Varicella-associated thrombocytopenia: autoantibodies against platelet surface glycoprotein V

Glycoprotein V : the unsolved GPV puzzle

Glycoprotein V (GPV) is a highly expressed 82 KDa platelet surface transmembrane protein which is loosely attached to the GPIb-IX complex. Despite remaining questions concerning its function, GPV presents several unique features which have repercussions in hematology, atherothrombosis, immunology and transfusion. GPV is specifically expressed in platelets and megakaryocytes and is an ideal marker and reporter gene for the late stages of megakaryopoiesis. The ectodomain of GPV can be released by a number of proteases, namely thrombin, elastase and ADAM10 and 17. Although it was originally proposed as a thrombin receptor, this hypothesis was abandoned since thrombin activation was preserved after blockade of GPV cleavage and in Gp5 knockout mice. The combined potential of GPV to reflect the direct action of thrombin, platelet exposure to strong agonists and inflammatory conditions has led one to evaluate its utility as a marker in the context of atherothrombosis. Increased plasma levels of soluble GPV have notably been recorded in myocardial infarction, stroke and venous thromboembolism. It is also highly valued in transfusion to monitor platelet storage lesions. GPV presents several polymorphisms, which are a possible source of alloantibodies, while autoantibodies have been frequently detected in immune thrombocytopenia. The real biological function of this glycoprotein nevertheless remains an enigma, despite the respectively decreased and increased responses to low concentrations of collagen and thrombin observed in Gp5 knockout mice. Current studies are exploring its role in modulating general or VWF-induced platelet signaling, which could bear relevance in thrombosis and platelet clearance.

Diagnosis and management of coronavirus disease-associated immune thrombocytopenia: a case series

Data on health problems and fatal complications associated with coronavirus disease (COVID-19) have consistently been reported. Although immune thrombocytopenia has been associated with multiple viral infections, only few studies have shown its association with COVID-19. Here, we have reported a case series of two cases pertaining to patients diagnosed with COVID-19-associated immune thrombocytopenia, elaborating on the clinical course, management, and response to treatment.

Integrative Genomic Analysis Reveals Four Protein Biomarkers for Platelet Traits

RATIONALE

Mean platelet volume (MPV) and platelet count (PLT) are platelet measures that have been linked to cardiovascular disease (CVD) and mortality risk. Identifying protein biomarkers for these measures may yield insights into CVD mechanisms.

OBJECTIVE

We aimed to identify causal protein biomarkers for MPV and PLT among 71 CVD-related plasma proteins measured in FHS (Framingham Heart Study) participants.

METHODS AND RESULTS

We conducted integrative analyses of genetic variants associated with PLT/MPV with protein quantitative trait locus variants associated with plasma proteins followed by Mendelian randomization to infer causal relations of proteins for PLT/MPV. We also tested protein-PLT/MPV association in FHS participants. Using induced pluripotent stem cell-derived megakaryocyte clones that produce functional platelets, we conducted RNA-sequencing and analyzed expression differences between low- and high-platelet producing clones. We then performed small interfering RNA gene knockdown experiments targeting genes encoding proteins with putatively causal platelet effects in megakaryocyte clones to examine effects on platelet production. In protein-trait association analyses, ten proteins were associated with MPV and 31 with PLT. Mendelian randomization identified 4 putatively causal proteins for MPV and 4 for PLT. GP-5 (Glycoprotein V), GRN (granulin), and MCAM (melanoma cell adhesion molecule) were associated with PLT, while MPO (myeloperoxidase) showed significant association with MPV in both analyses. RNA-sequencing analysis results were directionally concordant with observed and Mendelian randomization-inferred associations for GP-5, GRN, and MCAM. In siRNA gene knockdown experiments, silencing GP-5, GRN, and MPO decreased PLTs. Genome-wide association study results suggest several of these may be linked to CVD risk.

CONCLUSIONS

We identified 4 proteins that are causally linked to PLTs. These proteins may also have roles in the pathogenesis of CVD via a platelet/blood coagulation-based mechanism.

