Involvement of divalent cations in the complex between the platelet glycoproteins IIb and IIIa

Dissociation of the alphaIIbbeta3-integrin by EGTA stimulates the tyrosine kinase pp72(syk) without inducing platelet activation

Incubation of human platelets with EGTA under conditions that dissociate the alphaIIbbeta3-integrin stimulated tyrosine phosphorylation of pp72(syk) (6.8-fold) and of proteins of 62 (2. 2-fold), 68 (2.5-fold) and 130 kDa (1.4-fold). Stimulation of tyrosine phosphorylation of pp72(syk) was associated with an increase of pp72(syk) kinase activity. In contrast to pp72(syk), tyrosine phosphorylation of the focal adhesion kinase pp125(FAK) was not stimulated by EGTA. Preincubation of platelets with the monoclonal antibody P2, which binds to the alphaIIbbeta3 complex and thus stabilizes it, strongly reduced the increase of tyrosine phosphorylation of pp72(syk), p62, and p68 induced by EGTA. The Y2/51 monoclonal antibody, which recognizes only the beta3 glycoprotein, did not inhibit the stimulation of protein tyrosine phosphorylation evoked by EGTA. Stimulation of tyrosine phosphorylation of pp72(syk), p62, p68, and p130 induced by EGTA was not observed in thrombasthenic platelets, which lack the alphaIIbbeta3-integrin. The results indicate that the dissociation of the alphaIIbbeta3 complex in intact platelets activates pp72(syk). The mechanism of activation was found to be insensitive to inhibition by cAMP and cGMP and only partially dependent on cytosolic Ca2+, suggesting a close functional coupling of alphaIIbbeta3-integrin and pp72(syk). Since platelets retain their discoid shape after EGTA treatment, we further conclude that pp72(syk) stimulation alone is not sufficient for platelet activation.

Roles of myosin light-chain kinase in platelet shape change and aggregation

We examined the roles of myosin light-chain kinase in platelet responses to ADP using wortmannin, which almost completely inhibited myosin light-chain kinase at 3-6 microM. This concentration of wortmannin did not affect ADP-induced changes in the shape of the platelets, but it markedly inhibited aggregation in platelet-rich plasma and washed platelets. ML-9, another inhibitor of myosin light chain kinase, elicited similar effects on the platelet responses to wortmannin. Electron microscopic studies showed that there was no wortmannin effect on the ADP-induced spheration of discoid platelets, pseudopod formation, or granule centralization. Wortmannin at concentrations which prevented myosin light-chain kinase also inhibited platelet aggregation induced by ADP in the presence of U46619, an analogue of thromboxane A2, which is a prerequisite for ADP-induced irreversible aggregation. Although wortmannin partially inhibited protein kinase C, the protein kinase C inhibitor Ro-31-7549 (5 microM) prevented neither ADP- or ADP/U46619-induced changes in the shape of the platelets nor aggregation. These results suggest that myosin light-chain kinase activation is a prerequisite for ADP-induced platelet aggregation, but not for changes in their shape.

Platelet antibodies in serum of patients with human immunodeficiency virus (HIV) infection

Between 10 and 15% of human immunodeficiency virus (HIV) seropositive individuals develop an immune thrombocytopenic purpura; however, the mechanism involved in platelet destruction is not yet established. In the present work, we have analyzed 208 sera from HIV seropositive individuals, including 85 thrombocytopenic patients, for the presence of autoantibodies against platelet proteins by using the Western blot technique. Our results indicate that: (1) antibodies against platelet proteins were found in 8 of 123 (6.5%) nonthrombocytopenic patients, as compared with 17 of 85 (20%) of thrombocytopenic patients (p less than 0.03); (2) these antibodies appeared to be more frequently found in advanced stages of disease (p less than 0.02); (3) the reactivity of positive sera with antigenic determinants implicated several distinct platelet proteins; (4) antigens thus recognized are unrelated to the major membrane glycoproteins IIb and IIIa, as well as absent in vero cells and trypsin-sensitive cells. Such results underscore the difficulties in establishing the mechanisms involved in platelet destruction during HIV infection.

