Neutrophils emigrate from venules by a transendothelial cell pathway in response to FMLP

Heterogeneity of endothelial junctions is reflected by differential expression and specific subcellular localization of the three JAM family members

Endothelial cells are linked to each other through intercellular junctional complexes that regulate the barrier and fence function of the vascular wall. The nature of these intercellular contacts varies with the need for permeability: For example, in brain the impervious blood-brain barrier is maintained by "tight" contacts between endothelial cells. By contrast, in high endothelial venules (HEVs), where lymphocytes continuously exit the bloodstream, the contacts are generally leaky. The precise molecular components that define the type of junction remain to be characterized. An immunoglobulin superfamily molecule named JAM-2, specifically expressed in lymphatic endothelial cells and HEVs, was recently identified. JAM-3 was cloned and characterized in the current study, and JAM-1, -2, and -3 were shown to form a novel protein family belonging to the larger cortical thymocyte Xenopus (CTX) molecular family. Using antibodies specific for each of the 3 family members, their specific participation in different types of cell-cell contact in vivo and their specific and differential localization in lateral contacts or tight junctions were demonstrated. Furthermore, it was shown that JAM-1 and JAM-2 differentially regulate paracellular permeability, suggesting that the presence of JAM-1, -2, or -3 in vascular junctions may play a role in regulating vascular function in vivo.

Organ-specificity of the extravasation process: an ultrastructural study

The process of extravasation of the high metastatic Lewis lung carcinoma line was examined in different organs. Four of the five organs (liver, lungs, brain and adrenals) represent the most frequent metastatic sites in humans. In the case of each organ 150-350 tumor cells were analysed. The interaction of tumor cells with endothelial cells and the basement membrane showed significant differences between the organs. In the liver and lungs, endothelial cells were found to migrate onto the surface of the tumor cells, resulting in the removal of tumor cells from the circulation. The process was initiated by development of cytoplasmic projections on the luminal surface of the endothelial cells. In the liver only half of the tumor cells showed basement membrane degradation even after 24 h, although 6 h after injection 40% of the tumor cells were sequestered from the circulation. In the adrenals and brain, tumor cells were not covered by endothelial cells instead, limited retraction of endothelial cells was followed by penetration of the basement membrane. In the kidney both types of tumor cell-endothelial cell interactions were observed, but the process of extravasation was not completed, stopping as the tumor cells reached the basement membrane or the mesangial matrix. The time course of tumor cell extravasation also showed significant differences between the organs. The process was most rapid in case of the liver and adrenals. By 6 h 40-50% of the tumor cells were in the process of extravasation or were in an extracapillary position. These organs are preferential metastatic sites of this tumor line. The time of extravasation was much longer in the other organs (lungs 16 h, brain 48 h), for which this tumor line shows no preference.

CONCLUSIONS

(1) Type and duration of tumor cell extravasation differ between the organs. (2) The time needed to reach extraluminal position, but not the type of extravasation correlates with the organ preference. (3) Endothelial cells of the lungs and liver can play a much more active role in the process of extravasation than previously suggested. (4) Tumor cells can complete the metastatic process without reaching a complete extracapillary position; contact with the basement membrane or extracellular matrix seems to be sufficient.

Real-time imaging of vascular endothelial-cadherin during leukocyte transmigration across endothelium

Vascular endothelial-cadherin (VE-cadherin) is a component of the adherens junctions of endothelial cells whose role in endothelial transmigration of leukocytes has been controversial. Using a VE-cadherin/green fluorescent protein fusion construct (VEcadGFP) that mimics the native molecule, we visualized alterations in endothelial junctional structure in real time during transmigration of human neutrophils and monocytes in an in vitro flow model. We observed abundant transmigration occurring exclusively at the cell borders (paracellularly). Surprisingly, transmigration occurred both through de novo formation of transient gaps in VEcadGFP junctional distribution, and also through preexisting gaps. De novo gaps 4-6 microm in size were formed after a leukocyte arrived at a junction, whereas preexisting gaps were present even before the leukocyte had interacted with the endothelial cells contributing to a junction. Gaps rapidly resealed within 5 min after leukocyte transmigration. Migrating leukocytes appeared to push aside VEcadGFP in the plane of the junction, and this displaced material subsequently diffused back to refill the junction. To our knowledge, this is the first example where molecular events at the lateral junction have been tracked in real time during transmigration.

