A site involving the "hybrid" and PSI homology domains of GPIIIa (beta 3-integrin subunit) is a common target for antibodies associated with quinine-induced immune thrombocytopenia

Major Reservoir for Heparin-Releasable TFPIα (Tissue Factor Pathway Inhibitor α) Is Extracellular Matrix

[Figure: see text].

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Naturally occurring point mutation Cys460Trp located in the I-EGF1 domain of integrin β3 alters the binding of some anti-HPA-1a antibodies

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by the destruction of platelets in the fetus or newborn by maternal platelet alloantibodies, mostly against human platelet antigen (HPA)-1a. Recent studies indicate that two anti-HPA subtypes exist: Type I reacts with epitopes residing on the plexin-semaphorin-integrin (PSI) and type II with plexin-semaphorin-integrin/integrin epidermal growth factor 1 (I-EGF1) domains of the β3 integrin. Here, we evaluated whether a Cys460Trp mutation in the I-EGF1 domain found in a patient with Glanzmann thrombasthenia can alter the binding of anti-HPA-1a.

METHODS

Stable HEK293 cell lines expressing wild-type and mutant αIIbβ3 and αvβ3 were generated to prove the reactivity of different antibodies against HPA-1a.

RESULTS

Flow cytometry analysis of wild-type (Cys460) and mutant (Trp460) expressed on HEK293 cells showed equal surface expression of αIIbβ3 and αvβ3. When tested with mutant αIIbβ3 cells, reduced binding was observed in Type II but not in Type I anti-HPA-1a. These results could be confirmed with platelets carrying Cys460Trp mutation. Interestingly, reduced binding of Type I antibodies was detected with mutant αvβ3 cells. Both antibody types were found in maternal sera from FNAIT cases by an antigen-capture assay with use of HEK293 transfected cells.

CONCLUSIONS

These observations confirm the existence of Type I and Type II anti-HPA-1a. Furthermore, this study underlines different immunogenicity of HPA-1a antigen(s) residing on either αIIbβ3 or αvβ3. Further analysis of FNAIT cases from mothers having a fetus with and without intracranial bleedings with use of such an approach may highlight the functional relevance of different anti-HPA-1a subtypes.

Drug-induced immune thrombocytopenia: incidence, clinical features, laboratory testing, and pathogenic mechanisms

Drug-induced immune thrombocytopenia (DIIT) is a relatively uncommon adverse reaction caused by drug-dependent antibodies (DDAbs) that react with platelet membrane glycoproteins only when the implicated drug is present. Although more than 100 drugs have been associated with causing DIIT, recent reviews of available data show that carbamazepine, eptifibatide, ibuprofen, quinidine, quinine, oxaliplatin, rifampin, sulfamethoxazole, trimethoprim, and vancomycin are probably the most frequently implicated. Patients with DIIT typically present with petechiae, bruising, and epistaxis caused by an acute, severe drop in platelet count (often to <20,000 platelets/pL). Diagnosis of DIIT is complicated by its similarity to other non-drug-induced immune thrombocytopenias, including autoimmune thrombocytopenia, posttransfusion purpura, and platelet transfusion refractoriness, and must be differentiated by temporal association of exposure to a candidate drug with an acute, severe drop in platelet count. Treatment consists of immediate withdrawal of the implicated drug. Criteria for strong evidence of DIIT include (1) exposure to candidate drug-preceded thrombocytopenia; (2) sustained normal platelet levels after discontinuing candidate drug; (3) candidate drug was only drug used before onset of thrombocytopenia or other drugs were continued or reintroduced after resolution of thrombocytopenia, and other causes for thrombocytopenia were excluded; and (4) reexposure to the candidate drug resulted in recurrent thrombocytopenia. Flow cytometry testing for DDAbs can be useful in confirmation of a clinical diagnosis, and monoclonal antibody enzyme-linked immunosorbent assay testing can be used to determine the platelet glycoprotein target(s), usually GPIIb/IIIa or GPIb/IX/V, but testing is not widely available. Several pathogenic mechanisms for DIIT have been proposed, including hapten, autoantibody, neoepitope, drug-specific, and quinine-type drug mechanisms. A recent proposal suggests weakly reactive platelet autoantibodies that develop greatly increased affinity for platelet glycoprotein epitopes through bridging interactions facilitated by the drug is a possible mechanism for the formation and reactivity of quinine- type drug antibodies.

