Polar secretion of von Willebrand factor by endothelial cells

Lifecycle of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are rod or cigar-shaped secretory organelles that are formed by the vascular endothelium. They contain a diverse set of proteins that either function in haemostasis, inflammation, or angiogenesis. Biogenesis of the WPB occurs at the Golgi apparatus in a process that is dependent on the main component of the WPB, the haemostatic protein von Willebrand Factor (VWF). During this process the organelle is directed towards the regulated secretion pathway by recruiting the machinery that responds to exocytosis stimulating agonists. Upon maturation in the periphery of the cell the WPB recruits Rab27A which regulates WPB secretion. To date several signaling pathways have been found to stimulate WPB release. These signaling pathways can trigger several secretion modes including single WPB release and multigranular exocytosis. In this review we will give an overview of the WPB lifecycle from biogenesis to secretion and we will discuss several deficiencies that affect the WPB lifecycle.

von Willebrand factor multimerization and the polarity of secretory pathways in endothelial cells

The von Willebrand factor (VWF) synthesized and secreted by endothelial cells is central to hemostasis and thrombosis, providing a multifunctional adhesive platform that brings together components needed for these processes. VWF secretion can occur from both apical and basolateral sides of endothelial cells, and from constitutive, basal, and regulated secretory pathways, the latter two via Weibel-Palade bodies (WPB). Although the amount and structure of VWF is crucial to its function, the extent of VWF release, multimerization, and polarity of the 3 secretory pathways have only been addressed separately, and with conflicting results. We set out to clarify these relationships using polarized human umbilical vein endothelial cells (HUVECs) grown on Transwell membranes. We found that regulated secretion of ultra-large (UL)-molecular-weight VWF predominantly occurred apically, consistent with a role in localized platelet capture in the vessel lumen. We found that constitutive secretion of low-molecular-weight (LMW) VWF is targeted basolaterally, toward the subendothelial matrix, using the adaptor protein complex 1 (AP-1), where it may provide the bulk of collagen-bound subendothelial VWF. We also found that basally-secreted VWF is composed of UL-VWF, released continuously from WPBs in the absence of stimuli, and occurs predominantly apically, suggesting this could be the main source of circulating plasma VWF. Together, we provide a unified dataset reporting the amount and multimeric state of VWF secreted from the constitutive, basal, and regulated pathways in polarized HUVECs, and have established a new role for AP-1 in the basolateral constitutive secretion of VWF.

Evaluation of von Willebrand factor and von Willebrand factor propeptide in models of vascular endothelial cell activation, perturbation, and/or injury

Pharmacologically, vasoactive agents targeting endothelial and/or smooth muscle cells (SMC) are known to cause acute drug-induced vascular injury (DIVI) and the resulting pathology is due to endothelial cell (EC) perturbation, activation, and/or injury. Alteration in EC structure and/or function may be a critical event in vascular injury and, therefore, evaluation of the circulatory kinetic profile and secretory pattern of EC-specific proteins such as VWF and VWFpp could serve as acute vascular injury biomarkers. In rat and dog models of DIVI, this profile was determined using pharmacologically diverse agents associated with functional stimulation/perturbation (DDAVP), pathological activation (lipopolysaccharide [LPS]/endotoxin), and structural damage (fenoldopam [FD], dopamine [DA], and potassium channel opener (PCO) ZD6169). In rats, FD caused moderate DIVI and time-related increase in plasma VWF levels ∼33% while in control rats VWF increased ∼5%. In dogs, VWF levels transiently increased ∼30% when there was morphologic evidence of DIVI by DA or ZD6169. However, in dogs, VWFpp increased >60-fold (LPS) and >6-fold (DDAVP), respectively. This was in comparison to smaller dynamic 1.38-fold (LPS) and 0.54-fold (DDAVP) increases seen in plasma VWF. Furthermore, DA was associated with a dose-dependent increase in plasma VWFpp. In summary, VWF and VWFpp can discriminate between physiological and pathological perturbation, activation, and injury to ECs.

