Leukocyte Diapedesis Research Articles

CD99 Is Essential for Leukocyte Diapedesis In Vivo

Recruitment of leukocytes into inflamed tissue requires migration of leukocytes from the blood stream across the endothelial lining and the basement membrane of the local blood vessels. CD99 in humans is a 32-kDa highly O-glycosylated cell surface protein expressed on most leukocytes. The authors recently found CD99 to be expressed in leukocytes and at human endothelial cell contacts. Human CD99 is involved in homophilic interaction between the two cell types and participates in the transendothelial migration of monocytes and polymorphonuclear neutrophils (PMNs) in vitro. To test the role of CD99 in vivo, the authors cloned murine CD99 (muCD99), expressed it in vitro, and generated a blocking monoclonal antibody against it. We first showed that muCD99 is expressed on mouse leukocytes as well as enriched at the endothelial cell borders. Transfection of cells with muCD99 imparts on them the ability to aggregate in a CD99-dependent homophilic manner. Cells expressing muCD99 did not bind to cells expressing murine or human platelet endothelial call adhesion molecule (PECAM) or human CD99. In the thioglycollate peritonitis model of inflammation, anti-CD99 monoclonal antibody blocked the recruitment of neutrophils and monocytes by over 40% and 80%, respectively, at 18 h. Microscopy showed that this blocking occurred at the luminal surface of venules. The authors conclude that CD99 plays a major role in the emigration of leukocytes in vivo.

Nano-surgery at the leukocyte–endothelial docking site

The endothelium has an important role in controlling the extravasation of leukocytes from blood to tissues. Endothelial permeability for leukocytes is influenced by transmembrane proteins that control inter-endothelial adhesion, as well as steps of the leukocyte transmigration process. In a cascade consisting of leukocyte rolling, adhesion, firm adhesion, and diapedesis, a new step was recently introduced, the formation of a docking structure or “transmigratory cup.” Both terms describe a structure formed by endothelial pseudopods embracing the leukocyte. It has been found associated with both para- and transcellular diapedesis. The aim of this study was to characterize the leukocyte–endothelial contact area in terms of morphology and cell mechanics to investigate how the endothelial cytoskeleton reorganizes to engulf the leukocyte. We used atomic force microscopy (AFM) to selectively remove the leukocyte and then analyze the underlying cell at this specific spot. Firmly attached leukocytes could be removed by AFM nanomanipulation. In few cases, this exposed 8–12 μm wide and 1 μm deep footprints, representing the cup-like docking structure. Some of them were located near endothelial cell junctions. The interaction area did not exhibit significant alterations neither morphologically nor mechanically as compared to the surrounding cell surface. In conclusion, the endothelial invagination is formed without a net depolymerization of f-actin, as endothelial softening at the site of adhesion does not seem to be involved. Moreover, there were no cases of phagocytotic engulfment, but instead the formation of a transmigratory channel could be observed.

Primary chronic venous disorders

Primary chronic venous disorders, which according to the CEAP classification are those not associated with an identifiable mechanism of venous dysfunction, are among the most common in Western populations. Varicose veins without skin changes are present in about 20% of the population while active ulcers may be present in as many as 0.5%. Primary venous disorders are thought to arise from intrinsic structural and biochemical abnormalities of the vein wall. Advanced cases may be associated with skin changes and ulceration arising from extravasation of macromolecules and red blood cells leading to endothelial cell activation, leukocyte diapedesis, and altered tissue remodeling with intense collagen deposition. Laboratory evaluation of patients with primary venous disorders includes venous duplex ultrasonography performed in the upright position, occasionally supplemented with plethysmography and, when deep venous reconstruction is contemplated, ascending and descending venography. Primary venous disease is most often associated with truncal saphenous insufficiency. Although historically treated with stripping of the saphenous vein and interruption and removal of major tributary and perforating veins, a variety of endovenous techniques are now available to ablate the saphenous veins and have generally been demonstrated to be safe and less morbid than traditional procedures. Sclerotherapy also has an important role in the management of telangiectasias; primary, residual, or recurrent varicosities without connection to incompetent venous trunks; and congenital venous malformations. The introduction of ultrasound guided foam sclerotherapy has broadened potential indications to include treatment of the main saphenous trunks, varicose tributaries, and perforating veins. Surgical repair of incompetent deep venous valves has been reported to be an effective procedure in nonrandomized series, but appropriate case selection is critical to successful outcomes.