Effects of EDTA on the membrane glycoproteins IIb-IIIa complex--analysis using flow cytometry

We developed new monoclonal antibodies (NNKY1-32, NNKY2-6, NNKY2-18) that react with a determinant specific to the GPIIb-IIIa complex, and studied the distribution of GPIIb/IIIa on the platelet surface in vivo and some factors that influence the structure of GPIIb/IIIa using flow cytometry. 1. The surface of large platelets is richer in GPIIb/IIIa than that of small platelets. 2. Incubation of intact platelets with EDTA at 37 degrees C causes progressive dissociation of GPIIb-IIIa complexes, and this influence is marked in small platelets. 3. Addition of Ca2+ or Mg2+ to dissociated GPIIb-IIIa complexes causes reassociation, and additionary ADP after the cation treatment further increases the binding of monoclonal antibodies. 4. Prolonged incubation of intact platelets with EDTA at 37 degrees C induces a peculiar change of structure in the GPIIb/IIIa.

Identification of two distinct regions within the binding sites for fibrinogen and fibronectin on the IIb-IIIa human platelet membrane glycoprotein complex by monoclonal antibodies P2 and P4

The effect of two monoclonal antibodies P2 (LyP 2) or P4 (LyP 4), specific for the platelet membrane glycoprotein IIb/IIIa complex, on binding of 125I-labelled fibrinogen or 125I-labelled fibronectin to thrombin-stimulated platelets was studied. These monoclonal antibodies are directed against different determinants on the IIb-IIIa complex and react only with the complex and not with the individual glycoproteins. Fibrinogen binding to thrombin-stimulated platelets was significantly inhibited by P2 but not by P4. Fibronectin binding to thrombin-stimulated platelets was significantly inhibited by P4 but only poorly by P2. These results indicate the presence of specific regions on the glycoprotein IIb-IIIa complex which act as binding sites for fibrinogen or fibronectin. Other authors [Haverstick et al. (1985) Blood 66, 946-952; Ginsberg et al. (1985) J. Biol. Chem. 260, 4133-4138] have shown that a tetrapeptide, Arg-Gly-Asp-Ser, inhibited the binding of fibrinogen, fibronectin, and von Willebrand factor (vWf) to stimulated platelets and that fibrinogen competes with vWf and fibronectin for binding. These findings, together with previous studies, therefore indicate the presence of specific regions as well as a common region in the binding sites for fibrinogen and fibronectin on the IIb-IIIa complex.

Binding of 125I-labelled fibrin(ogen) fragments to platelets and to immunoprecipitated glycoprotein IIb-IIIa complex

To further investigate which parts of the fibrinogen molecule that are responsible for its binding to the fibrinogen receptor on human platelets, the following approaches were made: The glycoprotein IIb-IIIa complex (the putative fibrinogen receptor) was immunoprecipitated in crossed immunoelectrophoresis of Triton X-100-extracts of platelets against antibodies to whole platelet proteins. Subsequently, the immunoplates were incubated with 125I-labelled, plasmin- or CNBr-cleaved fibrinogen fragments (pre-X,X,Y,D,Degta,Efg,N-DSK) or fibrin fragments (E1,N-dsk), characterized by partial sequenation. The immunoplates were exposed to X-ray films, and binding of the fragments to the glycoprotein IIb-IIIa complex was examined. The findings were compared to the results obtained from studies on binding of the same fragments to intact gel-filtered platelets after ADP-stimulation. The following conclusions were made: All fragments except Efg and Degta bound to the immunoprecipitated GPIIb-IIIa complex as well as to ADP-stimulated platelets suggesting that at least two sequences in the E domain and one in each of the D domains of fibrinogen are involved in binding to the platelet receptor. The GPIIb-IIIa complex is the only surface-located platelet antigen that binds fibrinogen and the aforementioned fragments. The binding of the fragments to the receptor is dependent on divalent cations.

