The alloimmune thrombocytopenic syndromes

Passenger lymphocyte thrombocytopenia due to human platelet antigen 3a antibodies: Case report and review of literature

We report a case of severe acute thrombocytopenia occurring within days after a cadaveric liver transplant, received from a female patient with aplastic anemia who died of intracranial bleeding. The donor, who was homozygous for the ITGA2B*002 (HPA-3b) gene, had developed human platelet antigen (HPA)-3a antibodies, whereas the recipient was homozygous for the ITGA2B*001 (HPA-3a) gene. Thrombocytopenia responded to an infusion of immunoglobulin G. This is the first report of a passenger lymphocyte syndrome manifesting with thrombocytopenia due to anti-HPA-3a. We review the literature on thrombocytopenia in the setting of PLS and discuss the differential diagnosis.

How do we approach thrombocytopenia in critically ill patients?

A low platelet count is a frequently encountered haematological abnormality in patients treated in intensive treatment units (ITUs). Although severe thrombocytopenia (platelet count <20 × 10⁹ /l) can be associated with bleeding, even moderate-degree thrombocytopenia is associated with organ failure and adverse prognosis. The aetiology for thrombocytopenia in ITU is often multifactorial and correcting one aetiology may not normalise the low platelet count. The classical view for thrombocytopenia in this setting is consumption associated with thrombin-mediated platelet activation, but other concepts, including platelet adhesion to endothelial cells and leucocytes, platelet aggregation by increased von Willebrand factor release, red cell damage and histone release, and platelet destruction by the complement system, have recently been described. The management of severe thrombocytopenia is platelet transfusion in the presence of active bleeding or invasive procedure, but the risk-benefit of prophylactic platelet transfusions in this setting is uncertain. In this review, the incidence and mechanisms of thrombocytopenia in patients with ITU, its prognostic significance and the impact on organ function is discussed. A practical approach based on the authors' experience is described to guide management of a critically ill patient who develops thrombocytopenia.

Newer anticoagulants in critically ill patients

Critically ill patients are at increased risk for development of thrombosis. In addition, thrombosis is often unrecognized in this population. Furthermore, these patients are particularly susceptible to bleeding complications from anticoagulants. Herein the authors review the pharmacology, data from clinical trials, management of bleeding complications, and perioperative use of these agents in the intensive care unit population. Well-designed clinical trials are needed to improve our understanding of the safety and efficacy of these newer agents in critically ill patients.

Birds, babies and blood

This is an autobiographical review describing the author's career in immunology research and summarizing his current understanding of the areas involved. Contributions to autoimmunity, immune deficiency, transfusion immunology, HLA-disease associations, reproductive immunology, cellular therapies, and innate immunity are included; also discussion of medical research ethics and various research-related activities.

Foetal/neonatal alloimmune thrombocytopenia in Egypt; human platelet antigen genotype frequencies and antibody detection and follow-up in pregnancies

BACKGROUND AND OBJECTIVES

Foetal and neonatal alloimmune thrombocytopenia (FNAIT) is studied mainly in Caucasian populations. Severe thrombocytopenia (<50×10(9)/L) gives risk of haemorrhage and the most feared complication is intracranial haemorrhage (ICH). In Caucasian populations anti-human platelet antigen (HPA)-1a antibodies are the cause of FNAIT in >80% of the cases. The aims of this project were to study the gene frequencies of HPA-1-5 and 15 alleles in an Egyptian population (Arabic), and to determine the frequency of HPA-1a and -5b immunisations in a cohort of Egyptian pregnant women.

MATERIALS AND METHODS

Altogether 6974 pregnant women were included in the study. Genotyping was performed by polymerase chain reaction and antibodies were detected by flow cytometry and enzyme-linked immunosorbent assay. HPA-1-5 and 15 alleles were studied in 367 individuals.

RESULTS

The HPA genotypes differed from genotypes published from different Caucasian and Chinese (Han) populations in HPA-1, -2, -3, and -5 systems with significant higher frequency of HPA-1b, -2b and -5b. The rate of HPA-1a alloimmunisation was found comparable to Caucasian populations. Severe thrombocytopenia was found in two newborns. No bleeding complication was reported. Anti-HPA-5b antibodies were detected in 4.4% of the pregnant women. Clinical consequences of these antibodies were not studied.

