On the association of the platelet-specific alloantigen, Pena, with glycoprotein IIIa. Evidence for heterogeneity of glycoprotein IIIa

Clinical and laboratory predictors of fetal and neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is the most common cause of intracranial hemorrhage (ICH) in thrombocytopenic term infants. We investigated clinical and laboratory predictors of severe FNAIT in a tertiary care referral center.

STUDY DESIGN AND METHODS

Retrospective cohort study over a 30-year period. We defined FNAIT as recurrence of neonatal thrombocytopenia in a subsequent pregnancy; and severe outcomes as any of: (1) a birth platelet count below 20 × 10⁹ /L; (2) ICH or (3) fetal death. We used a generalized estimating equations analysis and classification tree analysis to identify risk factors for severe FNAIT in a subsequent pregnancy.

RESULTS

During index pregnancies (n = 135 in 131 mothers), 71 infants (52.6%) had severe outcomes including a platelet count <20 × 10⁹ /L (n = 45), fetal or neonatal ICH (n = 32), or fetal death (n = 4). During subsequent pregnancies (n = 72), 15 infants (20.8%) had severe outcomes including birth platelets <20 × 10⁹ /L (n = 10), ICH (n = 2), or death (n = 3). Forty-two women (58.3%) received antenatal intravenous immune globulin (IVIG) during subsequent pregnancies. Eight mothers (n = 9 infants) had severe FNAIT outcomes despite receiving antenatal IVIG. Maternal antibodies to human platelet antigens (HPA) was the only independent predictor of severe FNAIT in a subsequent pregnancy (OR = 25.3, p = .004). Nevertheless, one of 43 infants from antibody-negative mothers had a severe outcome.

CONCLUSIONS

The presence of anti-HPA is highly indicative of the diagnosis of severe FNAIT; however, we observed one infant who had severe FNAIT recurrence, defined using strict clinical criteria, without a maternal antibody. Improved diagnostic and therapeutic strategies are needed to prevent severe FNAIT in high-risk mothers.

Evolution and Utility of Antiplatelet Autoantibody Testing in Patients with Immune Thrombocytopenia

To this day, immune thrombocytopenia (ITP) remains a clinical diagnosis made by exclusion of other causes for thrombocytopenia. Reliable detection of platelet autoantibodies would support the clinical diagnosis, but the lack of specificity and sensitivity of the available methods for platelet autoantibody testing limits their value in the diagnostic workup of thrombocytopenia. The introduction of methods for glycoprotein-specific autoantibody detection has improved the specificity of testing and is acceptable for ruling in ITP but not ruling it out as a diagnosis. The sensitivity of these assays varies widely, even between studies using comparable assays. A review of the relevant literature combined with our own laboratory's experience of testing large number of serum and platelet samples makes it clear that this variation can be explained by variations in the characteristics of the tests, including in the glycoprotein-specific monoclonal antibodies, the glycoproteins that are tested, the platelet numbers used in the assay and the cutoff levels for positive and negative results, as well as differences in the tested patient populations. In our opinion, further standardization and optimization of the direct autoantibody detection methods to increase sensitivity without compromising specificity seem possible but will still likely be insufficient to distinguish the often very weak specific autoantibody signals from background signals. Further developments of autoantibody detection methods will therefore be necessary to increase sensitivity to a level acceptable to provide laboratory confirmation of a diagnosis of ITP.

Progress and development of platelet antibody detection

Human platelet antibody (HPA) detection is necessary for the diagnosis and therapeutic decisions for refractoriness to platelet transfusions, post transfusion purpura and fetal and neonatal alloimmune thrombocytopenia. In the last four to five decades many new developments, both in knowledge and methods, have increased the quality of platelet serology. However, the quest for the optimal antibody detection method(s) encountered, sometimes unexpected, difficulties. In this review the various aspects concerning platelet antibody test methods and detection of platelet antibodies both for the diagnostic and screening setting are discussed.

Detection and identification of platelet antibodies using a sensitive multiplex assay system-platelet antibody bead array

BACKGROUND

Tests for platelet-specific antibodies are important in the diagnosis of immune platelet disorders. Monoclonal antibody glycoprotein capture assays have been the gold standards in platelet antibody detection for almost 30 years. However, such assays are complex and cumbersome to perform, which limits their routine use. We report the performance of a newly developed, easy to perform platelet antibody bead array (PABA) for the detection of platelet-specific antibodies.

STUDY DESIGN AND METHODS

PABA is the equivalent of the monoclonal antigen capture enzyme-linked immunosorbent assay (ELISA) (MACE) on a bead and instead with fluorescent detection of immunoglobulin (Ig)G platelet antibodies by Luminex. Antibodies against platelet glycoproteins (GP) GPIIb/IIIa, GPIb/IX, GPIa/IIa, GPIV, and class I human leukocyte antigen (HLA) could be detected in a patient's serum simultaneously in a single well of a microplate. Results from 80 patient samples and 20 normal donor samples were compared using PABA, MACE, and a commercial ELISA kit.

RESULTS

PABA detected all antibodies, with specificity for human platelet antigens (HPAs) HPA-1a, HPA-1b, HPA-2a, HPA-2b, HPA-3a, HPA-3b, HPA-4a, HPA-4b, HPA-5a, HPA-5b, HPA-15b, and HLA. Overall, compared with MACE, the sensitivity and specificity of PABA were 99% and 97%, respectively, and both were significantly better than those of the commercial ELISA. PABA had greater analytic sensitivity than MACE for HPA-1a, HPA-3a, and HPA-5b antibodies. In addition, PABA detected HPA-5b and HPA-3b antibodies that were missed by MACE. The overall false-positive rate of PABA compared with MACE was 2.7%.

CONCLUSIONS

The PABA is a rapid, highly sensitive and specific, multiplex bead-based assay for detecting human platelet antibodies. Such a simple yet high-throughput platform will facilitate practical, routine testing for the identification of platelet-specific antibodies.

New low-frequency platelet glycoprotein polymorphisms associated with neonatal alloimmune thrombocytopenia

BACKGROUND

Recent reports suggest that maternal immunization against low-frequency, platelet (PLT)-specific glycoprotein (GP) polymorphisms is a more common cause of neonatal alloimmune thrombocytopenia (NATP) than previously thought.

STUDY DESIGN AND METHODS

Serologic and molecular studies were performed on PLTs and DNA from three families in which an infant was born with apparent NATP not attributable to maternal immunization against known PLT-specific alloantigens.

RESULTS

Antibodies reactive only with paternal PLTs were identified in each mother. In Cases 2 (Kno) and 3 (Nos), but not Case 1 (Sta), antibody recognized paternal GPIIb/IIIa in solid-phase assays. Unique mutations encoding amino acid substitutions in GPIIb (Case 2) or GPIIIa (Cases 1 and 3) were identified in paternal DNA and in DNA from two of the affected infants. Antibody from all three cases recognized recombinant GPIIIa (Case 1 [Sta] and Case 3 [Nos]) and GPIIb (Case 2, Kno) mutated to contain the polymorphisms identified in the respective fathers. None of 100 unselected normal subjects possessed the paternal mutations. Enzyme-linked immunosorbent assay and flow cytometric studies suggested that failure of maternal serum from Case 1 (Sta) to react with paternal GPIIIa in solid-phase assays resulted from use of a monoclonal antibody AP2, for antigen immobilization that competed with the maternal antibody for binding to the Sta epitope.

