Novel function for beta 1 integrins in keratinocyte cell-cell interactions.

We have examined the expression, localization, and function of beta 1 integrins on cultured human epidermal keratinocytes using polyclonal and monoclonal antibodies against the beta 1, alpha 2, alpha 3, and alpha 5 integrin subunits. The beta 1 polypeptide, common to all class 1 integrins, was localized primarily in areas of cell-cell contacts of cultured keratinocytes, as were alpha 2 and alpha 3 polypeptides, suggesting a possible role in cell-cell adhesion for these integrin polypeptides. In contrast, the fibronectin receptor alpha 5 subunit showed no such accumulations in regions of cell-cell contact but was more diffusely distributed in the keratinocyte plasma membrane, consistent with the absence of fibronectin at cell-cell contact sites. Colonies of cultured keratinocytes could be dissociated by treatment with monoclonal antibody specific to the beta 1 polypeptide. Such dissociation of cell-cell contacts also occurred under conditions where the monoclonal antibody had no effect on cell-substrate adhesion. Therefore, beta 1 integrin-dependent cell-cell adhesion can be inhibited without affecting other cell-adhesive interactions. Antibody treatment of keratinocytes maintained in either low (0.15 mM) or high (1.2 mM) CaCl2 also resulted in the loss of organization of intracellular F-actin filaments and beta 1 integrins, even when the anti-beta 1 monoclonal antibody had no dissociating effect on keratinocyte colonies at the higher calcium concentration. Our results indicate that beta 1 integrins play roles in the maintenance of cell-cell contacts between keratinocytes and in the organization of intracellular microfilaments. They suggest that in epithelial cells integrins can function in cell-cell interactions as well as in cell-substrate adhesion.

Carbohydrate deficiency of the factor VIII/von Willebrand factor Protein in von Willebrand's disease variants.

Study of the normal human factor VIII/von Willebrand factor reveals a macromolecular glycoprotein composed of apparently identical subunits. This purified glycoprotein has procoagulant, antigen, and von Willebrand factor activities. In three patients with a variant of the von Willebrand's disease syndrome, their factor VIII/von Willebrand factor protein was present in normal amounts and had normal procoagulant and antigen activities; however, this protein was deficient in both carbohydrate and von Willebrand factor activity. The carbohydrate portion of the factor VIII/von Willebrand factor glycoprotein is of major importance in its interactions with platelets or the blood vessel wall, or both.

Factor VIII/von Willebrand factor protein. Galactose a cryptic determinant of von Willebrand factor activity.

The normal Factor VIII/von Willebrand factor protein has the ability to agglutinate or aggregate normal platelets in the presence of ristocetin (von Willebrand factor activity). Removal of greater than 95% of the sialic acid from this protein by neuraminidase did not affect the von Willebrand factor or procoagulant activity. However, oxidation of the penultimate galactose of the asialo Factor VIII/von Willebrand factor protein with galactose oxidase resulted in a progressive loss of von Willebrand factor activity with no effect on procoagulant activity. Reduction of the 6-aldehydo intermediate by potassium borohydride caused full regeneration of von Willebrand factor activity. These studies confirm the identification of the intact penultimate galactose moiety as a critical determinant of von Willebrand factor activity.

Studies on the mechanism of ristocetin-induced platelet agglutination. Effects of structural modification of ristocetin and vancomycin.

The mechanism by which ristocetin induces platelet agglutination in the presence of the von Willebrand factor was studied by chemically altering ristocetin and a similar antibiotic, vancomycin, by reaction with a water-soluble carbodiimide in the presence of glycine methyl ester at pH 4.75. Altering ristocetin's phenolic groups (which are thought to be important in its peptide-binding properties) resulted in a loss of both platelet-agglutinating and antibiotic activities. Restoring the phenolic groups with hydroxylamine restored both activities. Vancomycin has antibiotic and peptide-binding properties similar to ristocetin's, but differs structurally in having a free carboxyl group and thus a less positive charge at neutral pH. It does not induce platelet agglutination and actually inhibits ristocetin-induced agglutination. Reacting vancomycin with the water-soluble carbodiimide resulted in alteration of phenolic groups and permanent conversion of the carboxyl to a neutral derivative. Restoring the phenolic groups with hydroxylamine (but leaving the carboxyl neutralized) produced a compound with charge properties similar to ristocetin's which induced platelet agglutination as ristocetin does. These data suggest both a binding requirement (mediated through phenolic groups) and a strong positive charge requirement for ristocetin-induced agglutination. The data are consistent with a model wherein positively charged ristocetin binds, via its phenolic groups, to sites on the platelet surface and reduces the platelet's negative charge. This could reduce the electrostatic repulsion between platelets and/or between platelets and the negatively charged von Willebrand factor, and permit the macromolecular von Willebrand factor to cause agglutination by bridging between platelets.