The vesiculo-vacuolar organelle (VVO). A new endothelial cell permeability organelle

A newly defined endothelial cell permeability structure, termed the vesiculo-vacuolar organelle (VVO), has been identified in the microvasculature that accompanies tumors, in venules associated with allergic inflammation, and in the endothelia of normal venules. This organelle provides the major route of extravasation of macromolecules at sites of increased vascular permeability induced by vascular permeability factor/vascular endothelial growth factor (VPF/VEGF), serotonin, and histamine in animal models. Continuity of these large sessile structures between the vascular lumen and the extracellular space has been demonstrated in kinetic studies with ultrastructural electron-dense tracers, by direct observation of tilted electron micrographs, and by ultrathin serial sections with three-dimensional computer reconstructions. Ultrastructural enzyme-affinity cytochemical and immunocytochemical studies have identified histamine and VPF/VEGF bound to VVOs in vivo in animal models in which these mediators of permeability are released from mast cells and tumor cells, respectively. The high-affinity receptor for VPF/VEGF, VEGFR-2, was localized to VVOs and their substructural components by pre-embedding ultrastructural immunonanogold and immunoperoxidase techniques. Similar methods were used to localize caveolin and vesicle-associated membrane protein (VAMP) to VVOs and caveolae, indicating a possible commonality of formation and function of VVOs to caveolae.

The neutrophil: function and regulation in innate and humoral immunity

The neutrophil is a critical effector cell in humoral and innate immunity and plays vital roles in phagocytosis and bacterial killing. Discussed here are the neutrophil components necessary for these processes and the diseases in which these components are either lacking or dysfunctional, illustrating that normal neutrophil function is vital for health.

CXC chemokine receptor 2 but not C-C chemokine receptor 1 expression is essential for neutrophil recruitment to the cornea in helminth-mediated keratitis (river blindness)

Infiltration of neutrophils and eosinophils into the mammalian cornea can result in loss of corneal clarity and severe visual impairment. To identify mediators of granulocyte recruitment to the corneal stroma, we determined the relative contribution of chemokine receptors CXC chemokine receptor (CXCR)-2 (IL-8R homologue) and CCR1 using a murine model of ocular onchocerciasis (river blindness) in which neutrophils and eosinophils migrate from peripheral vessels to the central cornea. CXCR2(-/-) and CCR1(-/-) mice were immunized s.c. and injected into the corneal stroma with Ags from the parasitic helminth Onchocerca volvulus. We found that production of macrophage-inflammatory protein (MIP)-2, KC, and MIP-1 alpha was localized to the corneal stroma, rather than to the epithelium, which was consistent with the location of neutrophils in the cornea. CCR1 deficiency did not inhibit neutrophil or eosinophil infiltration to the cornea or development of corneal opacification. In marked contrast, neutrophil recruitment to the corneas of CXCR2(-/-) mice was significantly impaired (p < 0.0001 compared with control, BALB/c mice) with only occasional neutrophils detected in the central cornea. Furthermore, CXCR2(-/-) mice developed only mild corneal opacification compared with BALB/c mice. These differences were not due to impaired KC and MIP-2 production in the corneal stroma of CXCR2(-/-) mice, which was similar to BALB/c mice. Furthermore, although MIP-1 alpha production was lower in CXCR2(-/-) mice than BALB/c mice, eosinophil recruitment to the cornea was not impaired. These observations demonstrate the critical role for CXCR2 expression in neutrophil infiltration to the cornea and may indicate a target for immune intervention in neutrophil-mediated corneal inflammation.

Neutrophil diapedesis: paracellular or transcellular?

To reach an inflammatory site in the interstitium, circulating neutrophils (PMN) must first traverse the endothelial barrier. Whether PMN emigrate between endothelial cells (paracellular pathway) or through the endothelial cells proper (transcellular pathway) is controversial. Herein, we present anatomic, functional, and teleological arguments that support both points of view. An attempt is also made to reconcile this apparent controversy.

Neutrophil-platelet interactions and their relevance to bovine respiratory disease

Respiratory disease is a serious and significant health problem for the bovine industry. Classically, the clinical and research focus has been on the putative causative agents and conditions, and their interactions with host inflammatory cells, particularly alveolar macrophages and blood neutrophils. There is, currently, growing acceptance of the concept that blood platelets play a primary role in the inflammatory process. This review explores the implications of such pro-inflammatory activity, especially in the context of neutrophil-platelet interactions, and the species specificity of cellular responses. The relevance of these issues for the treatment and prevention of bovine respiratory disease is also discussed.

