Cutting edge: evidence for a signaling partnership between urokinase receptors (CD87) and L-selectin (CD62L) in human polymorphonuclear neutrophils.

Leukocyte urokinase plasminogen activator receptors (uPARs) cluster at adhesion interfaces and at migratory fronts where they participate in adhesion, chemotaxis, and proteolysis. uPAR aggregation triggers activation signaling even though this glycolipid-anchored protein must associate with membrane-spanning proteins to access the cell interior. This study demonstrates a novel partnership between uPAR and L-selectin in human polymorphonuclear neutrophils. Fluorescence resonance energy transfer demonstrated a direct physical association between uPAR and L-selectin. To examine the role of L-selectin in uPAR-mediated signaling, uPAR was cross-linked and intracellular Ca(2+) concentrations were measured by spectrofluorometry. A mAb reactive against the carbohydrate binding domain (CBD) of L-selectin substantially inhibited uPAR-mediated Ca(2+) mobilization, whereas mAbs against the beta(2) integrin complement receptor 3 (CR3), another uPAR-binding adhesion protein, had no effect. Similarly, fucoidan, a sulfated polysaccharide that binds to L-selectin CBD, inhibited the Ca(2+) signal. We conclude that uPAR associates with the CBD region of L-selectin to form a functional signaling complex.

Clustering of urokinase receptors (uPAR; CD87) induces proinflammatory signaling in human polymorphonuclear neutrophils.

Leukocytes use urokinase receptors (uPAR; CD87) in adhesion, migration, and proteolysis of matrix proteins. Typically, uPAR clusters at cell-substratum interfaces, at focal adhesions, and at the leading edges of migrating cells. This study was undertaken to determine whether uPAR clustering mediates activation signaling in human polymorphonuclear neutrophils. Cells were labeled with fluo-3/AM to quantitate intracellular Ca2+ ([Ca2+]i) by spectrofluorometry, and uPAR was aggregated by Ab cross-linking. Aggregating uPAR induced a highly reproducible increase in [Ca2+]i (baseline to peak) of 295 +/- 37 nM (p = 0.0002). Acutely treating cells with high m.w. urokinase (HMW-uPA; 4000 IU/ml) produced a response of similar magnitude but far shorter duration. Selectively aggregating uPA-occupied uPAR produced smaller increases in [Ca2+]i, but saturating uPAR with HMW-uPA increased the response to approximate that of uPAR cross-linking. Cross-linking uPAR induced rapid and significant increases in membrane expression of CD11b and increased degranulation (release of beta-glucuronidase and lactoferrin) to a significantly greater degree than cross-linking control Abs. The magnitude of degranulation correlated closely with the difference between baseline and peak [Ca2+]i, but was not dependent on the state of uPA occupancy. By contrast, selectively cross-linking uPA-occupied uPAR was capable of directly inducing superoxide release as well as enhancing FMLP-stimulated superoxide release. These results could not be duplicated by preferentially cross-linking unoccupied uPAR. We conclude that uPAR aggregation initiates activation signaling in polymorphonuclear neutrophils through at least two distinct uPA-dependent and uPA-independent pathways, increasing their proinflammatory potency (degranulation and oxidant release) and altering expression of CD11b/CD18 to favor a firmly adherent phenotype.

Urokinase receptor (CD87) aggregation triggers phosphoinositide hydrolysis and intracellular calcium mobilization in mononuclear phagocytes.

Leukocytes utilize urokinase receptors (uPAR; CD87) in adhesion, migration, and matrix proteolysis. uPAR aggregate at cell-substratum interfaces and at leading edges of migrating cells, so this study was undertaken to determine whether uPAR aggregation is capable of initiating activation signaling. Monocyte-like U937 cells were labeled with fluo-3-acetoxymethyl ester to quantitate intracellular Ca2+ concentrations ([Ca2+]i) by spectrofluorometry, and uPAR was aggregated by mAb cross-linking. uPAR aggregation induced highly reproducible increases in [Ca2+]i of 103.0 +/- 10.9 nM (p < 0.0001) and >3-fold increases in cellular d-myoinositol 1,4,5-trisphosphate (Ins(1,4,5)P3) levels. Similar increases in [Ca2+]i were also elicited by uPAR aggregation in human monocytes, but cross-linking a control IgG2a had no effect on [Ca2+]i. Selectively cross-linking uPA-occupied uPAR with an anti-uPA mAb produced smaller increases in [Ca2+]i, but fully saturating uPAR with exogenous uPA enhanced the [Ca2+]i response to equal the effect of aggregating uPAR directly. Increased [Ca2+]i was inhibited by thapsigargin, herbimycin A, and U73122, but only partially reduced by low extracellular [Ca2+], indicating that uPAR aggregation increases [Ca2+]i by activating phospholipase C through a tyrosine kinase-dependent mechanism, generating Ins(1,4,5)P3 and releasing Ca2+ from Ins(1,4, 5)P3-sensitive intracellular stores. Cross-linking the beta2 integrin CR3 could not duplicate the effect of uPAR cross-linking, and uPAR-triggered Ca2+ mobilization was not blocked by anti-CR3 mAbs. These results indicate that uPAR aggregation initiates phosphoinositide hydrolysis by mechanisms that are not strictly dependent on associated uPA or CR3.