Inverse Correlation Between Plasma Sphingosine-1-Phosphate and Ceramide Concentrations in Septic Patients and Their Utility in Predicting Mortality

Supplemental Digital Content is available in the text

Introduction:

The aim of this study was to investigate the correlation between plasma sphingosine-1-phosphate (S1P) and ceramide concentrations in sepsis, and the possible mechanisms for altered expression.

Methods:

Plasma S1P and ceramide concentrations were measured by HPLC-ESI-MS/MS. HLA-DR (human leukocyte antigen-DR) expression on peripheral blood mononuclear cells was examined by flow cytometry. Platelet sphingosine kinases 1/2 (SphK1/2) mRNA expression, protein content, and enzyme activities were determined by qRT-PCR, western blot, and commercial enzyme assay kits, respectively.

Results:

Compared with healthy and ICU controls, septic patients had significantly decreased plasma S1P but increased ceramide concentrations (P < 0.05). S1P concentration was negatively associated with the ceramide concentration in the septic patients (r = −0.36, P < 0.05). Linear regression analysis found that plasma S1P and ceramide were linked not only to sequential (sepsis-related) organ failure assessment (SOFA) score but also the HLA-DR expression on circulating monocytes. An receiver operating characteristic analysis, including S1P, ceramide, SOFA score and HLA-DR, showed integrated analysis of S1P and ceramide as the better powerful predictors of septic lethality with area under the curve value of 0.95. More importantly, we found the platelet SphKs activities and the expression levels of SphK1 were significantly decreased in septic patients (P < 0.05). Linear regression analysis revealed platelet SphKs activity was positively associated with the plasma S1P concentration of the septic patients (r = −0.41, P = 0.02).

Conclusions:

Integrated analysis of plasma S1P and ceramide predict septic mortality with high accuracy. The decreased platelet SphK1 expression and subsequent reduced SphKs activity might be responsible for the decreased plasma S1P levels during sepsis.

Severe Immune Thrombocytopenia Complicated by Intracerebral Haemorrhage Associated with Coronavirus Infection: A Case Report and Literature Review

Immune thrombocytopenic purpura (ITP) is an autoimmune disorder that causes isolated thrombocytopenia. Many viruses have been identified as triggering the autoimmune process, including HIV, MCV, EBV, parvovirus, rubella and measles. However, ITP in association with coronavirus infection has not previously been reported. We describe the case of a healthy man who presented with severe ITP complicated by intracranial haemorrhage following upper respiratory tract infection. An infection screen revealed coronavirus infection.

Glycoprotein V is a relevant immune target in patients with immune thrombocytopenia

Platelet autoantibody-induced platelet clearance represents a major pathomechanism in immune thrombocytopenia (ITP). There is growing evidence for clinical differences between anti-glycoprotein IIb/IIIa and anti-glycoprotein Ib/IX mediated ITP. Glycoprotein V is a well characterized target antigen in Varicella-associated and drug-induced thrombocytopenia. We conducted a systematic study assessing the prevalence and functional capacity of autoantibodies against glycoprotein V. A total of 1140 patients were included. In one-third of patients, platelet-bound autoantibodies against glycoproteins Ib/IX, IIb/IIIa, or V were detected in a monoclonal antibody immobilization of platelet antigen assay; platelet-bound autoantiglycoprotein V was present in the majority of samples (222 out of 343, 64.7%). Investigation of patient sera revealed the presence of free autoantibodies against glycoprotein V in 13.5% of these patients by an indirect monoclonal antibody immobilization of platelet antigen assay, but in 39.6% by surface plasmon resonance technology. These antibodies showed significantly lower avidity (association/dissociation ratio 0.32±0.13 vs. 0.73±0.14; P<0.001). High- and low-avidity antibodies induced comparable amounts of platelet uptake in a phagocytosis assay using CD14⁺ positively-selected human macrophages [mean phagocytic index, 6.81 (range, 4.75-9.86) vs. 6.01 (range, 5.00-6.98); P=0.954]. In a NOD/SCID mouse model, IgG prepared from both types of anti-glycoprotein V autoantibodies eliminated human platelets with no detectable difference between the groups from the murine circulation [mean platelet survival at 300 minutes, 40% (range, 27-55) vs. 35% (16-46); P=0.025]. Our data establish glycoprotein V as a relevant immune target in immune thrombocytopenia. We would suggest that further studies including glycoprotein V will be required before ITP treatment can be tailored according to platelet autoantibody specificity.

