IMMUNOREACTIONS INVOLVING PLATELETS

Pathophysiology and Diagnosis of Drug-Induced Immune Thrombocytopenia

Drug-induced immune thrombocytopenia (DITP) is a life-threatening clinical syndrome that is under-recognized and difficult to diagnose. Many drugs can cause immune-mediated thrombocytopenia, but the most commonly implicated are abciximab, carbamazepine, ceftriaxone, eptifibatide, heparin, ibuprofen, mirtazapine, oxaliplatin, penicillin, quinine, quinidine, rifampicin, suramin, tirofiban, trimethoprim-sulfamethoxazole, and vancomycin. Several different mechanisms have been identified in typical DITP, which is most commonly characterized by severe thrombocytopenia due to clearance and/or destruction of platelets sensitized by a drug-dependent antibody. Patients with typical DITP usually bleed when symptomatic, and biological confirmation of the diagnosis is often difficult because detection of drug-dependent antibodies (DDabs) in the patient’s serum or plasma is frequently not possible. This is in contrast to heparin-induced thrombocytopenia (HIT), which is a particular DITP caused in most cases by heparin-dependent antibodies specific for platelet factor 4, which can strongly activate platelets in vitro and in vivo, explaining why affected patients usually have thrombotic complications but do not bleed. In addition, laboratory tests are readily available to diagnose HIT, unlike the methods used to detect DDabs associated with other DITP that are mostly reserved for laboratories specialized in platelet immunology.

Entecavir-Associated Thrombocytopenia: A Case Report and Review of the Pathophysiology, Diagnosis, and Treatment of a Rare but Reversible Cause of Thrombocytopenia

Drug-associated thrombocytopenia is often unrecognized. We report a 76-year-old female with lymphoma who presented with easy bruising and oral bleeding. She had undergone screening for hepatitis B virus (HBV) prior to starting rituximab and was found to have hepatitis B core serum antibody (IgG anti-HBc). She was therefore treated with prophylactic entecavir 0.5 mg daily to prevent reactivation of HBV. Her initial platelet count was 136,000/mm³. Five days after starting entecavir, she presented with bruising and oral bleeding and was found to have a platelet count of 7,000/mm³. A coagulation profile and the rest of the blood parameters (RBC and WBC counts) were normal. Entecavir was stopped, and she was given 3 units of apheresed platelets followed by intravenous immunoglobulin (1 g/kg) for 5 consecutive days. Her platelet counts improved and normalized in one week. She was diagnosed with entecavir-induced thrombocytopenia based on the temporal relationship and after carefully excluding alternate causes of thrombocytopenia. This case highlights the importance of recognizing drug-induced thrombocytopenia (DITP) as a reversible cause of thrombocytopenia.

Mechanism of quinine-dependent monoclonal antibody binding to platelet glycoprotein IIb/IIIa

Drug-dependent antibodies (DDAbs) that cause acute thrombocytopenia upon drug exposure are nonreactive in the absence of the drug but bind tightly to a platelet membrane glycoprotein, usually α(IIb)/β3 integrin (GPIIb/IIIa) when the drug is present. How a drug promotes binding of antibody to its target is unknown and is difficult to study with human DDAbs, which are poly-specific and in limited supply. We addressed this question using quinine-dependent murine monoclonal antibodies (mAbs), which, in vitro and in vivo, closely mimic antibodies that cause thrombocytopenia in patients sensitive to quinine. Using surface plasmon resonance (SPR) analysis, we found that quinine binds with very high affinity (K(D) ≈ 10⁻⁹ mol/L) to these mAbs at a molar ratio of ≈ 2:1 but does not bind detectably to an irrelevant mAb. Also using SPR analysis, GPIIb/IIIa was found to bind monovalently to immobilized mAb with low affinity in the absence of quinine and with fivefold greater affinity (K(D) ≈ 2.2 × 10⁻⁶) when quinine was present. Measurements of quinine-dependent binding of intact mAb and fragment antigen-binding (Fab) fragments to platelets showed that affinity is increased 10 000- to 100 000-fold by bivalent interaction between antibody and its target. Together, the findings indicate that the first step in drug-dependent binding of a DDAb is the interaction of the drug with antibody, rather than with antigen, as has been widely thought, where it induces structural changes that enhance the affinity/specificity of antibody for its target epitope. Bivalent binding may be essential for a DDAb to cause thrombocytopenia.