Regulation of neutrophil adhesion by pituitary growth hormone accompanies tyrosine phosphorylation of Jak2, p125FAK, and paxillin

Neutrophil adhesion is fundamentally important during the onset of inflammatory responses. The adhesion signaling pathways control neutrophil arrest and extravasation and influence neutrophil shape and function at sites of inflammation. In the present study the intracellular signaling pathways for the adhesion of human neutrophils by pituitary growth hormone (GH) were examined. Pituitary GH triggered the tyrosine phosphorylation of Janus kinase 2 (Jak2) and STAT3 in neutrophils. In addition, pituitary GH treatment resulted in the morphological changes and the tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin. Preincubation with genistein, a tyrosine kinase inhibitor, blocked the GH-stimulated adhesion and Jak2, STAT3, p125FAK, and paxillin phosphorylation. Confocal microscopy revealed that pituitary GH stimulates the focal localization of p125FAK, paxillin, phosphotyrosine, and filamentous actin filament into the membrane rufflings and uropods of human neutrophils. Immunoprecipitation experiments revealed a physical association of Jak2 with p125FAK via STAT3 in vivo. Also an in vitro kinase assay showed an augmentation of p125FAK autophosphorylation as a result of pituitary GH treatment. These results suggest that pituitary GH modulates neutrophil adhesion through tyrosine phosphorylation of Jak2, p125FAK, and paxillin and actin polymerization.

An anti-inflammatory property of aprotinin detected at the level of leukocyte extravasation

BACKGROUND

Aprotinin is a serine protease inhibitor used extensively in cardiac operations to reduce postoperative bleeding. It has also been used in trials aimed at reducing the systemic inflammatory response to cardiopulmonary bypass. It remains unclear whether the anti-inflammatory action of aprotinin is related to its general ability to suppress leukocyte activation or whether aprotinin can exercise effects during the leukocyte-endothelial cell adhesion cascade.

METHODS

We used intravital microscopy to study the 3 main stages of the adhesion cascade (leukocyte rolling, firm adhesion, and extravasation) within the mesenteric microcirculation of rats. This in vivo technique allows leukocyte recruitment to be viewed directly through the transparent mesentery of anesthetized animals.

RESULTS

Aprotinin, given by continuous infusion at a clinically relevant dose, exerted no effect on the rolling or firm adhesion responses toward local chemoattractant N -formyl-methyl-leucyl-phenylalanine but significantly inhibited extravasation of leukocytes (73% at 40 minutes, P =.04) into surrounding tissues. In parallel in vitro experiments, aprotinin (used at 200, 800, and 1600 kIU/mL) dose dependently inhibited neutrophil transmigration through cultured endothelial cells in response to 3 different chemoattractants: N -formyl-methyl-leucyl-phenylalanine (P <.001 at 800 and 1600 kIU/mL), interleukin 8 (P <.05 at 200 kIU/mL and P <.001 at 800 and 1600 kIU/mL), and platelet-activating factor (P <.05 at 1600 kIU/mL).

CONCLUSIONS

Our studies have therefore revealed a novel anti-inflammatory mechanism of aprotinin operating at the level of leukocyte extravasation. These findings may be relevant in the prevention of systemic inflammation after cardiopulmonary bypass through the use of protease inhibitors.

Deletion of the murine Duffy gene (Dfy) reveals that the Duffy receptor is functionally redundant

All of the antigenic determinants of the Duffy blood group system are in a glycoprotein (gp-Fy), which is encoded by a single-copy gene (FY) located on chromosome 1. gp-Fy is also produced in several cell types, including endothelial cells of capillary and postcapillary venules, the epithelial cell of kidney collecting ducts, lung alveoli, and the Purkinje cells of the cerebellum. This protein, which spans the cell membrane seven times, is a member of the superfamily of chemokine receptors and a malarial parasite receptor. The mouse Duffy gene (Dfy) homolog of human FY is also a single-copy gene, which maps in a region of conserved synteny with FY and produces a glycoprotein with 60% homology to the human protein. The mouse Duffy-like protein also binds chemokines. To study the biological role of gp-Fy, we generated a mouse strain in which Dfy was deleted. These homozygous Dfy(-/-) mice were indistinguishable in size, development, and health from wild-type and heterozygous littermates. We also examined components of the immune system and found no differences in lymph nodes or peripheral blood leukocyte levels between knockout and wild-type mice. The gross and histological anatomy of the thymus, spleen, lung, and brain showed no significant differences between mutants and wild-type mice. There was no indication of an overall difference between the knockout and wild-type mice in systematic neurological examinations. The only significant difference between Dfy(-/-) and Dfy(+/+) mice that we found was in neutrophil migration in peritoneal inflammations induced by lipopolysaccharide and thioglycolate. In mice homozygous for the deletion, there was less neutrophil recruitment into the peritoneal cavity and neutrophil influx in the intestines and lungs than in wild-type mice. Despite this, the susceptibility to Staphylococcus aureus infection was the same in the absence and in the presence of gp-Fy. Our results indicate that gp-Fy is functionally a redundant protein that may participate in the neutrophil migratory process.