Cloning and structural analysis of equine platelet endothelial cell adhesion molecule (PECAM, CD31) and vascular cell adhesion molecule-1 (VCAM-1, CD106)

Platelet endothelial cell adhesion molecule (PECAM, CD31) and vascular cell adhesion molecule-1 (VCAM-1, CD106) are essential for leukocyte emigration and diapedesis. PECAM is an essential histologic marker of endothelial cells; VCAM-1 is a prototype marker for endothelial cell activation. In this study, equine PECAM and VCAM mRNA were cloned and sequenced. Both genes are highly conserved amongst several species. This study also revealed conserved structural and regulatory motifs, emphasizing the importance of these genes’ physiological roles in immunological responses.

Stepping out of the flow: capillary extravasation in cancer metastasis

In order for cancer cells to successfully colonize a metastatic site, they must detach from the primary tumor using extracellular matrix-degrading proteases, intravasate and survive in the circulation, evade the immune response, and extravasate the vasculature to invade the target tissue parenchyma, where metastatic foci are established. Though many of the steps of metastasis are widely studied, the precise cellular interactions and molecular alterations associated with extravasation are unknown, and further study is needed to elucidate the mechanisms inherent to this process. Studies of leukocytes localized to inflamed tissue during the immune response may be used to elucidate the process of cancer extravasation, since leukocyte diapedesis through the vasculature involves critical adhesive interactions with endothelial cells, and both leukocytes and cancer cells express similar surface receptors capable of binding endothelial adhesion molecules. Thus, leukocyte extravasation during the inflammatory response has provided a model for transendothelial migration (TEM) of cancer cells. Leukocyte extravasation is characterized by a process whereby rolling mediated by cytokine-activated endothelial selectins is followed by firmer adhesions with beta1 and beta2 integrin subunits to an activated endothelium and subsequent diapedesis, which most likely involves activation of Rho GTPases, regulators of cytoskeletal rearrangements and motility. It is controversial whether such selectin-mediated rolling is necessary for TEM of cancer cells. However, it has been established that similar stable adhesions between tumor and endothelial cells precede cancer cell transmigration through the endothelium. Additionally, there is support for the preferential attachment of tumor cells to the endothelium and, accordingly, site-specific metastasis of cancer cells. Rho GTPases are critical to TEM of cancer cells as well, and some progress has been made in understanding the specific roles of the Rho GTPase family, though much is still unknown. As the mechanisms of cancer TEM are elucidated, new approaches to study and target metastasis may be utilized and developed.

Involvement of intraplaque hemorrhage in atherothrombosis evolution via neutrophil protease enrichment

The pathological remodeling of the arterial wall in atherosclerosis involves protease activities, which play a major role in complications via plaque rupture. Circulating leukocytes and particularly neutrophils have been shown to be an independent predictor of recurrent ischemic events. However, neutrophils are poorly documented within atherosclerotic plaques. We hypothesized that intraplaque hemorrhage could convey neutrophils into the lesion, spreading into the necrotic core, thus participating in its protease enrichment. One hundred human carotid endarterectomy specimens were dissected into culprit-stenosing plaques (CPs) and adjacent noncomplicated plaques. Half of CPs exhibited hemorrhage, which was confirmed by the release of hemoglobin. Pro- and active forms of matrix metalloproteinase-9 (MMP-9) were increased in media conditioned by hemorrhagic plaques. Higher levels of lipocalin [neutrophil gelatinase-associated lipocalin (NGAL)]/MMP-9 complexes, specifically released by neutrophils, were also found in conditioned media from plaques with hemorrhage. Immunohistochemical analysis of the corresponding carotid samples showed that neutrophil markers such as elastase, NGAL/MMP-9, CD66b, and proteinase 3 colocalized with blood constituents (i.e., hemoglobin, plasminogen). All markers of neutrophil degranulation were positively correlated in CP-conditioned media (alpha1-antitrypsin/elastase complexes, myeloperoxidase, and alpha-defensins), and higher levels came from CPs containing intraplaque hemorrhages. Addition of an elastase inhibitor at the time of incubation led to a decrease in the proMMP-9 activation in CPs, suggesting cross-talk between proteases released by neutrophils. Finally, we found that neovessels observed at the interface between cap and core exhibit an activated endothelium, which may favor leukocyte diapedesis. Our study thus provides evidence for the involvement of neutrophils in plaque vulnerability.