Antibodies causing thrombocytopenia in patients treated with RGD-mimetic platelet inhibitors recognize ligand-specific conformers of αIIb/β3 integrin

Arginine-glycine-aspartic acid (RGD)-mimetic platelet inhibitors act by occupying the RGD recognition site of α(IIb)/β(3) integrin (GPIIb/IIIa), thereby preventing the activated integrin from reacting with fibrinogen. Thrombocytopenia is a well-known side effect of treatment with this class of drugs and is caused by Abs, often naturally occurring, that recognize α(IIb)/β(3) in a complex with the drug being administered. RGD peptide and RGD-mimetic drugs are known to induce epitopes (ligand-induced binding sites [LIBS]) in α(IIb)/β(3) that are recognized by certain mAbs. It has been speculated, but not shown experimentally, that Abs from patients who develop thrombocytopenia when treated with an RGD-mimetic inhibitor similarly recognize LIBS determinants. We addressed this question by comparing the reactions of patient Abs and LIBS-specific mAbs against α(IIb)/β(3) in a complex with RGD and RGD-mimetic drugs, and by examining the ability of selected non-LIBS mAbs to block binding of patient Abs to the liganded integrin. Findings made provide evidence that the patient Abs recognize subtle, drug-induced structural changes in the integrin head region that are clustered about the RGD recognition site. The target epitopes differ from classic LIBS determinants, however, both in their location and by virtue of being largely drug-specific.

Overview: structural biology of integrins

Integrins are cell adhesion molecules that play important roles in many biological processes including hemostasis, immune responses, development, and cancer. Their adhesiveness is dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces outside-in signals from the extracellular domain to the cytoplasm. Advances in the past several years have shed light on structural basis for integrin regulation and signaling, especially how the large-scale reorientations of the ectodomain are related to the inter-domain and intra-domain shape shifting that changes ligand-binding affinity. Experiments have also shown how the conformational changes of the ectodomain are linked to changes in the α- and β-subunit transmembrane and cytoplasmic domains.

Infections, antigen-presenting cells, T cells, and immune tolerance: their role in the pathogenesis of immune thrombocytopenia

In the last 20 years, many publications have shed new light on the complex immunopathogenesis of immune thrombocytopenic purpura. They are associated with 3 interrelated areas of environmental autoimmunity, for example, infectious influences, antigen-presenting cell (APC) function, and T-cell abnormalities, particularly tolerance induction. This article highlights the recent literature and argues that infectious agents and platelets can significantly modulate APCs, which create an environment that dysregulates autoreactive T cells, leading to the production of autoantibodies.

Fine specificity of drug-dependent antibodies reactive with a restricted domain of platelet GPIIIA

Drug-induced immune thrombocytopenia is caused by drug-dependent antibodies (DDAbs) that bind tightly to platelet glycoproteins only when drug is present. How drugs mediate this interaction is not yet resolved. Several studies indicate that sites recognized by DDAbs tend to cluster in specific structural domains, suggesting they may recognize a limited number of distinct epitopes. To address this issue, we characterized the binding sites for 16 quinine-dependent antibodies thought on the basis of preliminary studies to be possibly specific for a single epitope on glycoprotein IIIa (GPIIIa). Fourteen of the antibodies reacted with a 29-kDa GPIIIa fragment comprising only the GPIIIa hybrid and plextrin-semaphorin-integrin homology domains. However, studies with mutant GPIIIa and the blocking monoclonal antibody AP3 showed that the 14 DDAbs recognize at least 6 and possibly more distinct, but overlapping, structures involving GPIIIa residues 50 to 66. The findings suggest that even antibodies specific for restricted domains on a target glycoprotein may each have a slightly different fine specificity; ie, "unique" epitopes recognized by DDAbs may be rare or nonexistent. The observations are consistent with a recently proposed model in which drug reacts noncovalently with both target protein and antibody to promote binding of an otherwise nonreactive immunoglobulin.

Drug-induced thrombocytopenia

Drug-induced thrombocytopenia (DIT) is a relatively common clinical disorder. It is imperative to provide rapid identification and removal of the offending agent before clinically significant bleeding or, in the case of heparin, thrombosis occurs. DIT can be distinguished from idiopathic thrombocytopenic purpura, a bleeding disorder caused by thrombocytopenia not associated with a systemic disease, based on the history of drug ingestion or injection and laboratory findings. DIT disorders can be a consequence of decreased platelet production (bone marrow suppression) or accelerated platelet destruction (especially immune-mediated destruction).

Structural basis of integrin regulation and signaling

Integrins are cell adhesion molecules that mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. They play critical roles for the immune system in leukocyte trafficking and migration, immunological synapse formation, costimulation, and phagocytosis. Integrin adhesiveness can be dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces signals from the extracellular domain to the cytoplasm in the classical outside-in direction. Recent structural, biochemical, and biophysical studies have greatly advanced our understanding of the mechanisms of integrin bidirectional signaling across the plasma membrane. Large-scale reorientations of the ectodomain of up to 200 A couple to conformational change in ligand-binding sites and are linked to changes in alpha and beta subunit transmembrane domain association. In this review, we focus on integrin structure as it relates to affinity modulation, ligand binding, outside-in signaling, and cell surface distribution dynamics.