Vaccines and autoimmunity

Vaccines have eradicated or controlled many infectious diseases, saving each year millions of lives and quality of life of many other millions of people. In spite of the success of vaccines over the last two centuries, parents (and also some health care workers) gloss over the devastating consequences of diseases, which are now avoided thanks to vaccines, and direct their attention to possible negative effects of immunization. Three immunological objections are raised: vaccines cause antigenic overload, natural immunity is safer and better than vaccine-induced immunity, and vaccines induce autoimmunity. The last point is examined in this review. Theoretically, vaccines could trigger autoimmunity by means of cytokine production, anti-idiotypic network, expression of human histocompatibility leukocyte antigens, modification of surface antigens and induction of novel antigens, molecular mimicry, bystander activation, epitope spreading, and polyclonal activation of B cells. There is strong evidence that none of these mechanisms is really effective in causing autoimmune diseases. Vaccines are not a source of autoimmune diseases. By contrast, absolute evidence exists that infectious agents can trigger autoimmune mechanisms and that they do cause autoimmune diseases.

Review: Viral infections and mechanisms of thrombosis and bleeding

Viral infections are associated with coagulation disorders. All aspects of the coagulation cascade, primary hemostasis, coagulation, and fibrinolysis, can be affected. As a consequence, thrombosis and disseminated intravascular coagulation, hemorrhage, or both, may occur. Investigation of coagulation disorders as a consequence of different viral infections have not been performed uniformly. Common pathways are therefore not fully elucidated. In many severe viral infections there is no treatment other than supportive measures. A better understanding of the pathophysiology behind the association of viral infections and coagulation disorders is crucial for developing therapeutic strategies. This is of special importance in case of severe complications, such as those seen in hemorrhagic viral infections, the incidence of which is increasing worldwide. To date, only a few promising targets have been discovered, meaning the implementation in a clinical context is still hampered. This review discusses non‐hemorrhagic and hemorrhagic viruses for which sufficient data on the association with hemostasis and related clinical features is available. This will enable clinicians to interpret research data and place them into a perspective. J. Med. Virol. 84:1680–1696, 2012. © 2012 Wiley Periodicals, Inc.

Clinical features, prognostic factors, and their relationship with antiplatelet antibodies in children with immune thrombocytopenia

We investigated and evaluated the demographics, clinical and laboratory features, treatment responses, and disease duration of 25 children with immune thrombocytopenia (ITP) eligible for detection of antiplatelet antibodies. We found that patients without antecedent of preceding infection (API) were more likely to have anti-GPIa/IIa than those with API (42.9% vs. 5.5%, P=0.048). Age groups of <2 years and 2 to 10 years were more likely to show response (R) or complete response (CR) to given treatments, whereas none of the patients whose onset age >10 years showed R or CR to given treatments (88.9% and 100% vs 0%, P=0.001). The percentage of newly diagnosed ITP was higher in age groups of <2 years (100%) and in 2 to 10 years (90%) than the age group of >10 years (16.7%, P=0.001). Patients without API (71.4%) were more likely to develop chronic ITP than those with API (5.6%, P=0.002). In conclusion, younger age was a favorable prognostic factor, especially in patients <2 years of age with respect to treatment responses and disease duration. In addition, API was associated with a short disease course as well as absence of anti-GPIa/IIa.

