Binding and release of factor VIII/von Willebrand's factor by human endothelial cells

Shear Stress and Endothelial Mechanotransduction in Trauma Patients with Hemorrhagic Shock: Hidden Coagulopathy Pathways and Novel Therapeutic Strategies

Massive trauma remains a leading cause of death and a global public health burden. Post-traumatic coagulopathy may be present even before the onset of resuscitation, and correlates with severity of trauma. Several mechanisms have been proposed to explain the development of abnormal coagulation processes, but the heterogeneity in injuries and patient profiles makes it difficult to define a dominant mechanism. Regardless of the pattern of death, a significant role in the pathophysiology and pathogenesis of coagulopathy may be attributed to the exposure of endothelial cells to abnormal physical forces and mechanical stimuli in their local environment. In these conditions, the cellular responses are translated into biochemical signals that induce/aggravate oxidative stress, inflammation, and coagulopathy. Microvascular shear stress-induced alterations could be treated or prevented by the development and use of innovative pharmacologic strategies that effectively target shear-mediated endothelial dysfunction, including shear-responsive drug delivery systems and novel antioxidants, and by targeting the venous side of the circulation to exploit the beneficial antithrombogenic profile of venous endothelial cells.

TGF-β1 induces the formation of vascular-like structures in embryoid bodies derived from human embryonic stem cells

Human embryonic stem cells (ESCs) can differentiate into endothelial cells in response to stimuli from extracellular cytokines. Transforming growth factor (TGF)-β1 signaling is involved in stem cell renewal and vascular development. Previously, human ESCs were isolated from inner cell mass and a stable ESC line was developed. In the present study, the effects of extracellular TGF-β1 were investigated on human ESC-derived embryoid bodies (EB) in suspension. The structures of the EBs were analyzed with light and electron microscopy, while the cellular composition of the EBs was examined via the expression levels of specific markers. Vascular-like tubular structures and cardiomyocyte-like beating cells were observed in the EBs at day 3 and 8, respectively. The frequencies of vascular-like structures and beating cells in the TGF-β1 treated group were significantly higher compared with the control group (84.31 vs. 12.77%; P<0.001; 37.25 vs. 8.51%; P<0.001, respectively). Electron microscopy revealed the presence of lumens and gap junctions in the sections of the tubular structures. Semiquantitative polymerase chain reaction revealed elevated expression levels of CD31 and fetal liver kinase-1 in EBs cultured with TGF-β1. In addition, extensive staining of von Willebrand factor was observed in the vascular-like structures of TGF-β1-treated EBs. Therefore, the results of the present study may aid the understanding of the underlying mechanisms of human ESC differentiation and improve the methods of propagating specific cell types for the clinical therapy of cardiovascular diseases.

Restoration versus reconstruction: cellular mechanisms of skin, nerve and muscle regeneration compared

In tissues characterized by a high turnover or following acute injury, regeneration replaces damaged cells and is involved in adaptation to external cues, leading to homeostasis of many tissues during adult life. An understanding of the mechanics underlying tissue regeneration is highly relevant to regenerative medicine-based interventions. In order to investigate the existence a leitmotif of tissue regeneration, we compared the cellular aspects of regeneration of skin, nerve and skeletal muscle, three organs characterized by different types of anatomical and functional organization. Epidermis is a stratified squamous epithelium that migrates from the edge of the wound on the underlying dermis to rebuild lost tissue. Peripheral neurons are elongated cells whose neurites are organized in bundles, within an endoneurium of connective tissue; they either die upon injury or undergo remodeling and axon regrowth. Skeletal muscle is characterized by elongated syncytial cells, i.e. muscle fibers, that can temporarily survive in broken pieces; satellite cells residing along the fibers form new fibers, which ultimately fuse with the old ones as well as with each other to restore the previous organization. Satellite cell asymmetrical division grants a reservoir of undifferentiated cells, while other stem cell populations of muscle and non-muscle origin participate in muscle renewal. Following damage, all the tissues analyzed here go through three phases: inflammation, regeneration and maturation. Another common feature is the occurrence of cellular de-differentiation and/or differentiation events, including gene transcription, which are typical of embryonic development. Nonetheless, various strategies are used by different tissues to replace their lost parts. The epidermis regenerates ex novo, whereas neurons restore their missing parts; muscle fibers use a mixed strategy, based on the regrowth of missing parts through reconstruction by means of newborn fibers. The choice of either strategy is influenced by the anatomical, physical and chemical features of the cells as well as by the extracellular matrix typical of a given tissue, which points to the existence of differential, evolutionary-based mechanisms for specific tissue regeneration. The shared, ordered sequence of steps that characterize the regeneration processes examined suggests it may be possible to model this extremely important phenomenon to reproduce multicellular organisms.