Polarization and interaction of adhesion molecules P-selectin glycoprotein ligand 1 and intercellular adhesion molecule 3 with moesin and ezrin in myeloid cells

In response to the chemoattractants interleukin 8, C5a,N-formyl-methionyl-leucyl-phenylalanine, and interleukin 15, adhesion molecules P-selectin glycoprotein ligand 1 (PSGL-1), intercellular adhesion molecule 3 (ICAM-3), CD43, and CD44 are redistributed to a newly formed uropod in human neutrophils. The adhesion molecules PSGL-1 and ICAM-3 were found to colocalize with the cytoskeletal protein moesin in the uropod of stimulated neutrophils. Interaction of PSGL-1 with moesin was shown in HL-60 cell lysates by isolating a complex with glutathione S-transferase fusions of the cytoplasmic domain of PSGL-1. Bands of 78- and 81-kd were identified as moesin and ezrin by Western blot analysis. ICAM-3 and moesin also coeluted from neutrophil lysates with an anti-ICAM-3 immunoaffinity assay. Direct interaction of the cytoplasmic domains of ICAM-3 and PSGL-1 with the amino-terminal domain of recombinant moesin was demonstrated by protein-protein binding assays. These results suggest that the redistribution of PSGL-1 and its association with intracellular molecules, including the ezrin-radixin-moesin actin-binding proteins, regulate functions mediated by PSGL-1 in leukocytes stimulated by chemoattractants.

Polarization and interaction of adhesion molecules P-selectin glycoprotein ligand 1 and intercellular adhesion molecule 3 with moesin and ezrin in myeloid cells

In response to the chemoattractants interleukin 8, C5a,N-formyl-methionyl-leucyl-phenylalanine, and interleukin 15, adhesion molecules P-selectin glycoprotein ligand 1 (PSGL-1), intercellular adhesion molecule 3 (ICAM-3), CD43, and CD44 are redistributed to a newly formed uropod in human neutrophils. The adhesion molecules PSGL-1 and ICAM-3 were found to colocalize with the cytoskeletal protein moesin in the uropod of stimulated neutrophils. Interaction of PSGL-1 with moesin was shown in HL-60 cell lysates by isolating a complex with glutathione S-transferase fusions of the cytoplasmic domain of PSGL-1. Bands of 78- and 81-kd were identified as moesin and ezrin by Western blot analysis. ICAM-3 and moesin also coeluted from neutrophil lysates with an anti-ICAM-3 immunoaffinity assay. Direct interaction of the cytoplasmic domains of ICAM-3 and PSGL-1 with the amino-terminal domain of recombinant moesin was demonstrated by protein-protein binding assays. These results suggest that the redistribution of PSGL-1 and its association with intracellular molecules, including the ezrin-radixin-moesin actin-binding proteins, regulate functions mediated by PSGL-1 in leukocytes stimulated by chemoattractants.

The parting of the endothelium: miracle, or simply a junctional affair?

Leukocyte extravasation from the blood across the endothelium is vital for the functioning of the immune system. Our understanding of the early steps of this process has developed rapidly. However, it is still unclear how leukocytes undergo the final step, migrating through the junctions that mediate adhesion between adjacent endothelial cells, while preserving the barrier function of the endothelium. The first stage of transmigration - tethering and rolling - is mediated by interactions between selectins on the surface of leukocytes and glycosylated proteins such as GlyCAM-1 on the surface of endothelial cells. Stimulation of the leukocyte by chemokines then induces tight adhesion, which involves binding of activated leukocyte integrins to endothelial ICAM-1/VCAM-1 molecules. Passage of the leukocyte across the endothelium appears to require delocalization of certain endothelial cell molecules and proteolytic degradation of junctional complexes.

