Rabbit Platelets do not Express Fc Receptors for IgG

Human platelets are activated by IgG immune complexes and undergo platelet aggregation and the release of granular contents. Recently, the receptor for IgG on human platelets has been identified and characterised. In contrast, it is not known if rabbit platelets carry Fc receptors. In this report we describe investigations to detect the presence of an Fc receptor on rabbit platelets. The Fc receptor was investigated using functional assays (platelet aggregation and (14)C-serotonin release), a direct binding assay, and an affinity precipitation assay. All of these methods gave consistently positive results using human platelets and consistently negative results using rabbit platelets, indicating that rabbit platelets do not carry Fc receptors. This inconsistency in the interactions of immune complexes with rabbit and human platelets indicates that rabbits are not ideal models for studying a number of human diseases that could be caused by immune complexes, including heparin-induced thrombocytopenia and certain immunological disorders such as systemic lupus erythematosus.     

Measurement of platelet-associated IgG in animal models of immune and nonimmune thrombocytopenia

Platelet-associated IgG (PAIgG) is elevated in idiopathic thrombocytopenic purpura (ITP), but it also is elevated in other thrombocytopenic disorders traditionally considered to be nonimmune. Consequently it is possible that elevated PAIgG is a nonspecific finding secondary to thrombocytopenia. To study this issue we developed a rabbit model of immune and nonimmune mediated thrombocytopenia. The mechanism of the thrombocytopenia was validated by platelet survival studies. Immune thrombocytopenia was produced by injection of antirabbit platelet serum that was raised in guinea pigs. Nonimmune aregenerative thrombocytopenia was produced by irradiation of the animals; nonimmune consumptive thrombocytopenia was produced by injection of adenosine diphosphate (ADP). PAIgG was measured in a direct binding assay using 125I-labeled staphylococcal protein A (SpA). Washed platelets from normal, nonthrombocytopenic rabbits bound an average of 81 molecules of SpA per platelet (81 +/- 168, mean +/- 2 SD, n = 39). Infusion of the antiplatelet antiserum produced thrombocytopenia with a rise in PAIgG that was closely correlated with the level of PAIgG (r = 0.86, n = 12). The thrombocytopenia was consumptive, as shown by a very short platelet life span using 111In- labeled platelets. In contrast, both nonimmune thrombocytopenic states resulted in an equal or greater drop in the platelet count but no change in the level of PAIgG. The animals with aregenerative thrombocytopenia had normal or only moderately reduced platelet life spans; however, in every animal the level of PAIgG was not different from the nonthrombocytopenic controls, irrespective of the platelet count. Similarly, the level of PAIgG was unchanged in those rabbits with nonimmune consumptive thrombocytopenia following infusion of ADP (82 +/- 55 molecules of SpA per platelet, mean +/- SD, n = 6). These studies indicate that elevated PAIgG is a specific finding of immune thrombocytopenia and is not secondary to thrombocytopenia itself. Indirectly these results support our hypothesis that immune mechanisms contribute to more thrombocytopenic disorders than was once thought likely.     

Detection of drug-dependent antibodies by the 51cr platelet lysis test: Documentation of immune thrombocytopenia induced by diphenylhydantoin,diazepam, and sulfisoxazole

Sudden, severe thrombocytopenia developed in each of three patients receiving diphenylhydantoin, diazepam, and sulfisoxazole, respectively. Recovery followed discontinuance of the drugs. An antiplatelet antibody requiring the presence of an appropriate drug for interaction with platelets was demonstrated in each case by the ⁵¹Cr platelet lysis test using normal, paroxysmal nocturnal hemoglobinuric, or enzyme-treated normal platelets as target cells. These antibodies could not be detected by techniques that depend on clot retraction inhibition, complement fixation, or platelet factor-3 activation. Quinine- and quinidine-dependent antiplatelet antibodies in the serum of 16 patients who developed acute thrombocytopenia while taking either quinine or quinidine could be demonstrated readily with the ⁵¹Cr platelet lysis test and could also be detected by other methods employed.     