CONCLUSION

The HPA-1bb and -5bb genotypes are more frequent in the Egyptian Arabic population studied compared to Caucasian populations. FNAIT due to anti-HPA-1a and -5b antibodies must be suspected in cases of neonatal thrombocytopenia. Further large prospective studies are needed to increase the knowledge of clinical complications related to HPA alloantibodies in populations with different genetic backgrounds.

HITlights: a career perspective on heparin-induced thrombocytopenia

Two decades of research into heparin-induced thrombocytopenia (HIT) permit a personal historical perspective on this fascinating syndrome. Previously, the frequency of HIT was unknown, although complicating thrombosis was believed to be rare and primarily arterial. The opportunity to apply a remarkable test for "HIT antibodies"--the (14) C-serotonin-release assay (SRA)--to serial plasma samples obtained during a clinical trial of heparin thromboprophylaxis, provided insights into the peculiar nature of HIT, such as, its prothrombotic nature--including its strong association with venous thrombosis (RR = 11.6 [95%CI, 6.4-20.8; P < 0.0001); its more frequent occurrence with unfractionated versus low-molecular-weight heparin; the "iceberg" model, which states that among the many patients who form anti-PF4/heparin antibodies during heparin therapy, only a minority whose antibodies evince strong platelet-activating properties develop HIT; and the characteristic HIT timeline, whereby serum/plasma antibodies are readily detectable at or prior to the HIT-associated platelet count fall. Applying the SRA also to patients encountered in clinical practice led to recognition of warfarin-induced venous limb gangrene (for which HIT is a major risk factor via its extreme hypercoagulability) and delayed-onset HIT (whereby thrombocytopenia begins or worsens following heparin discontinuation, due to the ability of HIT antibodies strongly to activate platelets even in the absence of heparin--so-called heparin-"independent" platelet activation). Recent concepts include the increasing recognition of HIT "overdiagnosis" (due to the low diagnostic specificity of the widely-applied PF4-dependent immunoassays), and the observation that HIT-associated consumptive coagulopathy is a risk factor for treatment failure with PTT-adjusted direct thrombin inhibitor therapy ("PTT confounding" secondary to HIT-associated coagulopathy).

Bleeding risk and the management of bleeding complications in patients undergoing anticoagulant therapy: focus on new anticoagulant agents

For more than 60 years, heparin and coumarin have been mainstays of anticoagulation therapy. They are widely available, inexpensive, effective, and have specific antidotes but are regarded as problematic because of their need for careful monitoring. In addition, coumarin has a delayed onset of action, interacts with many medications, has a narrow therapeutic window, and is paradoxically prothrombotic in certain settings (ie, can precipitate “coumarin necrosis”). Heparin may require monitoring of its therapeutic effect and can also cause thrombosis (heparin-induced thrombocytopenia/thrombosis syndrome). These limitations have led to the development of new anticoagulants with the potential to replace current agents. These newer agents fall into 2 classes, based on whether they are antithrombin dependent (low-molecular-weight heparin, fondaparinux) or antithrombin independent (direct inhibitors of factor Xa and thrombin [factor IIa]). This paper addresses newer anticoagulants, reviewing their efficacy and limitations, and focuses on the risk of major bleeding that may complicate their use. In contrast to heparin and coumarin, none of these newer agents has a specific antidote that completely reverses its anticoagulant effect. Available data on the efficacy and safety of current and experimental agents for anticoagulant reversal are reviewed, and a plan for management of anticoagulant-induced bleeding is presented.

Postnatal management of fetal and neonatal alloimmune thrombocytopenia: the role of matched platelet transfusion and IVIG

We evaluated the effects of platelet transfusions and intravenous immunoglobulin (IVIG) in neonates with fetal and neonatal alloimmune thrombocytopenia (FNAIT) with and without antenatal treatment with IVIG. Records of neonates with FNAIT admitted between January 2000 and November 2005 were reviewed. The patients were divided into group I, treated antenatally with IVIG for known FNAIT, and group II, postnatally diagnosed with FNAIT. The primary outcome was the time interval to reach a platelet level above 100 x 10(9)/L in relation to the type of treatment. Nineteen neonates with FNAIT were identified, 13 in group I and 6 in group II. In group I, four children were born with a platelet count above 100 x 10(9)/L and never needed treatment, and four received a single matched platelet transfusion at birth with a maintained response. Five neonates received IVIG and one matched transfusion, with all but one rapidly responding. In antenatally treated cases, postnatal IVIG had no apparent effect on the platelet count. In group II, two neonates died on day 1 with severe intracranial hemorrhage. Two of the four other patients responded to a number of unmatched platelet transfusions, with one neonate rapidly responding after one matched transfusion, while another needed nine matched transfusions before a persistent adequate platelet count was reached after 9 weeks. Postnatal IVIG had no apparent effect on the platelet count in any of our cases. In neonates with FNAIT treated antenatally with IVIG, neonatal management using a single matched platelet transfusion was adequate in all cases. In neonatally diagnosed cases not treated before birth, multiple matched platelet transfusions may be required. We found no evidence to support the use of IVIG in neonates with FNAIT.

