Platelet-specific Monoclonal Antibodies Research Articles

A Potential Protective Role of Platelets during Septic Shock Does Not Depend on Their Purinergic Receptors

Thrombocytopenia frequently occurs in septic shock patients and is associated with a worsened outcome. Experimental data indicate that platelets may have a protective role during sepsis. Our aims were to i) assess the potential protective role of platelets during septic shock and ii) evaluate the involvement of platelet P2Y1 and P2Y12 purinergic receptors.We used a model of polymicrobial-induced septic shock in mice by cecal ligation below the ileocecal valve and double puncture (CLP). The impact of moderate to severe thrombocytopenia was evaluated using mice deficient for the thrombopoietin receptor (Mpl-R-/-, 80% reduction in platelet count) and intravenous administration of a platelet specific rat anti-GPIbα monoclonal antibody (RAM.6, 2 mg/kg, 93% reduction in platelet count), respectively. To investigate the role of the P2Y12 receptor, WT mice were treated with clopidogrel, an irreversible inhibitor of the P2Y12 receptor, administered per os at 10 mg/kg, the day before and 2 h prior to surgery. Contribution of the P2Y1 receptor was evaluated using mice deficient for the P2Y1 receptor (P2Y1-/-). Mice underwent sham or CLP surgery (6 animals/group). Twenty hours post-surgery, blood and tissue samples were collected to evaluate critical parameters of septic shock. Mortality was recorded in a second group of animals.Twenty hours after CLP, mice with either normal or reduced platelet counts displayed signs of septic shock such as apathy, fur ruffling, conjunctivitis and diarrhea. Their mean arterial blood pressure (MAP) fell by 30% indicating systemic hypotension and organ failure characteristic of shock. Mpl-R-/- mice, displaying moderate thrombocytopenia, showed enhanced plasma concentrations of the pro-inflammatory cytokines TNF-α and IL-6, and of myeloperoxidase (MPO), indicating increased systemic inflammation relative to WT mice. These effects were more pronounced in RAM.6-treated mice displaying severe thrombocytopenia. Thrombin-antithrombin (TAT) complexes, reflecting in vivo thrombin generation, were enhanced in RAM.6-treated mice as compared to Mpl-R-/-mice or WT mice. Thrombocytopenia was also associated with increased plasma levels of aspartate aminotransferase (ASAT) relative to mice with normal platelet count, an effect also more pronounced in severely thrombocytopenic mice. These results indicate that depletion of platelets worsened organ injury during septic shock. Finally, severely thrombocytopenic mice succumbed more rapidly than control mice after CLP.Twenty hours after CLP, clopidogrel-treated mice and P2Y1-/- mice displayed signs of shock attested by a 30% decrease in MAP, similar to that observed in WT mice. Plasma concentrations of TNF-α, IL-6 and MPO were similarly increased in WT, clopidogrel-treated and P2Y1-/- mice. In addition, no differences in TAT levels were observed between the three groups of mice. Finally, the extent of liver damage, as evaluated by measurement of plasma ASAT levels, was similar between clopidogrel-treated mice, P2Y1-/-mice and WT mice.Overall, these results provide evidence for a beneficial role of platelets during experimental septic shock in mice. However, the platelet ADP receptors do not contribute to this effect, suggesting that anti-platelet therapy with P2Y12 receptor antagonists may not be beneficial in patients with septic shock. The mechanisms by which platelets modulate the host response to septic shock remain to be elucidated. DisclosuresNo relevant conflicts of interest to declare.

Detection and characterization of arterial thromboses using a platelet-specific monoclonal antibody (P256 Fab')

Arteriography does not reliably distinguish between acute and chronic arterial occlusions. Seventeen patients with acute lower limb ischaemia were investigated by arteriography and by imaging with a platelet-specific monoclonal antibody (P256 Fab'); 20 MBq 111In-labelled P256 Fab' was administered intravenously and patients were imaged at intervals of between 20 min and 24 h. Thirteen patients were subsequently treated with intra-arterial thrombolysis. In six the images showed foci of increased uptake of 111In-labelled P256 Fab' and the corresponding arterial segment was recanalized. Patency to 30 days was maintained in four cases. Seven patients had negative scans, only four of whom achieved lysis, and two of these suffered early rethrombosis. The remaining four patients were excluded from thrombolysis by the arteriographic appearances. 111In-labelled P256 Fab' imaging can identify sites of acute arterial thrombosis and may have clinical applications in the management of peripheral vascular disease. Further studies are required to test whether the technique has a role to play in patient selection for thrombolysis.

