Thrombotic thrombocytopenic purpura and autoimmunity: a tale of shadows and suspects. (1/157)

BACKGROUND AND OBJECTIVE: The key pathogenic feature of TTP is the formation of platelet aggregates within the microcirculation; however, the etiology of such aggregates has been elusive for years. A large amount of evidence points to an abnormal interaction between damaged vascular endothelium and platelets, although the cause of the primary microvascular endothelial cell injury is seldom clear. The autoimmune hypothesis often recurs, and this is based on a number of observations: the claimed superiority of plasma-exchange over plasma infusion, the anecdotal report of the presence of immunocomplexes and autoantibodies in TTP patients, the efficacy of the administration of corticosteroids and other immunosuppressant agents, and the concomitant occurrence of TTP in association with autoimmune diseases, especially systemic lupus erythematosus (SLE). This review will focus on the complex relationships between TTP and humoral autoimmunity; in particular, similarities and differences between TTP, SLE and antiphospholipid (aPL) antibodies syndrome, as well as the putative role of several other antibodies directed towards endothelial cells and/or platelets, including the recently discovered anti-CD36 antibodies and antivWF-cleaving metalloprotease, will be discussed. DESIGN AND METHODS: The authors have been involved in the study and treatment of TTP and autoimmune diseases for years; furthermore, the PubMed data base of the National Library of Congress has been extensively searched using the Internet. CONCLUSIONS: Although over the years evidence has increased in favor of the autoimmune hypothesis for TTP etiopathogenesis, TTP should not yet be considered an autoimmune disease. Autoantibodies should be regarded as only one of the many different insults which can trigger microvascular thrombosis even though the autoimmune theory of the pathogenesis of TTP is gaining more and more strength. As far as concerns the relationship between TTP, SLE and aPL antibodies-related disorders, these diseases should be distinguished on the basis of both different clinical presentations and accurate antibody screening, although this approach should definitely not delay the prompt start of treatment.  (+info)

Increased platelet aggregability associated with platelet GPIIIa PlA2 polymorphism: the Framingham Offspring Study. (2/157)

The platelet glycoprotein IIb/IIIa (GP IIb/IIIa) plays a pivotal role in platelet aggregation. Recent data suggest that the PlA2 polymorphism of GPIIIa may be associated with an increased risk for cardiovascular disease. However, it is unknown if there is any association between this polymorphism and platelet reactivity. We determined GP IIIa genotype and platelet reactivity phenotype data in 1422 subjects from the Framingham Offspring Study. Genotyping was performed using PCR-based restriction fragment length polymorphism analysis. Platelet aggregability was evaluated by the Born method. The threshold concentrations of epinephrine and ADP were determined. Allele frequencies of PlA1 and PlA2 were 0.84 and 0.16, respectively. The presence of 1 or 2 PlA2 alleles was associated with increased platelet aggregability as indicated by incrementally lower threshold concentrations for epinephrine and ADP. For epinephrine, the mean concentrations were 0.9 micromol/L (0.9 to 1.0) for homozygous PlA1, 0.7 mmol/L (0.7 to 0.9) for the heterozygous PlA1/PlA2, and 0.6 micromol/L (0.4 to 1.0) for homozygous PlA2 individuals, P=0.009. The increase in aggregability induced by epinephrine remained highly significant (P=0.007) after adjustment for covariates. For ADP-induced aggregation, the respective mean concentrations were 3.1 micromol/L (3.0 to 3.2), 3.0 micromol/L (2.9 to 3.2), and 2.8 micromol/L (2.4 to 3.3); P=0.19 after adjustment for covariates. Our findings indicate that molecular variants of the gene encoding GP IIIa play a role in platelet reactivity in vitro. Our observations are compatible with and provide an explanation for the reported association of the PlA2 allotype with increased risk for cardiovascular disease.  (+info)

Construction of a human platelet alloantigen-1a epitope(s) within murine glycoprotein IIIa: identification of residues critical to the conformation of the antibody binding site(s). (3/157)