Narrow window in nanoscale dependent activation of endothelial cell growth and differentiation on TiO2 nanotube surfaces

Critical features of biomimetic materials used for vascular grafts and stents are surface structure and chemical features of the implant material supporting adhesion, proliferation, and differentiation of endothelial cells and smooth muscle cells, the major cell types of blood vessels. Recently, experimental evidence from several laboratories have indicated a strong stimulation of cellular activities on vertically aligned TiO(2) nanotube surfaces in comparison to amorphous TiO(2) surfaces. Conflicting reports exist, however, concerning the nanoscale dimension, and the role of the chemistry and crystallinity of the nanotubes in eliciting cell responses. Here we demonstrate that 15 nm nanotubes provide a substantially stronger stimulation of differentiation of mesenchymal cells to endothelial cells and smooth muscle cells than 70-100 nm nanotubes, while high rates of apoptosis were seen on 100 nm nanotubes. Also endothelial cell adhesion, proliferation, and motility were several-fold higher on 15 nm than on 100 nm nanotubes. Furthermore, our data indicate a clear dominance of the nanoscale geometry on endothelial cell behavior over surface chemistry and crystallinity of the TiO(2) nanotube surface. These findings indicate that fine-tuning of TiO(2) surfaces at nanoscale will be an essential parameter in optimizing endothelial cell and smooth muscle cell responses to vascular implants.

The effects of aspirin and nonselective beta blockade on the acute prothrombotic response to psychosocial stress in apparently healthy subjects

We hypothesized that the 2 cardiovascular drugs aspirin and propranolol attenuate the prothrombotic response to acute psychosocial stress relative to placebo medication. We randomized 56 healthy subjects, double-blind, to 5-day treatment with an oral dose of either 100 mg of aspirin plus 80 mg of propranolol combined, single aspirin, single propranolol, or placebo medication. Thereafter, subjects underwent a 13-minute psychosocial stressor. Plasma levels of von Willebrand factor antigen (VWF:Ag), fibrinogen, coagulation factor VII (FVII:C) and XII (FXII:C) activity, and D-dimer were determined in blood samples collected immediately pre- and post-stress and 45 minutes post-stress. The stress-induced changes in prothrombotic measures were adjusted for gender, age, body mass index, mean arterial blood pressure, smoking status, and sleep quality. There was an increase in VWF:Ag levels from immediately pre-stress to 45 minutes post-stress in the placebo group relative to the 3 subject groups with verum medication (P's </= 0.019; relative increase in VWF:Ag between 17% and 21%); however, the VWF:Ag response to stress was not significantly different between the three groups with verum medication. The stress responses in fibrinogen, FVII:C, FXII:C, and D-dimer were similar in all 4 medication groups. The combination of aspirin with propranolol, single aspirin, and single propranolol all attenuated the acute response in plasma VWF:Ag levels to psychosocial stress. This suggests that these cardiovascular drugs might exert limited protection from the development of stress-triggered coronary thrombosis.

Platelets enhance endothelial adhesiveness in high tidal volume ventilation

Although platelets induce lung inflammation, leading to acute lung injury (ALI), the extent of platelet-endothelial cell (EC) interactions remains poorly understood. Here, in a ventilation-stress model of lung inflammation, we show that platelet-EC interactions are important. We obtained freshly isolated lung endothelial cells (FLECs) from isolated, blood-perfused rat lungs exposed to ventilation at low tidal volume (LV) or stress-inducing high tidal volume (HV). Immunofluorescence and immunoprecipitation studies revealed HV-induced increases in cell-surface von Willebrand factor (vWf) expression on FLEC. This increased expression was inhibited by platelet removal from the lung perfusion and by including a P-selectin-blocking antibody in the lung perfusion. The expression was also blocked in lungs from P-selectin knockout (P sel(-/-)) mice perfused with autologous blood, but not with heterologous wild-type blood containing P-selectin-expressing platelets. These findings indicate that in ventilation stress, platelets transfer vWf to the EC surface and that platelet P-selectin plays a critical role in this transfer. Further evidence for such intercellular transfers was the HV-induced FLEC expressions of platelet glycoprotein 1b and of platelet P-selectin. We conclude that in ventilation stress, platelets deposit leukocyte- and platelet-binding proteins on the EC surface, thereby establishing the proinflammatory phenotype of the vascular lining.