Roles of alpha(4) integrins/VCAM-1 and LFA-1/ICAM-1 in the binding and transendothelial migration of T lymphocytes and T lymphoblasts across high endothelial venules

Several cell adhesion molecules that mediate the binding of lymphocytes to high endothelial venules (HEV) from flowing blood have been identified but the regulation of lymphocyte migration across the HEV wall into the lymph node (LN) is far from understood. In this study we have used an in vitro model of lymphocyte migration across HEV, and analysed the roles of two integrins in the binding and transendothelial migration of T lymphocytes and T lymphoblasts. The adhesion of T lymphocytes to high endothelial cells (HEC) cultured from rat LN HEV differed from that of T lymphoblasts since the percentage of T lymphoblasts that adhered and transmigrated was higher and was not increased by IFN-gamma pretreatment of HEC. Antibodies to alpha(4) integrins, VCAM-1 or LFA-1 maximally inhibited T lymphocyte adhesion by 40-50%, whereas antibodies to ICAM-1 were less effective (<20% inhibition). The effects of alpha(4) integrin and LFA-1 antibodies were additive, giving >90% inhibition. T lymphocytes which adhered in the presence of LFA-1 antibody showed reduced levels of transmigration and, in the presence of alpha(4) integrin antibody, slightly increased transmigration. Antibodies to alpha(4) integrins, VCAM-1, LFA-1 or ICAM-1 had little effect on T lymphoblast adhesion (maxima of 10-30% inhibition) and T lymphoblasts transmigrated normally in the presence of either alpha(4) integrin or LFA-1 antibodies. However, the effects of alpha(4) integrin and LFA-1 antibodies on T lymphoblast adhesion were synergistic, giving >90% inhibition of adhesion. These results suggest that the majority of T lymphoblasts use either alpha(4) integrins or LFA-1 to bind and transmigrate HEV, and the roles of these integrins on activated T cells are overlapping and redundant. In contrast, either integrin supports half-maximal binding of unactivated T lymphocytes to the surface of HEV and LFA-1 makes a larger contribution than alpha(4) integrins to transendothelial migration.

Selective initial in vivo homing pattern of 5T2 multiple myeloma cells in the C57BL/KalwRij mouse

One of the main characteristics of multiple myeloma cells is their predominant localization in the bone marrow. It is, however, unclear whether this is due to a selective initial entry, or whether this entry is more random and other processes like survival and/or growth stimulation, only present in the medullar microenvironment, are unique. To investigate this, in vivo homing kinetics of murine 5T2MM cells shortly after injection were assessed in bone marrow, liver, spleen, lungs, heart, intestines, kidney and testis by tracing of radiolabelled cells, by immunostaining of isolated cells and by polymerase chain reaction analysis. We demonstrated the presence of 5T2MM cells in bone marrow, spleen and liver with all other organs being negative. Adhesion assays of 5T2MM cells to different types of endothelial cells demonstrated a selective adhesion of 5T2MM cells to bone marrow and liver and not to lung endothelial cells. We here demonstrate that the specific in vivo localization of the 5T2MM cells is a result of the combination of a selective entry/adhesion of the 5T2MM cells in the bone marrow, spleen and liver, and a selective survival and growth of these tumour cells in the bone marrow and spleen but not in the liver.

Molecular mechanisms that control endothelial cell contacts

Endothelial cell contacts control the permeability of the blood vessel wall. This allows the endothelium to form a barrier for solutes, macromolecules, and leukocytes between the vessel lumen and the interstitial space. Loss of this barrier function in pathophysiological situations can lead to extracellular oedema. The ability of leukocytes to enter tissue at sites of inflammation is dependent on molecular mechanisms that allow leukocytes to adhere to the endothelium and to migrate through the endothelial cell layer and the underlying basal lamina. It is a commonly accepted working hypothesis that inter-endothelial cell contacts are actively opened and closed during this process. Angiogenesis is another important process that requires well-controlled regulation of inter-endothelial cell contacts. The formation of new blood vessels by sprouting from pre-existing vessels depends on the loosening of established endothelial cell contacts and the migration of endothelial cells that form the outgrowing sprouts. This review focuses on the molecular composition of endothelial cell surface proteins and proteins of the cytoskeletal undercoat of the plasma membrane at sites of inter-endothelial cell contacts and discusses the current knowledge about the potential role of such molecules in the regulation of endothelial cell contacts.