Prediction of the effect of acute normovolemic hemodilution on the hematological constituents of sequestered autologous whole blood

During acute normovolemic hemodilution (ANH), autologous whole blood is collected in a series of collection bags containing anticoagulant. The effect of hemodilution on the actual hematological constituents of this sequestered whole blood product has never been examined. We developed a mathematical model that predicts how whole blood bag constituents change during ANH to elucidate the theoretical basis for ANH efficacy. Formulas were derived to calculate the effect of ANH on [X], the blood constituent of interest. An exponential envelope was defined so that the projected impact of ANH on each constituent could be computed while initial blood volume and whole blood bag volume (WB(ANH)) were manipulated. Equivalency of autologous whole blood hemoglobin, platelets, and fibrinogen were determined by comparison with standard allogeneic blood products. We determined that the concentration of blood constituent X in a particular unit of collected blood ([X](n)) is provided as a fraction of the initial concentration ([X](0)). As WB(ANH) increases relative to estimated blood volume, the decrement in [X](n) increases in successive blood collection bags. Irrespective of initial blood volume, the equivalence of a 450-mL autologous whole blood bag to 1 U of packed red cells and 1 U of whole blood-derived platelet concentrate is 13.3 g/dL and 123 x 10(3)/microL, respectively. The impact of ANH on autologous whole blood constituents may be accurately predicted using this model. Conversion of WB(ANH) into equivalent allogeneic blood products could provide a useful method of comparing outcome in various ANH studies. The exponential envelope may be used to assess the actual ANH technique performed by the anesthesiologist, which in turn may impact quality assurance standards.

Alloimmune thrombocytopenias

Alloimmunization against platelet antigens may take place during pregnancy and following transfusion or transplantation. The resulting antibodies bind to the target platelet antigens and may lead to a rapid sequestration of incompatible platelets. The alloimmune thrombocytopenias are classified into neonatal alloimmune thrombocytopenia, post-transfusion purpura, passive alloimmune thrombocytopenia, transplantation-associated alloimmune thrombocytopenia, and platelet transfusion refractoriness. Neonatal alloimmune thrombocytopenia occurs in roughly one of 1,000 births and is the most important type in children. The management of affected infants during pregnancy is yet not established, and the most rapidly effective treatment after delivery is the transfusion of compatible platelets.

A functional platelet fibrinogen receptor with a deletion in the cysteine-rich repeat region of the beta(3) integrin: the Oe(a) alloantigen in neonatal alloimmune thrombocytopenia

This report describes a new low-frequency alloantigen, Oe(a), responsible for a case of neonatal alloimmune thrombocytopenia (NAIT). In a population study none of 600 unrelated blood donors was an Oe(a) carrier. By immunochemical studies the Oe(a) antigen could be assigned to platelet glycoprotein (GP) IIIa. Sequencing of GPIIIa complementary DNA from an Oe(a) (+) individual showed deletion of a lysine residue at position 611 (DeltaLys(611)). Analysis of 20 Oe(a) (-) and 3 Oe(a) (+) individuals showed that the DeltaLys(611) form of GPIIIa was related to the phenotype. Anti-Oe(a) reacted with the DeltaLys(611), but not with the wild-type isoforms on stable transfectants expressing GPIIIa, indicating that DeltaLys(611) directly induces the expression of Oe(a) epitopes. Under nonreducing conditions the Pro(33)DeltaLys(611) variant migrated with a slightly decreased molecular weight compared to the Pro(33)Lys(611) isoform suggesting that DeltaLys(611) has an influence on the disulfide bonds of GPIIIa. The Pro(33)DeltaLys(611) GPIIIa could undergo conformational changes and bind to fibrinogen in a similar manner as the Pro(33)Lys(611) isoform. No difference was found in the tyrosine phosphorylation of pp125(FAK), suggesting that DeltaLys(611) has no effect on integrin function. In contrast to all other low-frequency antigens, the DeltaLys(611) isoform was associated with the HPA-1b, but not with the high frequency HPA-1a allele. Comparison with GPIIIa DNA from nonhuman primates indicated that the HPA-1a allele represents the ancestral form of GPIIIa. It can be assumed that the Oe(a) form did arise as a result of a mutational event from an already mutated GPIIIa allele.