Endothelial von Willebrand factor release due to eNOS deficiency predisposes to thrombotic microangiopathy in mouse aging kidney

Endothelial dysfunction is critical in the decline of renal function with. By using endothelial nitric oxide synthase knockout (eNOSKO) mice, we tested the hypothesis that a lack of endothelial nitric oxide synthase accelerates renal injury in the aging kidney. In contrast to control mice and young eNOSKO mice, aging eNOSKO mice showed greater renal injury and in particular developed a thrombotic microangiopathy, with mesangiolysis, endothelial swelling, endothelial cell loss, double-contour appearance of glomerular basement membrane (GBM), and thrombus formation. Thrombi, which were composed of fibrin, platelets, and von Willebrand factor (vWF), were identified predominantly in glomerular capillaries and rarely in arterioles, but not in larger vessels. In the tubulointerstitium, tubular degeneration and macrophage infiltration were also prominent in aging eNOSKO mice. Intraluminal vWF deposition was accompanied with thrombus formation, whereas mesangial deposition of vWF was associated with mesangial matrix expansion. Furthermore, the mesangial vWF deposition was detectable in young eNOSKO mice in which severe glomerular injury had not yet developed. Finally, a higher level of serum P-selectin in eNOSKO mice was consistent with the vWF behavior and suggested exocytosis of the Weibel-Palade body by the endothelium. In conclusion, a lack of endothelial nitric oxide synthase resulted in the development of glomerular thrombotic microangiopathy. A lack of nitric oxide likely contributed to the release of vWF, leading to thrombus formation in this model.

[Formation and function of Weibel-Palade bodies]

Weibel-Palade bodies (WPB) are specialized cigar-shaped secretory organelles in endothelial cells, which contain a variety of biologically active molecules. These contents can be released rapidly by stimulation and involved in hemostasis, inflammation and angiogenesis. The main component of WPB is von Willebrand factor (vWF), whose expression and tubulation are necessary for the formation of the unique rod-like WPBs. Different molecules such as vWF, P-selectin, CD63, Rab27A and Rab3D are recruited into WPB mediated by the AP-1, AP-3 or other transport machinery. The underlying mechanism of the formation of WPB remains further investigation, which will gain insights into its function. The molecular mechanism of WPB formation and its function were discussed in this review.

Oligomerized Tie2 localizes to clathrin-coated pits in response to angiopoietin-1

The tyrosine kinase receptor Tie2 is expressed on endothelial cells, and together with its ligand angiopoietin-1 (Ang1), is important for angiogenesis and vascular stability. Upon activation by Ang1, Tie2 is rapidly internalized and degraded, a mechanism most likely necessary to attenuate receptor activity. Using immunogold electron microscopy, we show that on the surface of endothelial cells, Tie2 is arranged in variably sized clusters containing dimers and higher order oligomers. Clusters of Tie2 were expressed on the apical and basolateral plasma membranes, and on the tips of microvilli. Upon activation by Ang1, Tie2 co-localized with the clathrin heavy chain at the apical and basolateral plasma membranes and within endothelial cells indicating that Tie2 internalizes through clathrin-coated pits. Inhibiting cellular endocytosis by depleting cellular potassium or by acidifying the cytosol blocked the internalization of Tie2 in response to Ang1. Our results suggest that one pathway mediating the internalization of Tie2 in response to Ang1 is through clathrin-coated pits.

Fungal invasion of normally non-phagocytic host cells

Many fungi that cause invasive disease invade host epithelial cells during mucosal and respiratory infection, and subsequently invade endothelial cells during hematogenous infection. Most fungi invade these normally non-phagocytic host cells by inducing their own uptake. Candida albicans hyphae interact with endothelial cells in vitro by binding to N-cadherin on the endothelial cell surface. This binding induces rearrangement of endothelial cell microfilaments, which results in the endocytosis of the organism. The capsule of Cryptococcus neoformans is composed of glucuronoxylomannan, which binds specifically to brain endothelial cells, and appears to mediate both adherence and induction of endocytosis. The mechanisms by which other fungal pathogens induce their own uptake are largely unknown. Some angioinvasive fungi, such as Aspergillus species and the Zygomycetes, invade endothelial cells from the abluminal surface during the initiation of invasive disease, and subsequently invade the luminal surface of endothelial cells during hematogenous dissemination. Invasion of normally non-phagocytic host cells has different consequences, depending on the type of invading fungus. Aspergillus fumigatus blocks apoptosis of pulmonary epithelial cells, whereas Paracoccidioides brasiliensis induces apoptosis of epithelial cells. This review summarizes the mechanisms by which diverse fungal pathogens invade normally non-phagocytic host cells and discusses gaps in our knowledge that provide opportunities for future research.