Fibrinogen activates NF-kappa B transcription factors in mononuclear phagocytes.

Adhesion to extracellular matrices is known to modulate leukocyte activation, although the mechanisms are not fully understood. Mononuclear phagocytes are exposed to fibrinous provisional matrix throughout migration into inflammatory foci, so this study was undertaken to determine whether fibrinogen triggers activation of NF-kappa B transcription factors. U937 cells differentiated with PMA in nonadherent culture were shown to express two fibrinogen-binding integrins, predominately CD11b/CD18, and to a lesser extent, CD11c/CD18. Cells stimulated with fibrinogen (10-100 microg/ml)/Mn2+ (50 microM) for 2 h were examined by electrophoretic mobility shift assay. NF-kappa B activation, minimal in unstimulated cells, was substantially up-regulated by fibrinogen. Fibrinogen also caused activation of AP-1, but not SP1 or cAMP response element-binding protein (CREB) factors. Blocking mAbs against CD18 and CD11b abrogated fibrinogen-induced NF-kappa B activation. To determine the effects on transcriptional regulation, U937 cells were transfected with a plasmid containing the HIV-1 enhancer (bearing two NF-kappa B sites) coupled to a chloramphenicol acetyltransferase (CAT) reporter. Cells were subsequently stimulated with 1) PMA for 24 h, inducing CAT activity by 2.6-fold, 2) fibrinogen/Mn2+ for 2 h, inducing CAT activity by 3.2-fold, or 3) costimulation with fibrinogen and PMA, inducing 5.7-fold the CAT activity induced by PMA alone. We conclude that contact with fibrinogen-derived proteins may contribute to mononuclear phagocyte activation by signaling through CD11b/CD18, resulting in selective activation of transcriptional regulatory factors, including NF-kappa B.

The urokinase receptor (CD87) facilitates CD11b/CD18-mediated adhesion of human monocytes.

Urokinase receptors (uPAR; CD87) from complexes with complement receptor 3 (CR3) (CD11b/CD18), a beta2 integrin. In this study, we sought to determine if this association modulates the adhesive function of CR3. Both CR3 and uPAR concentrate at the ventral surface of fibrinogen-adherent human monocytes, and CR3-uPAR coupling increases substantially upon adhesion to fibrinogen. Pretreatment with anti-uPAR monoclonal antibody reduced adhesion to CR3 counterligands (fibrinogen and keyhole limpet hemocyanin) by 50%, but did not affect adhesion to fibronectin, a beta1 integrin counterligand. Antisense (AS) oligonucleotides were used to determine if selectively suppressing uPAR expression also modulates CR3 adhesive function. AS-uPAR oligo reduced CR3-dependent adhesion by 43+/-9% (P<0.01), but did not affect CR3-independent adhesion. To determine if the effects of uPAR are mediated through its ligand, monocytes were pre-treated with AS oligo to block uPA expression. Unlike the effects of blocking uPAR expression, AS-uPA oligo increased adhesion by 46% (P<0.005), and exogenous intact uPA, but not uPA fragments, reversed this effect. We conclude that complex formation with uPAR facilitates the adhesive functions of CR3. This function of uPAR is not dependent upon its occupancy with uPA, which negatively influences adhesion.

Endogenously produced urokinase amplifies tumor necrosis factor-alpha secretion by THP-1 mononuclear phagocytes.