Inability of Probiotic Bacterial Strains Lactobacillus rhamnosus HN001 and Bifidobacterium lactis HN019 To Induce Human Platelet Aggregation In Vitro

Platelet aggregation contributes to the pathogenesis of infective endocarditis, and aggregation of platelets induced by lactobacilli is thought to be an important contributory factor in the development and progression of Lactobacillus endocarditis. The main purpose of this study was to examine the effect of immunity-enhancing probiotic strains Lactobacillus rhamnosus HN001 and Bifidobacterium lactis HN019 on the activation and aggregation of human blood platelets. Whole blood samples from healthy individuals were incubated in vitro with HN001 or HN019 and subsequently labeled with platelet-specific monoclonal antibodies, fluorescein isothiocyanate–conjugated anti-CD41a (expressed on normal platelets), and phycoerythrin-streptavidin–conjugated anti-CD62p (expressed on activated platelets) before analysis by flow cytometry. Platelet-rich plasma was used to assist the gating of the platelet cluster. ADP and epinephrine were used as the physiological platelet activation agonists. Platelet aggregation–inducing strain Streptococcus sanguis 133-79 was used as a positive control strain. The mean fluorescence intensity of phycoerythrin and the percentage of platelets expressing the CD62p marker were used to assess the degree of platelet activation. The percentage of CD62p-positive platelets and the light scatter profiles of the agonist-activated platelets were used to identify the occurrence and degree of platelet aggregation. HN001 and HN019 had no effect on spontaneous platelet activation and aggregation; they also failed to exacerbate the platelet aggregation activity induced by ADP and epinephrine. Therefore, these test probiotic strains HN001 and HN019 are less likely to participate in the pathogenesis of infective endocarditis or other thrombotic disorders with regard to platelet aggregation factors.

In vitro Biological Characteristics of Human Cord Blood-derived Megakaryocytes

Introduction: Umbilical cord blood (CB) has been used as an alternative source for haematopoietic stem cell transplantation (HSCT) in recent years. However, delayed platelet recovery is frequently associated with CB HSCT. Megakaryocytes (Mk) are the specialised precursors of platelets and they are among the rarest haemopoietic cell types. Despite the rapid expansion of our knowledge of megakaryopoiesis in recent years, many questions, such as the molecular regulatory mechanisms in Mk differentiation and maturation, platelet formation and release, remain unanswered in CB-derived megakaryopoiesis. Variations can be seen from the literature by individual investigators using different approaches for Mk-specific differentiation and maturation induction. The development of in vitro culture methods to obtain sufficient numbers of Mks from readily available haematopoietic stem cells is of value for both basic research and clinical applications. Materials and Methods: The CD34+ cells from cord blood samples were cultured in serum-free medium with haematopoietic growth factors (GFs), such as IL-3, stem cell factor (SCF), and thrombopoietin (Tpo). The differentiation of Mk was monitored using Mk- and platelet-specific monoclonal antibodies and flow cytometric analysis. The morphology of the cultured cells was studied by both light and electronic microscopy (LM and EM). The involvement of the human Notch gene family members was studied by real time-polymerase chain reaction (RT-PCR). Maturation of the cultured Mks was studied using flow cytometric analysis for both platelet-specific surface markers and enodomitosis. Platelet activation was assessed in the cytoplasmic fragments harvested from the cultures. Results: Specific Mk differentiation of >70% resulted from a 2-step culture approach using IL-3, SCF and Tpo for 7 days followed by Tpo only for another 14 days. RT-PCR showed high-level expression of both Notch-1 and its ligand, Jagged-1, in the cultured Mks. Limited levels of polyploidy (>4N, endomitosis, EnM) were observed in the cultured Mks. The results also showed that the cytoplasmic fragments from the cultures responded to platelet activation reagents, including ADP and collagen, marked by upregulation of platelet-specific activation markers, such as CD62P (P-selectin) and PAC-1 (gpαIIbβ3). Conclusion: The methods used in this study are specific for differentiation of Mk from CB CD34+ cell, which can partially mature and produce functional platelets in vitro. This approach for human Mk differentiation could be further optimised and may be adapted on larger scales for clinical purposes.

