scholarly journals Monoclonal antibodies against porcine platelet membrane glycoproteins Ib and IIb/IIIa

Blood ◽  
1988 ◽  
Vol 72 (5) ◽  
pp. 1740-1747 ◽  
Author(s):  
H Takami ◽  
WL Nichols ◽  
SE Kaese ◽  
RS Miller ◽  
JA Katzmann ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Polyacrylamide Gel Electrophoresis ◽  
Sodium Dodecyl ◽  
Human Platelets ◽  
Platelet Membrane ◽  
In Vivo Studies ◽  
Membrane Glycoproteins ◽  
Fab Fragments

Abstract We prepared murine monoclonal antibodies to porcine platelet membrane glycoprotein (GP) Ib and GP IIb/IIIa for further study of the porcine hemostatic mechanism. One monoclonal antibody, designated PP3–4C, blocked Ristocetin-induced platelet agglutination and caused 80% inhibition of Ristocetin-induced 125I-von Willebrand factor (vWF) binding to porcine platelets at a concentration of greater than or equal to 12 micrograms IgG/mL. PP3–4C did not affect adenosine diphosphate (ADP)- or collagen-induced platelet aggregation. Binding of 125I-Fab fragments of PP3–4C to platelets was saturable at 3.7 x 10(4) +/- 0.8 x 10(4) molecules per platelet. Another monoclonal antibody, designated PP3–3A, blocked ADP- or collagen-induced platelet aggregation at 6 micrograms IgG/mL. At a concentration of 10 micrograms IgG/mL, PP3–3A completely inhibited binding either of 125I-fibrinogen or of 125I-vWF to ADP-stimulated platelets. PP3–3A did not affect Ristocetin-induced platelet agglutination nor 125I-vWF binding to platelets in the presence of Ristocetin. Binding of 125I-Fab' fragments of PP3–3A to platelets was saturable at 9.8 x 10(4) +/- 1.2 x 10(4) molecules per platelet. PP3–4C antibody (anti-GP Ib) did not bind to human platelets; however, PP3–3A antibody (anti-GP IIb-IIIa) had partial cross-reactivity with human platelets. Immunoaffinity chromatography of solubilized surface-radiolabeled porcine platelets and subsequent sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis demonstrated that PP3–4C recognized a GP with an apparent molecular weight of 160,000 (nonreduced), and 140,000 (reduced). PP3–3A recognized GPs with apparent molecular weights of 130,000 and 80,000 (nonreduced), and 115,000 and 95,000 (reduced). These monoclonal antibodies to porcine platelet membrane GPs, which are structural and functional analogues of human GP Ib and GP IIb/IIIa, will be useful for in vitro and in vivo studies of the mammalian hemostatic mechanism.

scholarly journals Monoclonal antibodies against porcine platelet membrane glycoproteins Ib and IIb/IIIa

Blood ◽  
1988 ◽  
Vol 72 (5) ◽  
pp. 1740-1747
Author(s):  
H Takami ◽  
WL Nichols ◽  
SE Kaese ◽  
RS Miller ◽  
JA Katzmann ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Polyacrylamide Gel Electrophoresis ◽  
Sodium Dodecyl ◽  
Human Platelets ◽  
Platelet Membrane ◽  
In Vivo Studies ◽  
Membrane Glycoproteins ◽  
Fab Fragments

We prepared murine monoclonal antibodies to porcine platelet membrane glycoprotein (GP) Ib and GP IIb/IIIa for further study of the porcine hemostatic mechanism. One monoclonal antibody, designated PP3–4C, blocked Ristocetin-induced platelet agglutination and caused 80% inhibition of Ristocetin-induced 125I-von Willebrand factor (vWF) binding to porcine platelets at a concentration of greater than or equal to 12 micrograms IgG/mL. PP3–4C did not affect adenosine diphosphate (ADP)- or collagen-induced platelet aggregation. Binding of 125I-Fab fragments of PP3–4C to platelets was saturable at 3.7 x 10(4) +/- 0.8 x 10(4) molecules per platelet. Another monoclonal antibody, designated PP3–3A, blocked ADP- or collagen-induced platelet aggregation at 6 micrograms IgG/mL. At a concentration of 10 micrograms IgG/mL, PP3–3A completely inhibited binding either of 125I-fibrinogen or of 125I-vWF to ADP-stimulated platelets. PP3–3A did not affect Ristocetin-induced platelet agglutination nor 125I-vWF binding to platelets in the presence of Ristocetin. Binding of 125I-Fab' fragments of PP3–3A to platelets was saturable at 9.8 x 10(4) +/- 1.2 x 10(4) molecules per platelet. PP3–4C antibody (anti-GP Ib) did not bind to human platelets; however, PP3–3A antibody (anti-GP IIb-IIIa) had partial cross-reactivity with human platelets. Immunoaffinity chromatography of solubilized surface-radiolabeled porcine platelets and subsequent sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis demonstrated that PP3–4C recognized a GP with an apparent molecular weight of 160,000 (nonreduced), and 140,000 (reduced). PP3–3A recognized GPs with apparent molecular weights of 130,000 and 80,000 (nonreduced), and 115,000 and 95,000 (reduced). These monoclonal antibodies to porcine platelet membrane GPs, which are structural and functional analogues of human GP Ib and GP IIb/IIIa, will be useful for in vitro and in vivo studies of the mammalian hemostatic mechanism.