Platelet turnover and kinetics in immune thrombocytopenic purpura: results with autologous 111In-labeled platelets and homologous 51Cr- labeled platelets differ

Mean platelet survival and turnover were simultaneously determined with autologous 111In-labeled platelets (111In-AP) and homologous 51Cr- labeled platelets (51Cr-HP) in ten patients with chronic immune thrombocytopenic purpura (ITP). In vivo redistribution of the 111In-AP was quantitated with a scintillation camera and computer-assisted image analysis. The patients were divided into two groups: those with splenic platelet sequestration (spleen-liver 111In activity ratio greater than 1.4), and those with diffuse sequestration in the reticuloendothelial system. The latter patients had more severe ITP reflected by pronounced thrombocytopenia, decreased platelet turnover, and prominent early hepatic platelet sequestration. Mean platelet life span estimated with 51Cr-HP was consistently shorter than that of 111In-AP. Platelet turnover determined with 51Cr-HP was thus over-estimated. The difference in results with the two isotope labels was apparently due to greater in vivo elution of 51Cr. Although the limitations of the techniques should be taken into account, these findings indicate that platelet turnover is not always normal or increased in ITP, but is low in severe disease. We suggest that this may be ascribed to damage to megakaryocytes by antiplatelet antibody. The physical characteristics in 111In clearly make this radionuclide superior to 51Cr for the study of platelet kinetics in ITP.     

Transient vinblastine-induced thrombocytopenia during chemotherapy with vinblastine,cis-platinum and bleomycin*

An early, transient drop in peripheral blood platelets to a mean nadir value of 49% (range 31–83%) of the pretreatment value was seen in 31 patients during chemotherapy with cis-platinum, vinblastine and bleomycin (PVB). The mean number of platelets dropped by 22% during the first 24 h after the start of chemotherapy, nadir value was seen after 3 d, with recovery to 107% of pretreatment level on d 14–15. Platelet survival studies during PVB chemotherapy showed shortened platelet survival time and indicated increased destruction of platelets. Identical patterns of early, transient thrombocytopenia were seen in 6 patients treated with vinblastine only. The early fall in circulating platelets did not predict subsequent serious thrombocytopenia. There were no bleeding episodes in the 37 patients studied here, but the early thrombocytopenia seen after vinblastine therapy may possibly be of clinical importance in critically ill patients at risk for bleeding episodes. In rats, an early drop in circulating platelets occurred after a high dose of vinblastine, but not after bleomycin. Addition of bleomycin to vinblastine did not increase the vinblastine-induced thrombocytopenia. It is concluded that the early drop in peripheral blood platelets during chemotherapy with PVB is due to vinblastine.     

Quinidine purpura: evidence that glycoprotein V is a target platelet antigen

Quinidine-induced thrombocytopenia has been associated with both immune complex and autoantibody binding to platelets. In the present study, serum antibody from six of six patients with quindine purpura was shown by immunoblotting to bind to a single platelet membrane protein of mol wt 80,000. This target protein was absent from Bernard-Soulier (BSS) platelets. F(ab)2 prepared from one patient's serum also bound to this protein, indicating autoantibody rather than immune complex binding to the target antigen. Antibody binding to the 80-kd protein was preserved after treatment of platelets with concentrations of trypsin or chymotrypsin that completely removed glycoprotein Ib (GPIb). Preincubation of platelet proteins with one patient's serum blocked binding of a polyclonal rabbit antibody against glycoprotein V (GPV), indicating that these antibodies recognize the same antigen. By wheat germ affinity chromatography, GPV was shown to copurify with GPIb. Quinidine-induced antibody bound to the wheat germ-purified GPV but not to GPIb. We conclude that quinidine purpura is associated with autoantibody directed against platelet GPV.     

Drug-induced Immune Thrombocytopenia

SUMMARY. Immune thrombocytopenia is a relatively common problem associated with the clinical usage of drugs. Drugs frequently implicated include quinine, quinidine, heparin, penicillins, cephalosporins, co-trimoxazole, gold and D-penicillamine. Bleeding including bruising and purpura is the usual clinical manifestation except in immune heparin-induced thrombocytopenia in which thrombosis occurs more frequently than bleeding. Cessation of the offending drug is the important step in the treatment but other measures may also be required such as platelet transfusion and steroid therapy for patients with clinical bleeding or antithrombotic therapy with warfarin and dextran or low molecular weight heparin/heparinoid for patients with heparin-induced thrombocytopenia and thrombosis. Idiosyncratic drug-induced thrombocytopenia is mediated by an antibody which binds to platelets only in the presence of the drug resulting in the clearance of sensitised platelets by the reticuloendothelial system. In quinine/quinidine-induced thrombocytopenia, the antibodies recognise drug-dependent epitopes on platelet membrane glycoproteins Ib-IX and/or glycoproteins IIb-IIIa. In immune heparin-induced thrombocytopenia the current data suggest a mechanism which probably involves the binding of heparin-antibody complexes to the platelet Fc receptors but the precise mechanism is yet to be fully characterised. The associated thrombosis in this condition is likely to be due to platelet activation and possibly endothelial cell damage induced by the heparin-related antibody.     