Adhesion and signaling molecules controlling the transmigration of leukocytes through endothelium

Migration of leukocytes into tissue is a key element of innate and adaptive immunity. While the capturing of leukocytes to the blood vessel wall is well understood, little is known about the mechanisms underlying the actual transmigration of leukocytes through the vessel wall (diapedesis). Even a basic question such as whether leukocytes migrate through openings between adjacent endothelial cells (junctional pathway) or through single endothelial cells (transcellular pathway) is still a matter of intensive debate. It is generally accepted that both pathways exist; however, whether they are of equal physiological significance is unclear. Several endothelial adhesion and signaling molecules have been identified, most of them at endothelial cell contacts, which participate in leukocyte diapedesis. A concept is evolving suggesting that transendothelial migration of leukocytes is a stepwise process. Blocking or eliminating some of the different adhesion and signaling proteins results in very different effects, such as trapping of leukocytes above endothelial cell contacts, in between endothelial cells, or between the endothelium and the underlying basement membrane. Other proteins are involved in the opening of endothelial cell contacts and yet others in their maintenance providing the barrier for extravasating leukocytes. The various molecular players and the functional steps involved in diapedesis are discussed.

A CD99-related antigen on endothelial cells mediates neutrophil but not lymphocyte extravasation in vivo

AbstractCD99 is a long-known leukocyte antigen that does not belong to any of the known protein families. It was recently found on endothelial cells, where it mediates transendothelial migration of human monocytes and lymphocyte recruitment into inflamed skin in the mouse. Here, we show that CD99L2, a recently cloned, widely expressed antigen of unknown function with moderate sequence homology to CD99, is expressed on mouse leukocytes and endothelial cells. Using antibodies, we found that CD99L2 and CD99 are involved in transendothelial migration of neutrophils in vitro and in the recruitment of neutrophils into inflamed peritoneum. Intravital and electron microscopy of cremaster venules revealed that blocking CD99L2 inhibited leukocyte transmigration through the vessel wall (diapedesis) at the level of the perivascular basement membrane. We were surprised to find that, in contrast to CD99, CD99L2 was not relevant for the extravasation of lymphocytes into inflamed tissue. Although each protein promoted cell aggregation of transfected cells, endothelial CD99 and CD99L2 participated in neutrophil extravasation independent of these proteins on neutrophils. Our results establish CD99L2 as a new endothelial surface protein involved in neutrophil extravasation. In addition, this is the first evidence for a role of CD99 and CD99L2 in the process of leukocyte diapedesis in vivo.

Regulated Shedding of Transmembrane Chemokines by the Disintegrin and Metalloproteinase 10 Facilitates Detachment of Adherent Leukocytes

CX3CL1 (fractalkine) and CXCL16 are unique members of the chemokine family because they occur not only as soluble, but also as membrane-bound molecules. Expressed as type I transmembrane proteins, the ectodomain of both chemokines can be proteolytically cleaved from the cell surface, a process known as shedding. Our previous studies showed that the disintegrin and metalloproteinase 10 (ADAM10) mediates the largest proportion of constitutive CX3CL1 and CXCL16 shedding, but is not involved in the phorbolester-induced release of the soluble chemokines (inducible shedding). In this study, we introduce the calcium-ionophore ionomycin as a novel, very rapid, and efficient inducer of CX3CL1 and CXCL16 shedding. By transfection in COS-7 cells and ADAM10-deficient murine embryonic fibroblasts combined with the use of selective metalloproteinase inhibitors, we demonstrate that the inducible generation of soluble forms of these chemokines is dependent on ADAM10 activity. Analysis of the C-terminal cleavage fragments remaining in the cell membrane reveals multiple cleavage sites used by ADAM10, one of which is preferentially used upon stimulation with ionomycin. In adhesion studies with CX3CL1-expressing ECV-304 cells and cytokine-stimulated endothelial cells, we demonstrate that induced CX3CL1 shedding leads to the release of bound monocytic cell lines and PBMC from their cellular substrate. These data provide evidence for an inducible release mechanism via ADAM10 potentially important for leukocyte diapedesis.