Identification of the thrombin receptor on human platelets by chemical crosslinking.

To identify the molecular site of thrombin binding to the platelet membrane, we covalently linked 125I-thrombin to platelets by using the bifunctional chemical cross-linking agents disuccinimidyl suberate and dithiobis(succinimidyl propionate). The proteins cross-linked to 125I-thrombin by this method were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and followed by autoradiography. Two radiolabeled thrombin complexes were identified, a major species of Mr approximately 200,000 and a minor one of Mr approximately 400,000. Hirudin prevented the formation of both complexes. The radioactivity of the approximately 200,000-Mr complex was always 7-10-fold greater than the radioactivity of the approximately 400,000-Mr complex regardless of the thrombin concentration to which the platelets were exposed (0.1-29 nM). Although 125I-thrombin complexes generated with thrombasthenic platelets (lacking glycoprotein IIb/IIIa) were indistinguishable from normal, no complexes appeared when Bernard-Soulier platelets (lacking glycoprotein Ib [GPIb]) were used. Complex formation was blocked by rabbit antiglycocalicin antiserum, but not by the monoclonal antibody 6D1, which is directed against the site on GPIb where von Willebrand factor (vWf) binds in the presence of ristocetin. Although cross-linking studies suggested that vWf might partially inhibit thrombin binding to platelets, this was not confirmed by equilibrium binding studies in the presence of vWf and ristocetin. The data suggest, therefore, that at all thrombin concentrations binding occurs at the same membrane site, despite evidence from equilibrium studies for high and low affinity classes of receptors, and that the approximately 400,000-Mr complex is simply a dimer of the approximately 200,000-Mr species. We conclude that the membrane site to which thrombin binds is the glycocalicin portion of platelet GPIb at a site remote from the point of ristocetin-dependent vWf binding.

Studies of the human factor VIII/von Willebrand factor protein. III. Qualitative defects in von Willebrand's disease.

The Factor VIII/von Willebrand factor protein was characterized in two unrelated patients with von Willebrand's disease in whom procoagulant and Factor VIII/von Willebrand factor antigen levels were normal. In both patients evidence of an abnormal protein was observed on crossed antigen-antibody electrophoresis. In one patient the Factor VIII/von Willebrand factor protein eluted from Sepharose 4B in a position and distribution identical to normal with normal levels of procoagulant activity and antigen. However, the partially purified Factor VIII/von Willebrand factor protein had markedly reduced von Willebrand factor activity in a ristocetin assay. In the second patient the peak of Factor VIII/von Willebrand factor protein, antigen, and procoagulant activity eluted from a Sepharose 4B column with an estimated molecular weight of approximately half that of normal. This protein had no von Willebrand factor activity. In both patients the reduced Factor VIII/von Willebrand factor protein subunit was indistinguishable from normal on polyacrylamide gel electrophoresis. These studies indicate that in some patients with von Willebrand's disease there is a qualitative defect of the Factor VII/von Willebrand factor protein; the total amount of protein, antigen, and procoagulant activity are normal while the von Willebrand factor activity is deficient.

Studies on the purification and characterization of human factor 8.

Factor VIII (antihemophilic globulin) has been prepared from Hyland method IV AHG and cryoprecipitate using limited chymotryptic digestion followed by Sepharose gel filtration. The activity of factor VIII is unaffected by the digestion procedure, while fibrinogen in converted to large noncoagulable fragments. The purified factor VIII has been found to be a macromolecular glycoprotein with a major subunit of 240,000, as shown by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Carbohydrate analysis of factor VIII gave values of 1% sialic acid, 2.8% hexosamine, and 1-2% hexose (mannose, galactose, and fucose). The lipid content was found to be less than 5% of the protein content, and included no detectable phospholipid. The amino acid content is also reported. Immunoelectrophoretic analysis using rabbit antibody to purified factor VIII produced a single precipitin line. The chymotrypsin digestion step facilitates the preparation of factor VIII by reducing the viscosity of fibrinogen in the crude starting material, thereby increasing fivefold the quantity of material which can be processed at one time. It also improves markedly the resolution between factor VIII and fibrinogen on gel filtration.