Thrombocytopenia associated with gold therapy: Observations on the mechanism of platelet destruction

Severe thrombocytopenia developed in a patient with rheumatoid arthritis during gold therapy. Increased numbers of marrow megakaryocytes, shortened ⁵¹Cr-labeled platelet survival and platelet phagocytosis by splenic macrophages indicated that thrombocytopenia was due to excessive platelet destruction. Aurothiomalate disodium antigenicity was demonstrated by increased lymphocyte blastogenesis, and accentuation of blood and splenic leukocyte migration in the presence of the gold salt. In vitro splenic immunoglobulin G (IgG) production was markedly increased, and a significant portion of the culture-produced IgG attached specifically to homologous platelets and platelet membranes. Serum antiplatelet antibodies could not be demonstrated, nor could it be shown that gold enhanced the binding of splenicsynthesized IgG to platelets. The data indicate an immunologic mechanism for gold-associated thrombocytopenia and permit speculation as to possible ways in which unidentified antigens may be involved in the pathogenesis in idiopathic thrombocytopenic purpura.     

Drug-induced Immune Thrombocytopenia

SUMMARY.

Immune thrombocytopenia is a relatively common problem associated with the clinical usage of drugs. Drugs frequently implicated include quinine, quinidine, heparin, penicillins, cephalosporins, co-trimoxazole, gold and D-penicillamine. Bleeding including bruising and purpura is the usual clinical manifestation except in immune heparin-induced thrombocytopenia in which thrombosis occurs more frequently than bleeding. Cessation of the offending drug is the important step in the treatment but other measures may also be required such as platelet transfusion and steroid therapy for patients with clinical bleeding or antithrombotic therapy with warfarin and dextran or low molecular weight heparin/heparinoid for patients with heparin-induced thrombocytopenia and thrombosis. Idiosyncratic drug-induced thrombocytopenia is mediated by an antibody which binds to platelets only in the presence of the drug resulting in the clearance of sensitised platelets by the reticuloendothelial system. In quinine/quinidine-induced thrombocytopenia, the antibodies recognise drug-dependent epitopes on platelet membrane glycoproteins Ib-IX and/or glycoproteins IIb-IIIa. In immune heparin-induced thrombocytopenia the current data suggest a mechanism which probably involves the binding of heparin-antibody complexes to the platelet Fc receptors but the precise mechanism is yet to be fully characterised. The associated thrombosis in this condition is likely to be due to platelet activation and possibly endothelial cell damage induced by the heparin-related antibody.     

Auto-immune thrombocytopenia related to interferon therapy

A patient with renal cell carcinoma developed reversible autoimmune thrombocytopenia while receiving leukocyte interferon (IFN) and subsequently fibroblast IFN. Bone marrow biopsies and elevated platelet-associated IgG were suggestive of immune thrombocytopenia. The patient's history of exposure to IFN and exclusion of other causes are most consistent with drug-induced immune thrombocytopenia. Rechallenge with acetaminophen, the only other drug the patient was receiving concurrently, failed to induce thrombocytopenia. With IFN alpha, acute thrombocytopenia resolved within 48 h, whereas after IFN beta it was 5 wk before platelet counts returned to normal. PaIgG, however, remained elevated for up to 30 wk. Sensitive in vitro tests failed to confirm drug-dependent binding of IgG to the patient's platelets. It is suggested that this patient developed interferon-induced autoimmune thrombocytopenia.     