Independent modulation of von Willebrand factor and fibrinogen binding to the platelet membrane glycoprotein IIb/IIIa complex as demonstrated by monoclonal antibody

In this study we have used two new monoclonal antibodies, designated LJP5 and LJP9, as well as a previously described one, AP2, all specific for the platelet membrane glycoprotein (GP)IIb/IIIa complex. None of them reacted with dissociated GPIIb or GPIIIa. The monovalent Fab fragment of both LJP5 and LJP9 bound to unstimulated platelets in a saturable manner, but binding was markedly decreased after platelets had been incubated at 37 degrees C in the absence of added extracellular calcium. The binding of LJP9 was not affected by AP2, but was blocked by excess LJP5. On the contrary, the binding of LJP5 was blocked in the presence of both AP2 and LJP9. Thus, these antibodies bound to distinct epitopes of GPIIb/IIIa. At saturation, the binding to unstimulated platelets was between 2.41 and 10.9 X 10(4) molecules/platelet for LJP5 and between 3.47 and 9.1 X 10(4) molecules/platelet for LJP9 (range of 11 and 10 experiments, respectively). Binding increased up to 50% after thrombin stimulation. The estimated association constant, Ka, was 2.7 X 10(7) M-1 for LJP5 and 3.85 X 10(7) M-1 for LJP9. Both LJP5 and LJP9 partially inhibited the association of 45Ca2+ with the surface of unstimulated platelets. Moreover, both antibodies blocked the binding of von Willebrand factor (vWF) to stimulated platelets, whereas only LJP9, but not LJP5, blocked fibrinogen binding. LJP9 was also a potent inhibitor of platelet aggregation, whereas LJP5 was without effect in this regard. The results of the present study demonstrate that independent modulation of vWF and fibrinogen binding to stimulated platelets can be attained with monoclonal antibodies directed against distinct epitopes of GPIIb/IIIa.

Platelet--von Willebrand factor interactions in type IIB von Willebrand's disease

Type IIB von Willebrand's disease (vWD) is a distinct form of this disorder in which the largest multimers of the von Willebrand factor (vWF) are lacking in plasma but present in platelets. When the vasopressin analogue, 1-deamino-8-D-arginine vasopressin (DDAVP), is given to patients with type IIB vWD, an abnormal vWF is released to plasma. This vWF causes thrombocytopenia in vivo and platelet aggregation in vitro. Aggregation occurs in the plasma milieu and thus at physiological fibrinogen concentration. In this study we demonstrate that IIB post-DDAVP vWF aggregated only metabolically active platelets. The platelet aggregation was completely inhibited by EDTA and PGE1, and either inhibited or greatly weakened by ASA, demonstrating the role of divalent cations and thromboxane A2 formation. In spite of inhibiting platelet aggregation, EDTA, PGE1 and ASA did not prevent platelet binding of IIB post-DDAVP vWF. An antiserum against GP Ib made normal platelets less responsive to the IIB vWF although neither platelet aggregation nor vWF binding were completely prevented. The aggregation was fibrinogen-dependent and platelets from patients with Glanzmann's thrombasthenia were unresponsive. The studies provide evidence that IIB post-DDAVP vWF is bound to unstimulated platelets and that the interaction between vWF and platelets in type IIB vWD is different from ristocetin-induced as well as thrombin- and epinephrine-induced binding to platelets of normal vWF.

Tandem separation of labelled human blood platelet membrane glycoproteins by anion-exchange and gel fast protein liquid chromatography

Studies were made of the separation of surface-labelled platelet glycoproteins, solubilized in a non-ionic detergent (Berol 185), on a Mono Q anion-exchange column, coupled to a Superose 12 gel fast protein liquid chromatographic column. Peaks eluted from the anion-exchange and gel columns were subjected to electrophoresis on sodium dodecyl sulphate-polyacrylamide gels in the presence (non-reducing conditions) or absence (reducing conditions) of disulphide bridges. Labelled electrophoresed glycoprotein bands were rendered visible by fluorography or indirect autoradiography. Platelet membrane glycoproteins Ib, IIb, IIIa, IIIb, V and IX were identified by their apparent molecular weights and their surface labelling characteristics. It was concluded that tandem chromatography can be used to separate platelet membrane proteins and glycoproteins rapidly and with good resolution.