Analysis of tight junctions during neutrophil transendothelial migration

Intercellular junctions have long been considered the main sites through which adherent neutrophils (PMNs) penetrate the endothelium. Tight junctions (TJs; zonula occludens) are the most apical component of the intercellular cleft and they form circumferential belt-like regions of intimate contact between adjacent endothelial cells. Whether PMN transmigration involves disruption of the TJ complex is unknown. We report here that endothelial TJs appear to remain intact during PMN adhesion and transmigration. Human umbilical vein endothelial cell (HUVEC) monolayers, a commonly used model for studying leukocyte trafficking, were cultured in astrocyte-conditioned medium to enhance TJ expression. Immunofluorescence microscopy and immunoblot analysis showed that activated PMN adhesion to resting monolayers or PMN migration across interleukin-1-treated monolayers does not result in widespread proteolytic loss of TJ proteins (ZO-1, ZO-2, and occludin) from endothelial borders. Ultrastructurally, TJs appear intact during and immediately following PMN transendothelial migration. Similarly, transendothelial electrical resistance is unaffected by PMN adhesion and migration. Previously, we showed that TJs are inherently discontinuous at tricellular corners where the borders of three endothelial cells meet and PMNs migrate preferentially at tricellular corners. Collectively, these results suggest that PMN migration at tricellular corners preserves the barrier properties of the endothelium and does not involve widespread disruption of endothelial TJs.

Identification and characterisation of human Junctional Adhesion Molecule (JAM)

It is widely believed that migrating immune cells utilise the intercellular junctions as routes of passage, and in doing so cause the transient disruption of junctional structures. Thus there is much interest in the molecules that have been identified at cell-cell contact points and their potential involvement in the control of leukocyte diapedesis. In this report we describe the human orthologue to Junctional Adhesion Molecule (JAM), a recently identified member of the immunoglobulin superfamily expressed at intercellular junctions (Martin-Padura et al., 1998). The human protein shares a highly conserved structure and sequence with the murine protein. However it is distinct in that it is constitutively expressed on circulating neutrophils, monocytes, platelets and lymphocyte subsets. This broad expression pattern is similar to another IgSF molecule, CD31, expressed at intercellular junctions, and may indicate further complexities in the control of leukocyte/ endothelial interactions.

Kinetics of the different steps during neutrophil migration through cultured endothelial monolayers treated with tumour necrosis factor-alpha

To enable a better understanding of the regulation of neutrophil migration, we investigated the kinetics of adhesion and migration over, through and under endothelial monolayers. Neutrophils were perfused over human umbilical vein endothelial cells (HUVEC) which had been treated with tumour necrosis factor-alpha (TNF; 2-1,000 U/ml) for 4 h. Videomicroscopy showed that transendothelial migration was complete within about 5 min of completion of perfusion of a bolus of neutrophils. Separate populations of adherent cells could then be observed, either rolling, migrating over the surface of the HUVEC or migrating underneath, at different characteristic speeds. Increasing concentration of TNF had little effect on the kinetics of migration, but shifted the balance from rolling adhesion to transendothelial migration. When individual neutrophils were followed from the moment they bound to HUVEC treated with 100 U/ml TNF, we found that approximately 40% immobilised essentially immediately on contact, while approximately 40% immobilised after rolling for varying periods (average 26 s) and approximately 20% rolled continuously. Most of the immobilised cells went on to migrate through the monolayer after spending 20-200 s migrating on top, and took about 60 s to pass through. Overall, the time from first binding to completion of transmigration averaged 152 s (range approximately 60-240 s). Interestingly, neutrophils moved relatively slowly on top of the monolayer (about 8 microm/min) but more rapidly underneath (about 16 microm/min). We suggest that the different stages during neutrophil transmigration have characteristic kinetics with separate control mechanisms, which critically influence the efficiency and rate of clearance from the vasculature.

Leukocyte recruitment in the cerebrospinal fluid of mice with experimental meningitis is inhibited by an antibody to junctional adhesion molecule (JAM)

The mechanisms that govern leukocyte transmigration through the endothelium are not yet fully defined. Junctional adhesion molecule (JAM) is a newly cloned member of the immunoglobulin superfamily which is selectively concentrated at tight junctions of endothelial and epithelial cells. A blocking monoclonal antibody (BV11 mAb) directed to JAM was able to inhibit monocyte transmigration through endothelial cells in in vitro and in vivo chemotaxis assays. In this study, we report that BV11 administration was able to attenuate cytokine-induced meningitis in mice. The intravenous injection of BV11 mAb significantly inhibited leukocyte accumulation in the cerebrospinal fluid and infiltration in the brain parenchyma. Blood-brain barrier permeability was also reduced by the mAb. We conclude that JAM may be a new target in limiting the inflammatory response that accompanies meningitis.