This study examined the effects of endogenous urokinase (uPA) on lipopolysaccharide (LPS)-stimulated tumor necrosis factor alpha (TNF-alpha) secretion in THP-1 mononuclear phagocytes. Anti-uPA monoclonal antibody (mAb) suppressed LPS-driven TNF-alpha secretion by 61.6 +/- 5.9% (P<.001), and PAI-1, a uPA inhibitor, suppressed it to 53.1 +/- 8.2% of the control value (P<.001). Up-regulation of TNF-alpha mRNA was suppressed in parallel with secreted TNF-alpha protein. TNF-alpha secretion was unaffected by depleting plasminogen or by aprotinin, a plasmin inhibitor. When endogenous uPA was displaced from the cell, exogenous high-molecular-weight (intact) uPA augmented LPS-driven TNF-alpha secretion. By contrast, a uPA fragment containing the catalytic domain was inhibitory, and the uPA receptor-binding domain had no effect. We conclude that endogenous uPA amplifies TNF-alpha neosynthesis of LPS-stimulated THP-1 mononuclear phagocytes. The effect requires intact uPA and is independent of plasmin activity. This represents a novel mechanism by which a mononuclear phagocyte-derived protease contributes to generating proinflammatory signals.

Wegener's granulomatosis in the elderly.

STUDY OBJECTIVE: To determine if elderly patients with Wegener's granulomatosis (WG) exhibit distinctive clinical features or outcomes compared with patients whose conditions were diagnosed at younger ages. DESIGN: Retrospective cohort study. SETTING: University medical center. PATIENTS: Thirty-three patients with WG diagnosed when 60 years old or older and 34 patients with WG diagnosed at age younger than 60 years, identified by record review of all WG patients seen over an 11-year period. RESULTS: The prevalence of specific clinical features, progression to end-stage renal disease, mortality rate, and infectious and noninfectious complications of therapy were examined. The prevalence of upper respiratory tract involvement (rhinitis, sinusitis, otitis, epistaxis) and hemoptysis were significantly less common as initial manifestations in the elderly patients, although pulmonary infiltrates were seen more commonly during the course of their disease. Renal insufficiency was more common at the time of diagnosis in the elderly patients (64% vs 35%; p < 0.05). Most notably, CNS involvement was 4.5-fold more common in elderly patients (27% vs 6%; p = 0.02). The overall incidence of infectious and noninfectious complications of therapy was similar between the groups, although the mortality rate was markedly higher in the elderly patients (54% vs 19%; p < 0.01). Almost all deaths were due to overwhelming infection. CONCLUSIONS: Elderly patients with WG present with distinctive clinical features, particularly a relatively low incidence of upper respiratory tract complaints and a high incidence of CNS involvement. The mortality risk from infectious complications of WG is substantially higher in elderly patients, although this cannot be attributed directly to adverse affects of therapy.

Function of the urokinase receptor (CD87) in neutrophil chemotaxis.

During recruitment, leukocytes respond to chemotaxins and traverse matrix barriers. Urokinase-type plasminogen activator (uPA), bound to its receptor (uPAR; CD87) facilitates plasmin formation, which promotes matrix proteolysis. Polymorphonuclear leukocytes (PMNs) are critical to the inflammatory response and express both uPA and CD87. To determine whether uPA and CD87 are required for PMN chemotaxis, PMNs were pretreated with an anti-CD87 monoclonal antibody (mAb), a neutralizing anti-uPA mAb, or uPA. PMN chemotaxis was profoundly suppressed by the anti-CD87 mAb but was unaffected by anti-uPA mAb or uPA. The role CD87 plays in chemotaxis may be related to its ability to associate with CR3. CD87/CR3 coupling can be disrupted by specific saccharides. The same saccharides that disrupt CD87/CR3 coupling (NADG, D-mannose, and mannoside) inhibit PMN chemotaxis. We conclude that CD87 plays a crucial role in PMN chemotaxis in vitro that is independent of uPA enzyme activity but may be related to the ability of CD87 to interact with CR3.

Enzyme-linked immunoabsorbent assay detection of a soluble form of urokinase plasminogen activator receptor in vivo.