Tests of the extension and deadbolt models of integrin activation

Despite extensive evidence that integrin conformational changes between bent and extended conformations regulate affinity for ligands, an alternative hypothesis has been proposed in which a "deadbolt" can regulate affinity for ligand in the absence of extension. Here, we tested both the deadbolt and the extension models. According to the deadbolt model, a hairpin loop in the beta3 tail domain could act as a deadbolt to restrain the displacement of the beta3 I domain beta6-alpha7 loop and maintain integrin in the low affinity state. We found that mutating or deleting the beta3 tail domain loop has no effect on ligand binding by either alphaIIbbeta 3 or alphaVbeta3 integrins. In contrast, we found that mutations that lock integrins in the bent conformation with disulfide bonds resist inside-out activation induced by cytoplasmic domain mutation. Furthermore, we demonstrated that extension is required for accessibility to fibronectin but not smaller fragments. The data demonstrate that integrin extension is required for ligand binding during integrin inside-out signaling and that the deadbolt does not regulate integrin activation.

Mapping early conformational changes in alphaIIb and beta3 during biogenesis reveals a potential mechanism for alphaIIbbeta3 adopting its bent conformation

Current evidence supports a model in which the low-affinity state of the platelet integrin alphaIIbbeta3 results from alphaIIbbeta3 adopting a bent conformation. To assess alphaIIbbeta3 biogenesis and how alphaIIbbeta3 initially adopts the bent conformation, we mapped the conformational states occupied by alphaIIb and beta3 during biogenesis using conformation-specific monoclonal antibodies (mAbs). We found that alphaIIbbeta3 complex formation was not limited by the availability of either free pro-alphaIIb or free beta3, suggesting that other molecules, perhaps chaperones, control complex formation. Five beta3-specific, ligand-induced binding site (LIBS) mAbs reacted with much or all free beta3 but not with beta3 when in complex with mature alphaIIb, suggesting that beta3 adopts its mature conformation only after complex formation. Conversely, 2 alphaIIb-specific LIBS mAbs directed against the alphaIIb Calf-2 region adjacent to the membrane reacted with only minor fractions of free pro-alphaIIb, raising the possibility that pro-alphaIIb adopts a bent conformation early in biogenesis. Our data suggest a working model in which pro-alphaIIb adopts a bent conformation soon after synthesis, and then beta3 assumes its bent conformation by virtue of its interaction with the bent pro-alphaIIb.

Patients with quinine-induced immune thrombocytopenia have both "drug-dependent" and "drug-specific" antibodies

Immune thrombocytopenia induced by quinine and many other drugs is caused by antibodies that bind to platelet membrane glycoproteins (GPs) only when the sensitizing drug is present in soluble form. In this disorder, drug promotes antibody binding to its target without linking covalently to either of the reacting macro-molecules by a mechanism that has not yet been defined. How drug provides the stimulus for production of such antibodies is also unknown. We studied 7 patients who experienced severe thrombocytopenia after ingestion of quinine. As expected, drug-dependent, platelet-reactive antibodies specific for GPIIb/IIIa or GPIb/IX were identified in each case. Unexpectedly, each of 6 patients with GPIIb/IIIa-specific antibodies was found to have a second antibody specific for drug alone that was not platelet reactive. Despite recognizing different targets, the 2 types of antibody were identical in requiring quinine or desmethoxy-quinine (cinchonidine) for reactivity and in failing to react with other structural analogues of quinine. On the basis of these findings and previous observations, a model is proposed to explain drug-dependent binding of antibodies to cellular targets. In addition to having implications for pathogenesis, drug-specific antibodies may provide a surrogate measure of drug sensitivity in patients with drug-induced immune cytopenia.

Maternal alloimmunization against the rare platelet-specific antigen HPA-9b (Max a) is an important cause of neonatal alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NATP) is an important cause of morbidity and mortality in the newborn. Optimal management of subsequent pregnancies requires knowledge of the alloantigen that caused maternal immunization, but this is possible only in a minority of cases. This study investigated whether this can be explained in part by maternal immunization against the "rare" alloantigen HPA-9b (Max(a)), implicated previously only in a single NATP case.

STUDY DESIGN AND METHODS

Archived paternal DNA from unresolved cases of NATP and normal individuals was typed for platelet (PLT)-specific antigens with real-time polymerase chain reaction and direct sequencing. PLT-specific alloantibodies were characterized by flow cytometry and solid-phase enzyme-linked immunosorbent assay. Recombinant GPIIb/IIIa was expressed in stably transfected Chinese hamster ovary cells. Clinical information was obtained directly from attending physicians.