CONCLUSION

NATP in the three cases was caused by maternal immunization against previously unreported, low-frequency GP polymorphisms. Maternal immunization against low-frequency PLT-specific alloantigens should be considered in cases of apparent NATP not resolved by conventional serologic and molecular testing.

Modification of solid phase red cell adherence assay for the detection of platelet antibodies in patients with thrombocytopenia

Platelet refractoriness is caused by HLA antibodies and platelet-specific antibodies. Current methods used to detect antiplatelet antibodies have limitations. Solid phase red cell adherence (SPRCA) lacks sensitivity and requires a second assay using chloroquine-treated intact platelets to specify the response due to anti-HLA. We modified SPRCA by using 2 types of antihuman platelet antibodies with different specificities toward platelet lysate and tested samples from 361 patients (69 with unexplained thrombocytopenia and 292 with poor response to platelet transfusions not explicable by alloimmunization or the clinical situation) and 50 from healthy volunteers. Our method compared favorably with platelet suspension direct immunofluorescence. All samples from healthy volunteers were negative; of the samples from the patient population, 240 were positive (147 samples had only antiplatelet and 3 samples had only anti-HLA antibodies). This modified technique had a sensitivity of 98% and a specificity of 91%.

[Detection of platelet specific antibodies by modified antigen capture ELISA test.]

BACKGROUND

Autoimmune thrombocytopenia (AITP) is characterized by autoantibody-induced platelet destruction. Although several studies have shown that pathogenic autoantibodies are mainly IgG directed platelet glycoproteins (GP), a platelet GP specific test is not available in clinical laboratories. The aim of this study was to evaluate the clinical usefulness of a Modified Antigen Capture Enzyme-linked immunosorbent assay (MACE) test in the diagnosis of AITP.

METHODS

We investigated fifty-seven patients who showed a platelet count lower than 100 x 10(9)/L and underwent a bone marrow examination. They were classified into primary AITP (P-AITP) (n=21), secondary AITP (S-AITP) (n=15), and non-immune thrombocytopenia (NITP) (n=21) by bone marrow findings and clinical diagnosis. Platelet GP (IIb/IIIa, Ia/IIa, Ib/IX, IV)-specific antibodies and anti-HLA class I antibody were detected by MACE test.

RESULTS

Among 57 samples, platelet GP specific antibodies were detected in 8 (22.2%) of 36 patients with AITP and 1 (4.8%) of 21 patients with NITP. The specificities were as follows: GP IIb/IIIa (n=4), GP Ia/IIa (n=5), GP Ib/IX (n=3) and GPIV (n=2). Of the nine patients with platelet GP specific antibodies, four (44.4%) had more than two platelet GP specific antibodies. The sensitivity, specificity, positive predictive value and negative predictive values of the MACE test for AITP were 22.2%, 95.2%, 88.9%, 41.7%, respectively. A previous transfusion history was associated with a higher detection rate of anti-HLA class I antibodies (P<0.05).

CONCLUSIONS

The MACE test is a convenient method to detect platelet GP specific antibody and is very specific to diagnose AITP. In clinical practice, even though it is not sensitive, the MACE test would be useful in differentiating AITP from NITP.

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.

Human platelet antigen-specific alloantibodies implicated in 1162 cases of neonatal alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NATP) caused by fetomaternal mismatch for human platelet (PLT) alloantigens (HPAs) complicates approximately 1 in 1000 to 1 in 2000 pregnancies and can lead to a serious bleeding diathesis, intracranial hemorrhage, and sometimes death of the fetus or neonate. As a national reference center for NATP investigations, our experience with this entity over a 12-year period was reviewed.

STUDY DESIGN AND METHODS

The laboratory records of all cases of suspected NATP referred for evaluation from January 1, 1990, to December 31, 2002, were analyzed. The spectrum of PLT alloantibody specificities identified was compared with an earlier reported series of serologically verified NATP cases.

RESULTS

HPA-specific alloantibodies were identified in 1162 (31%) of 3743 sera of mothers of infants with clinically suspected NATP. Maternal HPA-1a (PlA1) alloimmunization accounted for the majority (79%) of confirmed NATP cases, with HPA-5b (Bra), HPA-3a (Baka), and HPA-1b (PLA2) alloantibodies accounting for 9, 2, and 4 percent of cases, respectively. In addition, an increase in the number of cases in which multiple HPA-specific alloantibodies were present in maternal sera was observed during the study period.

CONCLUSION

Although, as with the earlier series, maternal HPA-1a alloimmunization was the dominant cause of NATP, the identification of an increasing number of cases due to alternative HPA polymorphisms suggests that investigation for HPA-1 incompatibility alone is no longer sufficient to fully evaluate clinically suspect NATP cases.

Severe neonatal alloimmune thrombocytopenia caused by antibodies to human platelet antigen 3a (Baka) detectable only in whole platelet assays

BACKGROUND

Maternal antibodies that cause neonatal alloimmune thrombocytopenia are commonly identified by solid-phase assays that detect the causative antibodies on the basis of their reactions with specific PLT glycoproteins. Two cases of severe neonatal alloimmune thrombocytopenia caused by maternal antibodies specific for human PLT antigen 3a (HPA-3a [Baka]) that failed to give the expected reactions in some solid-phase assays were recently encountered.

STUDY DESIGN AND METHODS

PLT-reactive antibodies were characterized by three different solid-phase assays and by flow cytometry.

RESULTS

The two maternal antibodies gave negative reactions in the antigen capture ELISA, modified antigen capture ELISA, and MoAb immobilization of PLT antigens tests but reacted strongly in flow cytometry with intact PLTs that were HPA-3a+. Other sera samples specific for HPA-3a reacted equally well in all assays.

CONCLUSIONS

The two antibodies appear to recognize an epitope on the HPA-3a+ form of glycoprotein IIb that is lost when PLTs are solubilized in detergent, as required for solid-phase assays. The diagnosis was made in these cases because no HLA antibodies were present, allowing an HPA-3a-specific reaction to be identified with intact PLTs as targets. Such antibodies are likely to be overlooked when HLA antibodies are also present.

Prevalence of antiphospholipid and antiplatelet antibodies in human immunodeficiency virus (HIV)-infected Chilean patients

Antiphospholipid (aPL) and antiplatelet (aPlt) antibodies, found in patients with autoimmune diseases, are also detected in infectious diseases. The purpose of this study was to examine the prevalence of these antibodies in HIV patients and to evaluate an association of these antibodies with thrombocytopenia and/or thrombosis. Sixty-three HIV-seropositive patients and 52 normal controls were studied. Anti-cardiolipin (aCL), anti-beta(2) glycoprotein I (anti-beta(2)GPI), and antiprothrombin (aPT) antibodies were determined and the lupus anticoagulant (LA) test was performed. Antiplatelet antibodies (aPlt) were also determined. Seven out of 63 (12.7%) HIV patients were positive for aCL, four of 63 (6.3%) for anti-beta(2)GPI, and five of 63 (7.9%) for aPT. No patients studied were LA positive. Six out of 63 (9.5%) patients were positive for aPlt. One of them showed weak reactivity for GPIb-IX. The platelet count of patients (202+/-63 x 10(3) platelets/microL) was significantly lower than in the controls (343+/-6 x 10(3) platelets/microL) (P<0.001). There was no correlation between the presence of aPL and/or aPlt and thrombocytopenia. Of the HIV-infected patients, 22.2% presented aPL and 9.4% aPlt antibodies. In this study, the presence of aPL and aPlt antibodies was not associated with the development of thrombosis and/or thrombocytopenia.