The receptor for urokinase plasminogen activator (uPA-R, CD87) is a glycosylphosphatidylinositol (GPI)-anchored 50 to 65 kD glycoprotein that, by regulating membrane-associated plasmin activity, may facilitate the invasion of inflammatory and malignant cells. Certain other GPI-anchored glycoproteins are shed from the cell membrane and exist as soluble products in vitro and in vivo. To determine if uPA-R undergoes a similar phenomenon, we have developed a sensitive enzyme-linked immunoabsorbent assay (ELISA) (using a rabbit antiserum as both capture and detection reagents) to measure the quantity of soluble uPA-R (suPA-R) in tissue culture supernatants and biologic fluids. Using this ELISA, we have detected suPA-R in the culture supernatants of U-937 cells and human monocytes stimulated in vitro by certain soluble inflammatory mediators (Sitrin et al, Blood 84:1268, 1994; Mizukami et al., Clin Res 42:115A, 1994). To determine if suPA-R exists in vivo, we have screened the plasma of 20 normal volunteers (mean +/- SD, 3 +/- 3 ng/mL; median, 2 ng/mL; range, 1 to 11 ng/mL [serum values slightly higher]); the plasma of 13 ICU patients with clinical sepsis syndrome (mean +/- SD, 30 +/- 11 ng/mL; median, 11 ng/mL; range, 4 to 221 ng/mL); and the extravascular fluids (pleural, pericardial, and peritoneal) of 84 individuals with presumed inflammatory or malignant conditions (mean +/- SD, 21 +/- 39 ng/mL; median, 10 ng/mL; range, 2 to 253 ng/mL). Among the latter specimens, most were inflammatory exudates (only six were malignant by positive cytology) with the highest quantities of suPA-R associated with neutrophilic exudates. The solubility of suPA-R contained within these fluids was confirmed by reanalysis after ultracentrifugation to remove particulate material. When tested in a uPA ligand capture ELISA, representative specimens of extravascular body fluids and sepsis plasma contained suPA-R capable of binding uPA ligand (generally representing a small fraction of the immunoreactive material). We conclude from these data that suPA-R is immunologically detectable in vitro and in vivo with high concentrations of receptor found under conditions of inflammatory stimulation. The possibility of suPA-R's biologic activity is suggested by its partial retention of ligand binding capacity.

Human skin in organ culture. Elaboration of proteolytic enzymes in the presence and absence of exogenous growth factors.

Proteinase levels were assessed in organ culture fluids from human neonatal foreskin maintained under growth factor-free conditions and in the presence of a combination of growth factors (ie, epidermal growth factor, insulin, hydrocortisone, pituitary extract, and all-trans-retinoic acid). Analysis of culture fluids by gelatin zymography revealed the presence of 92-kd and 72-kd gelatinases. There was a greater amount of 92-kd gelatinase activity in the presence of growth factors whereas the levels of 72-kd gelatinase were similar in growth factor-free and growth factor-containing media. Experiments with keratinocytes and fibroblasts in monolayer culture and with isolated dermal tissue in organ culture indicated that the epithelial component was responsible for most of the 92-kd gelatinase activity whereas fibroblasts were primarily responsible for the 72-kd gelatinase activity. Activation with aminophenyl mercuric acetate, requirement for divalent cations, inhibition with EDTA, and insensitivity to inhibition with phenylmethyl sulfonyl fluoride indicated that both gelatinases were metalloproteinases. In additional studies, culture fluids were examined for the presence of plasminogen activator activity. This was detected in culture fluids from tissues maintained under both conditions but was increased in the growth factor-containing medium. The increased amount seen in the growth factor-containing medium appeared to be due almost entirely to a single factor, ie, all-trans-retinoic acid. In monolayer culture, both keratinocytes and fibroblasts produced plasminogen activator; the level was higher in keratinocyte culture fluids than in culture fluids from fibroblasts.

The approach to nonresolving pneumonia.

Pneumonias that fail to resolve at the expected rate may reflect derangements in host defenses, inadequate or inappropriate antimicrobial therapy, highly virulent pathogens, or myriad noninfectious causes. In this article, noninfectious causes of pulmonary infiltrates mimicking community-acquired pneumonia are discussed. The salient clinical, radiographic, and histopathologic features of diverse immune-mediated syndromes are reviewed, and an approach to diagnosis and therapy of nonresolving pneumonias is presented.

Expression of serine proteinases and metalloproteinases in organ-cultured human skin. Altered levels in the presence of retinoic acid and possible relationship to retinoid-induced loss of epidermal cohesion.