Temporal aspects of heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia is a relatively common antibody-mediated drug reaction. We studied the temporal relation between previous or current heparin therapy and the onset of heparin-induced thrombocytopenia.

METHODS

We examined the time between the start of heparin therapy and the onset of thrombocytopenia in 243 patients with serologically confirmed heparin-induced thrombocytopenia. We also investigated the persistence of circulating heparin-dependent antibodies by performing a platelet serotonin-release assay and an assay for antibodies against platelet factor 4. The outcome in seven patients who had previously had an episode of heparin-induced thrombocytopenia and were later treated again with heparin was also examined.

RESULTS

A fall in the platelet count beginning four or more days after the start of heparin therapy occurred in 170 of the 243 patients (70 percent); in these patients, a history of previous heparin treatment did not influence the timing of the onset of thrombocytopenia. In the remaining 73 patients (30 percent), the onset of thrombocytopenia was rapid (median time of onset, 10.5 hours after the start of heparin administration); all these patients had been treated with heparin within the previous 100 days. During recovery from thrombocytopenia, heparin-dependent antibodies in the serum fell to undetectable levels at a median of 50 to 85 days, depending on the assay performed. In the seven patients who were given heparin again after the disappearance of heparin-dependent antibodies, a new episode of heparin-induced thrombocytopenia did not occur.

CONCLUSIONS

Heparin-induced thrombocytopenia can begin rapidly in patients who have received heparin within the previous 100 days. Heparin-dependent antibodies do not invariably reappear with subsequent heparin use.

Drug-induced thrombocytopenia in the coronary care unit

Drug-induced thrombocytopenia is a phenomenon that causes significant morbidity and mortality among patients. Practitioners should be able to recognize the clinical manifestations of drug-induced thrombocytopenia, differentiate it from other causes, and manage it appropriately. Numerous case reports have documented drug-induced causes of thrombocytopenia. The following article focuses on the characteristics and management of drug-induced thrombocytopenia secondary to medications commonly encountered in the coronary care unit. Pharmacotherapeutic agents that are most commonly implicated in this setting include ticlopidine, unfractioned heparin, glycoprotein (GP)IIb/IIIa inhibitors, H(2)-receptor antagonists, quinidine and antibiotics. Case reports were obtained through a comprehensive search of the Medicine database and subsequently complemented by bibliographic reviews of the agents just specified. Reports that exhibited possible, probable, and definite associations with drug-induced thrombocytopenia are included in the article.

Drug-Induced Thrombocytopenia: Is it a Serious Concern for Glycoprotein IIb/IIIa Receptor Inhibitors?

Over the past decade, several glycoprotein IIb/IIIa receptor antagonists have been developed and tested clinically as adjuncts to coronary intervention and/or treatment of acute coronary syndromes. Thrombocytopenia associated with this class of compounds has been described in most large studies to date and when it occurs in combination with bleeding represents a major safety concern. Cases of thrombocytopenia caused by GP IIb/IIIa antagonists vary in their clinical presentation according to time of onset (following the first dose or delayed), severity (profound, i.e., <20,000 cells/mm(3), or mild), and may or may not be associated with clinically important bleeding. More than one etiology appears responsible for thrombocytopenia associated with GP IIb/IIIa antagonists, including acute, idiosyncratic, as well as delayed immune-mediated mechanisms. Comparison of the incidence of thrombocytopenia across the different agents currently being studied and the one agent commercially available is complicated by varying definitions of thrombocytopenia used to date; different clinical settings in which GP IIb/IIIa antagonists have been studied; use of concomitant medications such as heparin, which itself may cause thrombocytopenia; relatively infrequent occurrence of thrombocytopenia; and the limited number of patients exposed to these agents. Review of the large studies presented and published to date suggests that thrombocytopenia due specifically to GP IIb/IIIa receptor antagonists occurs in less than 5% of treated patients and may vary depending on the type of agent, concomitant therapy, and clinical scenario. Current standard management includes immediate cessation of the GP IIb/IIIa antagonist and, in severe cases, platelet transfusions. In cases with associated hemorrhage, other anticoagulants and antiplatelet agents should be discontinued and possibly reversed. There may be a role for IV IgG and steroids, especially for cases of thrombocytopenia that are immune-mediated; however, further investigations are necessary.

Multiple oral petechiae and ecchymoses in a patient with osteoarthritis

Ibuprofen is a frequently used medication, and possible drug reactions should be familiar to the clinician. Because oral manifestations of thrombocytopenia are often the initial finding and possibly represent the only clinical evidence of this disease, the dentist should be able to recognize the significance of these lesions and refer the patient for appropriate evaluation and therapy.