Plasticity of integrin expression by nerve-derived connective tissue cells. Human Schwann cells, perineurial cells, and fibroblasts express markedly different patterns of beta 1 integrins during nerve development, neoplasia, and in vitro.

Strikingly selective expression patterns of beta 1, alpha 2, alpha 3, and alpha 5 integrin subunits were revealed in endoneurium, perineurium, and epineurium of fetal and adult human peripheral nerve by immunostaining with specific antibodies. The alpha 2 subunit was expressed only on Schwann cells both in fetal and adult nerve, whereas the alpha 3 epitopes were expressed exclusively in the adult tissue and were primarily present on perineurial cells. The alpha 5 epitopes were expressed only on the innermost cell layer of perineurium of fetal and adult nerve. The tumor cells within schwannomas and cutaneous neurofibromas expressed both alpha 2 and alpha 3 subunits, indicating that Schwann cells have the potential to express also the alpha 3 subunit in vivo. Cell cultures established from human fetal nerve and neurofibromas revealed expression of the alpha 2 and alpha 5 epitopes on Schwann cells, perineurial cells, and fibroblasts, whereas only Schwann cells contained the alpha 3 epitopes which were occasionally concentrated on the adjacent Schwann cells at cell-cell contacts. Our findings emphasize that nerve connective tissue cells change their profiles for expression of extracellular matrix receptors under conditions which have different regulatory control signals exerted by, for example, axons, humoral factors, or the extracellular matrix of the peripheral nerve. This plasticity may play an important role during nerve development and in neoplastic processes affecting the connective tissue compartments of peripheral nerve.

Fibrinogen Bethesda: a congenital dysfibrinogenemia with delayed fibrinopeptide release.

A dysfibrinogenemia (fibrinogen Bethesda) was detected in a 9 yr old male of Mexican-English extraction who had a lifelong history of mild bleeding diathesis. The prothrombin and partial thromboplastin times were moderately prolonged; the thrombin and Reptilase times were markedly prolonged. The plasma fibrinogen level was normal by conventional methods but was markedly reduced by the Clauss method. Results of all other tests for clotting factors, fibrinolysis, antithrombin levels, clot stabilization, and fibrin(ogen) degradation products were normal. The patient's plasma and fibrinogen inhibited the clotting of normal plasma or fibrinogen by thrombin. Family studies revealed that the propositus' mother and two siblings exhibited these abnormalities to a lesser degree and indicated an autosomal dominant inheritance. Fibrinogen Bethesda was similar to normal fibrinogen in the following respects: metabolic turnover time (measured in the propositus' mother); immunodiffusion, ultracentrifugal, electrophoretic (on cellulose acetate or polyacrylamide gel), and chromatographic (on DEAE-cellulose) characteristics; sialic acid content; and aggregation of fibrin monomers. By contrast, fibrinogen Bethesda gave an abnormal immunoelectrophoretic pattern especially when whole plasma (as opposed to purified fibrinogen) was examined, and it showed a pronounced decrease in the rate of fibrinopeptide release by thrombin. This decrease, which was shown to involve both fibrinopeptides A and B, distinguishes fibrinogen Bethesda from previously reported dysfibrinogenemias.

Localization of integrin receptors for fibronectin, collagen, and laminin in human skin. Variable expression in basal and squamous cell carcinomas.

VLA integrins in human skin were examined by indirect immunofluorescence utilizing antibodies recognizing the beta 1, alpha 2, alpha 3, or alpha 5 subunits. Staining of fetal, newborn, or adult skin with antibodies to beta 1, alpha 2, or alpha 3 subunits gave essentially similar staining patterns: intense staining was associated with the basal layer of the epidermis, hair follicles, and blood vessel walls. The alpha 5 subunit could be detected only in epidermis and the inner root sheath of hair follicles in fetal skin. In epidermis, the staining reaction for the beta 1 subunit was not only found in sites interfacing with the basement membrane zone, but also around the entire periphery of these cells. We speculate that these receptors might have previously unrecognized functions in cell-cell interactions or that these findings may suggest the presence of previously unrecognized ligands in the intercellular spaces of keratinocytes. Examination of nine nodular basal cell carcinomas revealed a prominent staining reaction with anti-beta 1 and anti-alpha 3 antibodies at the periphery of the tumor islands. In contrast, staining of five squamous cell carcinomas revealed either the absence of integrins or altered and variable expression. Thus, matrix components and their receptors may participate in modulation of growth, development, and organization of human skin.