All-trans retinoic acid regulates adhesion mechanism and transmigration of the acute promyelocytic leukaemia cell line NB-4 under physiologic flow

The success of all-trans retinoic acid (ATRA) in the therapy of acute promyelocytic leukaemia (APL) has received increased attention. Unfortunately, life-threatening multiorgan failure commonly occurs, i.e. retinoic acid syndrome, and is thought to be the result of organ infiltration by leukaemic cells. We hypothesized that ATRA-induced differentiation of APL cells leads to adhesion receptor alterations responsible for leucocyte extravasation from the blood into tissue. Changes in adhesive properties of the APL cell line NB-4 in response to ATRA were investigated using a parallel plate flow chamber under conditions that recapitulate physiologic flow conditions. Untreated NB-4 cells initially tether and roll on activated human umbilical vein endothelial cell monolayers using a combination of E-selectin, P-selectin and alpha4 integrin. After ATRA treatment, > 80% of initial NB-4 cell attachment to endothelial cells was E-selectin dependent. Stable arrest (firm adherence) of NB-4 cells on activated endothelium was also altered by ATRA treatment. Untreated NB-4 cells used alpha4 integrin to arrest on endothelium, but beta2 integrin dependent arrest was induced by ATRA. With the acquisition of beta2 integrin function, ATRA-treated cells acquired the ability to transmigrate through activated endothelium. Thus, ATRA dramatically altered the adhesion phenotype on NB-4 cells: ATRA induced rolling largely attributable to E-selectin, abrogated alpha4 integrin dependent rolling, and promoted acquisition of beta2 integrin dependent firm adherence and transmigration. These findings represent novel cellular and differentiation effects of ATRA, and, to our knowledge, are the first demonstration that a therapeutic agent differentially regulates alpha4 and beta2 integrin on the same leucocyte.

Scanning electron microscopic studies on the route of neutrophil extravasation in the mouse after exposure to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP)

The present study was performed to demonstrate three-dimensionally the process of neutrophil extravasation induced by the bacterial chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) in mice. Thirty to 40 min after the injection of fMLP to the mouse lip, the tissues were fixed with glutaraldehyde and examined by scanning electron microscopy (SEM) as well as by transmission electron microscopy (TEM). Observation of fMLP-injected tissues showed many neutrophils adhering to the inner wall of postcapillary venules. Some of these adherent neutrophils attached to each other to form groups of two to six. There were also many neutrophils migrating through the endothelium. Most of these neutrophils took a transcellular route, in that they penetrated the cytoplasm of the endothelial cells. The junction of two neighboring endothelial cells did not open, and endothelial pores free from migrating neutrophils were scarcely observed. There were bulging portions on the vascular wall which were probably produced by the presence of underlayed neutrophils. Thus, the present study gave direct evidence of neutrophil migration via a transcellular route in response to fMLP. Our findings also indicate that the pores of the endothelium close quickly after neutrophil extravasation.

PAF-induced elastase-dependent neutrophil transendothelial migration is associated with the mobilization of elastase to the neutrophil surface and localization to the migrating front

One of the cardinal signs of acute inflammation is neutrophil (PMN) emigration across the endothelium and into the affected tissue. We have previously shown that human PMN migration across human umbilical vein endothelial cell (HUVEC) monolayers is dependent on PMN-derived elastase. However, whether migrating PMN release elastase into the extracellular milieu or retain it on the cell surface is unclear. In the present study, we show that when PMN are activated by platelet activating factor (PAF), elastase was mobilized to and retained in the cell membrane; no elastase activity was detected in the supernatant. Neutroplasts (enucleated cells devoid of granules) prepared from PAF-activated PMN contained twice as much elastase as did neutroplasts prepared from unstimulated PMN. Neutroplasts from PAF-activated PMN migrated across HUVEC monolayers in response to a chemotactic gradient (PAF), while those prepared from unstimulated PMN did not. The neutroplast transendothelial migration was inhibited (80%) by a monoclonal antibody against elastase. Using confocal microscopy, we noted that the localization of elastase on the cell surface of PMN, which were adherent to HUVEC but not migrating, was largely confined to the apical aspect of the PMN. There was little or no elastase detectable on the basal aspect of the PMN membrane in contact with the endothelium. By contrast, in migrating PMN the membrane-bound elastase was primarily localized to the migrating front, i.e. pseudopodia penetrating the HUVEC monolayers. Taken together, our findings indicate that migrating PMN localize their membrane-bound elastase to the migrating front where it facilitates transendothelial migration.