Identification and characterization of fragments of major glycoproteins from platelet membrane after chymotrypsin treatment

Human platelets were surface-labeled by the periodate/NaB3H4 method or by lactoperoxidase-catalysed iodination with 125I. The labeled platelets were treated with chymotrypsin under conditions known to give platelets which aggregate with fibrinogen without stimulation with ADP. Platelets and supernatant were then analysed by various gel electrophoretic techniques including isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing or non-reducing conditions and two-dimensional non-reduced/reduced sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by fluorography or indirect autoradiography. Chymotrypsin-treatment of surface-labeled platelets degraded the major glycoproteins Ib, IIb and IIIa but also GP120(4.9-5.4), GPIc and GPV. The membrane-bound fragments of GPIb, IIb and IIIa could be identified and also the supernatant fragments of GPIb and GPV. GPIIIa was also cleaved within a loop structure formed by disulfide bond(s). The fact that remnants of both GPIIb and IIIa are left on chymotrypsin-treated platelets which aggregate spontaneously with fibrinogen may indicate that a complex formed by these remnants constitutes the fibrinogen-binding site on platelets.

Immunochemical characterization of platelet-specific alloantigens

Immunoprecipitation was performed with platelet-specific alloantisera (anti-Zwa, -Zwb, -Baka and antiserum Luc) and 125I-labelled platelets of a panel of donors typed for these platelet-specific alloantigens. This was done by sensitization of intact, radiolabelled platelets with the antisera, solubilization of the sensitized platelets with Nonidet P-40 and recovery of the immune complexes from the lysate with heat-killed, formalin-fixed Staphylococcus aureus, strain Cowan I. In the case of platelet antibodies of the IgM class, bacteria also preincubated with rabbit-IgG anti-human IgM before treatment of the lysate were applied. Radiolabelled antigens were then eluted from the bacteria by boiling in an SDS-mercaptoethanol-containing buffer. Subsequently, the eluted antigens were analyzed by SDS-polyacrylamide gel electrophoresis. It revealed that both the Zwa and the Baka antigens are located on the glycoprotein-IIb/IIIa complex. The location of the Zwb antigen could not be established in this way. From the intensity of the precipitated glycoprotein bands, it appeared that Zwa is probably a marker of glycoprotein IIIa, Baka or glycoprotein IIb. However, immunoprecipitates of anti-Zwa and anti-Baka sera both carried, to some extent, complexes of glycoproteins IIb and IIIa. Iodinated platelets of a Glanzmann thrombasthenia patient failed to show any material precipitated by these antisera or the antiserum Luc, known to contain antibodies against both glycoproteins.

Glycoprotein IIb-IIIa complex in platelets of patients and heterozygotes of Glanzmann's thrombasthenia

The concentration of the glycoprotein (GP) IIb-IIIa complex in thrombasthenic platelets of 8 patients of 6 families has been estimated. In the thrombasthenic platelets of 3 patients this complex is absent (thrombasthenia type I and subtype I). In 2 patients only traces are detectable and in 3 patients GP IIb-IIIa complex is strongly reduced (less than 5%). On the basis of the haemostatic data as well as the content of GP IIb-IIIa complex and platelet fibrinogen the classification of these types as subtypes of thrombasthenia type II is discussed. The diagnostic applicability of GP IIb-IIIa complex determination for heterozygote detection in types of thrombasthenia with absent or extremely reduced GP IIb-IIIa complex is shown.

A monoclonal anti-platelet antibody with decreased reactivity for autoimmune thrombocytopenic platelets

Two monoclonal anti-platelet antibodies, 3B2 and 8G11, have been raised that are specific for normal human platelets. 3B2 is unique in that it has decreased reactivity for platelets from 16 patients with autoimmune thrombocytopenic purpura [mean platelet count, 65,000 +/- 6,000 (SEM)]. With 8G11 in an enzyme-linked immunosorbent assay, the mean of the ratios of patient platelet OD to control platelet OD was 0.95 +/- 0.07, whereas with 3B2, the mean of the ratios of patient platelet OD to control OD was 0.24 +/- 0.04, P less than 0.001. With 3B2 the mean of the OD ratios of five patients with autoimmune thrombocytopenic purpura in remission (greater than 150,000 platelets per mm3) compared to controls was 0.80 +/- 0.14. 3B2 did not react with platelets from a patient with Glanzmann's thrombasthenia, in which membranes lack glycoproteins IIb and IIIa (GPIIb and GPIIIa). Platelet membranes were run on crossed immunoelectrophoresis against a rabbit polyclonal anti-human platelet membrane antibody with 125I-labeled purified 3B2 in an intermediate spacer gel. 3B2 reacted with the GPIIb-GPIIIa-Ca2+ complex in the presence of excess Ca2+ and with GPIIb alone in the presence of excess EGTA. When Triton X-100-solubilized platelet membranes were immunoprecipitated with 3B2 plus rabbit anti-mouse IgG, reduced, and run on NaDodSO4/polyacrylamide gel electrophoresis, a single protein band was obtained with a molecular weight of 120,000 (the molecular weight of GPIIb). Thus, the reactivity of monoclonal antibody 3B2 with GPIIb or the GPIIb-GPIIIa-Ca2+ complex appears to be inhibited by the presence of autoantibody on platelets.