Arginine-glycine-aspartic acid- and fibrinogen gamma-chain carboxyterminal peptides inhibit platelet adherence to arterial subendothelium at high wall shear rates. An effect dissociable from interference with adhesive protein binding.

Arg-Gly-Asp (RGD)- and fibrinogen gamma-chain carboxyterminal (GQQHHLGGAKQAGDV) peptides inhibit fibrinogen, fibronectin (Fn), vitronectin, and von Willebrand factor (vWF) binding to the platelet glycoprotein IIb-IIIa complex (GP IIb-IIIa). GP IIb-IIIa, vWF, and Fn are essential for normal platelet adherence to subendothelium. We added peptides to normal citrated whole blood before perfusion over human umbilical artery subendothelium and evaluated platelet adherence morphometrically at high (2,600 s-1) and low (800 s-1) wall shear rates. We also examined the effects of the peptides on platelet adhesion to collagen in a static system. At the high wall shear rate, RGDS and GQQHHLGGAKQAGDV caused dose-dependent reduction in the surface coverage with spread and adherent platelets. Amino acid transposition and conservative substitutions of RGD peptides and the AGDV peptide significantly inhibited platelet adherence at 2,600 s-1. By contrast, the modified RGD peptides and AGDV do not affect adhesive protein binding to platelets. None of the native or modified RGD- or fibrinogen gamma-chain peptides significantly inhibited either platelet adherence to subendothelium at 800 s-1 or platelet adhesion to collagen. Our findings demonstrate that peptides that interfere with adhesive protein binding to GP IIb-IIIa inhibit platelet adherence to vascular subendothelium with flowing blood only at high wall shear rates. Platelet adherence to subendothelium at high wall shear rates appears to be mediated by different recognition specificities from those required for fluid-phase adhesive protein binding or static platelet adhesion.

Physiologic manifestations of human anaphylaxis.

In the course of a controlled study to evaluate different forms of immunotherapy for subjects with insect-sting hypersensitivity, we observed 11 subjects who had systemic cutaneous urticarial reactions and 3 subjects who experienced systemic anaphylaxis. With the exception of tachycardia, there were no cardiopulmonary changes in the subjects with urticaria, whereas the major manifestation of anaphylactic shock in the other three subjects was severe hypotension that was probably secondary to peripheral vasodilation. Significant abnormalities in gas exchange developed in two subjects. In one, bronchospasm precipitated a respiratory arrest followed by endotracheal intubation with mechanical ventilation. Although plasma histamine levels were not related to the development of cutaneous reactions, the plasma histamine levels correlated with the severity and duration of the cardiopulmonary changes observed during anaphylactic shock. The two subjects with the most severe shock showed evidence of intravascular coagulation characterized by a diminution of Factor V, Factor VIII, fibrinogen, and high molecular weight kininogen, as well as changes in components of the complement system. Standard therapy with epinephrine and fluids, usually recommended for the treatment of systemic anaphylaxis, did not immediately reverse either the hemodynamic or the respiratory abnormalities in the two subjects with the most severe anaphylactic shock. Hemodynamic recovery was gradual and did not seem directly related to any specific therapeutic intervention.

The molecular defect in type IIB von Willebrand disease. Identification of four potential missense mutations within the putative GpIb binding domain.

Type IIB von Willebrand Disease (vWD) is characterized by the selective loss of large von Willebrand Factor (vWF) multimers from plasma, presumably due to their increased reactivity with platelets and subsequent clearance from the circulation. Using the PCR, one of a panel of four potential missense mutations was identified in each of the 14 patients studied from 11 unrelated families. None of these substitutions was encountered in a large panel of normal DNAs. These changes all represent C----T transitions at CpG dinucleotides, proposed "hot spots" for mutation in the human genome. The four resulting amino acid substitutions, Arg543----Trp, Arg545----Cys, Val553----Met, and Arg578----Gln, are all clustered within the GpIb binding domain of vWF. Disruption of this latter functional domain may explain the pathogenesis of Type IIB vWD. By sequence polymorphism analysis, the Arg543----Trp substitution was shown to have occurred as at least two independent mutational events. This latter observation, along with the identification of mutations in all 14 patients studied and their localization to the GpIb binding domain, all strongly suggest that these substitutions represent the authentic defects responsible for Type IIB vWD. This panel of mutations may provide a useful diagnostic tool for the majority of patients with Type IIB vWD.

Monoclonal antibody and synthetic peptide inhibitors of human tumor cell migration.