An experience of oseltamivir phosphate (tamiflu™) in a pediatric patient with chronic idiopathic thrombocytopenic purpura: a case report

We report a pediatric patient with chronic idiopathic thrombocytopenic purpura who suffered from an influenza A (H1N1) virus infection. A 13-year-old girl presented with fever, coughing, and generalized petechiae. The influenza A antigen was positive in her pharyngeal aspirate. She was successfully treated with neuraminidase inhibitor oseltamivir phosphate. Her platelet counts progressively increased with oseltamivir phosphate without another blood product or agent. We suggest that oseltamivir phosphate can be used in patients with immune thrombocytopenic purpura who present with influenza virus A infection. We also suggest that oseltamivir phosphate can be effective in patients with immune thrombocytopenic purpura. Large study groups are needed to confirm the relationship between idiopathic thrombocytopenic purpura and neurominidase inhibitors.

Post-varicella thrombocytopenic purpura

AIMS

The aim of the study was to characterize the clinical course of post-varicella idiopathic thrombocytopenic purpura (ITP) and to asses the risk of acquiring ITP after varicella infection.

METHODS

A retrospective study of all children diagnosed with ITP in a tertiary medical centre during 1998-2008. Findings were compared with the Intercontinental Childhood ITP Study Group database. The risk of acquiring ITP after a varicella infection was assessed.

RESULTS

Ten children were diagnosed with post-varicella ITP. The incidence of post-varicella ITP was 1.9% amongst children diagnosed with ITP and 1.1% amongst children hospitalized for varicella. ITP was diagnosed, on average, 8.5 days after the onset of the varicella rash. The female-to-male ratio was 1:1.5. The average minimal platelet count was 9.5 × 10⁹ platelets/L. Post-varicella ITP had an acute course in 80% of cases and a chronic course in the remaining 20%. Bleeding episodes occurred in three patients. During the follow-up period, 11 patients with previously diagnosed ITP developed varicella. The infection had no apparent affect on the platelet count of the children with acute ITP, but caused a relapse in 71% of the patients with chronic ITP.

CONCLUSIONS

Post-varicella ITP has similar clinical features and course to non-varicella associated ITP. The calculated risk of ITP as a complication of varicella infections is approximately 1:25,000.

Varicella zoster virus-induced hemolytic crisis in an infant with severe vitamin B 12 deficiency

BACKGROUND

Vitamin B 12 deficiency is an uncommon disorder in infancy. Most cases are because of maternal deficiency resulting from insufficient storage and/or reduced intake and are generally seen in exclusively breast-fed infants. Accentuation of the hemolytic process has never been described in association with Varicella Zoster Virus (VZV) infections.

OBSERVATION

We describe a 9-months-old breast-fed infant with megaloblastic anemia secondary to maternal vitamin B 12 deficiency. He presented severe pancytopenia and regression of motor functions and developed hemolytic crisis during a VZV infection.

CONCLUSIONS

Nutritional cobalamin deficiency should be considered in anemic infants with a history of prolonged exclusive breastfeeding and delayed developmental milestones. VZV infection can trigger a hemolytic process in infants with severe megaloblastic anemia secondary to B12 deficiency. A normal mean corpuscular volume does not rule out megaloblastic anemia, when the condition is combined with severe hemolysis.

Controversies in the diagnosis and management of childhood acute immune thrombocytopenic purpura

Acute immune thrombocytopenic purpura (ITP) occurs most commonly in young children who present with severe isolated thrombocytopenia and purpura. A marrow examination is not required unless glucocorticoids are used, lest treatment mask incipient acute lymphoblastic leukemia, but controversy exists here. The recommendations for evaluation and management remain controversial, since prospective controlled trials have not been done. There is some consensus based on experience and empiric data. Almost all children with acute ITP will recover completely without therapy. Although the various treatments may increase the platelet count, they do not influence the outcome of the illness, may increase cost, and cause significant side effects. Therefore, careful observation may be the best management option for the patient with ITP, in the absence of severe bleeding. The data available relevant to these issues are discussed.