A new platelet polymorphism Duv(a+), localized within the RGD binding domain of glycoprotein IIIa, is associated with neonatal thrombocytopenia

We report here the identification and characterization of a new platelet alloantigen, Duv(a+), implicated in a case of neonatal thrombocytopenia. Immunochemical studies demonstrated that the epitope was localized on glycoprotein (GP) IIIa. Sequencing of the exons 2 to 15 of GP IIIa gene polymerase chain reaction products from both parents revealed a single base substitution 517C>T (complementary DNA) present in a heterozygous state in DNA from the father leading to amino acid substitution Thr140Ile (ACC>ATC) within the Arg-Gly-Asp binding domain of GP IIIa. Flow cytometry and immunoprecipitation studies of IIb-C517 or T517 IIIa transfected Cos cells allowed us to demonstrate this mutation was responsible for expression of the Duv(a+) epitope. By polymerase chain reaction-single-strand conformational-polymorphism analysis, the mutated allele could not be detected in a population of 100 healthy unrelated donors, indicating a low frequency of occurrence. The Thr140/Ile dimorphism, localized 3 amino acids upstream from the Arg143 involved in the expression of HPA-4a, did not interfere with the binding of an anti-HPA-4a antibody in flow cytometry. Results of functional analysis of wild-type or mutated transfected CHO cells-(1) aggregation in the presence of Ca(++) and soluble fibrinogen after complex activation by dithiothreitol, (2) adhesion on coated fibrinogen, (3) binding of monoclonal antibody PAC-1 or LIBS antibody D3, and (4) outside-in signaling-all suggest that the Thr140Ile polymorphism localized in the Arg-Gly-Asp binding domain of GP IIIa does not affect significantly, if at all, the integrin function. We have shown that the anti-Duv(a+) antibody may inhibit platelet GP IIb-IIIa function.

Platelet glycoprotein IIIa Pl(a) polymorphism, fibrinogen, and platelet aggregability: The Framingham Heart Study

BACKGROUND

Recent data suggest that the Pl(A2) allele of the platelet glycoprotein IIIa receptor may be a genetic risk factor for cardiovascular disease. We previously reported that the Pl(A2) allele was associated with increased platelet aggregability, as indicated by lower epinephrine threshold concentrations. Paradoxically, however, it has been reported that Pl(A2)-positive platelets have reduced fibrinogen binding. Because fibrinogen mediates platelet aggregability, we hypothesized that plasma fibrinogen levels may interact with Pl(A) genotype in modulating platelet aggregability. Methods and Results-- Glycoprotein IIIa Pl(A) genotype, fibrinogen level, and platelet aggregability were ascertained in 1340 subjects enrolled into the Framingham Offspring Study. Platelet aggregability was evaluated by the Born method. Higher fibrinogen levels were associated with increased epinephrine-induced aggregation (P=0.002) and a trend for ADP-induced aggregation (P=0.07). The fibrinogen effect was genotype specific, however, in that the increase in platelet aggregability with higher fibrinogen was present for the Pl(A1/A1) genotype (P=0.0005 and P=0.03 for epinephrine- and ADP-induced aggregation, respectively) but not for the Pl(A2)-positive genotype (P>0.90).

CONCLUSION

Higher fibrinogen levels were associated with increased platelet aggregability. However, the association between fibrinogen and platelet aggregability was genotype specific. This interaction may be responsible for the conflicting findings regarding Pl(A) genotype and platelet aggregability. Further study of this gene-environment interaction may provide insight into cardiovascular disease risk.

Blood group A and B antigens are strongly expressed on platelets of some individuals

It is widely thought that expression of ABH antigens on platelets is insufficient to materially affect the survival of ABH-incompatible platelets in transfusion recipients, but anecdotal reports of poor survival of A and B mismatched platelets suggest that this is not always the case. The A and B antigen expression on platelets of 100 group A1 and group B blood donors was measured, and 7% and 4%, respectively, had platelets whose A and B antigen levels consistently exceeded the mean plus 2 SD. On the basis of flow cytometric and statistical analysis, donors whose platelets contained higher than normal levels of A antigen were subdivided into 2 groups, designated Type I and Type II (“high expressers”). Serum A1- and B-glycosyltransferase levels of A and B high expressers were significantly higher than those of group A1 and B individuals with normal expression. H antigen levels were low on the red cells of high expressers, indicating that the anomaly affects other cell lineages. Immunochemical studies demonstrated high levels of A antigen on various glycoproteins (GPs) from high-expresser platelets, especially GPIIb and PECAM (CD31). The A1 Type II high-expresser phenotype was inherited as an autosomal dominant trait in one family. The sequences of exons 5, 6, and 7 of the A1-transferase gene of one Type II A1 high expresser and exon 7 from 3 other genes were identical to the reported normal sequences. Further studies are needed to define the molecular basis for the high-expresser trait and to characterize its clinical implications.

Blood group A and B antigens are strongly expressed on platelets of some individuals

It is widely thought that expression of ABH antigens on platelets is insufficient to materially affect the survival of ABH-incompatible platelets in transfusion recipients, but anecdotal reports of poor survival of A and B mismatched platelets suggest that this is not always the case. The A and B antigen expression on platelets of 100 group A1 and group B blood donors was measured, and 7% and 4%, respectively, had platelets whose A and B antigen levels consistently exceeded the mean plus 2 SD. On the basis of flow cytometric and statistical analysis, donors whose platelets contained higher than normal levels of A antigen were subdivided into 2 groups, designated Type I and Type II (“high expressers”). Serum A1- and B-glycosyltransferase levels of A and B high expressers were significantly higher than those of group A1 and B individuals with normal expression. H antigen levels were low on the red cells of high expressers, indicating that the anomaly affects other cell lineages. Immunochemical studies demonstrated high levels of A antigen on various glycoproteins (GPs) from high-expresser platelets, especially GPIIb and PECAM (CD31). The A1 Type II high-expresser phenotype was inherited as an autosomal dominant trait in one family. The sequences of exons 5, 6, and 7 of the A1-transferase gene of one Type II A1 high expresser and exon 7 from 3 other genes were identical to the reported normal sequences. Further studies are needed to define the molecular basis for the high-expresser trait and to characterize its clinical implications.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Evaluation of a new antigen capture ELISA for detection and characterization of platelet alloantibodies

The monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay is considered as a possible reference method for the detection of maternal alloantibodies when a foetomaternal alloimmunization is suspected. However, this method is tedious. In this study, we have compared the MAIPA results of 54 samples of mothers with (n=34) or without (n=20) alloantibodies with those obtained with a new antigen capture ELISA. Using the cutoff of 2.0 given by the manufacturer, the new kit had a sensitivity of 88.2% (95% CI 72.6-96.7) and a specificity of 100% (95% CI 98.0-100). From a receiver-operating characteristic curve analysis, the more appropriate cutoff for a screening assay would be 1.6, which gives a sensitivity of 100% (95% CI 89.7-100) with a specificity of 94.0% (95% CI 89.4-96.9). In conclusion, this new simple assay appears promising and could be used, with the modified cutoff, as a screening assay for the detection of platelet alloantibodies.