Effect of intraperitoneally administered plant lectins on leukocyte diapedesis and visceral organ weight in rats and mice

The effects of intraperitoneally administered plant lectins were examined in rats and mice. Intraperitoneally injected ConA transiently decreased the leukocyte count in the peritoneal cavity, due to the agglutination and attachment of cells to the peritoneal lining. Subsequently the total cell count was increased for hours, exceeding initial values. Peritoneal fluid aspartate transaminase (AST) concentration showed little change during the accumulation of ascitic fluid. The most marked histological alterations were found when wheat germ lectin was injected ip. (WGA, 10 mg/kg, 6 h). Neutrophil granulocytes migrated across the wall of both arterioles and venules, but the response was highly variable among adjacent vessels. The wall of the arterioles may have impeded the migration of neutrophil granulocytes, resulting in their accumulation in the muscular layer. Granulocyte accumulation was also observed in patches under the mesothelium and in other sites of the interstitium. Marked dilatation and thrombosis of a few venules were also observed. Kidney bean lectin (PHA) induced similar but less pronounced changes. The neutrophil diapedesis suggests the release of mediator(s) from mesothelial cells and/or peritoneal white cells. The cytokine-induced neutrophil chemoattractant CINC-1, injected as control, resulted in the diapedesis of predominantly mononuclear cells in the omentum within 40 minutes. In rats ip. injected ConA increased the wet weight of spleen and liver within 6 and 10 h, respectively, but kidney weight did not change. Intravascular clumping of red blood cells, thrombosis and organ weight changes also suggest the absorption of ConA into the circulation. The experiments show that plant lectins, used as models of bacterial lectins, can reproduce some aspects of peritonitis.

Suppression of endotoxin-induced inflammation by taxol

The pathogenesis of acute lung injury includes transendothelial diapedesis of leukocytes into lung tissues and disruption of endothelial/epithelial barriers leading to protein-rich oedema. In vitro studies show that the microtubule network plays a role in the regulation of endothelial permeability as well as in neutrophil locomotion. It was hypothesised that the microtubule-stabilising agent, taxol, might attenuate inflammation and vascular leak associated with acute lung injury in vivo. The effect of intravenously delivered taxol was assessed using a model of murine lung injury induced by intratracheal lipopolysaccharide (LPS) administration. Parameters of lung injury and inflammation were assessed 18 h after treatment. Intravenously delivered taxol significantly reduced inflammatory histological changes in lung parenchyma and parameters of LPS-induced inflammation: infiltration of proteins and inflammatory cells into bronchoalveolar lavage fluid, lung myeloperoxidase activity, and extravasation of Evans blue-labelled albumin into lung tissue. Taxol alone (in the absence of LPS) had no appreciable effect on these parameters. In addition to lung proteins, intravenous taxol reduced accumulation of leukocytes in ascitic fluid in a model of LPS-induced peritonitis. Taken together, the present data demonstrate that microtubule stabilisation with taxol systemically attenuates lipopolysaccharide-induced inflammation and vascular leak.

Histopathological Observations on Protective Effects of Vitamin E on Endosulfan Induced Cardiotoxicity in Rats

The protective effects of vitamin E was investigated on the cardiotoxicity induced by endosulfan administration. Male rats in different groups were given endosulfan (2 mg per kg body weight per day in corn oil through gavage), vitamin E (200 mg per kg body weight twice a week in corn oil through gavage) and endosulfan and vitamin E at the same dose and route, to the control group corn oil is given at the dose rate of 2 mL per rat per day through gavage, for a period of28 days. The animals were sacrificed and heart tissues were collected and subjected to histopathology. The result indicated, sever congestion, haemorrhages with interstitial oedema. In some places there was diapedesis of leukocytes. Myocardium showed different degrees of degeneration, some of the myofibrils were found to be granular with pyknotic nuclei. Thickening of wall of arteries were seen. In the Vitamin E and endosulfan treated group the above mentioned lesions were significantly decreased in their severity. This study brought to light the protective effects of vitamin E on the toxic pathological lesions caused by endosulfan administration.