Origin and characterization of multipotential mesenchymal stem cells derived from adult human trabecular bone

Much of the knowledge regarding the regulatory pathways for adult stem cell self-renewal and differentiation has been obtained from the results of in vitro cultures. However, it is unclear if adult stem cells are controlled in the same way under physiological conditions. We examined this issue with respect to the migration of stem cells to tissue injury and how switch from a migratory state to one of proliferation wherein they participate in development. Building on our previous identification of multipotent stem cells in trabecular bone, we have examined the in vitro behavior of these cells within the bone milieu. We found that cell proliferation is inhibited within the trabecular bone niche as cells migrate out of the trabecular bone prior to proliferation. Additionally, multiple cell types were detected in adult trabecular bone, including osteoblasts, osteoclasts, endothelial cells, and Stro-1-positive mesenchymal stem cells. Furthermore, we demonstrated that Stro-1-positive cells migrated out of their native bone niche to generate multipotential stem and progenitor cells during in vitro culture. We conclude that self-renewal and differentiation of adult stem cells in connective tissues are tightly controlled and separately orchestrated processes. A regulatory network of extrinsic factors and intrinsic signals acts to stimulate the exit of stem cells from their niche so that they can localize to sites of wound healing, where they participate in development after functional differentiation.

Beta-receptor blockade decreases elevated plasma levels of factor VIII:C in patients with deep vein thrombosis

BACKGROUND

An elevated plasma level of factor VIII:C (FVIII:C) is a strong and dose-dependent risk factor for venous thromboembolism (VTE). The cause of elevated FVIII:C in patients with thrombophilia is as yet unknown. FVIII:C increases significantly after infusion of epinephrine, vasopressin or physical exercise. The aim of the present study was to investigate whether beta-receptor blockade will lower sustained elevated FVIII:C in patients with VTE.

METHODS AND RESULTS

Two cohorts of patients with documented deep vein thrombosis and an elevated FVIII:C (>175 IU dL(-1)) and healthy volunteers, were studied. One cohort was treated with the beta-receptor blocker, whereas the other cohort served as non-treatment controls. The patient treatment group and healthy volunteers were given 40 mg propranolol, thrice daily, for 14 days. The mean baseline level of FVIII:C was 220 IU dL(-1) and 102 IU dL(-1) in patients and healthy volunteers, respectively. After 2 weeks of propranolol a significant 23% reduction of FVIII:C (- 52 IU dL(-1); 95%CI:[-65; -39]) compared with no change over time in the patient no- treatment group (-1.8 IU dL(-1); 95%CI:[-34; 30]). After discontinuation of propranolol FVIII:C returned to its initial high level. In healthy volunteers propranolol had no effect on the plasma concentration of FVIII:C.

CONCLUSION

This study demonstrates that in patients with VTE a sustained elevated FVIII:C concentration can be decreased with the use of propranolol. This observation may be of potential clinical relevance, since it has been shown that each increase of 10 IU dL(-1) in FVIII:C concentration enhanced the risk of a recurrent VTE by 24%.

Effects of nonspecific beta-adrenergic stimulation and blockade on blood coagulation in hypertension

A hypercoagulable state might contribute to increased atherothrombotic risk in hypertension. The sympathetic nervous system is hyperactive in hypertension, and it regulates hemostatic function. We investigated the effect of nonspecific beta-adrenergic stimulation (isoproterenol) and blockade (propranolol) on clotting diathesis in hypertension. Fifteen hypertensive and 21 normotensive subjects underwent isoproterenol infusion in two sequential, fixed-order doses of 20 and then 40 ng. kg(-1). min(-1) for 15 min/dose. Thirteen subjects were double-blind studied after receiving placebo or propranolol (100 mg/day) for 5 days each. In hypertensive subjects, isoproterenol elicited a dose-dependent increase in plasma von Willebrand factor (vWF) antigen [F(2,34) = 5.02; P = 0.032] and a decrease in D-dimer [F(2,34) = 4.57; P = 0.040], whereas soluble tissue factor remained unchanged. Propranolol completely abolished the increase in vWF elicited by isoproterenol [F(1,12) = 10.25; P = 0.008] but had no significant effect on tissue factor and D-dimer. In hypertension, vWF is readily released from endothelial cells by beta-adrenergic stimulation, which might contribute to increased cardiovascular risk. However, beta-adrenergic stimulation alone may not be sufficient to trigger fibrin formation in vivo.