Inhibitory and activating human antiplatelet antibodies

Platelets are essential for the maintenance of haemostasis and, on the other hand, play a pivotal role in the formation of a thrombus. It is clear that reduced platelet activity will result in a bleeding tendency, whereas stimulation of platelets can lead to thrombosis. Human antiplatelet antibodies may not only result in thrombocytopenia, but they have also been found either to inhibit or activate platelets. Inhibition by antibodies of the function of different receptors on platelets, such as collagen receptors, the glycoprotein (GP) Ib/IX (acquired Bernard-Soulier syndrome) or the GPIIb/IIIa complex (acquired Glanzmann's thrombasthenia), results in a haemorrhagic disorder very similar to the situation where the respective receptors are absent. On the other hand, reports have described a number of antibodies that activate platelets. The mechanism by which they do so varies and can involve interaction with the Fc receptor present on platelets, activation of the complement system or direct activation by binding to a signal-transducing antigen. Although the presence of such antibodies is expected to aggravate the problems due to the frequently occurring immune thrombocytopenia, treatment of these patients essentially relies on classical immunosuppressive therapy. In the case of activating antibodies, antithrombotic measures, such as anticoagulants or antiplatelet agents, can be envisaged.

BRa (HPA-5b) incompatibility may cause thrombocytopenia in neonates of mothers with immune thrombocytopenic purpura

PURPOSE

The association of anti-Br(a) immunoglobulin G (IgG) platelet alloantibodies with the development of thrombocytopenia in neonates of mothers with autoimmune thrombocytopenic purpura (ITP) is reported.

METHODS

Between March 1994 and July 1997, 28 consecutive pregnant women with ITP seen at New York Hospital were screened for platelet-reactive antiglycoprotein (glycoprotein [GP] IIb/IIIa, Ib/IX, and Ia/IIa) antibodies.

RESULTS

The sera from 6 of these 28 women contained IgG alloantibodies to GP Ia/IIa directed against the Br(a) (HPA-5b) antigen. Only three families each had at least one Br(a)+ and at least one Br(a)- infant. Platelet typing in these families revealed that the mothers were Br(b/b) (Br(a)-) and the fathers were Br(a/b) (Br(a)+). Platelet counts < 100,000/microl occurred only in 2 of the 3 infants who were Br(a)+. The platelet counts were significantly lower in the three Bra+ infants compared to the five Br(a)- infants (p = 0.038).

CONCLUSION

Platelet alloimmunization with anti-Br(a) can cause neonatal thrombocytopenia in infants of mothers with ITP. Platelet antibody testing in the pregnant women with ITP is recommended.

Leukocyte reduction and ultraviolet B irradiation of platelets to prevent alloimmunization and refractoriness to platelet transfusions

BACKGROUND

We conducted a multi-institutional, randomized, blinded trial to determine whether the use of platelets from which leukocytes had been removed by a filter or that had been treated with ultraviolet B irradiation would prevent the formation of antiplatelet alloantibodies and refractoriness to platelet transfusions.

METHODS

Patients who were receiving induction chemotherapy for acute myeloid leukemia were randomly assigned to receive one of four types of platelets transfusions: unmodified, pooled platelet concentrates from random donors (control); filtered, pooled platelet concentrates from random donors (F-PC); ultraviolet B-irradiated, pooled platelet concentrates from random donors (UVB-PC); or filtered platelets obtained by apheresis from single random donors (F-AP). All patients received transfusions of filtered, leukocyte-reduced red cells.

RESULTS

Of 530 patients with no alloantibodies at base line, 13 percent of those in the control group produced lymphocytotoxic antibodies and their thrombocytopenia became refractory to platelet transfusions, as compared with 3 percent in the F-PC group, 5 percent in the UVB-PC group, and 4 percent in the F-AP group (P< or =0.03 for each treated group as compared with the controls; there were no significant differences among the treated groups). Lymphocytotoxic antibodies were found in 45 percent of the controls, as compared with 17 to 21 percent in the treated groups (P<0.001 for each treated group as compared with the controls; there were no significant differences among the treated groups). Antibodies against platelet glycoproteins developed in 6 to 11 percent of the patients, with no significant differences among the four groups.

CONCLUSIONS

Reduction of leukocytes by filtration and ultraviolet B irradiation of platelets are equally effective in preventing alloantibody-mediated refractoriness to platelets during chemotherapy for acute myeloid leukemia. Platelets obtained by apheresis from single random donors provided no additional benefit as compared with pooled platelet concentrates from random donors.

Molecular determinants of arg-gly-asp ligand specificity for beta3 integrins

The Arg-Tyr-Asp (RYD) and Arg-Gly-Asp (RGD) sequences within the third complementarity-determining region of the heavy chain (H3) of murine recombinant Fab molecules OPG2 and AP7, respectively, are responsible for their specific binding to the platelet integrin alphaIIbbeta3. In this study, we evaluated the influence of divalent cation composition and single amino acid substitutions at key positions within H3 on the selectivity of these Fab molecules for integrin alphaIIbbeta3 versus the vitronectin receptor alphaVbeta3. The parent Fab molecule OPG2 (H3 sequence, HPFYRYDGGN) binds selectively to alphaIIbbeta3 and not at all to any other RGD-cognitive integrin, particularly alphaVbeta3, under any divalent cation conditions. The binding of the AP7 Fab molecule (HPFYRGDGGN) to alphaIIbbeta3 is not affected by the relative composition of calcium, magnesium or manganese. However, AP7 binding to alphaVbeta3, either expressed by M21 cells or as the purified integrin, is supported by manganese and inhibited by calcium. If the flanking asparagine 108 residue within the AP7 H3 loop is replaced by alanine (HPFYRGDGGA), the resulting Fab molecule AP7.4 binds selectively to alphaVbeta3 in a cation-dependent manner, but does not bind at all to alphaIIbbeta3 under any conditions. AP7.4 binding to alphaVbeta3 is supported by manganese, completely inhibited by calcium, and largely unaffected by magnesium. This behavior mimics that of the adhesive protein, osteopontin, another ligand that binds preferentially to alphaVbeta3. Despite these differences in specificity for alphaIIbbeta3 and alphaVbeta3, AP7 and AP7.4 remain selective for the beta3 integrins and do not bind to cell lines that express the RGD-cognitive integrins alphaVbeta5 or alpha5beta1. These results confirm that subtle changes in the amino acid composition immediately flanking the RGD or RYD motifs can have a profound effect on beta3 integrin specificity, most likely because they influence the juxtaposition of the arginine and aspartate side chains within the extended RGD loop sequence.

Evidence for an expression of blood group A antigen on platelet glycoproteins IV and V

Blood group ABO antigens are known to be carried by several platelet glycoproteins (GP), e.g. GPIb, GPIIa, GPIIb, GPIIa and PECAM. Beside these proteins, we recently observed that blood group A antigen was also expressed on some other uncharacterized platelet proteins (70-90 kDa) having electrophoretic mobilities closely resembling those of GPIV and GPV. These findings prompted us further to characterize these latter ABO-expressing platelet proteins. By antigen capture ELISA, wherein the monoclonal antibodies (mAbs) CLB-IVC7 and CLB-SWI6 were used to hold the corresponding antigens GPIV and GPV, human anti-A specifically bound to these proteins derived from A1-platelets; neither GPIV nor GPV derived from A2-, B- or O-platelets bound anti-A. In a Western blot assay using immunoprecipitated GPIV and GPV as antigens, mAb anti-A immunostained GPIV and GPV precipitated from A1, but not from A2 and O platelets. These results conclusively demonstrate that blood group A antigen is expressed on platelet GPIV and GPV.