The ITP syndrome: pathogenic and clinical diversity

Immune thrombocytopenia (ITP) is mediated by platelet autoantibodies that accelerate platelet destruction and inhibit their production. Most cases are considered idiopathic, whereas others are secondary to coexisting conditions. Insights from secondary forms suggest that the proclivity to develop platelet-reactive antibodies arises through diverse mechanisms. Variability in natural history and response to therapy suggests that primary ITP is also heterogeneous. Certain cases may be secondary to persistent, sometimes inapparent, infections, accompanied by coexisting antibodies that influence outcome. Alternatively, underlying immune deficiencies may emerge. In addition, environmental and genetic factors may impact platelet turnover, propensity to bleed, and response to ITP-directed therapy. We review the pathophysiology of several common secondary forms of ITP. We suggest that primary ITP is also best thought of as an autoimmune syndrome. Better understanding of pathogenesis and tolerance checkpoint defects leading to autoantibody formation may facilitate patient-specific approaches to diagnosis and management.

Clinico-pathologic conference: an 11-year-old girl with lupus erythematosus, venous thromboses, and purpura fulminans

An 11 year old girl with known diagnosis of lupus erythematosus with nephritis was admitted with respiratory distress during an episode in which she had infection with varicella-zoster virus complicated by purpura fulminans. She had a downhill course, terminating with a pulmonary embolus.

Relapse of idiopathic thrombocytopenic purpura caused by influenza A virus infection: a case report

We report a patient with idiopathic thrombocytopenic purpura (ITP) in remission, who relapsed as a result of an influenza A virus infection. A 41-year-old woman presented with fever elevation, coughing, and generalized petechiae. Her platelet count had decreased to 1 x 10(9)/l. She had been diagnosed with ITP at age 23, and continuous complete remission had followed steroid therapy and splenectomy. Influenza A antigen was positive in her pharyngeal aspirate, and oseltamivir was effective for her symptoms. Findings of a bone marrow smear were typical for ITP. Steroid therapy resulted in a second complete remission. Although the development of ITP caused by influenza infection and a relapse caused by an influenza vaccination have been previously described, a relapse caused by a sporadic infection has never been documented to our knowledge. Physicians should carefully monitor the hematological data of influenza patients, especially those with ITP, even in remission.

A case of varicella-associated idiopathic thrombocytopenic purpura in adulthood

We describe here an adult case of varicella-associated idiopathic thrombocytopenic purpura (ITP). A 23-year-old Japanese male presented with generalized hemorrhagic vesicles and fever. Laboratory examinations revealed severe thrombocytopenia associated with anti-platelet IgG auto-antibody. The patient rapidly recovered within a few days after antiviral agent (vidarabine) administration, and the platelet count returned to normal with skin lesion improvement. Idiopathic thrombocytopenic purpura associated with viral infection in adulthood is relatively rare.

Association of autoantibodies to nuclear lamin B1 with thromboprotection in systemic lupus erythematosus: lack of evidence for a direct role of lamin B1 in apoptotic blebs

OBJECTIVE

To demonstrate the association between autoantibodies to nuclear lamin B1 (aLB1) and protection against thrombosis ("thromboprotection") in patients with systemic lupus erythematosus (SLE), and to elucidate the mechanism by which aLB1 cause thromboprotection in vivo. Since a number of autoantigens in SLE have been localized specifically to the external surface of apoptotic blebs, it was hypothesized that circulating aLB1 may block the procoagulant effect of apoptotic blebs by binding to LB1 displayed at the external bleb surface.

METHODS

A cross-sectional study was performed using serum samples obtained at first evaluation of 259 English Canadian and French Canadian patients from SLE registries at 3 hospitals. A case-control study was performed to analyze the relationship between aLB1 and lupus anticoagulant (LAC) status and thrombotic manifestations between onset of disease and last followup. Reactivity of aLB1 with Jurkat or endothelial cells, which had been induced to undergo apoptosis, was determined by indirect immunofluorescence. Localization of LB1 in apoptotic cells and blebs was analyzed by confocal microscopy and surface labeling of cell membrane proteins.