Platelet GPIIb/IIIa Is Activated and Platelet-Leukocyte Coaggregates Formed in vivo during Hemodialysis

Background/Aim: During hemodialysis, platelets and leukocytes are activated and form platelet-leukocyte coaggregates in which GPIIb/IIIa (CD41/CD61) and CD62P (P-selectin) are involved. However, it is still controversial whether platelet activation and platelet-leukocyte coaggregate formation are dependent on the dialyzer membrane material. Method: We examined the appearance of activation-dependent antibody on platelets as an index of platelet activation, and the appearance of platelet-specific antigen on leukocytes as an index of platelet-leukocyte coaggregation, during hemodialysis in 7 patients treated using regenerated cellulose (RC) membrane and next using polysulfone (PS) membrane. In order to reduce the influence of factors other than dialyzer membrane material, this study was conducted in a prospective crossover fashion using a pyrogen-free bicarbonate dialysate. Moreover, flow cytometric techniques with whole blood were employed, which reduce artificial cell activation during the cell or plasma separation procedure. The platelet-specific monoclonal antibodies used in this study were anti-CD61, PAC-1 (which recognizes only the conformationally activated GPIIb/IIIa) and anti-CD62P. Results: Changes in the percentage of PAC-1-positive platelets were significantly greater during hemodialysis with RC than with PS. However, changes in the percentage of CD62P-positive platelets were not significantly different between hemodialysis with RC and PS. Changes in the percentage of CD61- or CD62P-positive leukocytes were significantly greater during hemodialysis with RC than with PS. Although changes in percentage of PAC-1-positive platelets did not parallel those of CD62P-positive platelets during hemodialysis, there was a significant positive correlation between the percentage of CD61-positive leukocytes and the percentage of CD62P-positive leukocytes. Conclusion: This study, conducted in a prospective crossover fashion using a pyrogen-free bicarbonate dialysate in order to reduce the influence of factors other than the dialyzer membrane material, demonstrated that both the degrees of GPIIb/IIIa activation and platelet-leukocyte coaggregation were greater during hemodialysis with RC than PS.

IVIg inhibits reticuloendothelial system function and ameliorates murine passive-immune thrombocytopenia independent of anti-idiotype reactivity.

Although the mechanism of action of intravenous immunoglobulin (IVIg) in treating antibody-dependent thrombocytopenia remains unclear, most studies have suggested that IVIg blocks the function of Fc receptors in the reticuloendothelial system (RES) and/or the protective effect may be due to the presence of variable region-reactive (anti-idiotype) antibodies within IVIg. We evaluated the effect of IVIg on platelet counts in a murine model of passively induced immune thrombocytopenia (PIT). Although IVIg was unable to neutralize the binding of two platelet-specific monoclonal antibodies to their target antigens either in vivo or in vitro, it was able to prevent PIT as well as ameliorate pre-established PIT mediated by these antibodies. IVIg adsorbed against the antibody used to induce thrombocytopenia or endogenous murine immunoglobulin also protected against PIT, indicating that antibodies with anti-idiotype activity present in IVIg are not necessary for its effective treatment of PIT. IVIg significantly blocked the ability of the RES to clear antibody-sensitized red blood cells. F(ab')2 fragments of IVIg, which are unable to block the RES but retain the idiotypic regions, were ineffective at protecting mice from PIT. Our data suggest that IVIg exerts its rapid effect by inhibiting RES function and that anti-idiotype interactions are not required.

Reticulated platelet counts in the diagnosis of acute immune thrombocytopenic purpura.

Bone marrow aspiration is often performed to diagnose childhood acute immune thrombocytopenic purpura (ITP) because no non-invasive investigation to confirm the diagnosis is routinely available. Reticulated platelets (RPs--young platelets characterized by a high RNA content--increase with increased platelet production and may be useful in the diagnosis of ITP. To assess the role of RP counts in distinguishing ITP, we compared counts from 15 consecutive patients with ITP with counts from 20 patients with acute lymphoblastic leukemia (ALL), 10 with aplasia, and 27 healthy normal children. Whole blood in edetic acid (EDTA) was labelled with a platelet-specific monoclonal antibody and incubated with thiazole orange (TO). A standard gate was set to achieve a fluorescence value of 1.3 +/- 0.5% for control lyophilized platelets. Patients with ITP had a mean (+/- 1 standard deviation) RP level of 32.9 +/- 10.2%; patients with ALL, 6.6 +/- 3.1%; patients with aplasia, 3.4 +/- 2.0%; and normal patients, 7.9 +/- 2.9%. The difference in RPs between the ITP group and the ALL, aplasia, and normal groups was highly significant (p < 0.0001 each), with no significant difference between the non-ITP groups (p = 0.12). Measuring RPs by this simple whole-blood cytometric technique discriminated very well between the acute ITP and non-ITP groups. This test has a strong positive predictive value and may prove very useful in the assessment of childhood acute ITP and the screening of candidates for bone marrow aspiration.