The processes of migration and invasion by human tumor cells are likely to involve specific cell surface receptors, such as receptors for the extracellular matrix molecules fibronectin, laminin, and collagen. We have examined the roles of several of these receptors using a set of monoclonal antibodies directed against the beta 1 integrin family, as well as a series of synthetic peptides reported to inhibit various interactions of each of these proteins with the cell surface. The most general inhibitor of tumor cell migration was found to be the anti-beta 1 monoclonal antibody 13, which inhibited the migration of human HT-1080 fibrosarcoma cells, 5637 bladder carcinoma cells, VA13 viral transformants, and HCT 116 colon carcinoma cells when fibronectin was the migration substrate. Moreover, this antibody was particularly effective in blocking cell migration on laminin, as well as migration within 3-dimensional collagen gels. It also inhibited in vitro invasiveness in a reconstituted basement membrane invasion assay (Matrigel assay) at concentrations as low as 1 microgram/ml. Integrins of the beta 1 class thus appear to play a central role in several types of migration by a variety of human tumor cell lines. Anti-alpha 5 fibronectin receptor monoclonal antibody 16 also significantly inhibited migration on fibronectin, but not on other substrates, in 3 of the 4 cell lines. Conversely, anti-alpha 2 monoclonal antibody F17 strikingly inhibited migration in 3-dimensional collagen gels, but not on other substrates, implicating the alpha 2 beta 1 integrin system in migration of tumor cells within collagenous matrices. A series of synthetic peptides previously reported to inhibit interactions of normal cells with fibronectin, laminin, and collagen were also tested as inhibitors of tumor cell migration. Peptides containing the Arg-Gly-Asp adhesive recognition signal were partially inhibitory, but with occasional exceptions, most other peptides had no effects on migration. Our results indicate the central importance of several specific beta 1 integrins in human tumor cell migration and show the effectiveness of monoclonal antibody treatment in blocking this process in vitro.

Mechanisms underlying the morning increase in platelet aggregation: a flow cytometry study.

OBJECTIVES: Mechanisms underlying the morning increase in platelet aggregation produced by arising and assuming the upright posture were studied by examining 1) the expression on the platelet surface of activation-dependent markers; 2) platelet aggregation in whole blood; and 3) hematologic factors likely to influence aggregation. BACKGROUND: The morning increase in thrombotic cardiovascular events has been attributed, in part, to the morning surge in platelet aggregability, but its mechanisms are poorly understood. METHODS: Expression of seven platelet surface antigens (including P-selectin, activated GPIIb,IIIa and GPIb-IX), whole-blood platelet aggregation, platelet count and hematocrit were measured before and after arising in 17 normal volunteers. The fibrinolytic variables, tissue-type plasminogen activator, plasminogen activator inhibitor 1 and catecholamine levels were also measured. RESULTS: On arising and standing, platelet aggregation increased by 71% (p < 0.01) and 27% (p < 0.03) in response to collagen and adenosine diphosphate, respectively. However, there was no change in any of the activation-dependent platelet surface markers. Whole-blood platelet count and hematocrit increased by 15% and 7% (both p < 0.0001), respectively. Norepinephrine and epinephrine levels increased by 189% (p < 0.0001) and 130% (p < 0.01), respectively. Tissue-type plasminogen activator antigen increased (31%, p < 0.01), but there was no significant increase in plasminogen activator inhibitor 1, suggesting an overall increase in fibrinolysis on standing. Prothrombin fragment 1.2 increased by 28% (p < 0.02), indicating a small increase in thrombin generation. The increases in hematocrit and platelet count that occurred on standing were carefully mimicked in vitro and resulted in a 115% (p < 0.05) increase in platelet aggregation in response to adenosine diphosphate. CONCLUSIONS: These data demonstrate that the morning increase in platelet aggregation is not accompanied by expression of activation-dependent platelet surface receptors and suggest that the increase in whole-blood aggregation may be primarily due to the increases in catecholamine levels, platelet count and hemoconcentration.

Hypergranular promyelocytic leukemia: correlation between morphology and chromosomal translocations including t(15;17) and t(11;17).

The FAB group has reviewed 32 cases of promyelocytic leukemia and variant forms. By utilizing published criteria the ability to make a correct diagnosis by morphology with molecular genetic confirmation and to eliminate cases that did not have the PML/RARalpha rearrangement was excellent.

Treatment of polyneuropathy in Waldenström's macroglobulinemia: role of paraproteinemia and immunologic studies.