How to roll an endothelial cigar: the biogenesis of Weibel-Palade bodies

Weibel-Palade bodies (WPB) are the regulated secretory organelles of endothelial cells. These cigar-shaped membrane-bound structures function in both hemostasis and inflammation but their biogenesis is poorly understood. Here, we review what is currently known about their formation. The content of WPBs is dominated by the hemostatic factor von Willebrand factor (VWF), whose complex biogenesis ends in the formation of high molecular weight multimers. VWF is also organized into proteinaceous tubules which underlie the striated interior of WPBs as seen in the EM. VWF expression is necessary for formation of WPBs, and its heterologous expression can even lead to the specific recruitment of WPB membrane proteins, including the leukocyte receptor P-selectin, the tetraspanin CD63, and Rab27a. Unusually, the VWF propeptide is implicated in the biogenesis of WPBs, being essential for formation of the storage compartment. The elongation of the cigars and the formation of the tubules are determined by non-covalent interactions between pro- and mature VWF proteins. Surprisingly, high molecular weight multimers seem neither necessary nor sufficient to trigger formation of a storage compartment, and do not seem to have any role in WPB biogenesis. Von Willebrand's disease, usually caused by mutations within VWF, has provided many of the insights into the way in which VWF drives the formation of these organelles.

Real-time imaging of the dynamics and secretory behavior of Weibel-Palade bodies

OBJECTIVE

Weibel-Palade bodies (WPBs) are specialized secretory granules found in endothelial cells. These vesicles store hormones, enzymes, and receptors and exhibit regulated exocytosis on cellular stimulation. Here we have directly visualized intracellular trafficking and the secretory behavior of WPBs in living cells by using a hybrid protein consisting of von Willebrand factor (vWF), a prominent WPB constituent, and green fluorescent protein (GFP).

METHODS AND RESULTS

Immunofluorescence microscopy demonstrated that this chimera was targeted into WPBs. In resting cells, some WPBs seemed motionless, whereas others moved at low speed in a stochastic manner. On stimulation of cells with [Ca2+]i- or cAMP-raising secretagogues, membrane-apposed patches were formed, suggesting fusion of WPBs with the plasma membrane. Patches remained visible for >20 minutes. This sustained, membrane-associated retention of vWF might play a role in focal adhesion of blood constituents to the endothelium after vascular injury. In addition, stimulation with cAMP-raising agonists resulted in clustering of a subset of WPBs in the perinuclear region of the cell. Apparently, these WPBs escaped secretion. This feature might provide a mechanism to control regulated exocytosis.

CONCLUSIONS

In conclusion, the fusion protein vWF-GFP provides a powerful tool to study, in real time, signal-mediated trafficking of WPBs.

Centrosome-directed translocation of Weibel-Palade bodies is rapidly induced by thrombin, calyculin A, or cytochalasin B in human aortic endothelial cells

To examine the possible role of the cytoskeleton in exocytosis of Weibel-Palade bodies (WPBs), we used double immunofluorescence and electron microscopy to study the spatial relationships between WPBs and main cytoskeletal elements in endothelial cells treated with secretagogue, such as thrombin, or cytoskeleton-damaging agents. Unexpectedly, we have found that WPBs undergo rapid translocation towards the centrosome both in cells treated with thrombin and in those treated with cytochalasin B or calyculin A. Typically, 3 or 5 min after agent addition compact cluster of WPBs became visible near the microtubule-organizing center (MTOC) in most endothelial cells in which a fivefold increase in WPBs localized in close proximity to the mother centriole had been detected. In both thrombin- and cytochalasin-treated cells that exhibit a noticeable depletion in WPBs compared to control cells, WPBs located at the cell periphery were found to colocalize with vimentin intermediate filaments, but not with microtubules. In contrast, there was precise colocalization observed between WPBs and microtubules in calyculin-treated cells in which all WPBs undergo centrosome-directed translocation within 15 min after the agent addition. When vimentin filaments were induced to collapse to a perinuclear location by the microtubule-disrupting agent demecolcine, WPBs also translocated to the perinuclear region, where numerous WPBs were found to be localized within the bundles of intermediate-sized filaments. The data provide the first direct evidence that secretory granules utilize microtubule-based transport system to move in retrograde direction, i.e., away from the plasma membrane, towards the centrosome. We suggest that anterograde movement of WPBs is primarily dependent on their interaction with vimentin intermediate filaments.