The current status of antiplatelet antibodies

The importance of antiplatelet antibodies in clinical medicine was first recognized in 1951. Since that time, a number of syndromes have been described, including autoimmune thrombocytopenia purpura, posttransfusion purpura, neonatal alloimmune thrombocytopenia, and drug-induced thrombocytopenia purpura, that fit into the category of immunologic thrombocytopenias. The laboratory methods for detecting the antiplatelet antibodies present in these diseases are enumerated and discussed along with the currently recognized platelet-specific antigens. Because of the complexities of performance and the lack of agreement among many of the available procedures, it is recommended that antiplatelet antibody testing remain primarily a research tool for the present time.

Binding of quinine- and quinidine-dependent drug antibodies to platelets is mediated by the Fab domain of the immunoglobulin G and is not Fc dependent

The antibody domain controlling reactions between platelet membranes and drug-dependent (dd) antibodies from patients with thrombocytopenia induced by cinchona alkaloids was studied using F(ab')2, Fab, and Fc fragments made from purified dd-IgG. By direct binding radioimmunoassay (RIA) measurements, 20,000 to 50,000 antibody molecules bound per platelet equivalent of purified platelet membranes at apparent saturation with three different antibodies. F(ab')2 and Fab fragments bound to platelet membranes drug dependently but Fc fragments did not. The ability of dd-IgG fragments to compete with intact IgG was quantitatively measured by RIA and by complement fixation. F(ab')2 and Fab competed with intact IgG at an 8:1 and greater than 50:1 molar ratio, respectively, in RIA, and at a 1.6-3:1 and 44-75:1 ratio, respectively, by complement fixation assays. Fc did not compete with IgG in either assay. We conclude that the Fab domain supports attachment of dd antibody to the platelet surface.

Drug-antibody-platelet interaction in quinine- and quinidine-induced thrombocytopenia

Binding of quinine- and quinidine-dependent antibodies to platelets was studied using an electroimmunoassay to measure platelet-bound IgG. Antibodies from four patients with drug-induced thrombocytopenia differed significantly in their interaction with platelets: association constants for binding to platelets at high drug concentrations ranged from 0.29 to 2.6 x 10(7) M(-1), the maximum number of antibody molecules bound ranged from 36,000 to 161,000/platelet, the amount of drug necessary to achieve half-maximum binding of antibodies to platelets ranged from 2 to 60 muM, and only one of the antibodies cross-reacted with the stereoisomer of the drug to which the patient was sensitized. Binding of the antibodies to platelets was enhanced at the highest achievable molar ratio of drug:antibody, 10,000:1, rather than being inhibited, as would be expected in a conventional, hapten-dependent reaction. The drug-antibody-platelet reaction was unaffected by Factor VIII/von Willebrand protein, nonspecifically aggregated IgG, or heat-labile complement components. After pretreatment with tritiated quinine, platelets retained several hundred thousand molecules of drug each, but failed to bind detectable amounts of antibody. However, platelets treated simultaneously with quinine-dependent antibody and tritiated quinine retained significantly more drug after repeated washes than platelets treated with drug and normal serum. These findings support the proposition that in quinine- and quinidine-induced thrombocytopenia, drug and antibody combine first in the soluble phase to form a complex, which then binds with high affinity to a receptor on the platelet surface (innocent bystander reaction), and demonstrate that these antibodies are heterogeneous in respect to the amount of drug required to promote their binding to platelets, the number of platelet receptors they recognize, and their binding affinities.

Drug-induced immune thrombocytopenia

A variety of drugs and other agents have been reported to cause immune-mediated platelet destruction. The cardinal features of this syndrome are acute, often alarming, purpura, closely related to drug exposure, which remits in one to two weeks after discontinuation of all suspect drugs. Quinidine and quinine have been most commonly implicated but, recently, both heparin and heroin have been the subject of numerous reports. Platelets are removed rapidly from the circulation, apparently as a result of the attachment of drug-antibody immune complexes. In vitro documentation of platelet injury by these complexes has inspired the development of many in vitro tests of differing sophistication. While valuable in confirming the clinical suspicion, none is sufficiently sensitive to exclude the diagnosis.