Allele-specific restriction analysis of human platelet antigen system 4

BACKGROUND

Diagnosis, and occasionally treatment, of disorders involving platelet-specific alloimmunization, including neonatal alloimmune thrombocytopenia and posttransfusion purpura, requires platelet allotyping. Allele-specific restriction analyses for convenient genotyping are available for the major human platelet antigen (HPA) systems except HPA-4.

STUDY DESIGN AND METHODS

An allele-specific restriction analysis for HPA-4 was developed by designing a polymerase chain reaction primer containing a single-base substitution (A for T) at glycoprotein IIIa cDNA position 529. The resulting mismatch did not interfere with DNA amplification. Digestion of the polymerase chain reaction product with BsmI allowed differentiation of HPA-4a and HPA-4b alleles.

RESULTS

A 126-bp polymerase chain reaction product was amplified with the novel primer. BsmI endonuclease cleaved product encoding HPA-4a into 104- and 22-bp fragments and left DNA encoding HPA-4b intact.

CONCLUSION

This rapid allele-specific restriction analysis for genotyping the HPA-4 system complements similar methods for other platelet alloantigenic determinants.

A molecular basis for affinity modulation of Fab ligand binding to integrin alphaIIb beta3

The Arg-Gly-Asp (RGD) sequence within the third complementarity-determining region (CDR3) of the heavy chain (H3) is responsible for the binding of the recombinant murine Fab molecules, AP7 and PAC1.1, to the platelet integrin alphaIIbbeta3. AP7 binding is minimally influenced by the conformational state of this receptor, whereas PAC1.1 binds preferentially to the activated state of the receptor induced by platelet agonists. To study the molecular basis for this functional difference, we replaced the AP7 H3 loop (HPFYRGDGGN) with all or segments of the analogous sequence from PAC1.1 (RSPSYYRGDGAGP). AP7 Fd (VH domain + Cgamma1 domain) segments containing these H3 loop sequences were expressed as active Fab molecules by coinfection of Spodoptera frugiperda cell lines with recombinant baculoviruses containing Fd and AP7 kappa chain cDNA. Replacement of the entire AP7 H3 loop with that from PAC1.1 generated the mutant AP7.3 Fab molecule, which bound selectively to either activated, gel-filtered platelets or to purified alphaIIbbeta3 in a manner identical to that of PAC1.1. Identical results were obtained when solely the sequences flanking the amino side of RGD within the respective H3 loops were exchanged. AP7.3 and PAC1.1 exhibited saturable but submaximal binding to activated gel-filtered platelets. Relative to AP7, the number of AP7.3 or PAC1. 1 Fab molecules bound per platelet was 17% in the presence of 1 m Ca2+ + 1 mM Mg2+ or 40% in the presence of 10 microM Mn2+. The ratio of Fab molecules bound after versus before activation (mean =/- S.D.; n = 3) was: for AP7.3, 9.8 =/- 0.6; for PAC1.1, 8.8 +/- 0.3; and for AP7, 1.4 =/- 0.2. In addition, AP7 bound to the stably expressed integrin mutant alphaIIbbeta3(S123A), whereas AP7.3 and PAC1 did not. Because AP7.3 behaves in every respect like PAC1.1, we conclude that the ability of RGD-based ligands to distinguish activated from resting conformations of the integrin alphaIIbbeta3 can be regulated by limited amino acid sequences immediately adjacent to the RGD tripeptide. Furthermore, those Fab molecules that exhibit increased selectivity for the activated conformation of alphaIIbbeta3 bind to a subpopulation of this integrin on platelets that is modulated by divalent cations.

Biochemical and functional characterization of a new murine monoclonal antibody against human platelet glycoprotein IIIa

A new murine monoclonal antibody, MDP-1, specific for human platelet glycoprotein IIIa has been produced and characterized. Following SDS-polyacrylamide gel electrophoresis, MDP-1 reacted with a 94kDa protein immobilized on a nitrocellulose membrane. Upon reduction, MDP-1 no longer bound to the 94kDa protein indicating an epitope requiring at least one disulfide bond. On crossed immunoelectrophoresis MDP-1 reacted to the same peak as the GP IIb-IIIa complex-specific antibody AP-2. After dissociation of the GP IIb-IIIa complex with EDTA, AP-2 showed no reactivity while MDP-1 bound to a new peak that was broader and anodal to the original GP IIb-IIIa peak, consistent with GP IIIa. MDP-1 inhibited ADP and thrombin induced aggregation. In addition, MDP-1 inhibited ADP induced release of ATP, but did not inhibit thrombin stimulated ATP release. Following chymotrypsin digestion, MDP-1 bound to a cleaved GP IIIa protein (nonreduced M, = 122 kDa) consistent with opening of the major disulfide loop. A second cleavage resulted in a 63 kDa species that reacted with MDP-1. Scatchard analysis revealed 22 000 molecules of MDP-1 bound per platelet, and indicated a type of binding consistent with positive cooperativity. The antibody bound equally well to stimulated and unstimulated platelets. MDP-1 binding was inhibited by a polyclonal anti-PI(A1) antibody, but bound to platelets from a PI(A1) negative individual indicating a binding site close to but not identical to the PI(A1) epitope. In addition, MDP-1 binding was not inhibited by Arg-Gly-Asp-Ser (RGDS) suggesting that it is not directed to the RGD binding site on GP IIIa.

Fab-mediated binding of glycoprotein Ib/IX and IIb/IIIa specific antibodies in chronic idiopathic thrombocytopenic purpura

The antibody domain responsible for the interactions between platelet glycoproteins (GP) and serum IgG autoantibodies in patients with chronic idiopathic thrombocytopenic purpura (ITP) was studied. Sera from nine non-transfused ITP patients and 20 normal controls and a serum containing an anti-PlA1 antibody were employed. Serum, purified IgG and F(ab')2 fragments were prepared and their binding to platelet GPIb/IX and GPIIb/IIIa were analysed using a modified MAIPA assay and an antigen capture ELISA. In all experiments most of the autoantibodies studied behaved identically to the anti-PlA1 antibody in that the IgG-F(ab')2 fragments retained their ability to bind to the respective glycoprotein. Substituting the enzyme-conjugated secondary antibody (Fab specific), in the MAIPA assay, with an Fc specific antibody removed all reactivities observed against platelet GPs, produced by IgG-F(ab')2 fragments. Furthermore, in an antigen-capture ELISA, IgG autoantibodies against platelet GPIb/IX and/or GPIIb/IIIa were blocked preferentially by pre-incubating the ITP sera with a goat anti-human IgG (F(ab')2 specific) antibody, but not with an anti-Fc antibody. We conclude that these ITP patients produced antibodies specific for platelet GPIb/IX and/or GPIIb/IIIa, and that the autoantibody-platelet interaction was mediated by the classic Fab binding.