Gαi2-mediated signaling events in the endothelium are involved in controlling leukocyte extravasation

The trafficking of leukocytes from the blood to sites of inflammation is the cumulative result of receptor-ligand-mediated signaling events associated with the leukocytes themselves as well as with the underlying vascular endothelium. Our data show that Galpha(i) signaling pathways in the vascular endothelium regulate a critical step required for leukocyte diapedesis. In vivo studies using knockout mice demonstrated that a signaling event in a non-lymphohematopoietic compartment of the lung prevented the recruitment of proinflammatory leukocytes. Intravital microscopy showed that blockade was at the capillary endothelial surface and ex vivo studies of leukocyte trafficking demonstrated that a Galpha(i)-signaling event in endothelial cells was required for transmigration. Collectively, these data suggest that specific Galpha(i2)-mediated signaling between endothelial cells and leukocytes is required for the extravasation of leukocytes and for tissue-specific accumulation.

Molecular events during leukocyte diapedesis

The recruitment of leukocytes from the circulation into tissues requires leukocyte migration through the vascular endothelium. The mechanisms by which leukocytes attach and firmly adhere to the endothelial cell surface have been studied in detail. However, much less is known about the last step in this process, the diapedesis of leukocytes through the vascular endothelium. This minireview focuses on the interactions between leukocyte and endothelial cell adhesion molecules that are important during leukocyte extravasation. In the past few years a series of endothelial cell surface and adhesion molecules have been identified that are located at endothelial cell contacts and found to participate in leukocyte diapedesis. These junctional cell adhesion molecules are believed to have an active role in controlling the opening and closure of endothelial cell contacts to allow the passage of leukocytes between adjacent endothelial cells. Alternatively, leukocytes can cross the endothelium at nonjunctional locations, with leukocytes migrating through a single endothelial cell. Further work is clearly needed to understand, in greater detail, the molecular mechanisms that allow leukocytes to cross the endothelium via either the paracellular or the transcellular pathway.