Monoclonal antibodies against human platelet glycoprotein IIIa

Two murine monoclonal antibodies specific for human platelets were prepared and characterized by immunofluorescence, immunoprecipitation and by studying their effect on platelet function. Immunoprecipitation with lysates of normal platelets and platelets from a patient with Glanzmann's thrombasthenia revealed that the monoclonal antibodies were directed against glycoprotein GP IIIa. One of these anti-GP-IIIa antibodies (C17) inhibited both ADP- and collagen-induced platelet aggregation as well as ADP-induced fibrinogen binding to platelets. The other anti-GP-IIIa antibody (C15) also caused a complete inhibition of aggregation with collagen. However, a small, and fully reversible, 'primary wave' was observed if the platelets were stimulated with ADP when platelet-rich plasma was used. The ability to bind fibrinogen was unimpaired for platelets incubated with C15. These findings show that C15 and C17 probably recognize different determinants on GP IIIa. Neither of the monoclonal anti-GP-IIIa antibodies blocked the binding to Zwa-positive platelets of human polyclonal anti-Zwa antibodies. This implies that Zwa is different from the epitopes recognized by C15 and C17.

Monocytes and platelets share the glycoproteins IIb and IIIa that are absent from both cells in Glanzmann's thrombasthenia type I

By means of an antiserum specific to the complex of the platelet membrane glycoproteins IIb and IIIa we demonstrate here that monocytes and purified monocyte membranes share these glycoproteins with platelets. The monocyte glycoprotein IIb-IIIa complex showed complete immunological identity with the platelet counterpart and, furthermore, dissociated after EDTA treatment exactly as did the platelet complex. In Glanzmann's thrombasthenia type I, monocytes as well as platelets lack this antigen completely.

Comparison of protein and lipid composition of the human platelet alpha-granule membranes and glycerol lysis membranes

Platelet glycerol lysis membranes and alpha-granule membranes were compared with respect to protein and lipid composition. Crossed immunoelectrophoresis using antibodies against whole platelets, and sodium dodecyl sulphate polyacrylamide gel electrophoresis, revealed the presence of the glycoproteins IIb and IIIa, myosin and an antigen termed G4 in both membrane fractions. The glycoproteins Ia, Ib and IIIb, in addition to beta 2-microglobulin and actin, appeared specific for the glycerol lysis membranes, whereas two antigens, termed G8 and G18, were observed only in the alpha-granule membranes. The localization of glycoprotein IIa was inconclusive. Comparison with the surface-located proteins revealed that the glycerol lysis membranes represented a reasonable approximation to a plasma membrane preparation. Radioactively labelled immunoprecipitates obtained after crossed immunoelectrophoresis of 125I-labelled platelets were cut out and applied to sodium dodecyl sulphate electrophoresis on polyacrylamide slab gels. Autoradiography of the dried gels revealed that antigen G4 represented a protein with an average molecular weight of 146 000 in its unreduced state and 132 000 in its reduced state. Antigen G18 represented a protein of molecular weight 130 000-135 000 in the reduced as well as unreduced state. Quantitation of protein and lipids showed that the alpha-granule membranes contained about one-third as much cholesterol and 2-times as much protein in relation to phospholipids as compared to the glycerol lysis membranes. No significant difference between the two membrane preparations was found as regards the composition of their phospholipids.