Very low platelet counts in post-transfusion purpura falsely diagnosed as heparin-induced thrombocytopenia. Report of four cases and review of literature

Differential diagnosis between post-transfusion purpura (PTP) and heparin-induced thrombocytopenia (HIT) can be difficult in the initial stages of thrombocytopenia, as the early clinical presentations are often similar. Four patients are described who were suspected clinically of suffering from HIT. All four patients had recent blood transfusions and platelet alloantibodies, thus the diagnosis of PTP was made. One lethal gastrointestinal and one retroperitoneal hemorrhage developed in two of the four patients. Unusually, one patient was male and two different platelet alloantibodies were present in his serum; in another patient platelet alloantibodies and HIT-antibodies were detectable. To arrive at the right diagnosis as quickly as possible is vitally important since treatment, which has to be initiated promptly, is very different for the two syndromes. Thus, we suggest that in patients where HIT is suspected, additional information should be sought. If features consistent with PTP (such as a recent blood transfusion or a marked drop in platelet count to below 15 Gpt/L) are present, we recommend parallel testing for platelet alloantibodies to rule out PTP.

Transfusion Medicine: An Overview and Update

The discovery of AIDS in the 1980s and its rapid evolution as a major concern for physicians and their patients have led to many questions about the safety of the blood supply. The attention placed on AIDS has led to new discoveries and technologies to reduce the risk of other transfusion complications such as hepatitis, bacterial contamination, and transfusion-associated graft-vs-host disease. Concerns about blood safety have focused much attention on alternative blood transfusion strategies such as autologous blood, viral inactivation, and artificial blood substitutes. This review describes the transfusion medicine delivery system in the United States, with special emphasis on evolving developments and their implications for the discipline of chemical pathology.

Development and validation of a genotyping kit for the eight principal human platelet alloantigen systems

Thrombocytopenia in newborns is often due to maternal alloimmunization against platelet alloantigens of the foetus which have been inherited from the father and are absent in the mother. Our aim was to develop a "ready-to-use" typing kit based on polymerase chain reactions, using sequence-specific primers for rapid and simultaneous genotyping of the eight principal human platelet alloantigens. The typing technique uses two specific primer pairs for each bi-allelic system and a monomorphic primer pair as amplification control, with a single temperature cycle programme and identical PCR stringency conditions for all pairs of primers. This kit allows typing of blood samples of small volume within three hours after reception. Validation criteria are essential to check the reliability and specificity of the test, and DNA controls carrying the targeted HPA alleles must be obtained from typed individuals or created in vitro by PCR.

Simultaneous genotyping of human platelet antigens (HPA) 1 through 6 using new sequence-specific primers for HPA-5

BACKGROUND

Polymerase chain reaction using sequence-specific primers is widely used for genotyping human platelet antigens (HPA). However, the results of HPA-5 genotyping are still problematic.

STUDY DESIGN AND METHODS

New sequence-specific primers were designed for HPA-5 that, together with already published primers, allow simultaneous genotyping of HPA-1, -2, -3, -4, -5, and -6. The reliability of the described protocol was determined by using reference DNA samples as well as samples from healthy blood donors.

RESULTS

All primers produced specific amplification products. The genotype and the previously ascertained phenotype of the tested specimens were in concordance in all cases.

CONCLUSION

The described polymerase chain reaction protocol allows rapid, reliable, simultaneous genotyping of HPA-1, -2, -3, -4, -5, and -6.