Human breast carcinoma cells synthesize a protein immunorelated to platelet glycoprotein-lbα with different functional properties

Although tumor cell-induced platelet aggregation is thought to mediate an early step in the metastatic process, little is known about tumor adhesive receptors responsible for the initial platelet-tumor attachments. Because our preliminary work demonstrated that a platelet-immunorelated glycoprotein Ib α (GPIb α) receptor expressed by the human breast carcinoma cell line MCF-7 participates in tumorinduced platelet aggregation, we examined the synthesis and functional characteristics of this MCF-7-immunorelated GPlbα. When 35S-cysteine-labeled, digitoninlysed MCF-7 cells were immunoprecipitated with platelet-specific monoclonal antibodies (mAbs) to GPlbα, major radioactive bands were observed. Northern blots showed MCF-7 transcripts for GPlbα under both high- and low-stringency hybridization conditions. In the presence of purified human iodine 125-labeled von Willebrand factor ( 125I-labeled vWf) with or without the addition of ristocetin, unlabeled vWf was observed to competitively bind to fixed MCF-7 cells (50% inhibitory concentration = 10 μg/ml, dissociation constant = −3.8 ± 1.9 nmol/L, 2.7 × 10 6 ± 445,000 binding sites/cell) in which non-GPlbα vWf binding sites were blocked. 125I-vWf binding to blocked MCF-7 cells could be selectively and completely inhibited by mAbs specific for the vWf binding domain of GPlbα but not by mAbs against the GPIX subunit, the GPIbβ subunit, or alternate GPlba epitopes other than the vWf-binding domain. Finally, when whole blood substrate was incubated with a mAb specific for the GPIb binding epitope of vWf, MCF-7-induced platelet aggregation was virtually abolished in comparison with control specimens (N = 8; p < 0.0009). These findings (1) confirm the synthesis and expression of an MCF-7 protein with homology to platelet GPlbα, (2) confirm that the functional activity of this MCF-7-immunorelated GPlbα differs from that of platelet GPIbα, and (3) suggest that MCF-7-immunorelated GPlbα in its adhesive interactions with plasma vWf may constitute an initial event in MCF-7-induced platelet aggregation.

Platelet-leukocyte aggregation during hemodialysis

Hemodialysis is associated with simultaneous changes in leukocytes and platelets, but it is unclear whether these alterations affect the interactions between these cell types. To evaluate this process, we examined the appearance of platelet specific antigens (CD41) on leukocytes as an index of platelet-leukocyte aggregation during hemodialysis using three different synthetic membranes. Patients with end-stage renal disease (ESRD) on long-term hemodialysis treatment were enrolled. Flow cytometric techniques and platelet specific monoclonal antibodies (MoAb) that recognize the glycoprotein complex on resting and activated platelets (anti-CD41), the activated GPIIb-IIIa complex receptor (anti-LIBS1), and the p selectin GMP140, that is exposed on platelet plasma membrane after activation and platelet degranulation (anti-CD62), were used. Subjects with ESRD had a lower predialysis platelet surface expression of CD41 and LIBS1 compared to normal controls, but unchanged CD62 expression. In parallel, patients with ESRD manifested a uniformly reduced platelet-leukocyte microaggregates predialysis compared to normal controls. When examined across the dialyzer, however, an increase in platelet-neutrophil and platelet-monocyte microaggregates was observed with all three synthetic membranes at both 15 and 30 minutes after initiation of dialysis. This phenomenon could be duplicated in vitro by physiologic concentrations of the platelet specific agonist ADP, but not by the complement factors C3a or C5a. We conclude that platelet-leukocyte aggregates occur during dialysis likely related to a primary platelet activation mechanism. This phenomenon may serve as a new biocompatibility parameter and may shed light on some of the biologic consequences of hemodialysis.

Changes in platelet membrane glycoproteins and platelet-leukocyte interaction during hemodialysis.