In vitro and in vivo expression of endothelial von Willebrand factor and leukocyte accumulation after fractionated irradiation

Previous investigations have demonstrated an increased release of von Willebrand factor (VWF; also known as vWF) in endothelial cells after high single-dose irradiation in vitro. We have also found increased levels of Vwf protein in mouse glomeruli after a high single dose of renal irradiation in vivo. In addition, increased numbers of leukocytes were observed in the renal cortex after irradiation in vivo. The aim of the present study was to investigate and quantify these biological processes after clinically relevant fractionated irradiation and to relate them to changes in renal function. A significantly greater increase in release of VWF was observed in cultured human umbilical vein endothelial cells (HUVECs) after fractionated irradiation (20 x 1.0 Gy) than after a single dose of 20 Gy (147% compared to 115% of control, respectively, P < 0.0005). In contrast with the in vitro observations, glomerular Vwf staining was lower after fractionated irradiation in vivo (20 x 2.0 Gy or 10 x 1.6 Gy +/- re-irradiation) than after a single dose of 16 Gy. The number of leukocytes accumulating in the renal cortex was also lower after fractionated in vivo irradiation than after a single radiation dose. The onset of these events preceded renal functional and histopathological changes by approximately 10 weeks. These data indicate that radiation-induced changes in endothelial VWF expression after in vivo irradiation may be distinct from the in vitro observations. Increased VWF expression may reflect pivotal processes in the pathogenesis of late radiation nephropathy and provide a clue to appropriate timing of pharmacological intervention.

Increased glomerular deposits of von Willebrand factor in chronic, but not acute, rejection of primate renal allografts

BACKGROUND

In our previously described primate renal allograft model, T cell ablation leads to long-term graft survival. The role of endothelial cell alteration in chronic rejection was examined in our model.

METHODS

Renal transplants were performed in rhesus monkeys using a T cell- depleting immunotoxin, FN18-CRM9. Sections from 10 rejected kidneys (5 acute and 7 chronic rejection) were examined after immunohistochemical staining for expression of endothelium-related proteins [von Willebrand factor (vWF), CD62P, and CD31], fibrinogen, and a macrophage marker (CD68). Glomerular staining for each antigen was graded on a semiquantitative scale.

RESULTS

Intense staining for vWF was consistently observed in glomerular endothelium, subendothelium, and mesangium in all kidneys removed due to chronic rejection. vWF staining was weak in kidneys showing acute rejection. The difference in glomerular staining was statistically significant. Staining for vWF in extraglomerular vessels was nearly identical in kidneys showing acute and chronic rejection. Expression of CD62P was increased in extraglomerular vessels in allografts with chronic rejection, but the glomeruli showed little or no staining. There was no significant difference in the glomerular staining for CD62P or CD31 in organs showing acute and chronic rejection. Fibrinogen staining of glomerular mesangium was seen in kidneys with chronic rejection. Macrophages (CD68+) infiltrating glomeruli were more numerous in kidneys showing chronic rejection.

CONCLUSION

Increased glomerular deposition of vWF in renal allografts showing chronic rejection, without increased staining for CD62P or CD31, suggests increased constitutive secretion of vWF from endothelial cells as a component of the mechanism of chronic rejection in our model.

Involvement of calcium and G proteins in the acute release of tissue-type plasminogen activator and von Willebrand factor from cultured human endothelial cells

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.

Polarity of constitutive and regulated von Willebrand factor secretion by transfected MDCK-II cells

Von Willebrand factor (vWF), synthesized by endothelial cells, is both rapidly secreted by the constitutive pathway and stored in Weibel-Palade bodies. Secretion from these organelles occurs upon activation of the protein kinase C signal transduction pathway and yields highly multimerized vWF. Highly multimerized vWF acts as a more effective adhesive ligand than the lower molecular weight forms that are constitutively secreted. We employed the extensively characterized polar Madin-Darby Canine Kidney II (MDCK-II) epithelial cell line, stably transfected with full-length vWF cDNA or deletion mutants thereof, to gain insight in the polarity of vWF secretion by either one of the two pathways. Immunofluorescence analysis and metabolic labeling experiments revealed that multimeric "wild-type" vWF is stored in MDCK-II cells and released upon stimulation with phorbol esters. Furthermore, we show that 62.0 +/- 3.8% of constitutively secreted and 83.2 +/- 6.6% of the regulated secreted wild-type vWF is encountered at the apical side of the cell. The polarity of the constitutive secretion of deletion mutant vWFdelD'D3 is similar to that of constitutively secreted wild-type vWF, whereas deletion mutant vWFdelD1D2 displays no polar secretion (50.1 +/- 5.7% apical).