A murine monoclonal antibody that completely blocks the binding of fibrinogen to platelets produces a thrombasthenic-like state in normal platelets and binds to glycoproteins IIb and/or IIIa

To define better the role of the fibrinogen receptor in platelet physiology and to characterize it biochemically, a murine monoclonal antibody that completely blocks the binding of fibrinogen to the platelet surface was produced by the hybridoma technique with the aid of a functional screening assay. Purified F(ab')2 fragments and/or intact antibody completely blocked aggregation induced by ADP, thrombin, or epinephrine and the binding of radiolabeled fibrinogen to platelets induced by ADP. The antibody did not block agglutination of formaldehyde-fixed platelets by ristocetin or shape change induced by either ADP or thrombin. ADP- and epinephrine-induced release of ATP was completely inhibited by the antibody, but inhibition of release induced by collagen and thrombin was dose dependent and partial. The antibody also dramatically inhibited platelet retention in glass-bead columns, platelet adhesion to glass, and clot retraction. Thus, the antibody induced a thrombasthenic-like state. Immunofluorescent studies confirmed the specificity of the antibody for normal platelets and megakaryocytes and suggested that there is a marked decrease in detectable antigen in thrombasthenic platelets. Radiolabeled antibody bound to an average of approximately 40,000 sites on normal platelets but it bound to less than 2,000 sites on the platelets of a patient with thrombasthenia. The antibody immunoprecipitated both glycoproteins IIb and IIIa, and both glycoproteins bound to an affinity column of the antibody. These studies indicate that there is probably a single anatomic site that is crucial to the binding of all fibrinogen molecules and that this site is most likely on the glycoprotein IIb/IIIa complex. It also suggests that the thrombasthenic phenotype can be completely accounted for on the basis of the inhibition of fibrinogen binding to platelets.

Calcium-binding proteins from human platelets. A study using crossed immunoelectrophoresis and 45Ca2+

Calcium-binding platelet proteins were examined by crossed immunoelectrophoresis of solubilized platelets against antibodies to whole platelets followed by incubation of the immunoplates with 45Ca2+ and autoradiography. When the immunoplates had been pretreated with EDTA at pH 9.0 in order to remove divalent cations, three immunoprecipitates were markedly labelled with 45Ca2+. These corresponded to the glycoprotein IIb-IIIa complex, glycoprotein Ia and a presently unidentified antigen termed G18. These antigens were membrane-bound and surface-oriented. When an excess of EDTA was introduced in the incubation media the results revealed that the glycoprotein IIb-IIIa complex and antigen G18, but not glycoprotein Ia, contained sites with a stronger affinity for calcium than has EDTA at pH 7.4. Immunoprecipitates of the separate glycoproteins IIb and IIIa both bound calcium in the same manner as the glycoprotein IIb-IIIa complex. As another approach, platelet-rich plasma was incubated with 45Ca2+ prior to crossed immunoelectrophoresis of the solubilized platelets. A single immunoprecipitate was weakly labelled. This did not correspond to any of the immunoprecipitates which were visible after staining with Coomassie blue. The labelling of this antigen was markedly increased when the platelet-rich plasma had been preincubated with EDTA and in this case a weak labelling of the glycoprotein IIb-IIIa precipitate also became apparent. No increased incorporation of calcium occurred in any of these immunoprecipitates when the platelets were aggregated with ADP in the presence of 45Ca2+.

Membrane proteins on human megakaryocytes and platelets identified by monoclonal antibodies

We describe five monoclonal antibodies that react with four discrete antigens present on human platelets. Antibodies B2.12 and B59.2 precipitate the glycoprotein IIb-IIIa complex from radiolabeled platelet membrane extracts and inhibit platelet aggregation induced by adenosine diphosphate (ADP), collagen, or epinephrine. The antigen recognized by the two antibodies is present on megakaryocytes but either absent entirely or expressed in small amounts on platelets from Glanzmann's thrombasthenic patients. The antigen recognized by antibody B37.3 is absent from thrombasthenic platelets. Antibody B1.12 reacts with an antigen shared by platelets and 20% of peripheral blood lymphocytes and is a potent inducer of platelet aggregation. Antibody B2.10 reacts specifically with platelets and megakaryocytes but does not affect platelet functions. Thus, these reagents are useful tools in diagnostic and functional studies of both normal and abnormal platelets.