FAB FRAGMENTS OF MONOCLONAL ANTIBODIES SPECIFIC FOR PORCINE PLATELET MEMBRANE GLYCOPROTEINS GP IB ANDGP IIB/IIIA

1987 ◽  
Author(s):  
H Takami ◽  
W L Nichols ◽  
S E Kaese ◽  
R S Miller ◽  
J A Katzmann ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Platelet Membrane ◽  
In Vivo Studies ◽  
Membrane Glycoproteins ◽  
Fab Fragments ◽  
Igg1 Subclass ◽  
Von Willebrand

For further study of the porcine hemostatic mechanism, we have prepared murine monoclonal antibodies, and F(ab')2 and Fab fragments, specific for porcine platelet membrane glycoproteins GP lb and GP Ilb/IIIa. To avoid production of antibodies to von Willebrand factor (vWF), mice were immunized with platelets obtained from pigs with severe von Willebrand,s disease. One monoclonal antibody (PP3-4C), of IgG1 subclass, caused 85% inhibition of Ristocetin-induced platelet binding of 125I-vWF (porcine) at ≥12 µg IgG/ml. PP3-4C did not affect ADP or collagen-induced platelet aggregation nor inhibit 125I-fibrinogen (porcine) binding. Pepsin and papain digestion, respectively, were used to prepare PP3-4C F(ab')2 and Fab fragments. PP3-4C F(ab')2 at concentrations ≥12 µg/ml caused 80% inhibition of washed platelet agglutination in the presence of vWF and Ristocetin, whereas Fab fragments at concentrations ≥10 µg/ml caused 60% inhibition. Another monoclonal antibody (PP3-3A), of IgG1 subclass, completely inhibited ADP or collagen-induced platelet aggregation at an IgG concentration of 6 µg/ml. At 10 µg IgG/ml PP3-3A completely inhibited binding either of 125I-fibrinogen or of 125I-vWF to ADP-stimulated porcine platelets. PP3-3A did not affect vWF-dependent Ristocetin-induced platelet agglutination, nor 125I-vWF binding to platelets in the presence of Ristocetin. PP3-3A did not bind to platelets which were treated with 10 mM EDTA at 37°C for 60 min. F(ab')2 and Fab fragments were isolated from PP3-3A pepsin or papain digests. Both types of PP3-3A fragments caused 100% inhibition of ADP-induced platelet aggregation, at concentrations ≥6 yg/ml. Immunoprecipitation of surface-radiolabeled porcine platelets and subsequent SDS-PAGE demonstrated that PP3-4C recognized a glycoprotein with molecular weight of 140,000 (under reducing conditions), and 165,000 (non-reduced). PP3-3A recognized glycoproteins with molecular weights of 115,000 and 100,000 (reduced), and 130,000 and 80,000 (non-reduced). Neither monoclonal antibody bound to human platelets. These monoclonal antibodies to porcine platelet membrane glycoproteins which are analogues of human GP lb and GP Ilb/IIIa will be useful for in vitro and in vivo studies to further understanding of mammalian hemostatic mechanisms.

scholarly journals A defect of platelet aggregation associated with an abnormal distribution of glycoprotein IIb-IIIa complexes within the platelet: the cause of a lifelong bleeding disorder