The control of production and release of haemostatic factors in the endothelial cell

Endothelial cell products contribute to many aspects of the regulation of haemostasis. They include potent inhibitors of platelet aggregation (prostacyclin and nitric oxide) rapidly released in response to agonists such as thrombin. Similar agonists also induce the formation of platelet-activating factor by endothelium. Endothelial cell surface ectonucleotidase enzymes control the catabolism of platelet-active adenine nucleotides. The main source of the circulating coagulant cofactor von Willebrand factor is the endothelium, where it is stored in granules for agonist-triggered exocytosis and also secreted constitutively. Surface anticoagulant activities are due to the presence of antithrombin and thrombomodulin. Endothelial cells also secrete plasminogen activator and its inhibitor. Many of these reactions are significantly modulated by exposure of endothelium to cytokines or bacterial endotoxin, the most striking example being the new synthesis and surface expression of the procoagulant tissue factor (thromboplastin).

Comparison of apical and basal surfaces of confluent endothelial cells: patch-clamp and viral studies

The distribution of inwardly rectifying (Ki) and calcium-activated (KCa) potassium channels on the apical and basal surfaces of bovine aortic endothelial cells (BAECs) was examined by inverting BAEC monolayers onto polylysine-coated cover slips. To monitor cellular polarity, we examined human red blood cell adherence (hemadsorption) to the influenza virus protein, hemagglutinin (HA), and virus budding on the surface of infected BAECs. Hemadsorption and virus budding occurred on the apical surface but were not apparent on the basal surface of monolayers 1 and 5 h after inversion, although cellular HA antigen localization confirmed that all monolayers were infected. In contrast, by 9.5 and 24 h after inversion, hemadsorption was evident on the "new" apical surface. Single-channel patch-clamp analysis revealed the presence of both Ki and KCa channels on the apical surface and basal surface of BAEC monolayers 2-5 h after inversion. K channel conductance and kinetics were similar regardless of the surface monitored. This nonenzymatic mechanical technique of exposing the basal surface of endothelium provides a useful tool to study the distribution of ion channels in endothelium and in other polarized cell types grown in tissue culture.

Monocytes enhance endothelial von Willebrand factor release and prostacyclin production with different kinetics and dependency on intercellular contact between these two cell types

Human umbilical vein endothelial cells cultured on a collagen lattice were used to study the effects of the interaction between human monocytes and endothelial cells on endothelial von Willebrand Factor (vWF) release and prostacyclin (PGI2) production by these cells. The effects of monocytes were compared with those of other leucocytes, conditioned media from monocytes, and agonists such as interleukin 1 (IL-1) and the phorbol ester PMA. Because the cell culture system used allows simultaneous analysis of the lumenal and ablumenal compartment of endothelial cell monolayers, we also studied into which direction these products were released by endothelial cells. Under quiescent conditions the concentration of vWF in the ablumenal compartment was about three-fold higher than that in the lumenal compartment, whereas PGI2 was equally distributed between the two compartments. Direct cell-cell contact between purified monocytes and endothelial cells strongly enhanced both vWF release and PGI2 synthesis, in a dose-dependent and monocyte-specific manner. The monocyte-induced enhancement of PGI2 production, however, was much earlier in onset than that of vWF. Secretory products from monocytes also enhanced endothelial PGI2 synthesis, although to a lesser extent than with monocytes that were in direct contact with endothelial cells. In contrast, the monocyte-induced enhancement of endothelial vWF release was completely dependent on the direct interaction between monocytes and endothelial cells.

von Willebrand factor and the blood vessel wall

The role of von Willebrand factor (vWF) in mediating platelet adhesion has been established with the use of in vitro perfusion systems. vWF binds to the subendothelium, changes in conformation, and is then able to interact with glycoprotein Ib. vWF deposited in the subendothelium is responsible for up to 40% of normal adhesion. The action of vWF is seen at high shear rates. It also acts at low shear rates, but other factors can assume its role. Binding of vWF to the subendothelium is not through the established collagen binding sites but involves a new domain on the vWF molecule. Collagen VI has been implicated as the vessel wall component involved. Deposition of vWF in the subendothelium is determined by growth conditions and activation state of the endothelial cell. Subendothelial vWF in von Willebrand's disease may support platelet adhesion in situations in which plasma vWF of the same patient does not.