Analysis of platelets by flow cytometry

Transfusion medicine has become a multi-disciplinary field with many recent technical developments and the flow cytometer has had a significant impact in transfusion medicine, especially at the level of platelet immunobiology. Many routine tests in platelet immunology are now performed by flow cytometry laboratories, including assessment of platelet-associated allo- and auto-antibodies and complement components. Platelet analysis by flow cytometry has been applied to detection of platelet antigens, platelet surface-bound proteins, platelet activation, measurement of reticulated platelets, intracellular calcium studies, and the measurement of platelet microparticles in vivo and in vitro. This review will focus on the use of the flow cytometer in these applications in investigations of platelet immunology.

Platelet autoantibodies in patients with chronic liver disease

The thrombocytopenia in chronic liver disease (CLD) has been attributed mainly to hypersplenism, although other factors such as reduced mean life span with increased platelet turnover have also been demonstrated. Immunological abnormalities have been described in the pathogenesis and progression of CLD. In this sense, many studies have reported elevated levels of platelet associated IgG (PAIgG) in patients with CLD, and it has been suggested that PAIgG could represent true antiplatelet antibody. In this study we used a glycoprotein (GP)-specific immunoassay (MACE) to determine whether PAIgG or circulating antiplatelet antibodies, reacted against the GPIIb/IIIa or GPIb/IX complexes, in patients with CLD. Thirty-six patients with CLD of diverse etiology were studied (20 female, mean age 53 years, range 38-75 years). 23 out of 36 patients (64%) had anti-GP antibodies in MACE. Particularly, 12 had anti-GPIb, 4 anti-GPIIb/IIIa, and 7 had both types of autoantibodies. The existence of these anti-GP antibodies was not related with the blood platelet count or etiology of CLD. These data show that in patients with CLD of diverse origin, there is a high prevalence of autoantibodies reacting specifically with platelet membrane GP, which constitutes the first evidence of the specific nature of platelet-bound IgG in CLD. These findings suggest that in patients with CLD, an immune mechanism may participate in inducing or aggravating the thrombocytopenia.

Effects of anti-platelet glycoprotein Ib and/or IIb/IIIa autoantibodies on the size of megakaryocytes in patients with immune thrombocytopenia

To determine whether anti-platelet autoantibodies react with megakaryocytes, as well as with platelets, in immune thrombocytopenia (ITP), 38 ITP patients were studied. They were classified into four groups; anti-platelet glycoprotein Ib-positive (group A, n = 5), anti-platelet glycoprotein II/b/IIIa-positive (group B, n = 2), positive to both antibodies (group C, n = 3), and negative to both antibodies (group D, n = 28). The number and size of megakaryocytes in each group were compared. The number of megakaryocytes in groups A, B, C, and D was 12.8 +/- 8.9, 75.2, 29.1, and 17.0 +/- 21.7/mm2, respectively. The mean cytoplasmic area of megakaryocytes in groups A, B, C, and D was 1001 +/- 26.3, 1621, 1109, and 1311 +/- 235.6/micrograms2, respectively. This finding indicated that, in the presence of anti-platelet glycoprotein Ib, megakaryocytes were not increased in number and were small in size, whereas, in the presence of anti-platelet glycoprotein IIb/IIIa, megakaryocytes were increased in number and in cytoplasmic area. Our study suggested that anti-platelet glycoprotein Ib may impair platelet production by megakaryocytes in ITP.

Megakaryopoiesis in patients with cyclic thrombocytopenia

Megakaryopoiesis was examined in 10 patients (eight females and two males) with cyclic thrombocytopenia (CT) to investigate the underlying pathogenesis. Numbers of CFU-Meg and megakaryocytes and the mean cytoplasmic area (mean area) of megakaryocytes at the peak, nadir, ascent mid phase, and descent mid phase of the platelet cycle were determined. The patients were classified as female cases group I (cases 1-4; previously diagnosed as ITP and CT occurred during remission), female cases group II (cases 5-8; persistent CT from initial diagnosis), and male CT (cases 9 and 10). In three of the four female cases in group I, numbers of CFU-Meg and megakaryocytes were normal or increased persistently during the platelet cycle, whereas the mean area fluctuated in synchrony with the platelet cycle, suggesting failure of cyclic production rather than platelet destruction. In the female cases in group II and one female case in group I, numbers of CFU-Meg and megakaryocytes were also normal or increased at four phases of the cycle, but the mean area did not fluctuate, remaining large during the cycle, suggesting cyclic destruction or platelet clearance. In contrast, in the male patients values for numbers of CFU-Meg, megakaryocytes and mean cytoplasmic area fluctuated during the platelet cycle, indicating that cyclic changes in megakaryopoiesis generated the platelet cycle. These findings indicate that the measurement of cytoplasmic area is useful for distinguishing cyclic platelet production from cyclic destruction or clearance in CT.

A platelet monoclonal antibody inhibition assay for detection of glycoprotein IIb/IIIa-related platelet alloantibodies

Post-transfusion purpura (PTP) and neonatal alloimmune thrombocytopenia (NAT) result from formation of alloantibodies to platelet membrane glycoprotein-associated antigens. The detection and identification of platelet-specific alloantibodies in patient sera is often complicated by the presence of co-existing HLA antibodies and/or more than one platelet specificity in the same serum. We describe a solid phase assay that specifically detects antibodies to platelet membrane associated alloantigens by measuring the ability of patient antisera to inhibit the binding of glycoprotein GPIIb or GPIIIa monoclonal antibodies to intact platelets. When tested in the GPIIIa assay against a panel of random platelet donors, the reactivities of two known PLAI antisera that also contained different HLA antibodies were highly correlated (r = 0.99) and allowed PLA phenotyping of the population. A standard direct binding platelet ELISA, on the other hand, was unable to accurately PLA phenotype the same population. The reactivities of two known Baka antisera (one containing additional anti-PLA2 and the other anti-Brb specificities) were highly correlated (r = 0.95) in the GPIIb assay, and Bak phenotype determination was similarly accomplished for a random platelet panel. Furthermore, a comparison of platelet phenotype results (using the monoclonal inhibition assay) and genotype results (using DNA analysis) for the PLA and Bak systems showed a concordance of 98% for 146 alleles tested. In conclusion, the platelet monoclonal antibody inhibition assay: (1) allows determination of platelet-specific alloantibodies in the presence of contaminating HLA antibodies and/or in sera containing multiple platelet alloantibodies; (2) allows accurate platelet phenotyping for the GPIIIa-associated PLA and GPIIb-associated Bak antigen systems; and (3) may be applicable to the detection of other known or even novel platelet glycoprotein-associated antigens.

An anti-alpha v-integrin antibody that blocks integrin function inhibits the development of a human melanoma in nude mice

A series of murine monoclonal antibodies were raised against purified human alpha v beta 3 integrin and against M21 human melanoma cells. Five notable hybridomas were identified by ELISA on purified integrins, and the isolated antibodies bound the alpha v-chain. These antibodies, 17E6, 20A9, 23G5, 14D9.F8 and 10G2, recognised the extracellular domains of the integrin, and were shown to be reactive in FACS, immunoprecipitation, ELISA, and ELISA on fixed cells with M21, M21-L4, and UCLA-P3, but not with the alpha v-deficient M21-L or M21-L-IIb (M21-L transfected with GpIIb integrin). One antibody, 17E6, strongly perturbed cell attachment mediated by alpha v integrins, reacting at least with alpha v beta 3, alpha v beta 5, and alpha v beta 1, and strongly inhibiting cell attachment to alpha v-ligands vitronectin and fibronectin with an IC50 of approximately 0.1 microgram ml-1. Furthermore, 17E6 at this concentration could induce cell retraction from the substrate, while LM609 (anti-alpha v beta 3) and control antibody 14E2 (anti-200 kDa melanoma surface protein) at 1,000-fold higher concentrations had minimal effects on cell morphology. The action of 17E6 was reversible and was not due to toxic effects: in vitro 17E6 at 0.1 mg ml-1 did not affect either cell proliferation or DNA synthesis. In two nude-mouse tumour models, subcutaneous tumour development and a lung colonisation (‘experimental metastasis’) assay, injection of 17E6 strongly inhibited tumour development, while isotype-matched controls had no effect. There was no obvious mechanism of cell or of complement-mediated tumour cytotoxicity; the antibody did not mediate ADCC or AECDC, or complement fixation. The data strongly support previous studies which have indicated the importance of alpha v-integrins, and especially alpha v beta 3, in the tumour progression of human melanoma.