RESULTS

High-titer aLB1 was restricted to a subset of SLE patients (46 patients), with an overall frequency of 17.8% (range 11.6-24.3% in the 3 centers). LB1 antibodies were significantly associated with LAC but not with antibodies to cardiolipin (aCL) or beta(2)-glycoprotein I (anti-beta(2)GPI). The frequency of thrombosis differed markedly depending on aLB1 and LAC status, as follows: presence of LAC and absence of aLB1 50%, presence of both LAC and aLB1 22.7%, absence of both LAC and aLB1 25.5%, absence of LAC and presence of aLB1, 20.8%. Further subclassification of patients based on aCL and anti-beta(2)GPI status revealed that, in the presence of LAC but in the absence of aCL, anti-beta(2)GPI, and aLB1, the frequency of thrombosis was 40%, whereas in the presence of aLB1, it decreased strikingly, to 9.1%. LB1 was found to be translocated into surface membrane blebs during apoptosis and to be entirely enclosed within the apoptotic bleb plasma membrane of Jurkat and endothelial cells.

CONCLUSION

The presence of aLB1 in SLE patients with LAC essentially nullifies the strong prothrombotic risk associated with LAC. Hence, aLB1 is associated with thromboprotection. Reactivity of aLB1 with apoptotic blebs does not seem to play a direct role in mediating this protection, since LB1 is buried within apoptotic blebs and inaccessible to circulating aLB1. The mechanism by which aLB1 confers thromboprotection in SLE remains to be elucidated.

Glycoprotein V: the predominant target antigen in gold-induced autoimmune thrombocytopenia

Autoimmune thrombocytopenia is generally caused by autoantibodies against glycoprotein (GP) IIb-IIIa or GPIb-IX and occasionally against GPIa-IIa or GPV. By investigating 38 rheumatoid arthritis (RA) patients on gold therapy, 10 with profound thrombocytopenia and 28 nonthrombocytopenic controls, we showed that in all 10 patients with thrombocytopenia, the platelet autoantibodies preferentially targeted GPV but the presence of gold was not required for their reactivity. Elevated levels of platelet-associated IgG (PAIgG) were observed in 8 of the 10 patients in whom the tests were performed. In 5 patients with sufficient autologous platelets, the GPV specificity of PAIgG was confirmed. Tests with GPV transfectants revealed that the antibodies reacted with GPV independent of GPIb alpha, GPIb beta, or GPIX. Autoantibodies recognizing GPV were not seen in the 28 nonthrombocytopenic control RA patients. Thus, GPV seems to be targeted in gold-induced autoimmune thrombocytopenia.

Postvaricella purpura fulminans caused by acquired protein s deficiency resulting from antiprotein s antibodies: search for the epitopes

Postvaricella purpura fulminans is a rare disease in children that is probably caused by an acquired protein S deficiency resulting from antiprotein S antibodies. The epitope of these antibodies is unknown. A 5-year-old girl is described with postvaricella purpura fulminans and an acquired protein S deficiency. In this patient and in her 3-year-old sister with uncomplicated varicella, the concentrations of antiprotein S antibodies were measured and followed with enzyme-linked immunosorbent assay techniques. The epitope of the antiprotein S antibodies was studied using miniprotein S, a recombinant variant of protein S that consists of the first 242 amino acids of protein S, lacking the sex hormone binding globulin-like domain. In the patient's plasma, concentrations of free protein S antigen and total protein S antigen reached normal levels in 4 months and 5 weeks, respectively. The concentrations of the antiprotein S antibodies decreased to 25% of the initial level in the course of 5 months. In the sister, antiprotein S antibodies were present as well, but the concentrations were lower than those in the patient. Most of the antiprotein S antibodies were directed against the first 242 amino acids of protein S. After varicella, a heterozygous autoantibody response may develop that may result in severe acquired protein S deficiency leading to purpura fulminans. Epitopes of these antiprotein S antibodies are situated on both the first 242 amino acids of protein S and the sex hormone binding globulin-like domain.