Platelet glycoprotein (GP) V polymorphisms in Japanese

The glycoprotein (GP) V is a subunit of a platelet receptor for von Willebrand factor, the GPIb/IX/V complex. Recently, polymorphisms in the GPV gene, four involving amino acid changes and five silent, were identified. We developed an allele-specific restriction method to determine the gene frequencies of the non-synonymous substitutions in Japanese and Caucasians. The gene frequencies for 341Gly/Arg polymorphism were 0.94 for 341Gly and 0.06 for 341Arg, and 0.95 for 341Gly and 0.05 for 341Arg, in Japanese and Caucasians, respectively. We could not find rare variants for 114Asp/Tyr, 273Met/Ile or 397Leu/Arg. Flow cytometric analysis showed that the 341Arg variant is expressed normally on the platelet membranes, suggesting the possibility of the involvement of the 341Gly/Arg polymorphism as a platelet alloantigen.

Use of HLA- and HPA--matched platelets in alloimmunized patients

Refractoriness for platelet transfusion is mostly due to clinical factors but may also be caused by alloimmunization. Use of leukocyte-depleted blood cells for transfusions of patients with hematological diseases has reduced if not eliminated HLA-alloimmunization. HLA-antibodies reduce the survival time of incompatible platelets complicating seriously the platelet transfusion support in at least 5% of patients. If consecutive transfusions of HLA matched platelets also fail without identifiable clinical causes, HPA-alloimmunization may have occurred. Platelets from donors phenotyped for both HLA and HPA may produce good platelet count increments and allow optimal treatment of the basic disease despite broad spectrum alloimmunization. Additional cross-matching of phenotyped platelets with patient serum may be needed to circumvent platelet-specific antibodies of unknown specificity.

Clinical aspects and typing of platelet alloantigens

Platelet alloantigens can induce the formation of corresponding alloantibodies when exposed to phenotypically negative individuals. These antibodies are responsible for fetal and neonatal alloimmune thrombocytopenia, posttransfusion purpura, passive alloimmune thrombocytopenia and transplantation-associated thrombocytopenia and may contribute to platelet transfusion refractoriness together with HLA antibodies. Besides antibody detection laboratory diagnosis of the clinical syndromes requires alloantigen typing. Furthermore, typed platelet donors are a prerequisite for effective platelet transfusion therapy. Different techniques for phenotyping are well established and easy to perform but they rely on the availability of antisera. Since the molecular genetic background of the clinically most relevant alloantigens has been elucidated during the last years various genotyping methods have been applied to the platelet membrane polymorphisms and thus facilitated widespread platelet alloantigen typing. Generation of antibodies from phage display libraries and of lymphoblastoid cell lines from donors with all genetic variants will allow further developments.

ABH antigens on human platelets: expression on the glycosyl phosphatidylinositol-anchored protein CD109

Platelets express alloantigens that are platelet specific (eg, the HPA antigens) and alloantigens that are shared with other blood cells (eg, the ABH antigens). The blood group A and B determinants are expressed on glycolipids and on some intrinsic platelet membrane glycoproteins. This report characterizes multiple platelet proteins reacting with blood group antibodies in serum samples from mothers of children born with neonatal alloimmune thrombocytopenia. ABH antigens on additional platelet proteins are identified, including the glycosyl phosphatidylinositol-anchored protein CD109. The proteins that carry ABH antigens were identified by using monoclonal antibodies to glycoproteins Ib, IIb/IIIa, Ia/IIa, CD31, and CD109 and immunoprecipitation/immunoblotting techniques with monoclonal antibodies to A and B antigens. The maternal serum samples and anti-A and anti-B monoclonal antibodies immunoprecipitated identical radiolabeled platelet proteins including proteins at 220 and 175 kd and proteins with mobilities corresponding to glycoproteins Ib, IIb/IIIa, IV, and V. Treatment of platelets with phosphatidylinositol-specific phospholipase C released into the supernatant a 175-kd protein that expressed the blood group determinants. This protein comigrated with the glycosyl phosphatidylinositol-anchored protein CD109. When platelet proteins were purified by immunoprecipitation with monoclonal antibodies and then tested by immunoblotting, anti-A reacted with the glycosyl phosphatidylinositol-anchored protein CD109 and to glycoproteins Ib, IIb, IIa, IIIa, and CD31 (PECAM). These results indicate that structures for modification by glycosyltransferases exist on platelet CD109, which also expresses the Gov alloantigen system. This study indicates that certain platelet proteins express both platelet-specific and blood group antigens that may contribute to platelet transfusion refractoriness and to neonatal alloimmune thrombocytopenia.