Leukocyte–endothelial cell interactions

Leukocytes constantly patrol the vascular system in order to react promptly to infections when and where it is necessary. To fulfil this task, the cells have to enter secondary lymphoid organs and to emigrate into inflamed tissues, which requires them to cross the barrier of endothelial cells that line blood vessels. Transendothelial migration into inflamed tissues is preceded by a sequence of leukocyte–endothelial cell interactions, generally referred to as the ‘multistep adhesion cascade’ (Fig. 1). The cascade is initiated by inflammatory signals that cause local activation of vascular endothelium. Activated endothelial cells express Eand P-selectin molecules that capture leukocytes from the bloodstream and mediate rolling of the cells by binding to glycoconjugate ligands on the leukocyte surface. Homing to secondary lymphoid organs is initiated by interactions of L-selectin (expressed by leukocytes) with glycoconjugates on high endothelial venules. Rolling allows the leukocytes to perceive and accumulate signals that are mediated by the binding of endothelial-bound chemokines to their leukocyte receptors. These signals lead to leukocyte activation, resulting in a transfer of leukocyte integrins into a high affinity conformation. The integrins now allow the leukocytes to adhere firmly to the endothelium by interactions with intercellular adhesion molecule-1 and ⁄or vascular cell adhesion molecule-1. The leukocytes then migrate to sites where they can cross the endothelial layer, using either a paracellular pathway through intercellular junctions, or a transcellular pathway through the endothelial cell body. This minireview series comprises five reviews that cover important aspects of leukocyte–endothelial cell interactions (indicated in Fig. 1). In the first review, Markus Sperandio considers in vivo results on selectinmediated leukocyte rolling. He describes the requirements for rolling interactions and reviews the current knowledge of the functional relevance of the various known selectin ligands. This review focuses in particular on the glycosyland sulfotransferases that are required for the post-translational modification of selectin ligands and for which a series of knockout animals is available. The second review, by Sviatlana Yakubenia & Martin K. Wild, deals with leukocyte adhesion deficiency II, a human congenital disease in which hypofucosylation prevents selectin binding and leukocyte capture ⁄ rolling, thus inducing an immunodeficiency. The review describes recent advances on the genetic defects in patients with leukocyte adhesion deficiency II. It also focuses on open questions concerning the molecular basis of the therapy for leukocyte adhesion deficiency II and the mechanisms of the developmental defects that accompany the immunodeficiency in this disease. The third review, by Bjorn Petri & Gabriele Bixel, discusses current knowledge on the interaction of cell adhesion molecules during leukocyte transendothelial migration. In contrast to leukocyte rolling and firm adhesion, leukocyte diapedesis, and the routes taken by migrating leukocytes, are less well understood. The article focuses on cell adhesion molecules that are known to participate in the process of leukocyte extravasation, including cell surface molecules such as platelet ⁄ endothelial cell adhesion molecule-1, members of the junctional adhesion molecule family and CD99. Junctional cell adhesion molecules are believed to be important for paracellular transmigration, with leukocytes squeezing through between adjacent endothelial cells. In addition, leukocytes can cross the endothelium via an alternative pathway whereby they migrate through the body of an endothelial cell. To date, not much is known about the molecular players involved in the latter pathway. However, endothelial components, such as intermediate filaments and caveolae, seem to be important for this process. The fourth review, by Peter Hordijk, summarizes recent progress made towards an understanding of the signaling pathways that are induced in endothelial cells during leukocyte extravasation. A series of studies clearly shows that endothelial cells play an active role in this process, allowing leukocytes to cross the vascular endothelium via the paracellular or the transcellular pathway. Adhesion of leukocytes to the luminal surface of the vascular endothelium induces the formation of a docking structure that is associated with the clustering of adhesion and signaling molecules. This local concentration of cell adhesion molecules, such as intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, is believed to be required for the initiation of endothelial signaling. Both intracellular calcium and the actin cytoskeleton, but also small GTPases, reactive oxygen species and protein kinases, are involved in these signaling events. doi: 10.1111/j.1742-4658.2006.05436.x

The p110δ Isoform of PI3K Differentially Regulates β1 and β2 Integrin‐Mediated Monocyte Adhesion and Spreading and Modulates Diapedesis

Leukocyte diapedesis is misregulated in inflammatory disease and depends on the binding of monocytic LFA-1 and VLA-4 to endothelial ICAM-1 and VCAM-1, respectively. The authors hypothesized that these different molecular interactions elicit specific signaling cascades within monocytes regulating specific steps in adhesion, motility, and diapedesis. The authors employed the PI3K p110delta catalytic subunit specific inhibitor IC87114 (2 microM) and the broad-spectrum PI3K inhibitory agents LY294002 (50 microM) and wortmannin (100 nM), to examine the role of PI3Kdelta in monocyte diapedesis through endothelial monolayers and its role in monocyte adhesion and spreading upon carpets of ICAM-1 or VCAM-1. They further explored the effects of PI3Kdelta inhibition on the activation state of beta1 and beta2 integrins with immunocytochemistry and flow cytometry. In human peripheral blood monocytes IC87114 was as effective as wortmannin and LY294002 at inhibiting diapedesis, however, in THP-1 cells LY294002 and wortmannin caused a 5-fold reduction in diapedesis, while IC87114 only decreased diapedesis 2-fold. PI3Kdelta activity was specifically required for THP-1 cell adhesion and spreading on VCAM-1, but not on ICAM-1 protein substrates. Flow cytometric analysis demonstrated that PI3Kdelta inhibition decreased the amount of conformationally active beta 1-integrins, while having no effect on the prevalence of conformationally active beta 2-integrins expressed on the cell surface. In addition, PI3Kdelta inhibition resulted in a 4-fold decrease in the activation state of Rac-1 and Cdc42. These results demonstrate the specific necessity of PI3Kdelta in regulating monocytic integrin activation and the general role of PI3K signaling during diapedesis, implicating PI3K as a target for therapeutic intervention.

