO released from the azurophilic granules. The colorimetrical evaluation of the amount of DAB oxidized allows for cheap, rapid quantification of MPO intraphagosomal secretion in whole cells. Using this method, we show that the degranulation process, involving azurophilic granules, can be monitored carefully during phagocytosis. It takes place after the ingestion of zymosan particles opsonized with normal human serum, as well as during IgG-mediated phagocytosis and under conditions where beta2 integrins are blocked. However our findings also show that the extent of intraphagosomal secretion depends on either the extent of opsonization or the type of receptor engaged during the phagocytic event.

Pathways and bottlenecks in the web of inflammatory adhesion molecules and chemoattractants

Leukocyte recruitment requires capture, rolling, activation, adhesion, and transmigration. Adhesion molecules and chemoattractants have been identified that mediate each of these steps. Their functions often overlap, but the combined absence of some molecules can lead to severe spontaneous phenotypes, as seen in CD18-/- mice, or early lethality, as in CD18-/- E-selectin-/- mice. These groups of molecules define bottlenecks that restrict the inflammatory response. Adhesion molecules and activation mechanisms can also form groups of preferential usage, or pathways. Based on these findings, a web-like model may represent the inflammatory process better than the linear cascade model. Bottlenecks and pathways depend on the degree and nature of overlapping functions, the disease process, tissue site, and the inflammatory stimulus.

Control of leukocyte rolling velocity in TNF-alpha-induced inflammation by LFA-1 and Mac-1

Previously it was shown that beta(2)-integrins are necessary for slow leukocyte rolling in inflamed venules. In this study, mice that are deficient for either one of the beta(2)-integrins, alpha(L)beta(2) (LFA-1) or alpha(M)beta(2) (Mac-1), were used to determine which of the beta(2)-integrins are responsible for slowing rolling leukocytes. The cremaster muscles of these mice were treated with tumor necrosis factor-alpha and prepared for intravital microscopy. The average rolling velocities in venules were elevated in LFA-1(-/-) mice (11.0 +/- 0.7 microm/s) and Mac-1(-/-) mice (10.1 +/- 1.1 microm/s) compared to wild-type mice (4.8 +/- 0.3 microm/s; P <.05), but were lower than in CD18(-/-) mice (28.5 +/- 2.1 microm/s). When both LFA-1 and Mac-1 were absent or blocked, rolling velocity became dependent on shear rate and approached that of CD18(-/-) mice. In addition, leukocyte adhesion efficiency was decreased in LFA-1(-/-) mice to near CD18(-/-) levels, but decreased only slightly in Mac-1(-/-) mice. Thus, both LFA-1 and Mac-1 contribute to slowing down rolling leukocytes, although LFA-1 is more important than Mac-1 in efficiently inducing firm adhesion.

Nervous system pathology: the fibrin perspective

Studies of extracellular matrix (ECM) biology in the nervous system have mainly focused on laminin, fibronectin and tenascin-R, proteins that are present during nervous system development and normal function. However, during disease, fibrin, which physiologically is not present in the nervous tissue, is detected at nervous tissue lesions. This review summarizes evidence that correlates fibrin deposition with neuropathology and presents recent findings on cellular mechanisms and intracellular signaling pathways regulated by fibrin that might contribute to nervous system disease.

Increased granulopoiesis through interleukin-17 and granulocyte colony-stimulating factor in leukocyte adhesion molecule-deficient mice

Many mutant mice deficient in leukocyte adhesion molecules display altered hematopoiesis and neutrophilia. This study investigated whether peripheral blood neutrophil concentrations in these mice are elevated as a result of accumulation of neutrophils in the circulation or altered hematopoiesis mediated by a disrupted regulatory feedback loop. Chimeric mice were generated by transplanting various ratios of CD18(+/+) and CD18(-/-) unfractionated bone marrow cells into lethally irradiated wild-type mice, resulting in approximately 0%, 10%, 50%, 90%, or 100% CD18 null neutrophils in the blood. The presence of only 10% CD18(+/+) neutrophils was sufficient to prevent the severe neutrophilia seen in mice reconstituted with CD18(-/-) bone marrow cells. These data show that the neutrophilia in CD18(-/-) mice is not caused by enhanced neutrophil survival or the inability of neutrophils to leave the vascular compartment. In CD18(-/-), CD18(-/-)E(-/-), CD18(-/-)P(-/-), EP(-/-), and EPI(-/-) mice, levels of granulocyte colony-stimulating factor (G-CSF) and interleukin-17 (IL-17) were elevated in proportion to the neutrophilia seen in these mice, regardless of the underlying mutation. Antibiotic treatment or the propensity to develop skin lesions did not correlate with neutrophil counts. Blocking IL-17 or G-CSF function in vivo significantly reduced neutrophil counts in severely neutrophilic mice by approximately 50% (P <.05) or 70% (P <.01), respectively. These data show that peripheral blood neutrophil numbers are regulated by a feedback loop involving G-CSF and IL-17 and that this feedback loop is disrupted when neutrophils cannot migrate into peripheral tissues.