Anti-platelet antibodies associated with the Canale-Smith syndrome bind to the same platelet glycoprotein complexes as those of idiopathic thrombocytopenic purpura patients

The Canale-Smith syndrome (CSS) is an inherited disease characterized by massive lymphadenopathy, hepatosplenomegaly and systemic autoimmunity to erythrocytes and platelets. Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease in which approximately 60-80% of patients have anti-platelet antibodies directed against specific platelet glycoprotein complexes (GPCs) located on their membrane: GP IIb/IIIa, GPIb/IX, and GPIa/IIa. Almost all (95-100%) of the antibody-positive patients have antibodies directed against GPIIb/IIIa alone, or in combination with other glycoprotein targets. Our objective was to determine the specificities of the anti-platelet antibodies in CSS patients. The detection of anti-platelet antibodies was performed using a commercially available ELISA, the Pak-AUTO (GTI, Brookfield, WI), in which highly purified GPIIb/IIIa, GPIb/IX, and GPIa/IIa are immobilized on microtitre plates, incubated with serum or plasma, and subsequently developed with an antihuman polyclonal immunoglobulin. Of 14 CSS patients tested, 11 (79%) had anti-platelet antibodies in their serum directed toward at least one of the three major GPC, nine (82%) of which were against GPIIb/IIIa alone or in combination. Antibodies detected in the sera of ITP patients had similar specificities. No such antibodies were detected in samples from 25 consecutive normal controls. These results demonstrate that a genetically defined defect in lymphocyte apoptosis results in a humoral autoimmune response with anti-platelet specificities very similar to the common idiopathic form of autoimmune thrombocytopenia.

Glycoprotein V-specific platelet-associated antibodies in thrombocytopenic patients

In autoimmune thrombocytopenia, platelet-associated IgG (PA-IgG) frequently displays specificity against glycoprotein (GP) IIbIIIa and/or GP IbIX. Because in a high proportion of patients positive PA-IgG may not be explained by these GP specificities, studies on other target proteins are needed. We studied the presence of GP V-specific PA-IgG by direct monoclonal antibody-specific immobilization of platelet antigens (MAIPA) with the monoclonal antibody SW16. We focused on 69 consecutive random patients with histories of thrombocytopenia who were strongly positive for PA-IgG detected by the direct platelet immunofluorescence test (PIFT). PA-IgG against GP V (ratio > or = 1.5) was noted in 15 (22%) patients. The degree of PA-IgG measured by PIFT, and of GP IIbIIIa-and/or GP IbIX-specific PA-IgG measured by direct MAIPA, correlated directly with the GP V-specific PA-IgG (P < 0.001). In one patient, GP V-specific antibodies were associated with quinidine-induced thrombocytopenia. Although this patient had strongly positive GP V-specific PA-IgG, she remained negative in GP IIbIIIa- and GP IbIX-specific direct MAIPA. Two patients studied because of thrombocytopenia associated with gold therapy had strongly positive GP V-specific PA-IgG. In one patient with rheumatoid arthritis and severe gold-induced thrombocytopenia, the amount of GP V-specific PA-IgG decreased during the recovery phase. Thus, GP V may represent an important target antigen in autoimmune-mediated thrombocytopenia, especially in drug-induced thrombocytopenia.