A patient with polyneuropathy due to Waldenström's macroglobulinemia (WM) was treated successfully with chlorambucil and prednisone. Before therapy, 60% of peripheral lymphocytes were B cells, the nerve had IgM-bearing B-cell infiltrates, and the circulating IgM had antibody-binding activity to autologous and homologous nerves. Neurologic improvement, sustained for 4 years, began 3 months after therapy and coincided with the return to normal of bone marrow and circulating B cells. Binding of IgM to autologous and homologous nerves persisted after therapy, suggesting that not the IgM alone but other B-cell factors, possibly complexed to IgM, may have been responsible for the nerve damage.

Erythroblastic transformation of chronic granulocytic leukemia.

Erythroblastic transformation of chronic granulocytic leukemia was found in seven of 67 unselected patients with blast crisis. This morphologic picture of erythroblastic transformation was indistinguishable from that in erythroleukemia or Di Guglielmo's syndrome. The median survival of the patients with erythroblastic transformation was two months, considerably less than the four-month median survival in the entire series of 67 patients. Only two brief partial remissions were obtained with combination chemotherapy. The causes of death were primarily hemorrhage and infection, related to thrombocytopenia and neutropenia. In this regard, the patients with erythroblastic transformation resembled all the patients with blast crisis and patients with acute leukemia in general. The erythroblastic transformation seems to represent a morphologic variant of chronic granulocytic leukemia blast crisis, without apparent prognostic or therapeutic implications.

Blast crisis of chronic granulocytic leukemia. Morphologic variants and therapeutic implications.

Chronic granulocytic leukemia (CGL) terminates in a disease similar to acute leukemia. Except for a study indicating an increased response rate to the drugs, vincristine and prednisone, therapy of this terminal phase has been universally disappointing. We have studied the bone marrows and clinical courses of 67 patients in the terminal phase of CGL to discern if any parameters were associated with an increased response rate or survival. The results of this study indicate that patients who have a lymphoblastic morphology or hypodiploid cytogenetics in the terminal phase respond better to treatment with the combination of vincristine and prednisone than those with myeloblastic morphology or hyperdiploid cytogenetics. Response rate and survival are significantly increased in those with lymphoblastic morphology. Recognition of the heterogeneity of the terminal phase of CGL may dictate specific therapeutic modalities.

Plasma and platelet von Willebrand factor defects in uremia.

PURPOSE: Several mechanisms have been proposed to explain the prolonged bleeding times and clinical bleeding in chronic renal failure. Recent evidence has implicated an abnormality in the structure or function of the von Willebrand factor or in its interaction with uremic platelets. We investigated this factor in 11 patients with chronic renal failure. PATIENTS AND METHODS: Blood samples for cell counts, chemistries, and coagulation studies were obtained from 11 patients with chronic renal failure and prolonged bleeding times. Concentrations of von Willebrand factor antigen and ristocetin cofactor activity were determined in plasma and platelets. Multimeric analysis of von Willebrand factor in plasma and platelets was conducted. In eight cases, the platelets of uremic patients were purified, and the thrombin- and ristocetin-induced binding of normal von Willebrand factor to these platelets was examined. RESULTS: The mean plasma von Willebrand factor antigen and activity (ristocetin cofactor assay) were elevated 2.77 mu/ml and 1.88 mu/ml, respectively (normal, 1.01 mu/ml and 1.07 mu/ml, respectively). The ratio of activity to antigen in uremic plasma was 0.67 (normal, 1.05). The mean platelet von Willebrand factor antigen and activity in the uremic patients was decreased (0.26 and 0.50 mu/10(9) platelets, respectively) compared with normal patients (0.46 and 0.93 mu/10(9) platelets, respectively). The oligomeric structure of the uremic plasma von Willebrand factor lacked the largest multimers. Collection of the blood for analysis in several protease inhibitors and/or EDTA did not change the multimeric structure. The von Willebrand factor multimeric structure of platelets from uremic patients was normal. The ristocetin-induced platelet aggregation of the uremic platelet-rich plasma was decreased compared with normal plasma samples. Thrombin and ristocetin-induced binding of normal von Willebrand factor to uremic patients' platelets was indistinguishable from the binding to normal platelets. CONCLUSION: These data suggest that the uremic platelet-binding sites for von Willebrand factor are intact and that the defect in ristocetin-induced platelet aggregation is most likely plasmatic in nature. At least one plasmatic defect was the observed reduction or absence of the largest plasma von Willebrand factor multimer in uremic patients. The platelet von Willebrand content was significantly decreased. These defects may play a role in the prolonged bleeding time and the clinical bleeding observed in patients with uremia.