The exchange of Arg-Gly-Asp (RGD) and Arg-Tyr-Asp (RYD) binding sequences in a recombinant murine Fab fragment specific for the integrin alpha IIb beta 3 does not alter integrin recognition

The murine monoclonal antibody OPG2 is an excellent paradigm of natural RGD ligands and binds specifically to alpha IIb beta 3 integrin. A reactive Arg103-Tyr104-Asp105 (RYD) tripeptide is located in an extended loop, the third complementarity-determining region of the heavy chain (H3). When compared to other RGD ligands, the RYD tripeptide of OPG2 is unique, in that the side chains are fixed in a stable orientation that we have defined by x-ray crystallography. In this study, we express OPG2 H chain segments (Fd) and kappa chains as components of active, Fab heterodimers by coinfection of Spodoptera frugiperda cell lines with recombinant baculoviruses containing cDNA specific for each protein. Recombinant AP7 Fd segments are generated from the parent OPG2 Fd segments by replacement of Tyr104 with Gly, while recombinant AP7E Fd segments are produced from AP7 Fd segments, by exchange of Asp105 with Glu. Neither the free Fd segments nor the free kappa chains of OPG2 or AP7 can bind to alpha IIb beta 3. The AP7 Fab fragment, like the parent OPG2 Fab, binds strongly to purified alpha IIb beta 3 but weakly, if at all, to purified alpha V beta 3. The affinity of OPG2 and AP7 Fab fragments for gel-filtered platelets, whether nonstimulated or activated by 0.2 microM phorbol 12-myristate 13-acetate, is identical. As with other natural RGD ligands, the binding of recombinant OPG2 Fab or AP7 Fab fragments to purified alpha IIb beta 3 or to gel-filtered platelets is completely inhibited by the peptide RGDW or by addition of EDTA, AP7E Fab fragments do not bind at all to either purified alpha IIb beta 3 or platelets. Our results demonstrate, for the first time within a natural protein ligand, that the tripeptides RGD and RYD exhibit equivalent binding capacity and specificity for the integrin alpha IIb beta 3.

Repertoire cloning of a human IgG inhibitor of alpha IIB beta 3 function. The OG idiotype

A patient (OG) with Glanzmann thrombasthenia became refractory to platelet transfusion following the production of IgG antibodies (Ab1) specific for the integrin subunit beta 3. We generated recombinant VH and VL cDNA libraries using IgG-specific mRNA isolated from OG peripheral blood B-lymphocytes that had been selected for binding to antigen (alpha IIb beta 3 adsorbed to plastic dishes). These antigen-specific B-lymphocytes contain rearranged VH DNA segments that belong exclusively to the VH4 gene family. Recombinant Fab were expressed on the surface of filamentous phage coinfected with VH and VL segments cloned into the phagemid pHEN1 or the phage fd-tet-DOG1. To facilitate selection of the desired recombinant Ab1 Fab, we developed a rabbit polyclonal antibody specific for affinity-purified OG anti-beta 3 Fab (Ab2). Ab2 reacts specifically with Ab1, and this interaction is inhibited by purified alpha IIb beta 3. Following three rounds of phage selection on Ab2 adsorbed to plastic dishes and random reassociation of heavy and light chains, we isolated Ab1 Fab and tested their binding to alpha IIb beta 3. Five Id-positive Fab were selected for further characterization. These Fab use one of two VH genes (H21 or H23) complexed with one of three V lambda genes. Subsequent sequence data demonstrated that all three lambda genes are the same clone L22 which uses a germline V lambda gene segment. Fab using H23 bind to alpha IIb beta 3, while those using H21 do not. Based on sequence homology, both H21 and H23 use VH gene segments belonging to the VH4 gene family. Thus, the idiotype OG is restricted to the VH4 gene family and is the first sequenced prototype of human antibodies that bind close to or at a functional epitope(s) of alpha IIb beta 3.

Further characterization of the thrombasthenia-related idiotype OG. Antiidiotype defines a novel epitope(s) shared by fibrinogen B beta chain, vitronectin, and von Willebrand factor and required for binding to beta 3

A patient (OG) with Glanzmann thrombasthenia became refractory to platelet transfusion after the production of an immunoglobulin G (IgG) isoantibody (Ab1) specific for the integrin subunit beta 3. To determine the frequency at which the OG idiotype is found in the general population and in immune-mediated disease states, we developed a rabbit polyclonal antibody (Ab2) specific for affinity-purified OG anti-beta 3 Fab. The binding of Ab2 to Ab1 is inhibited by purified alpha IIb beta 3. Ab2 als binds to IgG specific for alpha IIb beta 3 obtained from one nonrelated Glanzmann thrombasthenia patient ES who has developed isoantibodies of similar specificity. On the other hand, Ab2 does not recognize alpha IIb beta 3-specific antibodies produced by two Glanzmann thrombasthenia patients, AF and LUC, who have developed isoantibodies with specificities distinct from that of the OG isoantibody. Moreover, Ab2 does not recognize alpha IIb beta 3-specific antibodies developed by three representative patients with (autoimmune) thrombocytopenic purpura or six representative patients with alloimmune thrombocytopenias, nor does it bind to IgG from any of 13 nonimmunized individuals. We have found that Ab2 also binds to selected protein ligands of alpha IIb beta 3 namely, fibrinogen, vitronectin, and von Willebrand factor, but not to other protein ligands or control proteins, such a fibronectin, type I collagen, and albumin. The epitope(s) recognized by Ab2 on each adhesive protein are either very similar or identical since each protein can inhibit the binding of Ab2 to any of the other proteins. The epitope on fibrinogen recognized by Ab2 resides in the B beta chain, and is likely contained within the first 42 amino acids from the NH2 terminus. Since OG IgG inhibits fibrinogen binding to alpha IIb beta 3, the specificity of the OG idiotype defines a novel binding motif for the integrin alpha IIb beta 3 that is shared by fibrinogen, vitronectin, and von Willebrand factor, but distinct from previously described RGD-containing sites on the fibrinogen, A alpha chain or the fibrinogen gamma chain COOH-terminal decapeptide site. Our findings reported here represent an excellent example of molecular mimicry in which an antigen-selected, IgG inhibitor of alpha IIb beta 3 function shares a novel recognition sequence common to three physiologic protein ligands of that receptor.