RESULTS

Six of 217 fathers were positive for the presence of HPA-9b (Max(a)), an incidence about seven times that in the general population. In each of five cases studied, maternal serum samples reacted with intact paternal PLTs and paternal GPIIb/IIIa. Only one of three serum samples tested recognized recombinant GPIIb/IIIa carrying the HPA-9b (Max(a)) mutation. These seemingly discrepant reactions may reflect different requirements for oligosaccharides linked to residues close to the mutation in GPIIb that determines HPA-9b (Max(a)). NATP in the affected children was severe and was associated with intracranial hemorrhage in three of six infants on whom information was obtained.

CONCLUSIONS

Maternal immunization against HPA-9b (Max(a)) is an important cause of NATP and should be considered in cases of apparent NATP not resolved on the basis of maternal-fetal incompatibility for "common" PLT antigens.

A novel adaptation of the integrin PSI domain revealed from its crystal structure

Integrin beta-subunits contain an N-terminal PSI (for plexin-semaphorin-integrin) domain that contributes to integrin activation and harbors the PI(A) alloantigen associated with immune thrombocytopenias and susceptibility to sudden cardiac death. Here we report the crystal structure of PSI in the context of the crystallized alphaVbeta3 ectodomain. The integrin PSI forms a two-stranded antiparallel beta-sheet flanked by two short helices; its long interstrand loop houses Pl(A) and may face the EGF2 domain. The integrin PSI contains four cysteine pairs connected in a 1-4, 2-8, 3-6, 5-7 pattern. An unexpected feature of the structure is that the final, eighth cysteine is located C-terminal to the Ig-like hybrid domain and is thus separated by the hybrid domain from the other seven cysteines of PSI. This architecture may be relevant to the evolution of integrins and should help refine the current models of integrin activation.

Ceftriaxone causes drug-induced immune thrombocytopenia and hemolytic anemia: characterization of targets on platelets and red blood cells

BACKGROUND

Ceftriaxone, a third-generation cephalosporin, has been reported to occasionally cause fatal drug-induced immune hemolytic anemia (DIHA). A clinical and serologic analysis of the first two patients with severe drug-induced thrombocytopenia (DITP) due to ceftriaxone and one patient with fatal DIHA is reported.

STUDY DESIGN AND METHODS

Sera were assessed by the IAT, EIA, glycoprotein (GP)-specific immunoassay, flow cytometry, and immunoprecipitation using transfectants expressing GPIIb/IIIa and GPIb/IX and with different cephalosporins.

RESULTS

Sera from Patients 1 and 2 reacted strongly with PLTs in the presence of the drug, but not with RBCs. The binding sites of the drug-dependent antibodies (DDAbs) could be localized to GPIIb/IIIa and GPIb/IX, respectively. Inhibition studies indicated that DDAbs recognized epitopes residing on the GPIIb/IIIa complex and on the GPIX subunit, respectively. No cross-reactivity was observed with other cephalosporin derivatives. Serum 3 showed strong agglutination with RBCs of Rh(null) phenotype in the presence of ex-vivo metabolites of ceftriaxone, but no cross-reactivity with PLTs.

CONCLUSIONS

The first two cases of severe DITP and a third patient with DIHA are reported. DDAbs from all patients showed individual reaction patterns and clear cell lineage specificity. In addition, the DDAbs were dependent on the substitution at position 3 of the ceftriaxone molecule. Epitopes on GPIIb/IIIa and GPIX were involved on PLTs. The Rh protein was not the only target of DDAbs on RBCs.

Integrins: dynamic scaffolds for adhesion and signaling in platelets

The major platelet integrin, alphaIIbbeta3, is required for platelet interactions with proteins in plasma and the extracellular matrices (ECMs) that are essential for platelet adhesion and aggregation during hemo stasis and arterial thrombosis. Lig and binding to alphaIIbbeta3 is controlled by inside-out signals that modulate receptor conformation and clustering. In turn, ligand binding triggers outside-in signals through alphaIIbbeta3 that, when disrupted, can cause a bleeding diathesis. In the past 5 years there has been an explosion of knowledge about the structure and function ofalphaIIbbeta3 and the related integrin, alphaVbeta3. These developments are discussed here, and current models of bidirectional alphaIIbbeta3 signaling are presented as frameworks for future investigations. An understanding that alphaIIbbeta3 functions as a dynamic molecular scaffold for extracellular and intracellular proteins has translated into diagnostic and therapeutic insights relevant to hematology and cardiovascular medicine, and further advances can be anticipated.