Neonatal human foreskin obtained at circumcision was cut into 2 x 2-mm pieces and placed in organ culture. Culture medium consisted of a serum-free, growth factor-free basal medium containing either 0.15 mmol/L Ca2+ or 1.4 mmol/L Ca2+. Some cultures were left as control, whereas others were treated with 3 mumol/L all-trans retinoic acid (RA). In the presence of RA, epidermal cohesion was disrupted and the upper layers separated from the viable epidermis beneath. This effect was observed under both low Ca2+ and high Ca2+ conditions. At 2-day intervals, culture fluids were collected and analyzed for serine and metalloproteinase activities. Serine proteinase activity was detected in the culture fluids and virtually all of the detected activity was dependent on the presence of plasminogen. Activity was elevated in the RA-treated tissues and this was due to increased amounts of both urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA). Elastase and cathepsin G were not detected in either control or RA-treated cultures. Increased plasminogen activator levels were also detected in RA-treated keratinocytes and fibroblasts in monolayer culture. Significant amounts of t-PA (though not u-PA) were found in fibroblast culture fluids, whereas both t-PA and u-PA were detected in culture fluids from keratinocytes. Metalloproteinase activity was also detected in the culture fluids of control and RA-treated tissues but in contrast to plasminogen activator, metalloproteinase activity decreased in the presence of RA. Casein and gelatin zymographic studies indicated the presence of both 92- and 72-kd gelatinases and stromelysin-1 and suggested that the decreased activity was primarily due to reduction in the 92- and 72-kd gelatinases. When serine proteinase inhibitors (aprotinin and soybean trypsin inhibitor) were included in the culture medium throughout the incubation period, epidermal discohesion was reduced. A metalloproteinase inhibitor, tissue inhibitor of metalloproteinase-2, did not have this effect. Taken together, these data show that a number of proteolytic enzymes are produced during organ culture of human skin. They suggest that these proteases may influence the structural integrity of the tissue.

Cytokine-specific regulation of urokinase receptor (CD87) expression by U937 mononuclear phagocytes.

Mononuclear phagocytes concentrate urokinase-type plasminogen activator (uPA) at the cell surface by expressing membrane uPA receptors (uPAR). This study examines the ability of exogenous cytokines to alter expression of membrane-associated uPA and uPAR in U937 mononuclear phagocytes. Cells were stimulated with recombinant interferon gamma (IFN gamma) or tumor necrosis factor alpha (TNF alpha), followed by immunolabeling for uPA or uPAR and flow cytometry. IFN gamma increased surface uPA 2.2-fold relative to unstimulated controls (P < .001), whereas TNF alpha had no significant effect. Likewise, maximal uPA binding capacity was increased 2.8-fold by IFN gamma (P < .02), but was not affected by TNF alpha. In unstimulated cells, 50% of receptors were occupied by endogenously generated uPA, and this proportion was not affected by either cytokine. IFN gamma upregulated uPAR 2.1-fold relative to unstimulated controls (P < .001), whereas TNF alpha had no effect. In contrast to effects on surface protein, TNF alpha induced a substantial increase in uPAR mRNA, equaling the effect of IFN gamma. In addition, both cytokines doubled the intracellular uPAR pool (P < .01). By contrast, TNF alpha induced a 2.5-fold increase in the level of uPAR protein released into conditioned medium (compared with unstimulated cells), whereas IFN gamma had no effect. These results indicate that uPAR expression is regulated in a cytokine-specific fashion. Some stimuli, such as TNF alpha, may increase uPAR synthetic activity without a corresponding change in membrane expression, because of enhanced release of uPAR from the cell. Cytokine-specific modulation of uPAR may be important in regulating the function of mononuclear phagocytes in inflammation and tissue repair.

The urokinase receptor is required for human monocyte chemotaxis in vitro.