Blood ◽  
1992 ◽  
Vol 80 (3) ◽  
pp. 696-708 ◽  
Author(s):  
R Hardisty ◽  
D Pidard ◽  
A Cox ◽  
T Nokes ◽  
C Legrand ◽  
...  
Keyword(s):  
Flow Cytometry ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Platelet Rich Plasma ◽  
Polyacrylamide Gel Electrophoresis ◽  
Sodium Dodecyl ◽  
Adenosine Diphosphate ◽  
Membrane Glycoproteins ◽  
Clot Retraction ◽  
Platelet Size

Abstract A young Italian man (A.P.) has a lifelong history of bleeding from gums and mucocutaneous tissue. Electron microscopy showed a wide diversity of platelet size including giant forms. In citrated platelet-rich plasma (PRP), platelet aggregation induced by adenosine diphosphate (ADP) and other agonists was much reduced. Both secretion and clot retraction were normal. The aggregation of washed platelets with ADP was improved but remained subnormal, as was aggregation with collagen and thrombin. Fibrinogen-binding was analyzed by flow cytometry using platelets in whole blood or PRP and was markedly decreased. Crossed immunoelectrophoresis of Triton X-100 extracts of (A.P.) platelets showed that GP IIb-IIIa levels were 40% to 50% of normal. Glycoprotein (GP) IIb and GP IIIa were of usual migration in sodium dodecyl sulfate- polyacrylamide gel electrophoresis, but their labeling was much reduced during lactoperoxidase-catalyzed iodination. Binding to (A.P.) platelets of four different 125I-labeled monoclonal antibodies to GP IIb-IIIa complexes was reduced to 12% to 20% of normal levels. However, when the patient's platelets were stimulated with alpha-thrombin, monoclonal antibody binding showed the same increase (approximately 20,000 sites) as normal platelets. Both flow cytometry and immunocytochemical studies showed that the distribution of residual surface GP IIb-IIIa within the total (A.P.) platelet population was heterogeneous and not related to platelet size. Staining of ultrathin sections confirmed the presence of an internal pool of GP IIb-IIIa. Monoclonal antibodies to other membrane glycoproteins bound normally to (A.P.) platelets. The patient has a selective deficiency of the surface pool of GP IIb-IIIa complexes that is manifested clinically by a mild Glanzmann's thrombasthenia-like syndrome.

scholarly journals A defect of platelet aggregation associated with an abnormal distribution of glycoprotein IIb-IIIa complexes within the platelet: the cause of a lifelong bleeding disorder

Blood ◽  
1992 ◽  
Vol 80 (3) ◽  
pp. 696-708
Author(s):  
R Hardisty ◽  
D Pidard ◽  
A Cox ◽  
T Nokes ◽  
C Legrand ◽  
...  
Keyword(s):  
Flow Cytometry ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Platelet Rich Plasma ◽  
Polyacrylamide Gel Electrophoresis ◽  
Sodium Dodecyl ◽  
Adenosine Diphosphate ◽  
Membrane Glycoproteins ◽  
Clot Retraction ◽  
Platelet Size

A young Italian man (A.P.) has a lifelong history of bleeding from gums and mucocutaneous tissue. Electron microscopy showed a wide diversity of platelet size including giant forms. In citrated platelet-rich plasma (PRP), platelet aggregation induced by adenosine diphosphate (ADP) and other agonists was much reduced. Both secretion and clot retraction were normal. The aggregation of washed platelets with ADP was improved but remained subnormal, as was aggregation with collagen and thrombin. Fibrinogen-binding was analyzed by flow cytometry using platelets in whole blood or PRP and was markedly decreased. Crossed immunoelectrophoresis of Triton X-100 extracts of (A.P.) platelets showed that GP IIb-IIIa levels were 40% to 50% of normal. Glycoprotein (GP) IIb and GP IIIa were of usual migration in sodium dodecyl sulfate- polyacrylamide gel electrophoresis, but their labeling was much reduced during lactoperoxidase-catalyzed iodination. Binding to (A.P.) platelets of four different 125I-labeled monoclonal antibodies to GP IIb-IIIa complexes was reduced to 12% to 20% of normal levels. However, when the patient's platelets were stimulated with alpha-thrombin, monoclonal antibody binding showed the same increase (approximately 20,000 sites) as normal platelets. Both flow cytometry and immunocytochemical studies showed that the distribution of residual surface GP IIb-IIIa within the total (A.P.) platelet population was heterogeneous and not related to platelet size. Staining of ultrathin sections confirmed the presence of an internal pool of GP IIb-IIIa. Monoclonal antibodies to other membrane glycoproteins bound normally to (A.P.) platelets. The patient has a selective deficiency of the surface pool of GP IIb-IIIa complexes that is manifested clinically by a mild Glanzmann's thrombasthenia-like syndrome.