Cellular expression of the neurokinin 1 receptor in the human antrum

The localization of the neurokinin 1 receptor in rat and guinea pig gastrointestinal tract has been extensively studied but not in human tissues. The present study used antibodies to characterize the cellular expression of neurokinin 1 receptors in human antrum. Cryostat sections (40-80 microm) were immunostained for the neurokinin 1 receptor double labeled with substance P, von Willebrand's factor, c-kit, fibronectin, S-100, serotonin, gastrin and somatostatin. Neurokinin 1 receptor-immunoreactivity was observed on neurons within the myenteric and submucosal plexuses surrounded by substance P-immunoreactive fibers and on von Willebrand's factor-immunoreactive endothelial cells lining blood vessels throughout the antral wall. c-Kit-immunoreactive interstitial cells of Cajal and gastrin cells were co-stained by the monoclonal neurokinin 1 receptor antibody. Finally, there was no evidence for the presence of the neurokinin 1 receptor on fibroblasts, Schwann, somatostatin, serotonin or smooth muscle cells. This study clearly demonstrates an expanded cellular expression of the neurokinin 1 receptor in the human antrum.

Effects of histamine and nitric oxide synthase inhibition on plasma levels of von Willebrand factor antigen

Endothelial cells release von Willebrand factor (vWf) either constitutively or by a regulated pathway. Based on various studies in vitro, we hypothesized that the stimulatory action of histamine on vWf release could also be induced in vivo and that it may be inhibited by endogenous production of nitric oxide (NO). Nine healthy subjects received placebo or one of two dosages of a primed constant infusion of the NO-synthase inhibitor N-monomethyl-L-arginine (L-NMMA) in a randomized, double-blind, three-way crossover trial. Histamine was coinfused for 15 minutes at 0.16 microg/kg/min after 30 minutes of pretreatment with either placebo or L-NMMA. Thirty minutes after either the low or the high L-NMMA dose was started, which caused, respectively, a 40% decrease and a 60% decrease in exhaled end expiratory NO level (p = 0.008), there was no increase in von Willebrand factor antigen (vWf-Ag) level (p > 0.05). Histamine caused an 11% (95% confidence interval (CI): 0.4% to 22%; p = 0.011) increase in vWf-Ag level at 125 minutes. After pretreatment with the low and the high L-NMMA doses, vWf-Ag level increased by 18% (Cl: 5% to 31%; p = 0.011) and by 29% (CI: 15% to 42%; p = 0.008), respectively. At 125 minutes, vWf-Ag level was significantly higher after either L-NMMA pretreatment when compared with the results after histamine alone (p < 0.05). In conclusion, the infusion of histamine increased vWf-Ag level, and the inhibition of NO-synthase enhanced this effect, whereas it did not by itself elevate vWf-Ag level. Thus endogenously produced NO may dampen the regulated pathway of vWf secretion; it will be interesting to investigate whether endogenous NO production also inhibits vWf release caused by other stimulators.

Dietary copper in the physiology of the microcirculation

Dietary copper has long been known to be essential for cardiovascular homeostasis. However, the role of copper and cuproenzymes in the normal control of vascular physiology is not well understood. Most studies in the cardiovascular system have focused on copper deficiency-induced defects in the heart or large vessels. Recently, attention has also focused on the effects of copper deficiency in the microcirculation or the small blood vessels that control blood flow, nutrient and waste exchange, and peripheral vascular resistance. Studies in the microcirculation demonstrate that copper is important in mechanisms of macromolecular leakage, platelet-endothelial interactions and vascular smooth muscle reactivity. There is a significantly greater leakage of proteins from postcapillary venules in copper-deficient rats in response to mast cell-released histamine. This response appears to be the result of increased numbers of mast cells and thereby increased available histamine. Copper deficiency also causes an inhibition of in vivo thrombogenesis, which appears to be related to an inhibition of platelet adhesion. Subsequent studies have demonstrated that this is probably caused by a diminished concentration of the adhesion molecule von Willebrand factor. Nitric oxide (NO)-mediated arteriole vasodilation is also compromised in copper-deficient rats. This functional deficit to NO can be reversed by the addition of Cu, Zn-superoxide dismutase (SOD), suggesting that degradation of NO by superoxide anion occurs during copper deprivation. These observations demonstrate that dietary copper is necessary for several microvascular control mechanisms affecting inflammation, microhemostasis and regulation of peripheral blood flow.