Evidence of a Functional Role for Interaction between ICAM-1 and Nonmuscle α-Actinins in Leukocyte Diapedesis

ICAM-1 is involved in both adhesion and extravasation of leukocytes to endothelium during inflammation. It has been shown that the ICAM-1 cytoplasmic domain is important for transendothelial migration of leukocytes but the precise molecular mechanisms involving the intracytoplasmic portion of ICAM-1 is not known. To characterize precisely the molecular scaffolding associated with ICAM-1, we have used the yeast two-hybrid system, and we have identified six different proteins interacting with the ICAM-1 cytoplasmic domain. In this study, we report that the two forms of nonmuscle alpha-actinin (i.e., alpha-actinin 1 and alpha-actinin 4) associate with ICAM-1, and that these interactions are essential for leukocyte extravasation. These interactions were further confirmed by coimmunoprecipitation and immunofluorescence in endothelial cells and in ICAM-1-transfected Chinese hamster ovary cells. The function of these interactions was analyzed by point mutation of charged amino acids located on ICAM-1 cytoplasmic domain. We have identified three charged amino acids (arginine 480, lysine 481, and arginine 486) which are essential in the binding of alpha-actinins to the ICAM-1 cytoplasmic tail. Mutation of these amino acids completely inhibited ICAM-1-mediated diapedesis. Experiments with siRNA inhibiting specifically alpha-actinin 1 or alpha-actinin 4 on endothelial cells indicated that alpha-actinin 4 had a major role in this phenomenon. Thus, our data demonstrate that ICAM-1 directly interacts with cytoplasmic alpha-actinin 1 and 4 and that this interaction is required for leukocyte extravasation.

The role of JAM-A and PECAM-1 in modulating leukocyte infiltration in inflamed and ischemic tissues

Innate and adaptive immunological responses are accompanied by leukocyte adhesion to the blood-vessel wall and their subsequent infiltration into the underlying tissues. In the majority of the cases, leukocytes cross the endothelium by squeezing through the border of apposed endothelial cells, a process that is known as diapedesis. Many data suggest that proteins at endothelial junctions establish homophilic interactions with identical proteins, which are present on leukocytes. These interactions might then direct the passage of leukocytes through the endothelial border. In this review, we focus on two endothelial junctional proteins [junctional adhesion molecule-A (JAM-A) and PECAM], which play an important role in leukocyte diapedesis. In vivo data with blocking antibodies or inactivation of JAM-A and PECAM genes indicate that the role of these two proteins depends on the stimulus and the experimental model used.

Neisseria meningitidis infection of human endothelial cells interferes with leukocyte transmigration by preventing the formation of endothelial docking structures

Neisseria meningitidis elicits the formation of membrane protrusions on vascular endothelial cells, enabling its internalization and transcytosis. We provide evidence that this process interferes with the transendothelial migration of leukocytes. Bacteria adhering to endothelial cells actively recruit ezrin, moesin, and ezrin binding adhesion molecules. These molecules no longer accumulate at sites of leukocyte–endothelial contact, preventing the formation of the endothelial docking structures required for proper leukocyte diapedesis. Overexpression of exogenous ezrin or moesin is sufficient to rescue the formation of docking structures on and leukocyte migration through infected endothelial monolayers. Inversely, expression of the dominant-negative NH2-terminal domain of ezrin markedly inhibits the formation of docking structures and leukocyte diapedesis through noninfected monolayers. Ezrin and moesin thus appear as pivotal endothelial proteins required for leukocyte diapedesis that are titrated away by N. meningitidis. These results highlight a novel strategy developed by a bacterial pathogen to hamper the host inflammatory response by interfering with leukocyte–endothelial cell interaction.

The transcellular railway: insights into leukocyte diapedesis

Leukocyte recruitment from blood to areas of infection is a key step in both innate and adaptive immunological responses. The predominant model has been that leukocytes transmigrate through the junctions between adjacent endothelial cells. However, leukocytes can also migrate through the endothelial cells, and new insights suggest that both caveolae and intermediate filaments are important for this.