The human platelet alloantigen 1 system (HPA-1) is determined by a polymorphism at position 33 in the N-terminus of human glycoprotein IIIa (GPIIIa). This naturally occurring substitution creates a conformation in the HPA-1a allelic form that can be antigenic when presented to an individual expressing the HPA-1b form. Anti-HPA-1a antibodies generated by this immune response can lead to the destruction of platelets, as seen in the clinical disorders, neonatal alloimmune thrombocytopenia (NAIT) and posttransfusion purpura (PTP). To understand better the structural requirements for recognition by these pathogenic antibodies, we investigated the N-terminal 66 amino acids from the HPA-1a form of human GPIIIa and the analogous amino acids from the nonimmunogenic murine homolog. Our objectives were to define further the boundaries of the HPA-1a epitope(s) in the N-terminus of human GPIIIa, to isolate the murine 5' nucleotide sequence and compare the deduced murine N-terminal sequence to that of human, and to mutate the murine sequence systematically to include an HPA-1a epitope(s). Murine amino acids that differed from human were changed by site-directed mutagenesis to the analogous residues in the HPA-1a form of human GPIIIa, starting and radiating from murine position 33 (site of human polymorphism). This systematic approach allowed us to pinpoint amino acids critical to a conformation recognized by anti-HPA-1a antibodies. Our results show that an HPA-1a epitope can be created within the N-terminus of murine GPIIIa and raise the possibility that murine models of HPA-1a sensitization can be developed.  (+info)

Perinatal management of fetal hemolytic disease due to Rh incompatibility combined with fetal alloimmune thrombocytopenia due to HPA-5b incompatibility. (4/157)

We report out experience in the perinatal management of a complex case of fetal hemolytic disease primarily due to Rhesus incompatibility combined with fetal alloimmune thrombocytopenia. The lowest fetal hemoglobin and platelet levels were 2.6 g/dl and 13,000/microliter, respectively. Intrauterine treatment consisted of six transfusions of packed red cells into the umbilical vein and one transfusion of platelets. The neonate required four transfusions of packed red cells to correct her hyporegenerative erythropoiesis. Postnatal management also included one platelet transfusion, intravenous immunoglobulins and erythropoietin. Although some degree of fetal thrombocytopenia may invariably be found in fetal red cell incompatibility, other rare causes need to be excluded.  (+info)

Association of the platelet glycoprotein IIb HPA-3 polymorphism with survival after acute ischemic stroke. (5/157)

BACKGROUND AND PURPOSE: The role of polymorphisms of the platelet glycoprotein (GP) IIb/IIIa receptor in the development of cardiovascular disease has been the subject of intensive research. The aim of this study was to determine the association of the HPA-3 polymorphism of platelet GPIIb with ischemic stroke and subsequent survival and to identify possible interactions of HPA-3 with classic risk factors. METHODS: HPA-3 genotype was determined by restriction fragment length polymorphism in 515 patients with ischemic stroke and 423 healthy, age-matched control subjects. RESULTS: There was no significant difference in the genotype distribution of patients and controls, nor was there any difference when patients were subclassified into small- and large-vessel disease. The genotype distribution of the 231 patients subsequently dying during 2.8 years of follow-up (aa=45.0%, ab=46.8%, bb=8.2%) was significantly different from that of those still alive (aa=37.0%, ab=48.2%, bb=14. 8%) (P=0.03). In a Cox regression model, the relative risks for poststroke mortality in patients of aa and ab genotype compared with those of bb genotype were 2.42 (95% CI, 1.24 to 4.71) and 2.13 (95% CI, 1.09 to 4.17), respectively, after we accounted for confounding factors. In addition, significant interactions of HPA-3 with the Pl(A) polymorphism of GPIIIa (P=0.002) and with fibrinogen (P=0.01) were identified in relation to mortality. CONCLUSIONS: HPA-3 is related to poststroke mortality, and the significant interaction of HPA-3 with Pl(A) and fibrinogen suggests that it may in some way influence the interaction of GPIIb/IIIa with fibrinogen, particularly in the presence of high fibrinogen.  (+info)

A point mutation Thr(799)Met on the alpha(2) integrin leads to the formation of new human platelet alloantigen Sit(a) and affects collagen-induced aggregation. (6/157)