A modified method for application of indirect immunofluorescent staining for factor VIII/vWF to capillary endothelia

Immunofluorescent staining for factor VIII/von Willebrand factor (VIII/vWF) as applied to capillary endothelia has sources of false negativity and false positivity. False negativity occurs because capillary endothelia are not as rich as endothelia of larger vessels in this factor. To overcome this problem we enriched liver endothelium by preincubation with the factor. The source of false positivity is the presence of Fc receptors on freshly isolated capillary endothelia. These receptors may bind antibody to this factor through the Fc portion of the molecule. To overcome this problem we preincubated the cells with IgG not against VIII/vWF (irrelevant antibody) to cover Fc receptors. The results presented indicate that considerable improvement in the immunofluorescent staining of capillary endothelia can be achieved by application of our method.

An in vitro model for the study of acute release of von Willebrand factor from human endothelial cells

An in vitro model is described which utilizes human umbilical vein endothelial cells cultured on plastic microcarrier spheres and perfused with serum-free medium. This model was used to study the acute release of von Willebrand factor following stimulation of the cells with putative agonists. Thrombin, plasmin and interleukin-1 were found to release von Willebrand factor. Adrenaline and bradykinin also stimulated release but only at high dosage. 1-desamino-8-D-arginine vasopressin (DDAVP) was inactive.

von Willebrand disease

von Willebrand disease (vWD) is a bleeding disorder characterized by a complex hemostatic defect. Abnormal platelet function, usually reflected by a prolonged bleeding time, is the result of a quantitative or qualitative defect of von Willebrand factor (vWF). A secondary deficiency of factor VIII procoagulant protein (factor VIII) may occur leading to a coagulation defect as well. These two glycoprotein macromolecules circulate as a complex in plasma. This article will review current understanding of structures and functions of vWF factor and factor VIII as they relate to the pathogenesis, diagnosis, classification and therapy of vWD.

Immunocytochemical localization of factor VIII-related antigen in tumors of the human central nervous system

Using both immunohisto- and immunocytochemical techniques with periodate-lysine-paraformaldehyde (PLP) fixation, we have studied the distribution of Factor VIII-related antigen (FVIIIR:Ag) in 12 cases of tumors of the human central nervous system (CNS) and one sample of non-tumor brain tissue. FVIIIR:Ag was found both extracellularly and intracellularly. It was localized in the vascular lumen, between endothelial cells, and in the endothelial cell basement membrane. In the endothelial cell cytoplasm, FVIIIR:Ag was found in the endoplasmic reticulum, perinuclear space, and in intracytoplasmic vacuoles and vesicles. Characteristic of malignant tumors (six out of seven) was a strongly-positive dilated endoplasmic reticulum. This may reflect increased FVIIIR:Ag synthesis in the endothelial cells of malignant tumors. Only one of five benign tumors showed such staining. Six of 12 tumors and the non-tumor brain showed perinuclear FVIIIR:Ag. Both ad- and abluminal vesicles in the tumor endothelial cells contained FVIIIR:Ag suggesting that endocytosis, transcellular transport, and/or endocytosis, as well as FVIIIR:Ag synthesis occurs. The non-tumor brain showed normal capillary structure and very little FVIIIR:Ag immunoreactivity. The relationship of these FVIIIR:Ag abnormalities to the hypercoagulable state seen in some malignant brain tumor patients remains to be clarified.

Transplantation of normal bone marrow into a pig with severe von Willebrand's disease

Bone marrow from a normal male pig was transplanted into a related female pig with severe homozygous von Willebrand's disease (vWd). After engraftment the circulating leukocytes were of the male karyotype, and the platelets were strongly positive for von Willebrand factor (vWF) by indirect immunofluorescence. The average level of vWF was 1.96 U/dl and of ristocetin cofactor was 2.8 U/dl. The ear immersion bleeding time before transplantation was consistently more than 15 min and afterwards varied between 5 min and more than 15 min. Transfused vWF corrected the bleeding time at a level of 10 U/dl, which is lower than that required for a von Willebrand pig. We concluded that: the plasmatic compartment is only minimally replenished by the vWF from platelets and megakaryocytes; and the platelet vWF alone only partially corrects the abnormal tests of the hemostatic mechanism in severe vWd.