scholarly journals Further studies on the interaction between human platelet membrane glycoproteins IIb and IIIa in triton X-100

Blood ◽  
1982 ◽  
Vol 60 (4) ◽  
pp. 894-904 ◽  
Author(s):  
D Pidard ◽  
JP Rosa ◽  
TJ Kunicki ◽  
AT Nurden
Keyword(s):  
Membrane Proteins ◽  
Divalent Cation ◽  
Human Platelet ◽  
Polyacrylamide Gel Electrophoresis ◽  
Indirect Evidence ◽  
Human Platelets ◽  
Platelet Membrane ◽  
Membrane Glycoproteins ◽  
Nonionic Detergent ◽  
Triton X

Abstract Analysis of human platelet membrane proteins by crossed immunoelectrophoresis (CIE) in the presence of Triton X-100 (TX-100) has previously shown that glycoproteins (GP) IIb and IIIa are located in a single immunoprecipitate, band 16.2 To investigate whether IIb and IIIa are associated in a complex, we have analyzed TX-100-solubilized 125I-labeled membrane proteins by density gradient ultracentrifugation using 10%-40% sucrose gradients containing the nonionic detergent. studies were performed using soluble proteins derived from membranes isolated in the presence or absence of EDTA. Analysis of gradient fractions by SDS-polyacrylamide gel electrophoresis showed that in the absence of divalent cation chelation, GP IIb and IIIa penetrated well into the gradient (fractions 15–17). Analysis of fractions 15–17 by CIE revealed the presence of band 16. In contrast, when the membrane proteins were incubated with EDTA prior to or after TX-100 solubilization, IIb and IIIa remained near the top of the gradient (fractions 8–11) and gave separate immunoprecipitates during CIE. Incubation of washed platelet lysates with leupeptin, an inhibitor of the Ca2+-dependent protease of human platelets, had no effect on the shape of the band 16 immunoprecipitate. Thus, for the first time, direct evidence has been obtained that GP IIb and IIIa may form a divalent cation-mediated complex. Calibration of the sedimentation profiles using proteins of known molecular weight suggests that the complex is of limited size. Indirect evidence suggests that the complex is a heterodimer.

Antibodies Against Platelet Membrane Glycoproteins: Effect On Ristocetin-Induced Platelet Aggregation

1981 ◽  
Author(s):  
E F Ali-Briggs ◽  
C S P Jenkins ◽  
K J Clemetson
Keyword(s):  
Platelet Aggregation ◽  
Affinity Chromatography ◽  
Platelet Membrane ◽  
Receptor Activity ◽  
Free Medium ◽  
Membrane Glycoproteins ◽  
Lectin Affinity Chromatography ◽  
Lectin Affinity ◽  
Fab Fragments ◽  
Induced Aggregation

Some membrane glycoproteins (GPs) have been isolated by lectin-affinity chromatography and antibodies towards them have been raised. Platelets that have lost glycocalicin no longer respond to ristocetin-human VIII:WF, bovine VIIIR:WF, or to anti-glycocalicin or anti-GPs la and lb antibodies but are still agglutinated by anti-GPs lib and Ilia antibodies. Anti-GPs la and lb and anti-glycocalicin antibodies, IgG and Fab' fragments inhibited ristocetin- human VIIIR:WF- and bovine VIIIR:WF-induced aggregation of fixed, washed platelets and of platelets in plasma while anti-GPs Hb and Ilia antibodies were without effect.Crossed immunoelectrophorectic studies showed that glycocalicin was present on whole platelets in only trace amounts; anti-glycocalicin antibodies, however, recognized a slower migrating component. Platelets incubated in an EDTA-free medium no longer respond to ristocetin-human VIIIRrWF. Membranes isolated from such platelets contained glycocalicin which cross-reacted with a remnant of the slower migrating component. Anti-GPs la and lb antibodies gave more complex patterns but it was possible to identify the slower moving component recognized by the anti-glycocalicin antibodies.These results show that glycocalicin is not normally found as such on whole platelets but is present as a precursor which is most likely GP lb. On degradation of this precursor, glycocalicin is released from the membrane and VIIIRrWF-receptor activity is lost.

scholarly journals Further studies on the interaction between human platelet membrane glycoproteins IIb and IIIa in triton X-100