Platelet aggregation and interaction with other vascular cells play a key role in hemostatic events during hemodialysis. We studied seven patients with end-stage renal failure on long-term hemodialysis treatment. Flow-cytometric techniques and platelet-specific monoclonal antibodies were used to measure the platelet surface expression of glycoproteins-fibrinogen receptor on glycoprotein IIb-IIIa (GP IIb-IIIa; CD41) and alpha-granule membrane protein (GMP-140; CD62). In addition, adhesion of platelets or platelet microparticles with leukocytes was evaluated by appearance of the platelet-specific antigen (GP IIb-IIIa) on leukocytes. Blood samples were taken before the start of dialysis and 15, 60, and 240 min thereafter. There was a significant increase in fibrinogen receptor activation on circulating platelets after 15 min of dialysis treatment (P < 0.001) and enhanced degranulation of GMP-140 (P < 0.05). In parallel, the interaction of platelets with neutrophils and monocytes also increased with the duration of dialysis and was maximal after 15 min (P < 0.001). We conclude that the platelet fibrinogen receptor on GP IIb-IIIa in circulating platelets is activated during hemodialysis and is associated with increased adhesion of platelets or platelet microparticles with circulating leukocytes. Thus, the phenomenon described here of platelet-leukocyte interaction could be pathophysiologically important for the development of dialysis-associated leukopenia.

Platelet Imaging of Thromboembolism: Natural History of Postoperative Deep Venous Thrombosis and Pulmonary Embolism Illustrated Using the 111In-Labelled Platelet-Specific Monoclonal Antibody, P256

Deep venous thrombosis (DVT) and pulmonary thromboembolic disease are difficult to diagnose, particularly following surgery. This report demonstrates the use of 111In-labelled platelet-specific monoclonal antibody, P256 Fab', for the diagnosis and study of the time course of thromboembolic disease in a patient following total hip replacement. One hundred micrograms of pentetic acid (DTPA)-P256 Fab' was labelled with 8 to 10 MBq of 111In chloride by incubation at room temperature for 15 min. After dilution in physiologic saline, the tracer was injected intravenously on the third and sixth days postoperatively. Imaging of the chest, pelvis, and legs was carried out at 24, 48 and 72 h following each injection. The first image four days after surgery demonstrated activity in the right heart which moved to the right pulmonary artery on the following day. Activity was seen in both femoral veins; on the left, this increased over two days, followed by a reduction on the seventh day after surgery, at which time new activity was seen in the right heart. After a further two days, this activity moved to the left pulmonary artery. The DVT was confirmed by venography and the pulmonary embolism (PE) by ventilation perfusion scan. 111Indium-labelled platelet-specific monoclonal antibody, P256 Fab', provides a technique for studying the natural history of thromboembolic disease and its treatment.

86. P256 Fab?? platelet specific monoclonal antibody in the assessment of platelet deposition following angioplasty

86. P256 Fab?? platelet specific monoclonal antibody in the assessment of platelet deposition following angioplasty

Detection of antibiotic-induced platelet dysfunction in whole blood using flow cytometry.

Using flow cytometry and activation-dependent monoclonal antibodies, we have developed a technique based on forward angle-light scatter (FALS) and immunofluorescence that simultaneously detects human platelet activation, secretion, and aggregation in whole blood. To detect the effects of cefotetan and latamoxef, both of which contain an N-MTT side chain, and of free N-MTT and cefoxitin, which does not contain the N-MTT side chain, on platelet activation and secretion, platelets were stained by the indirect method using a murine-produced platelet specific activation-dependent monoclonal antibody, S12, and a goat anti-mouse IgG fluorescein-conjugated antibody. S12 binds to a 140kd alpha granule membrane protein (GMP-140) that is expressed during secretion. Single parameter, 256 channel, log integrated green fluorescence histograms were generated, and negative and positive fluorescent populations were defined. Latamoxef and cefotetan reduced the number of platelets expressing S12 by more than 43%. In contrast, cefoxitin reduced the number of platelets expressing S12 by only 13.5%. The inhibition of GMP-140 expression per platelet was calculated by converting the log data to linear fluorescence intensity. Latamoxef and cefotetan inhibited expression of GMP-140 by 88% and 87% respectively. Free N-MTT inhibited its expression by 68%. In contrast cefoxitin reduced GMP-140 expression per platelet by only 45%.