Reevaluation of the presence of the major antigen Ca++ complex in Bernard-Soulier syndrome platelets. Elastase degradation of the complex in Bernard-Soulier syndrome platelet preparations

Crossed immunoelectrophoresis of human platelet membranes reveals a major antigen GPIIb-GPIIIa-Ca++ complex which is absent to decreased in Glanzmann's thrombasthenia. In our initial report, we also noted its absence in two patients with Bernard-Soulier syndrome associated with the presence of two neoantigens, and postulated that this may be due to endogenous proteolysis. It now appears that this observation is due to exogenous proteolysis from granulocyte-elastase contamination in the platelet membrane preparation. Purified elastase is capable of degrading the major antigen-Ca++ complex, particularly in its dissociated form, to the pattern initially reported for Bernard-Soulier platelets. Of particular interest is the observation that the elastase-degraded dissociated complex can be reassociated in the presence of Ca++. This indicates that the elastase-sensitive antigenic determinants recognized by the rabbit anti-human platelet membrane antibody are not required for the bonds which reassociate the complex.

Monoclonal antibodies against platelet membrane glycoproteins. Characterization and effect on platelet function

The specificity of five monoclonal antibodies (P1-P6) against platelet surface components was determined by immunoprecipitation of surface-labelled platelets from normal donors and patients with known platelet glycoprotein defects, followed by analysis by gel electrophoresis. Three (P2, P4 and P6) precipitated glycoproteins IIb and IIIa and, in addition, P2 precipitated glycoprotein Ia. P1 precipitated normally only glycoprotein Ib also Ia when the platelets were pretreated with neuraminidase. P3 precipitated principally glycoprotein Ia but glycoprotein Ib was also weakly precipitated. The effects of the monoclonals on platelet function were tested. P1 and P2 completely inhibited and P3 slightly inhibited thrombin-induced platelet aggregation. P2 also inhibited collagen-induced aggregation and partially inhibited ADP-induced platelet aggregation. P3, P4 and P6 partially inhibited ADP-induced platelet aggregation. None had any effect on ristocetin-induced aggregation despite P1 and P3 binding to glycoprotein Ib. These results confirm the role of glycoproteins IIb and IIIa in aggregation induced by various agents and suggest that the function of glycoprotein Ib in thrombin-induced aggregation is more important than previously suspected and that glycoprotein Ia may also be involved in platelet functions.

Dissociation of the glycoprotein IIb-IIIa complex in isolated human platelet membranes. Dependence of pH divalent cations

The platelet membrane glycoproteins IIb and IIIa normally exist as a complex which forms a predominant immunoprecipitate after crossed immunoelectrophoresis of Triton-X-100-solubilized platelets. Dissociation of the complex occurs by solubilization in the presence of EDTA or EGTA at pH 8.7 and is readily verified by crossed immunoelectrophoresis. Incubations of isolated membranes with EDTA or EGTA at various pH levels were performed. Removal of the chelators and solubilization showed no dissociation of the glycoprotein IIb-IIIa complex in membranes incubated at pH below 8.0. At pH above 8.0 a dissociation which increased with increasing pH was seen. Under these conditions, dissociation appears to take place already in the intact membranes. The tendency of the glycoprotein IIb-IIIa complex to become dissociated with EDTA or EGTA at increasing pH seems to be due to increased chelating capacity of the chelators concomitant with a decreased chelating capacity of glycoprotein IIb and IIIa. The divalent cations Ca2+ and Mg2+, but not Cu2+, Zn2+, Mn2+ or Sr2+, in molar concentrations below that of EGTA were able to prevent the dissociation of the glycoprotein IIb-IIIa complex by the chelator at pH 9.0, indicating that Ca2+ as well as Mg2+ can be used to keep the complex together. In some experiments it was possible to reverse the dissociation in the membranes after removal of EDTA. At pH 7.5 reassociation occurred within 15 min whether divalent cations were added or not. At pH 9.0. reassociation occurred within 2 h provided Ca2+ was present. The tendency of glycoprotein IIb and IIIa to form a complex thus appeared to be most pronounced over the physiological pH range and to be a rapid process in platelet membranes under such conditions.