Blood ◽  
1982 ◽  
Vol 60 (4) ◽  
pp. 894-904 ◽  
Author(s):  
D Pidard ◽  
JP Rosa ◽  
TJ Kunicki ◽  
AT Nurden
Keyword(s):  
Membrane Proteins ◽  
Divalent Cation ◽  
Human Platelet ◽  
Polyacrylamide Gel Electrophoresis ◽  
Indirect Evidence ◽  
Human Platelets ◽  
Platelet Membrane ◽  
Membrane Glycoproteins ◽  
Nonionic Detergent ◽  
Triton X

Analysis of human platelet membrane proteins by crossed immunoelectrophoresis (CIE) in the presence of Triton X-100 (TX-100) has previously shown that glycoproteins (GP) IIb and IIIa are located in a single immunoprecipitate, band 16.2 To investigate whether IIb and IIIa are associated in a complex, we have analyzed TX-100-solubilized 125I-labeled membrane proteins by density gradient ultracentrifugation using 10%-40% sucrose gradients containing the nonionic detergent. studies were performed using soluble proteins derived from membranes isolated in the presence or absence of EDTA. Analysis of gradient fractions by SDS-polyacrylamide gel electrophoresis showed that in the absence of divalent cation chelation, GP IIb and IIIa penetrated well into the gradient (fractions 15–17). Analysis of fractions 15–17 by CIE revealed the presence of band 16. In contrast, when the membrane proteins were incubated with EDTA prior to or after TX-100 solubilization, IIb and IIIa remained near the top of the gradient (fractions 8–11) and gave separate immunoprecipitates during CIE. Incubation of washed platelet lysates with leupeptin, an inhibitor of the Ca2+-dependent protease of human platelets, had no effect on the shape of the band 16 immunoprecipitate. Thus, for the first time, direct evidence has been obtained that GP IIb and IIIa may form a divalent cation-mediated complex. Calibration of the sedimentation profiles using proteins of known molecular weight suggests that the complex is of limited size. Indirect evidence suggests that the complex is a heterodimer.

scholarly journals Anti-Lipid A Monoclonal Antibody Centoxin (HA-1A) Binds to a Wide Variety of Hydrophobic Ligands

1998 ◽  
Vol 66 (2) ◽  
pp. 870-873 ◽  
Author(s):  
E. J. Helmerhorst ◽  
J. J. Maaskant ◽  
B. J. Appelmelk
Keyword(s):  
Molecular Weight ◽  
Monoclonal Antibody ◽  
Monoclonal Antibodies ◽  
Bovine Serum Albumin ◽  
Gel Electrophoresis ◽  
Lipid A ◽  
Polyacrylamide Gel Electrophoresis ◽  
Sodium Dodecyl ◽  
Binding Properties ◽  
Antibody Epitopes

ABSTRACT This note describes the binding specificities of four lipid A monoclonal antibodies (MAbs) including Centoxin (HA-1A); these MAbs display similar binding properties. MAbs reacted with lipid A and heat-killed smooth bacteria, whereas no reactivity was observed with smooth lipopolysaccharide (LPS). Immunoblotting of bacterial extracts separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the MAbs bound to many polypeptide bands including the molecular weight markers. Denaturation of bovine serum albumin (BSA) by boiling or dithiothreitol treatment unmasked antibody epitopes. In addition, binding both to a hydrophobic aliphatic C12 chain covalently coupled to BSA and to single-stranded DNA was observed. The polyreactivity of these clones is most likely mediated by a preferential reactivity with hydrophobic molecular patches.

Aggregation to Thrombin and Collagen of Platelets from a Glanzmann Thrombasthenic Patient Lacking Glycoproteins IIb and IIIa

1989 ◽  
Vol 62 (03) ◽  
pp. 962-967 ◽  
Author(s):  
Lilian McGregor ◽  
Michel Hanss ◽  
Amal Sayegh ◽  
Juan J Calvette ◽  
Marie-Christine Trzeciak ◽  
...  
Keyword(s):  
Monoclonal Antibody ◽  
Monoclonal Antibodies ◽  
Platelet Aggregation ◽  
Membrane Glycoproteins ◽  
Albumin Solution ◽  
Western Blots ◽  
Glycoprotein Complex ◽  
Antigen Present ◽  
High Concentrations ◽  
Induced Aggregation