Mononuclear phagocytes (Mphi) produce urokinase-type plasminogen activator (uPA) and also express a specific cell-surface receptor for urokinase, uPAR. The concomitant expression of these proteins provides a mechanism by which Mphi can degrade extracellular matrix proteins during directed cell migration. In this study, we sought to determine if uPAR plays a role in Mphi chemotaxis that is distinct from its role in matrix proteolysis. Exposing adherent monocytes to a chemotactic gradient causes plasma membrane uPAR to localize strongly to the leading edge of cell migration. Adherence alone or exposure to FMLP had no effect on uPAR expression. Using Boyden chamber chemotaxis assays, we demonstrate that treating mononuclear cells with an anti-uPAR mAb (either as an intact mAb or F[ab']2) ablates chemotaxis induced by FMLP and monocyte chemotactic peptide-1 (P < 0.001). Inactivating the catalytic activity of uPAR-bound uPA had no effect on chemotaxis. Similarly, blocking uPAR expression with an antisense oligonucleotide to uPAR completely ablates chemotaxis, but blocking uPA expression with an antisense oligonucleotide to uPA has a minimal effect. We therefore demonstrate that expression and unimpeded function of uPAR plays an obligate role in M phi chemotaxis by mechanisms that are largely independent of its ligand, uPA. Combined with its known role in mediating pericellular proteolysis, these observations demonstrate that uPAR is essential for both locomotion and traversing tissue barriers during M phi migration.

Monocyte urokinase expression: modulation by interleukins.

This study delineates the regulatory effect of interleukin-1 (IL-1) and interleukin-2 (IL-2) on monocyte plasminogen activator (PA) activity. Mononuclear phagocytes regulate net PA activity by modulating the expression of urokinase-type PA (uPA) and a specific plasminogen activator inhibitor, PAI-2. To understand the regulation of mononuclear phagocyte PA activity, it is important to compare the expression of uPA and PAI-2. In this study, we determined the relative abundance of secreted PA and PA inhibitor activity in human monocyte-conditioned medium after stimulation with human recombinant IL-1 or IL-2. In agreement with our previous description of tumor necrosis factor-alpha and interferon-gamma stimulation of mononuclear phagocytes, we found no detectable PA activity in conditioned medium. Both IL-1 and IL-2 had dose-dependent effects, significantly up-regulating PA inhibitor activity in monocyte-conditioned medium (up to 11-fold). To further investigate the mechanism underlying this effect, Northern blot analysis was done to measure steady-state mRNA for uPA and PAI-2. Consistent with the increase in secreted PA inhibitor activity, we found that both IL-1 and IL-2 significantly increased steady-state mRNA for PAI-2. In addition, however, both IL-1 and IL-2 increased steady-state mRNA for uPA. IL-1 appears to increase mRNA for uPA to a greater extent than does IL-2. We conclude that IL-1 and IL-2 modulate monocyte proteolytic activity by increasing expression of uPA and PAI-2 with a resultant predominance of PAI-2. We further conclude that cytokine-specific regulation of plasminogen activity is achieved partly by varying the proportionate expression of uPA and PAI-2.

Noninfectious mimics of community-acquired pneumonia.

A broad spectrum of diseases and clinical syndromes can masquerade as community-acquired pneumonia (CAP). Many such disorders, such as hypersensitivity pneumonitis (HP), chronic eosinophilic pneumonia (CEP), bronchiolitis obliterans--organizing pneumonia (BOOP), reactions to drugs or exogenous agents, systemic vasculitis, and alveolar hemorrhage (AH; pulmonary-renal) syndromes, are immune-mediated and warrant treatment with corticosteroids or immunosuppressive agents. In addition, rare neoplastic and lymphoproliferative disorders, and conditions of uncertain etiology (eg, pulmonary alveolar proteinosis [PAP]) may have clinical and radiographic features that overlap with infectious causes of pneumonia. Distinguishing infectious from noninfectious causes of pneumonia may be difficult, and requires the use of ancillary serologic studies and often histologic material to establish a precise etiologic diagnosis. For some of these disorders (particularly Wegener's granulomatosis [WG], systemic necrotizing vasculitis [SNV], and antiglomerular basement antibody disease [anti-GBM disease]), serologic markers are invaluable in confirming the diagnosis and monitoring the course of the disease. In this report, we review the salient clinical and histologic features of these diverse diseases, and present a diagnostic and therapeutic approach.

Fibrin degradation by rat pulmonary alveolar epithelial cells.