Rifampin-Induced Immune Thrombocytopenia

In this report, a case of rifampin-induced immune thrombocytopenia with the following characteristics is described: (a) thrombocytopenia follows intermittent drug administration; (b) onset occurs within hours of drug ingestion; (c) IgG antirifampin antibody binds in vitro to normal platelets only in the presence of rifampin; (d) thrombocytopenia resolves quickly in the absence of rifampin; (e) using immunofluorescence microscopy, IgG binding to normal platelets was seen with the patient's serum only in the presence of rifampin, and (f) using fluorescence spectrofluorometry, an absence of rifampin binding to normal platelets was demonstrated. Although the serological studies are not definitive, the mechanism of thrombocytopenia in the patient can best be explained by the formation of immune complexes composed of rifampin-antirifampin antibody binding to platelets causing their rapid clearance from the circulation.     

Loss of 111Indium as indicator of platelet injury

We previously demonstrated that platelets can be labeled with 111Inoxine with high labeling efficiency and that 111In is not liberated from labeled platelets during the platelet release reaction or prolonged in vitro storage. In view of these findings, we examined the potential usefulness of loss of 111In from labeled platelets as an indicator or platelet damage by comparing the loss of 111In with that of 51Cr and LDH (in some experiments also with platelet factor 3 availability) under different conditions of platelet injury. When washed human platelets labeled with either 51Cr-chromate or 111In-oxine were exposed to increasing concentrations of detergents (Triton X-100, lysolecithin), threshold, rate, and extent of loss of 111In, 51Cr and, LDH were similar. In contrast, when labeled platelets were depleted of metabolic energy by incubation in glucose-free Tyrode albumin solution or glucose-depleted plasma in the presence of antimycin A and 2-deoxy-D- glucose, loss of 51Cr (and PF3a) occurred earlier and progressed at a faster rate than that of 111In or LDH. Similar results were obtained when platelets were exposed to increasing concentrations of PlA1 antibody, causing complement-mediated immune injury. The findings indicate that with certain agents that cause rapid platelet disruption (lysis), different platelet constituents are lost at similar rates. However, under conditions of more subtle or slowly progressive platelet injury, small molecules such as adenine nucleotides (51Cr) may escape earlier and at faster rates than larger molecules such as LDH or 111In- binding platelet protein. Thus, neither 111In loss nor LDH loss appear to be suitable indicators for sublytic or prelytic platelet injury.     

Correlation of a quinidine-induced platelet-specific antibody with development of thrombocytopenia

The rapidity by which drug-dependent antiplatelet antibodies can develop is not known, since patients are only studied during or after the episode of thrombocytopenia. This report describes the development of quinidine-induced immune thrombocytopenia in a healthy volunteer during a drug study. The thrombocytopenia developed within two weeks of initiation of quinidine therapy. During the thrombocytopenic episode, but not before receiving the drug, the patient had an IgG antiplatelet antibody that bound to control platelets in the absence of the drug. This antibody was absent when the drug was discontinued and the platelet count rose. The patient's acute serum also induced the release of serotonin from control platelets, and the reaction was enhanced by quinidine. This indicates that drug-dependent antiplatelet antibodies can develop rapidly and supports the hypothesis that quinidine-induced thrombocytopenia is due to a quinidine-dependent platelet-specific IgG.     

Acute Complement-mediated Haemolysis in the Rabbit: Relationship between the Thrombocytopenia and Neutropenia and Complement Consumption

S ummary . There appears to be a direct relationship between complement consumption in the rabbit (demonstrated by a fall in CH₅₀ and plasma C3 concentration) and the temporary disappearance of platelets and neutrophils from the circulation during IgM cold antibody induced intravascular haemolysis. Studies with ⁵¹Cr-labelled platelets showed that although the platelet counts returned to initial values following the thrombocytopenia, 18–38% of the labelled platelets failed to return, suggesting platelet loss at the site of sequestration (mainly the lungs) and dilution of the recirculating platelets with unlabelled platelets from another pool. There is no evidence that these events trigger intravascular coagulation. Complement-depleted rabbits are totally refractory to further attempts to produce lysis, thrombocytopenia and neutropenia by cold antibody, though, even when complement activity in the plasma is allowed to recover, the lytic response to doses of cold antibody similar to those originally given, is reduced. The surviving circulating red cells are coated with C₃ following an acute lytic episode and the immune-adherence of neutrophils and platelets to fixed-C3 on red-cell membranes in the pulmonary circulation provides one possible mechanism for the thrombocytopenia and neutropenia.     