A new private platelet antigen, Groa, localized on glycoprotein IIIa, involved in neonatal alloimmune thrombocytopenia

The serum of a Caucasian woman who gave birth to a child with neonatal alloimmune thrombocytopenia contained antibodies directed against a platelet antigen of the newborn. There was no incompatibility for the known platelet alloantigens HPA-1 to HPA-7 or for the private or low-frequency antigens Sra and Vaa, between the platelets of the parents. However, crossmatching with the serum of the mother and the platelets of the child and the father was strongly positive, suggesting a new platelet antibody specificity. To investigate the inheritance of the 'Groa' antigen involved, the available family members were tested in the platelet immunofluorescence test (PIFT) and the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. The Groa antigen was found to be inherited in an autosomal-codominant fashion. In the MAIPA, we localized the Groa antigen on the glycoprotein IIb/IIIa complex (alpha IIb beta 3). The GP IIb/IIIa localization was confirmed in immunoprecipitation studies. In Western blotting experiments, we further localized the Groa antigen on the GP IIIa (beta 3) subunit of the GP IIb/IIIa complex. Until now we have tested approximately 400 unrelated donors. None of these appeared to be positive for the Groa antigen, suggesting a phenotype frequency in the Dutch population of less than 0.01.

A case of severe neonatal thrombocytopenia with schizencephaly associated with anti-HPA-1b and anti-HPA-2a

We report a family with a neonate who was severely damaged by intracranial haemorrhages. These probably occurred before the 20th week of gestation. The neonate had a moderate thrombocytopenia. In the maternal serum anti-HPA-1b and anti-HPA-2a alloantibodies were detected. Third-generation assays were applied to identify the alloantibodies. No other cause for the bleeding was found. Probably the combination of anti-HPA-1b and anti-HPA-2a alloantibodies, directed against the platelet fibrinogen receptor and the von Willebrand receptor, respectively, induced a thrombocytopenia and a thrombocytopathy.

A new platelet alloantigen, Tua, on glycoprotein IIIa associated with neonatal alloimmune thrombocytopenia in two families

We describe immunization of two mothers against a new platelet alloantigen, designated Tua, in association with thrombocytopenia in their first born children. The platelet-specific antibodies were identified by a glycoprotein-specific platelet protein assay with husband's platelets. Monoclonal antibodies against glycoprotein complex IIb/IIIa (AP2) and against glycoprotein IIb (SZ22) could be used to immobilize the antigen bearing protein. When monoclonal antibodies against glycoprotein Ib/IX (FMC25) or Ia/IIa (Gi9) were used, no platelet-specific antibodies were detectable. The previously described alloantigens on the glycoprotein IIb/IIIa complex (HPA 1,3,4, Sra and Vaa) were not responsible for the reaction. Immunochemical analysis by an immunoblot assay showed that the Tua antigen resides on GPIIIa but the antigen was destroyed by reduction of the protein. Altogether 10 individuals belonging to three unrelated families were shown to carry the antigen. The family studies within three generations indicated autosomal codominant inheritance. Thus the Tua antigen is apparently different from all previously published platelet alloantigens. One Tua positive blood donor was identified in a population study of approximately 150 individuals. This indicates a low frequency in the Finnish population. Extended population studies will be required to determine a more exact frequency of Tua antigen.

Fetal and neonatal alloimmune thrombocytopenia: current trends in diagnosis and therapy

Neonatal thrombocytopenia affects 20-40% of the infants in intensive care units. The frequency of neonatal alloimmune thrombocytopenia (NAIT) is estimated at 1/1500 to 1/5000 live births. The risk of morbidity is significant with 20% neurological sequelae and the death rate is estimated at 10% of affected infants. During recent years considerable efforts have been made to prevent fetal bleeding and to avoid birth trauma, which have significantly changed the natural history of NAIT.

Anti-HPA-4b (anti-Yuk(a)) neonatal alloimmune thrombocytopenia: first report in a Caucasian family

Neonatal alloimmune thrombocytopenia (NAIT) is caused by platelet antigen incompatibility between the mother and fetus. NAIT is mainly due to alloimmunization; the frequency varying among ethnic groups. In Caucasians HPA-1a is the antigen most frequently implicated. In Japan, NAIT due to anti-HPA-4b antibody has already been described, but this is the first case to be reported in Caucasians.

An amino acid polymorphism within the RGD binding domain of platelet membrane glycoprotein IIIa is responsible for the formation of the Pena/Penb alloantigen system

The human Pena/Penb alloantigen system represents a naturally occurring polymorphism of human platelet membrane glycoprotein (GP) IIIa, and has previously been implicated in the onset of two important clinical syndromes, neonatal alloimmune thrombocytopenic purpura and posttransfusion purpura. To investigate the molecular basis of the polymorphism underlying the Pen alloantigen system, we used the polymerase chain reaction to amplify platelet-derived GPIIIa mRNA transcripts. DNA sequence analysis of amplified GPIIIa cDNAs from nucleotides 161 to 1341 (encompassing amino acid residues 22-414) revealed a G526<==>A526 polymorphism that segregated precisely with Pen phenotype in twelve other individuals examined. This nucleotide substitution results in an Arg (CGA) to Gln (CAA) polymorphism at amino acid 143 of GPIIIa. Interestingly, this polymorphic residue is located within the putative RGD binding site (residues 109-171) of GPIIIa. Platelet aggregation patterns of a Penb/b individual, however, were nearly normal in response to all physiological agonists tested, indicating that this polymorphism does not grossly affect integrin function. Short synthetic peptides encompassing residue 143 were unable to mimic either the Pena or Penb antigenic determinants, suggesting that the Pen epitopes are dependent upon proper folding of the polypeptide chain. Finally, we constructed allele-specific recombinant forms of GPIIIa that differed only at amino acid residues 143. Whereas anti-Pena alloantibodies were able to recognize the Arg143 recombinant form of GPIIIa, anti-Penb antibodies were not. Conversely, anti-Penb alloantibodies were reactive only with the Gln143 isoform of the GPIIIa molecule. It thus appears that amino acid 143 of GPIIIa is not only associated with Pen phenotype, but specifically controls the formation and expression of the Pen alloantigenic determinants.

Endoglycosidase H digestion of Yukb (Pena) alloantigen

We characterized a platelet specific alloantigen (Yukb). In immunoblotting, anti-Yukb antibody was found to react with both 110 kDa and 96 kDa bands under nonreducing condition. Immunoblotting followed by separation by two-dimensional electrophoresis (isoelectric focusing/SDS-PAGE) showed that the 96 kDa band had a pI of 5.1-5.8, while the 110 kDa band had a pI of 5.2-5.7. The 96 kDa band was identified as glycoprotein (GP) IIIa on the basis of periodic acid Schiff staining, but the 110 kDa band was not characterized. These results implied that it is difficult to determine differences in antigenic epitopes between Yukb and PlA1 antigens by the electrophoresis. Yukb antigen, unlike PlA1 antigen, was partially destroyed by chymotrypsin treatment. Furthermore, endoglycosidase H digestion resulted in loss of the Yukb epitope, while the PlA1 determinant was retained on the three bands with lower molecular weights after endoglycosidase H digestion. The transfer of Yukb antigens recognized by anti-Yukb antibody into the supernatant of platelets treated with endoglycosidase H was also found using mixed passive hemagglutination test. The results indicated that the Yukb epitope might be located to the endoglycosidase H digested N-linked high mannose carbohydrate chains of GP IIIa or hybrid type chains of GP IIIa, which is different from the location of PlA1 epitope.