Adhesion molecules in inflammatory diseases

Cell adhesion molecules (CAM) have a key role in the inflammatory response. Selectins, integrins and immunoglobulin (Ig) gene superfamily adhesion receptors mediate the different steps of the migration of leucocytes from the blood-stream towards inflammatory foci. The activation of endothelial cells (EC) upregulates the expression of several CAM and triggers the interaction of these cells with leucocytes. Selectins are involved in the initial interactions (tethering/rolling) of leucocytes with activated endothelium, whereas integrins and Ig superfamily CAM mediate the firm adhesion of these cells and their subsequent extravasation. During rolling, leucocytes are activated through the intracellular signals generated by CAM and chemokine receptors. Blockade of the function or expression of CAM has emerged as a new therapeutic target in inflammatory diseases. Different drugs are able to interfere with cell adhesion phenomena. In addition, new antiadhesion therapeutic approaches (blocking monoclonal antibodies, soluble receptors, synthetic peptides, peptidomimetics, etc.) are currently in development.

Mechanisms of acute eosinophil mobilization from the bone marrow stimulated by interleukin 5: the role of specific adhesion molecules and phosphatidylinositol 3-kinase

Mobilization of bone marrow eosinophils is a critical early step in their trafficking to the lung during allergic inflammatory reactions. We have shown previously that the cytokine interleukin (IL)-5, generated during an allergic inflammatory reaction in the guinea pig, acts systemically to mobilize eosinophils from the bone marrow. Here, we have investigated the mechanisms underlying this release process. Examination by light and electron microscopy revealed the rapid migration of eosinophils from the hematopoietic compartment and across the bone marrow sinus endothelium in response to IL-5. Using an in situ perfusion system of the guinea pig hind limb, we showed that IL-5 stimulated a dose-dependent selective release of eosinophils from the bone marrow. Eosinophils released from the bone marrow in response to IL-5 expressed increased levels of beta2 integrin and a decrease in L-selectin, but no change in alpha4 integrin levels. A beta2 integrin-blocking antibody markedly inhibited the mobilization of eosinophils from the bone marrow stimulated by IL-5. In contrast, an alpha4 integrin blocking antibody increased the rate of eosinophil mobilization induced by IL-5. In vitro we demonstrated that IL-5 stimulates the selective chemokinesis of bone marrow eosinophils, a process markedly inhibited by two structurally distinct inhibitors of phosphatidylinositol 3-kinase, wortmannin and LY294002. Wortmannin was also shown to block eosinophil release induced by IL-5 in the perfused bone marrow system. The parallel observations on the bone marrow eosinophil release process and responses in isolated eosinophils in vitro suggest that eosinophil chemokinesis is the driving force for release in vivo and that this release process is regulated by alpha4 and beta2 integrins acting in opposite directions.

Platelets exit venules by a transcellular pathway at sites of F-met peptide-induced acute inflammation in guinea pigs

Platelets maintain the integrity of vascular endothelium, but also appear outside of blood vessels in pathological states such as acute inflammation. However, it is widely believed that platelets extravasate from blood vessels only as the result of endothelial injury and that, on contacting extravascular collagen, they undergo a morphologically defined activation sequence and release their granule contents. We here report that platelets may cross intact venular endothelium without exhibiting this release reaction or injury. Platelets became adherent to the luminal surface of venular endothelium within approximately 15 min of intradermal injection of 10(-5) M N-formyl-methionyl-leucyl-phenylalanine in guinea pig flank skin. Individual intact platelets were noted in large endothelial cell cytoplasmic vacuoles from which they subsequently migrated abluminally. They then crossed the vascular basal lamina and entered the dermis without exhibiting evidence of a release reaction. Serial electron-microscopic sections confirmed that the cytoplasmic vacuoles within which platelets crossed endothelial cells were independent of interendothelial cell junctions which remained normally closed. Platelets extended pseudopods and gave other evidence of cell motility. These findings require a paradigm shift in our thinking about platelet movement and functions.