Evidence for the co-existence of immunocomplexes and platelet specific antibodies in HIV infection-related thrombocytopenia in narcotic drug addicts. Experimental results and immunopathogenetic discussion.

BACKGROUND. The development of symptomatic or asymptomatic thrombocytopenia is not uncommon in HIV seropositive drug addicts. METHODS. Platelet binding immunoglobulins, as immune complexes or as platelet specific antibodies, were studied in eleven patients. Whole sera anche serum factions obtained by PEG sedimentation and gel filtration were investigated using methods able to characterize the IgG and IgM immunoglobulin classes. Immune complexes-containing fractions were used in competition assays with some monoclonal antibodies with receptor specificities. RESULTS. Immune complexes able to bind both autologous and allogenic platelets were detected in all patients, while platelet specific IgG or IgM autoantibodies were detected in most, but not in all, patients. The results obtained with the various individual tests were sometimes discrepant, owing to the low avidity binding of platelet-reacting immunoglobulins. Anti-low-affinity FcRII monoclonal antibody moderately reduced the platelet binding of IgG containing immune-complex fractions. CONCLUSIONS. In thrombocytopenic, HIV seropositive drug addicts immunecomplexes and platelet specific antibodies usually coexist and possibly have different pathogenetic potential in the development of thrombocytopenia.     

Mechanisms of thrombocytopenia in varicella

51Cr-labeled platelet and 125I-labeled fibrinogen kinetic studies in four thrombocytopenic children with varicella infection revealed marked platelet destruction (platelet survival times 0.1, 0.4, 1.0, and 2.4 days) and relatively normal fibrinogen disappearance. The platelet count was directly related to the platelet survival time. Marrow megakaryocyte mass was increased twofold to fourfold. IgG or IgM antiplatelet antibody was present on autologous platelets in the three patients tested. These data suggest that thrombocytopenia in some patients with varicella is the consequence of immune mediated platelet destruction.     

Adhesion of blood platelets to human arterial subendothelium: role of factor VIII-Von Willebrand factor.

The adhesion of human platelets to the human arterial subendothelium was investigated using everted postmortem renal arteries after the first bifurcation and reconstituted blood containing 51Cr-labelled, aspirin-treated platelets in a perfusion chamber according to Baumgartner. The accumulation of 51Cr on the arterial segment was a reliable reflection of the number of platelets that adhered. The dependence of platelet adhesion on platelet number, perfusion time, haematocrit and perfusion rate were found to be similar to what was observed with rabbit aorta previously, notwithstanding the essentially different nature of the human subendothelium, with its layers of smooth muscle cells above the internal elastic lamina. The system was used to study the role of factor VIII-von Willebrand factor (VIII-VWF). VIII-VWF was found to be the only plasma factor enhancing adhesion of platelets fo the vessel wall. No suggestion of an inhibitory substance in plasma was found. Double perfusion experiments indicated that VIII-VWF binding to subendothelium supported subsequent platelet adhesion. A good correlation was found between the amount of VIII-VWF bound and platelet adhesion. Comparison of binding of albumin, fibrinogen, gamma-globulin and VIII-VWF showed that VIII-VWF bound less. This suggests that VIII-VWF does penetrate less easily in the subendothelium, which is in agreement with immunofluorescence studies of tissues. Studies with washed platelets not treated with aspirin showed parallelism between accumulation of VIII-VWF and platelet deposition in thrombi. These data may be explained by assuming a non-easily exchangeable pool of VIII-VWF on the platelet surface and increased binding of VIII-VWF to platelets in thrombi.     

Glycoprotein IIb/IIIa complex is the target in mirtazapine-induced immune thrombocytopenia