Immunological effect of co-trimoxazole on platelets

Diminished survival of transfused platelets occurred in two patients given co-trimoxazole, and a third patient taking this drug developed thrombocytopenia. By means of an indirect immunofluorescence assay antibodies against donor platelets coated with co-trimoxazole were found in the sera in all cases. These antibodies were directed against the trimethoprim component of co-trimoxazole and not against sulphamethoxazole. Co-trimoxazole is a potent antimicrobial agent and is advocated for treatment and prophylaxis in leukaemia. Hence its adverse effect on platelets is of great importance.

Increased fluidity of human platelet membranes during complement-mediated immune platelet injury

Complement appears to be involved in the destruction of platelets in certain clinical disorders, such as quinidine purpura and post-transfusion purpura. In both disorders, the classical complement sequence is activated by antigen-antibody complexes. It has been suggested that the terminal components of the complement sequence insert into the hydrophobic core of cell surface membranes and that this process leads to cell lysis. Fluidity is a fundamental property of lipids within the membrane's hydrophobic core. To examine the interaction of complement with membranes, we investigated the effect of complement activation on the fluidity of human platelet membranes. Complement was fixed to platelets using a post-transfusion purpura antibody, and membrane lipid fluidity was assessed in terms of fluorescence anisotropy using two fluorescent probes, 1,6-diphenyl-1,3,5-hexatriene and 9-(12-anthroyl) stearic acid. Microviscosity, expressed in poise, was derived from the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene.Post-transfusion purpura antibody plus complement made platelet membranes more fluid as evidenced by a 21% decrease in anisotropy and a 35% decrease in microviscosity of platelets at 37 degrees C, and this was associated with platelet lysis ((51)Cr release). Complement damage to platelets was accompanied by a 10-15% increase in DeltaE, the fusion activation energy for microviscosity, indicating that complement not only decreased membrane microviscosity but also made membrane lipids less ordered. These changes were consistent and rapid, with platelet lysis and the reduction in microviscosity being half-maximal by 6 min. They were prevented by inactivation of complement with heat or with EDTA, and they were not observed when C5-deficient plasma was used as the complement source. Qualitatively similar changes in platelet membrane fluidity were observed when complement was fixed to platelets by a quinidine-dependent anti-platelet antibody rather than by post-transfusion purpura antibody. Post-transfusion purpura antibody plus complement also decreased the microviscosity of isolated platelet membranes. Moreover, the lipids extracted from platelets lysed by complement had a 22% decrease in microviscosity (P < 0.01), with no associated changes in the amount of cholesterol relative to phospholipid or in the amounts of the various phospholipids. These studies demonstrate that lipids within the hydrophobic core of platelet membranes damaged by complement become more fluid, and this is associated with platelet lysis. These findings are consistent with the concept that the insertion of the terminal complement components into the platelet membrane bilayer perturbs lipid-lipid interactions within the membrane's hydrophobic core.

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 diphenylhydantion, 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 deomonstrated 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.

A platelet and granulocyte membrane defect in paroxysmal nocturnal hemoglobinuria: usefulness for the detection of platelet antibodies

The tendency of platelets and leukocytes to lyse after their interaction with antibody and complement was studied by measuring the release of (51)Cr from cells labeled with this isotope. Platelets from six patients with paroxysmal nocturnal hemoglobinuria (PNH) were 15-230 times more sensitive to antibodies and 10-32 times more sensitive to complement than normal platelets or platelets from patients with other types of thrombocytopenic or hemolytic disorders. Mixed white blood cell (WBC) preparations from patients with PNH were 3-20 times more sensitive to anti-WBC antibodies and 5-10 times more sensitive to C' than were WBC preparations from normal subjects, but PNH lymphocytes showed normal immunologic reactivity. PNH platelets, like PNH erythrocytes, lysed more readily than normal platelets in acidified serum and in media of reduced ionic strength, but these characteristics were not demonstrable with PNH WBC's under the conditions of study. In PNH, platelets appear to comprise a single population with respect to their sensitivity to immune lysis, yet their survival time as measured with (51)Cr falls within normal limits. PNH granulocytes likewise appear to consist of a single, uniformly sensitive population. It is concluded that, in PNH, platelets and granulocytes share the membrane defect characteristic of erythrocytes in this disorder. These observations support the concept that PNH arises as the result of a somatic mutation in a primitive cell capable of differentiating into erythroblast, myeloblast, and megakaryoblast lines. PNH platelets or enzymatically treated normal platelets permit the detection of some types of platelet antibodies in dilutions up to 2000-fold greater than is possible with currently available methods, a finding suggesting that the immune lysis technique will prove useful for the study of platelet immunology.