SummaryThe aim of this study was to investigate the platelets of a Glanzmann thrombasthenic patient, which in citrated PRP failed to respond to various agonists, but aggregated and secreted to high concentrations of thrombin (0.36, 0.72 and 1 U/ml) and collagen (4, 10 and 20 μg/ml) when washed and resuspended in a Tyrode-albumin solution (containing 2 mM Ca2+). Aggregation of the patient platelets was not affected by anti-IIb/IIIa monoclonal antibody (P18) which strongly inhibits thrombin or collagen induced aggregation of normal platelets. Washed platelets of this patient did not aggregate to ADP (10-100 μM) in the presence of added fibrinogen (2 mg/ml) nor bind 125I-labelled fibrinogen (40 to 320 μg/ml) when thrombin-stimulated. Different anti-IIb/IIIa monoclonal antibodies (P2, P18) when used in binding or crossed immunoelectrophoretic studies showed a complete absence of the IIb-IIIa glycoprotein complex on the patient platelets. Moreover, glycoproteins IIb or IIIa were absent on silver-stained twodimensional (non-reduced/reduced) polyacrylamide gel separations of the patient platelets and were not detected by Western blots used in combination with anti-PLA1 (antigen present on Ilia), anti-Leka (antigen present on IIb). This study shows that platelets lacking glycoproteins IIb or IIIa can aggregate in response to high concentrations of collagen or thrombin when resuspended in the presence of physiological concentrations of calcium. Results obtained in this study could indicate the existence of other mechanisms (other than the IIb-IIIa glycoprotein complex) involving glycolipids, heparans, proteoglycans, and/or unknown membrane glycoproteins to mediate platelet aggregation of stimulated thrombasthenic platelets.

scholarly journals Response of human platelets to activating monoclonal antibodies: importance of Fc gamma RII (CD32) phenotype and level of expression

Blood ◽  
1992 ◽  
Vol 80 (9) ◽  
pp. 2261-2268 ◽  
Author(s):  
Y Tomiyama ◽  
TJ Kunicki ◽  
TF Zipf ◽  
SB Ford ◽  
RH Aster
Keyword(s):  
Monoclonal Antibodies ◽  
Normal Subjects ◽  
Human Platelets ◽  
Fc Receptor ◽  
Platelet Membrane ◽  
The Other ◽  
Gene Products ◽  
Membrane Glycoproteins ◽  
Binding Studies

Abstract Certain monoclonal antibodies (MoAbs) specific for platelet membrane glycoproteins are known to be capable of activating platelets, and it is generally thought that platelets from normal subjects are equally susceptible to stimulation by such MoAbs. We found that platelets from 20 normal donors varied significantly in their sensitivity to three IgG1 murine MoAbs specific for membrane glycoproteins CD9, GPIV (CD36), and the GPIIb/IIIa complex (CD41), respectively. The response of platelets to these MoAbs was blocked by prior addition of MoAb IV.3 specific for the Fc gamma RII receptor, indicating that activation was Fc receptor mediated. Platelets that responded poorly to these MoAbs failed to bind the MoAb 41H.16, specific for the “responder” form of Fc gamma RII, but platelets that responded well reacted with this MoAb. The average number of Fc gamma RII receptors on platelets from “responders” and “non-responders” was approximately the same. However, the number of Fc gamma RII receptors expressed influenced sensitivity of a subgroup of “responder” platelets to the anti-CD41 MoAb. These platelets were judged on the basis of MoAb binding studies to be heterozygous for the two alleles of Fc gamma RIIA. In contrast to their varying sensitivity to IgG1 MoAbs, members of the platelet panel responded equally well to 50H.19, an IgG2a MoAb specific for CD9, and these responses could not be blocked by MoAb IV.3 in the presence of plasma. This appears to be because of dual actions of 50H.19 on platelets: one FcR-dependent and the other complement-dependent. Our findings confirm previous reports that certain IgG1 MoAbs activate platelets through binding of their Fc domains to Fc gamma RII receptors and demonstrate that this response is influenced both by Fc gamma RII phenotype and (in the case of the anti-CD41 MoAb) by the number of Fc gamma RII receptors expressed. The failure of nonresponding platelets to bind detectable amounts of MoAb 41H.16, which is thought to recognize all Fc gamma RII receptors except for one allele of the Fc gamma RIIA gene, is consistent with the possibility that Fc gamma RIIA gene products, but not Fc gamma RIIB or Fc gamma RIIC gene products, are expressed on platelets.

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