A three-dimensional model system to study the interactions between human leukocytes and endothelial cells

Leukocyte adhesion to endothelial cells and migration into the subendothelial matrix was studied with a three-dimensional model system, consisting of human endothelial cells cultured on a loose collagen matrix. We developed a new method to separate the endothelial cell monolayer and adhering leukocytes, from the subendothelial matrix, allowing simultaneous analysis of leukocyte adhesion and transendothelial migration. Monocytes adhered more avidly to untreated endothelial cells than did neutrophils (2.5 +/- 0.3 vs. 1.0 +/- 0.2 leukocytes per endothelial cell). Only a small fraction (10%-20%) of these leukocytes migrated into the subendothelium. Pretreatment of endothelial cells with interleukin 1 (IL 1) enhanced adhesion (20%), but not migration of monocytes. In contrast, neutrophil adhesion was markedly and in a time-dependent manner increased by IL 1 treatment (i.e. 200% after 6 h and 110% after 24 h of IL 1 treatment). Moreover, IL 1 pretreatment enhanced neutrophil migration twofold. Activation of leukocytes with formyl-methionyl-leucyl-phenylalanine (fMLP) enhanced both monocyte and neutrophil adhesion, but did not affect leukocyte migration. Under all conditions, monocyte adhesion was only partly (30%-40%) inhibited by monoclonal antibodies (mAb) against the common beta subunit of the leukocyte-cell adhesion molecules (LeuCAM: CD18) and 25%-30% by mAb against the alpha subunit of LFA-1 (CD11a). In contrast, mAb against the alpha subunits of Mac-1 (CD11b) and p150.95 (CD11c) were hardly effective. fMLP-mediated neutrophil adhesion was reduced to below baseline levels by anti-LeuCAM (CD18) mAb, whereas the LeuCAM contribution in IL 1-mediated neutrophil adhesion was less pronounced and varied in time. IL 1-mediated neutrophil migration, however, was completely blocked by anti-LeuCAM mAb. fMLP-mediated neutrophil adhesion was inhibited by mAb against the alpha subunits of Mac, while mAb against the alpha subunits of LFA-1 and Mac-1 both reduced IL 1-mediated adherence. In summary, we describe a novel leukocyte adhesion/migration method and demonstrate that the contribution of the LeuCAM complex in leukocyte-endothelium interaction varies depending on cell type and stimulus used.

Interleukin-1 and tumour necrosis factor alpha induced polymorphonuclear leukocyte-endothelial cell adhesion and transendothelial migration in vitro: the effect of apical versus basal monolayer stimulation

The cytokines interleukin-1 (IL-1) and tumour necrosis factor alpha (TNF alpha) enhance polymorphonuclear leukocyte (PMNL) adhesion to vascular endothelium by an endothelial cell dependent mechanism in vitro and induce PMNL infiltration in vivo In this study, we employed human umbilical vein endothelium (HUVE) cultured on microporous membrane filters to form a monolayer, a system in which PMNL adherence and PMNL transendothelial migration could be measured using 51Cr-labelled human PMNL. In this system, it was found that PMNL adhesion and migration were dependent on prior treatment of the HUVE monolayer with IL-1 or TNF alpha for at least 2 h and that cytokine could be removed prior to the addition of PMNL without any effect on the response. PMNL adherence to the HUVE was maximal by 30 min and was followed by progressive migration of PMNL across the monolayer and the membrane filter into the lower chamber. The effect of apical surface versus basal surface exposure of the HUVE monolayer to IL-1 alpha and TNF alpha on subsequent PMNL interaction with the HUVE monolayer in the absence of cytokine was examined. Apical or basal stimulation induced comparable PMNL adherence at 30 min following addition of PMNL (35.5% and 43.1%). However, basal (i.e., abluminal) exposure to IL-1 or TNF alpha of the HUVE induced significantly greater PMNL transendothelial migration (e.g., 27.8% vs. 15.4%; P less than 0.01). The expression of endothelial-leukocyte adhesion molecules ELAM-1 and ICAM-1 following apical versus basal stimulation was determined by ELISA on viable cells. These adhesion molecules were upregulated to a similar extent under both conditions. These observations suggest that spacial localization or orientation of adhesion molecules may be influenced by basal versus apical cytokine stimulation or that other mechanisms are responsible for the preferential PMNL migration with basal stimulation. These findings may have implications for the in vivo interactions of PMNL with vascular endothelium, depending on whether the endothelium is exposed to IL-1 of TNF alpha via the blood on the luminal (apical) surface or via the extravascular space on the abluminal (basal) surface.