A new platelet-specific alloantigen, termed Sit(a), was identified in a severe case of neonatal alloimmune thrombocytopenia. The Sit(a) alloantigen is of low frequency (1/400) in the German population. Immunochemical studies demonstrated that the Sit(a) epitopes reside on platelet glycoprotein (GP) Ia. Nucleotide sequence analysis of GPIa cDNA derived from Sit(a)-positive platelets showed C(2531)-->T(2531) point mutation, resulting in Thr(799)Met dimorphism. Analysis of genomic DNA from 22 Sit(a)-negative normal individuals showed that the Thr(799) is encoded by ACG(2532) (90.9%) or ACA(2532) (9.1%). To establish a DNA typing technique, we elucidated the organization of the GPIa gene adjacent to the polymorphic bases. The introns (421 bp and 1.2 kb) encompass a 142-bp exon with the 2 polymorphic bases 2531 and 2532. Polymerase chain reaction-restriction fragment length polymorphism analysis on DNA derived from 100 donors using the restriction enzyme Mae III showed that the Met(799) form of GPIa is restricted to Sit(a) (+) phenotype. Analysis of stable Chinese hamster ovary transfectants expressing allele-specific recombinant forms of GPIa showed that anti-Sit(a) exclusively reacted with the Glu(505)Met(799), but not with the Glu(505)Thr(799) and the Lys(505)Thr(799) isoforms. In contrast, anti-Br(a) (HPA-5b) only recognized the Lys(505)Thr(799) form, whereas anti-Br(b) (HPA-5a) reacted with both Glu(505)Thr(799) and Glu(505)Met(799) isoforms. These results demonstrated that the Met(799) is responsible for formation of the Sit(a) alloantigenic determinants, whereas amino acid 505 (Lys or Glu) specifically controls the expression of Br(a) and Br(b) epitopes, respectively. Platelet aggregation responses of Sit(a) (+) individuals were diminished in response to collagen, indicating that the Thr(799)Met mutation affects the function of the GPIa/IIa complex.  (+info)

Threonine-145/methionine-145 variants of baculovirus produced recombinant ligand binding domain of GPIbalpha express HPA-2 epitopes and show equal binding of von Willebrand factor. (7/157)

Glycoprotein (GP) Ibalpha is the functionally dominant subunit of the platelet GPIb-IX-V receptor complex, with the von Willebrand factor (vWF) binding site residing on the amino-terminus. A threonine for methionine-145 replacement of GPIbalpha is associated with the human platelet antigen (HPA)-2 system. To study the structural and functional consequences of this mutation, both forms of GPIbalpha were expressed as calmodulin fusion proteins in insect cells. Both recombinant proteins were recognized by their respective alloantibodies, independent of glycosylation or intactness of disulfide bonds, and gave similar results to platelet-derived GPIbalpha in antibody detection assays. Resonant mirror studies showed that vWF binding was not affected by the HPA-2 mutation; however, vWF binding was partially inhibited by IgG HPA-2 antibodies. Our data are compatible with an involvement of the leucine-rich repeat domain of GPIbalpha in vWF binding and indicate that recombinant GPIbalpha may be used to detect HPA-2 antibodies. (Blood. 2000;95:205-211)  (+info)

Requirement of leucine-rich repeats of glycoprotein (GP) Ibalpha for shear-dependent and static binding of von Willebrand factor to the platelet membrane GP Ib-IX-V complex. (8/157)

The platelet glycoprotein (GP) Ib-IX-V complex mediates adhesion to von Willebrand factor (vWf) in (patho)physiologic thrombus formation. The vWf-binding site on GP Ib-IX-V is within the N-terminal 282 residues of GP Ibalpha, which consist of an N-terminal flanking sequence (His-1-Ile-35), 7 leucine-rich repeats (Leu-36-Ala-200), a C-terminal flank (Phe-201-Gly-268), and a sulfated tyrosine sequence (Asp-269-Glu-282). We have used mammalian cell expression of canine-human chimeras of GP Ibalpha, corresponding to precise structural boundaries, to demonstrate the first specific requirement for individual leucine-rich repeats for binding of vWf either induced by a modulator, ristocetin, or under hydrodynamic flow. Implicit in this approach was that the GP Ibalpha chimeras retained a functional conformation, a supposition confirmed by analyzing restoration of function to reversed human-canine chimeras and demonstrating that all chimeras bound vWf activated by botrocetin, a modulator that is indiscriminate between species. Leucine-rich repeats 2, 3, and 4 of GP Ibalpha were identified as being critical for vWf adhesion to GP Ib-IX-V.  (+info)

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