Hybridoma antibodies to human von Willebrand factor. II. Relative role of intramolecular loci in mediation of platelet adhesion to the subendothelium

Hybridoma antibodies specific for seven independent topographical sites were used to characterize von Willebrand factor (vWF) and to relate the epitopes to functional loci required for vWF-mediated adhesion of platelets to subendothelium and ristocetin-induced platelet aggregation. The capacity of antibodies to influence the adhesion of human platelets to rabbit aortic subendothelium was analysed in annular perfusion chambers. At a high shear rate similar to that of the microcirculation, four monoclonal antibodies inhibited adhesion. In contrast, no inhibition was observed at low shear. Only one of the four antibodies that inhibited platelet adhesion also attenuated ristocetin-cofactor activity (VIIIR:RCo). Conversely, one antibody that inhibited VIIIR:RCo had no effect upon platelet adhesion. These data support the hypothesis that the molecular loci involved in the two biological functions of vWF are not identical. When these conclusions are considered within the context of a spatial map of the vWF protein surface developed by competitive displacement analysis, the epitopes related to platelet adhesion appear to be spaced and differ from those involved in ristocetin-induced platelet-platelet interaction.

Differential localisation and synthesis of porcine factor VIII related antigen (VIIIR:AG) in vascular endothelium and in endothelial cells in culture

Porcine and human umbilical vein and adult blood vessels were studied for the presence and synthesis of factor VIII related antigen (VIIIR:Ag) and fibronectin (Fn) by immunofluorescence histology and immunoautoradiography. Investigation of human tissue confirmed the widespread distribution of VIIIR:Ag on the endothelium of all blood vessels examined but observations on porcine tissue gave different results. Porcine umbilical vein and porcine adult veins were positively stained for VIIIR:Ag whilst porcine aorta and other pig arteries appeared to be negative or only weakly positive. Some blood vessels (?venous) in the adventitia of porcine aorta were positively stained whilst adjacent ones (?arterial) were negative. Radiolabelled methionine was added to culture medium and proteins synthesised by cultured EC were examined by two dimensional crossed immunoelectrophoresis and autoradiography. Identification of radiolabelled precipitin arcs provided a highly sensitive method for confirming the specificity of antisera and for detecting VIIIR:Ag and Fn. Examination of cultured human umbilical vein EC confirmed the synthesis of VIIIR:Ag and distinguished between VIIIR:Ag and Fn. Studies of porcine umbilical vein EC in culture gave similar results to those observed with corresponding human EC. However, cultures derived from porcine aorta did not demonstrate synthesis of VIIIR:Ag and microscopy failed to locate VIIIR:Ag in these cells with certainty. The results confirmed the synthesis of VIIIR:Ag by human and porcine umbilical vein EC but differences in staining reactions and the apparent inability to synthesise VIIIR:Ag by cells derived from porcine aorta suggested that porcine EC at different anatomical sites may subserve different functions.

Altered factor VIII in acute respiratory failure

Acute respiratory failure is characterized by pulmonary vascular injury and increased endothelial permeability. Since endothelial cells synthesize or release factor VIII, we studied factor VIII:antigen, factor VIII:von Willebrand's factor, and factor VIII:coagulant activity in 100 patients with acute respiratory failure, in 29 critically ill patients without evidence of lung disease, and in 60 normal subjects. As a group, patients with respiratory failure had factor VIII:antigen levels that were five times normal, whereas their factor VIII:coagulant and von Willebrand's activities were normal or above normal. However, there was a remarkable disproportion between the levels of factor VIII:antigen and those of factor VIII:coagulant and von Willebrand's factor. In the other ill patients and in the normal group, these levels were in close proportion. In patients with respiratory failure, both slowly and rapidly migrating components of factor VIII:antigen had abnormal electrophoretic patterns; in moderate and severe cases, there was a marked increase in the fast component. Patients who recovered from acute respiratory failure had electrophoretic patterns that returned toward normal. Factor VIII:antigen may be a sensitive circulating indicator of pulmonary endothelial injury and repair.