Genetic polymorphism in human platelet glycoprotein GP Ib/IX/V complex is enriched in GP V (CD42d)

Platelet glycoproteins Ib beta (CD42c), IX (CD42a), and V (CD42d), together with GP Ib alpha (CD42b), form a receptor whose interaction with the von Willebrand factor is essential in the initial stages of haemostasis. Genetic variation in these proteins can cause alloimmunization leading to neonatal alloimmune thrombocytopenia and platelet transfusion refractoriness. Defective mutations cause a rare bleeding disorder, Bernard-Soulier syndrome. Only two antigenic polymorphisms have thus far been established in these proteins: the HPA-2 in GP Ib alpha and the rare Iy variant in GP Ib beta. Recently, we reported that only a limited degree of polymorphism can be found in the GP Ib alpha gene; the level of variation in the other components is not known. We therefore systematically screened polymorphism in the GP Ib beta, GP IX, and GP V genes in 50 unrelated Finnish blood donors. Nine polymorphic sites were found in the GP V gene, of which four changed the amino acid code and five were silent. The gene frequencies for substitutions Asp114Tyr, Met273Ile, Gly341Arg, and Leu397Arg were 1%, 1%, 2%, and 1% respectively. The five silent polymorphisms also had low frequencies, 1-4%. No polymorphism was found in the GP Ib beta gene and only one mutation was found in the 3' untranslated region of the GP IX gene. Our results indicate that genetic variation in the GP Ib/IX/V complex is mostly tolerated in the GP V protein--whose function in the complex is not clear whereas the other components have only very limited genetic polymorphism.

Virus-associated idiopathic thrombocytopenic purpura

Two-thirds of children with acute idiopathic thrombocytopenic purpura (ITP) have a history of an infectious illness a few days to a few weeks before the onset of thrombocytopenia. In a subset of affected children, identification of a specific virus can be made, such as varicella zoster virus, rubella, Epstein-Barr virus, influenza, or human immunodeficiency type 1 virus, indicating an etiological role for preceding viral infection in these children with ITP. While inhibition of thrombopoiesis has been established to play a role in thrombocytopenia associated with infection with some viruses, it does not appear to play a major role in the etiology of most typical ITP cases. Rather, enhanced clearance of platelets by the reticuloendothelial system is considered to be, at least in part, responsible for the thrombocytopenia which occurs during the viremic phase of acute virus infection or which develops days to weeks following the virus illness. Molecular mimicry between viral antigens and host proteins has been implicated in a number of autoimmune phenomena, and may be involved in the enhanced platelet clearance in virus-associated ITP.

Autoimmune thrombocytopenic purpura (AITP) and acquired thrombasthenia due to autoantibodies to GP IIb-IIIa in a patient with an unusual platelet membrane glycoprotein composition

The subject (E.B.) is a 63-year-old woman with autoimmune thrombocytopenic purpura (AITP) who was first examined some 6 years ago with symptoms of epistaxis and gum bleeding, severe thrombocytopenia, and large platelets. Her serum tested positively with control platelets in the MAIPA assay performed using monoclonal antibodies (MoAb) to glycoprotein (GP) IIIa (XIIF9, Y2/51), yet was negative in the presence of MoAbs to GP IIb (SZ 22) or to the GP IIb-IIIa complex (AP2, P2). The patient's platelets failed to aggregate with all agonists tested except for ristocetin. IgG isolated from the patient's serum inhibited ADP-induced aggregation of control platelets. Unexpectedly, flow cytometry showed an altered expression of membrane glycoproteins on the patient's platelets. Levels of GP Ib-IX were much higher than previously located by us in platelets. In contrast, the expression of GP IIb-IIIa was about half that seen with control subjects. When Western blotting was performed, a striking finding was a strong band of 250 kDa recognized by a series of MoAbs to GP Ib alpha in addition to the band in the normal position of GP Ib alpha. Finally, ADP-stimulated (E.B.) platelets failed to express activation-dependent epitopes on GP IIb-IIIa as recognized by PAC-1, AP6, or F26 and additionally gave a reduced P-selectin expression after thrombin addition. In conclusion, we present a novel patient with a severely perturbed platelet function where an altered membrane GP profile is associated with the presence of an autoantibody recognizing a complex-dependent determinant on GP IIb-IIIa and inhibitory of platelet aggregation.