Down's Syndrome and Acute Leukemia in Children: An Analysis of Phenotype by Use of Monoclonal Antibodies and Electron Microscopic Platelet Peroxidase Reaction

The clinical, hematologic, and immunophenotypic features in 20 patients with Down's syndrome (DS) and acute leukemia were analyzed. Of the 20 patients, all 14 patients who were 3 years old and less were diagnosed as having acute megakaryoblastic leukemia (AMKL) by use of platelet-specific monoclonal antibodies and platelet peroxidase (PPO) reaction in electron microscopy. They were characterized by the presence of bone marrow fibrosis, having a history of myelodysplastic syndrome (MDS) and a poor response to chemotherapy. Only one patient has remained in continuous complete remission for more than 1 year. Acute leukemia in six patients who were older than 4 years was classified as common acute lymphoblastic leukemia antigen (CALLA)-positive acute lymphoblastic leukemia (ALL). In one of six patients classified as ALL, the leukemic blasts simultaneously expressed myeloid-associated surface antigens. All six patients achieved a complete remission and have remained in continuous complete remission and have remained in continuous complete remission from 10 to 52 months from the initial diagnosis. Although it has been suggested that the distribution of types of acute leukemia in patients with DS is similar to that in normal children, the present study shows that the distribution of acute leukemia types is quite different from that in patients without Down's syndrome.

111In labelling of post-operative thrombus

I am sure that I am not alone in being excited by the article from Stuttle et al (1989) on platelet specific monoclonal antibody that appeared in the November issue of the Journal. The first publication that I read on this topic, at the beginning of this year, led me to seek a source of MAB P256 for two important research projects in this department, using labelled platelets. I was told that the antibody was not available for general use and I am still unable to obtain a supply.

Sequential imaging of post-operative thrombus using the In-111-labelled platelet-specific monoclonal antibody P256.

Eleven patients undergoing total hip replacement were studied by radioimmunoscintigraph for development of postoperative thrombus. Images were obtained using the 111In-labelled Fab' fragment of a monoclonal antibody (P256) directed against human platelets. Six patients showed focal accumulation of radioactivity in the lower limbs. Three of these had contrast venograms that were all positive for thrombus. Two of these six patients showed evidence of pulmonary localization of P256-Fab' and this correlated with positive ventilation/perfusion scintigraphy in one of the patients. Sequential imaging with 111In-P256 Fab' not only established a diagnosis of deep venous thrombus but also provided useful information regarding its natural history.

In vitro studies with an anti-platelet monoclonal antibody; P256.

P256 is a platelet-specific monoclonal antibody. It has potential as a diagnostic agent for use in thrombus detection by radioimmunoscintigraphy. However, the whole antibody has been reported to cause platelet aggregation. The object of the present study was to determine the effects of the F(ab')2 and Fab' fragments of P256 on human platelets in vitro. Gel-filtered platelets were prepared from citrated venous blood, and aggregation was studied at 37 degrees C using the method of Born. The whole antibody caused aggregation over the concentration range 10(-9)-10(-7) M, with a maximum of 70% aggregation. The F(ab')2 fragment was less potent with a maximum response of only 45%. The monovalent Fab' fragment caused no aggregation at concentrations up to 10(-7) M. However, binding studies with 111In-labelled monovalent Fab' fragment demonstrated saturable displaceable binding (Kd = 9.1 x 10(-9) M). Aggregation induced by the divalent forms of the antibody was partially dependent upon added fibrinogen. It is concluded that the monovalent Fab' fragment of P256 may be used in platelet-labelling studies without risk of inducing aggregation irrespective of concentration. The whole antibody, or its divalent F(ab')2 fragment, may also be used provided their concentrations are kept below 10(-9) M.

Immunochemical characterization of a new platelet specific monoclonal antibody and its use to demonstrate the cytoskeletal association of the platelet glycoprotein IIb-IIIa complex.

We describe the production and characterization of a monoclonal antibody specific for platelets. This antibody reacts strongly with human and primate platelets, but does not recognise human monocytes, polymorphonuclear leucocytes, lymphocytes, erythrocytes, leukaemic nor fibroblast cell lines, nor rodent platelets. Immunoprecipitation studies using radiolabelled platelet membrane proteins showed that the monoclonal antibody binds to the platelet membrane glycoprotein IIb-IIIa complex. Affinity chromatography using immobilized monoclonal antibody allows purification of the antigen, but also co-purifies the cytoskeletal proteins actin and myosin. Our results demonstrate immunochemically that although the GP IIb-IIIa complex is an external structure, it is connected through the cell membrane to the microfilament system.