The persistence of intra-alveolar fibrin during acute and chronic inflammatory lung diseases indicates that the normally profibrinolytic environment of the alveolar space has been altered as part of the disease process. We have recently shown that alveolar epithelial cells may control fibrinolysis by expressing both urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor-1. In this study, monolayers of rat alveolar epithelial cells were used as a model of the alveolar surface and were found to lyse plasma-derived fibrin matrices by a process that was plasminogen and uPA dependent. Fibrinolysis was not achieved by fluid-phase epithelial products but required the presence of epithelial cells, optimally in close contact with the clot surface. Epithelial cell-mediated fibrinolytic activity was augmented 99% by endotoxin and suppressed 66% by dexamethasone. Fibrinolysis also increased 84% as cells aged in culture from day 1 to day 4, during which time the cells lose many type II cell characteristics and assume a type I cell-like phenotype. We conclude that alveolar epithelial cells actively participate in fibrin clearance through mechanisms that require close proximity between epithelial cell and clot surfaces. Alterations in these mechanisms may be partly responsible for the persistence of intraalveolar fibrin during lung inflammation.

Tissue factor procoagulant expression by rat alveolar epithelial cells.

Fibrin deposition in the alveolar space is characteristic of inflammatory lung injury. The formation of fibrin in the alveolus results from the coagulation of extravasated plasma. The cellular elements that promote intra-alveolar clotting have not been completely defined. We have investigated the capacity of alveolar epithelial cells (AEC) to promote coagulation through the expression of procoagulant activity (PCA) in tissue culture. Using a single-stage coagulation assay, rat AEC monolayers were found to contain 20,750 +/- 4,035 procoagulant units (PCU)/10(6) cells; 10- to 20-fold greater activity than that found in concomitantly isolated alveolar macrophages. The epithelial-derived procoagulant was shown to be tissue factor by a series of assays using clotting factor-deficient human plasmas. Freshly isolated AEC also possessed PCA (2,500 +/- 1,000 PCU/10(6) cells) and expressed a 2.1-kb mRNA that hybridized with a cDNA for murine tissue factor. Using a kinetic turbidometric assay of clot acceleration, PCA was found on the surface of unstimulated epithelial monolayers and could be increased to 170% of control by incubation with phorbol myristate acetate (PMA). This response to PMA was accompanied by a parallel increase in the relative abundance of tissue factor mRNA. AEC shed particulate PCA into the culture media that displayed a specific activity similar to that recovered from alveolar lining fluid. Therefore, by expressing both cell surface and particulate PCA, the alveolar epithelium likely contributes significantly to the modulation of intra-alveolar coagulation.

Urokinase expression in mononuclear phagocytes: cytokine-specific modulation by interferon-gamma and tumor necrosis factor-alpha.

This study delineates the regulatory effects of inflammatory cytokines on mononuclear phagocyte plasminogen activator (PA) activity. The mechanisms by which mononuclear phagocytes modulate PA activity are described. Mononuclear phagocytes regulate net PA activity by the balanced expression of urokinase-type PA (uPA), in either secreted or membrane-associated forms, and a specific plasminogen activator inhibitor, PAI-2. Therefore, understanding how immunomodulators regulate macrophage PA activity requires that the comparative effects of uPA and PAI-2 be elucidated. We determine how recombinant interferon-gamma (IFN) and tumor necrosis factor-alpha (TNF) regulate plasminogen activation in monoblast-like U937 cells and normal human monocytes. In U937 cells, both IFN and TNF induced concurrent increases in secreted PA and PA inhibitor activities. These effects were accompanied by increased immunoreactive uPA and PAI-2 in conditioned media (enzyme-linked immunosorbent assay) and steady-state levels of cellular uPA and PAI-2 mRNA (Northern analysis). To determine the relative abilities of IFN and TNF to either promote or inhibit plasmin generation, we directly compared the effects IFN and TNF, using optimal stimulating concentrations. IFN induced PA activity to 180% of the level achieved by TNF. In contrast, IFN elicited only 78% of the PA inhibitor produced by TNF stimulation. These differences in secreted activity can be explained by the shift in balance between uPA and PAI-2 proteins. Immunoreactive uPA was induced equally by IFN and TNF, but TNF generated higher levels of PAI-2. The same overall pattern of results was seen in normal human monocytes. IFN and TNF differ greatly in the ability to augment receptor-bound PA activity in U937 cells, as IFN induced a twofold increase but TNF had no effect. We conclude that IFN and TNF modulate mononuclear phagocyte proteolytic activity through coordinate regulation of secreted and receptor-bound uPA, balanced against concurrent expression of PAI-2. These effects are cytokine specific, as IFN is superior to TNF in stimulating expression of both secreted and receptor-associated PA activities. These properties suggest mechanisms by which mononuclear phagocytes control proteolysis in cytokine-rich inflammatory foci.