Mirtazapine (MW 265.36), a tetracyclic antidepressant of the piperazine-azapine group which augments central noradrenergic and serotonergic activity, is currently used as an oral antidepressant. We report a case of severe thrombocytopenia in a 66-year-old patient occurring after mirtazapine administration, suggesting an immune mechanism. This report documents the first case of mirtazapine-induced immune thrombocytopenia. The patient's serum was screened for drug-induced anti-platelet antibody with the chromium(51) (Cr(51)) platelet lysis technique. The drug-dependent antibody was characterized using flow cytometry, the monoclonal antibody immobilization of platelet antigens assay (MAIPA assay), and immunoprecipitation. By the Cr(51) platelet lysis technique, we obtained an equivocal result for the detection of mirtazapine-induced antibody. However, the patient's serum tested positive for mirtazapine-induced antibody by flow cytometry. The results showed that the binding ratio of 5.7 (mean fluorescence intensity) in the presence of the patient's serum and mirtazapine in a final concentration of 1.0 mmol/L was strongly positive. The antibody was found to bind the glycoprotein (GP) IIb/IIIa complex by MAIPA assay by using five different monoclonal antibodies against GP complexes Ib/IX, GPIIb/IIIa, or GPIa/IIa. Immunoprecipitation studies showed that the GPIIb/IIIa complex was precipitated by antibody in the presence, but not in the absence, of mirtazapine. These findings provide evidence that immune thrombocytopenia can be caused by sensitivity to the antidepressant mirtazapine. This is the first well-documented case of mirtazapine-induced immune thrombocytopenia.     

The effect of intravascular neutrophil chemotactic factors on blood neutrophil and platelet kinetics

Intravenous infusion of an analogue (f-met-leu-phe [FMLP]) of a bacterial-derived polymorphonuclear leukocyte (PMNL) chemotactic factor, or of the complement-derived chemotactic stimulus, zymosan-activated plasma (ZAP, containing C5ades Arg) into rabbits induces acute PMNL margination in the pulmonary vasculature. This process also occurs during hemodialysis and the adult respiratory distress syndrome. The pulmonary PMNL sequestration is accompanied by thrombocytopenia. Because of the role platelets and PMNLs play in hemostasis and defense against infection, we studied the fate of these blood elements following sequestration induced by chemotactic factors. By employing 111In-labelled platelets and external radioisotope scanning, platelets were found to sequester in the pulmonary vasculature during FMLP infusion. Simultaneous 51Cr PMNL and 111In-platelet studies showed that following sequestration, PMNLs returned to the circulation and disappeared with a normal half-life (T1/2) whereas the T1/2 of the platelets was markedly shortened (T1/2 of control = 49 +/- 3.0 hr; FMLP or ZAP infused T1/2 = 27 +/- 2.7 hr). Infusion of platelet-activating factor (PAF) induced PMN and platelet sequestration with similar abnormalities in platelet kinetics. Studies with 51Cr- and 14C-serotonin-labelled platelets showed that platelets did not release serotonin during FMLP, ZAP, or low dose PAF-induced sequestration. In contrast to platelet survival, platelet size, platelet aggregation responses, and platelet glycoproteins were not affected by transient sequestration. These results indicate that during PMNL margination induced by relatively "pure" PMNL stimuli such as FMLP, platelets may reversibly marginate and subsequently be cleared at an accelerated rate. The reason for accelerated platelet clearance is not a result of circulating platelet aggregates or detectable proteolytic modification of membrane glycoproteins. Such altered platelet kinetics may contribute to thrombocytopenia during sepsis, the adult respiratory distress syndrome, and other states in which excess PMNL margination occurs.     

Drug-induced thrombocytopenia is associated with increased binding of IgG to platelets both in vivo and in vitro

Thrombocytopenia is a common serious adverse effect of drug treatment. A variety of in vitro diagnostic techniques to confirm the diagnosis are available, but the majority lack sufficient sensitivity to detect all cases of drug-induced thrombocytopenia. We studied 19 patients with suspected drug-induced thrombocytopenia and demonstrated that platelet- associated IgG (PAIgG) was elevated in all at the time of thrombocytopenia, and PAIgG returned to normal levels as the thrombocytopenia resolved. In the majority of patients, the platelet count rapidly returned to normal after the drug was discontinued; however, in six patients, the thrombocytopenia persisted well beyond the period of time that the offending drug would be expected to be cleared from the blood. In 13 patients, serum obtained after recovery was used to identify the drug responsible for the thrombocytopenia in an in vitro assay. In all cases, the addition of the drug historically associated with the thrombocytopenic episode was associated with an increased binding of IgG to control platelets. For uncertain reasons, the concentration of drug required to increase the in vitro binding of IgG to test platelets was often more than the concentration usually achieved in vivo. Wider application of these techniques may provide better understanding of the clinical characteristics and mechanisms responsible for drug-induce thrombocytopenia.