Human Platelet Antigen Antibodies Research Articles

Post-Transfusion Purpura: A Literature Review

Introduction: Post-transfusion purpura (PTP) is a rare and occasionally life-threatening transfusion reaction characterized by severe thrombocytopenia usually within two weeks of blood transfusion. It is associated with the development of alloantibodies to human platelet antigens (HPAs). Due to its rarity, our knowledge of PTP is mostly based on reported cases or small cohorts. Methods: This is a systematic literature review of English language articles published in 2 large medical databases (Embase and Pubmed) using the search terms “post-transfusion purpura”, “posttransfusion purpura”, and “post transfusion purpura” between the years 1985 and 2023. Only articles reporting on patients older than 18 years were included. 684 articles were identified, 590 of them were excluded since they represented a combination of case reports of other conditions, laboratory, and epidemiological reports. The 94 articles left, represented case reports and case series of PTP encompassing 149 patients. 21 patients were removed since they represented duplicate cases. 17 patients were excluded due to missing information. 11 patients were excluded due to the presence of confounding clinical conditions. Results: A total of 100 cases were included in the study. The mean age was 56.7 years. 85% were female, 14% were male, and 1 was missing gender data. 62% of cases reported a baseline platelet count, ranging between 78,000 - 600,000 cells/µL. 65% of cases presented after a procedure- or surgery-related transfusion, of these 22 were related to cardiovascular procedures, while 16, 10, and 9 cases were related to gynecological, abdominal, and orthopedic surgeries respectively. A history of prior transfusions was only documented in 24% of cases. A prior pregnancy was reported in 63% of the cases. Red blood cell, fresh frozen plasma, and platelet transfusions were associated to 64%, 11%, and 9% of cases, respectively. The most common HPA antibody detected was HPA-1a (81%). HPA-1b, HPA-5b, and HPA-3a were the next most common antibodies, identified in 7%, 5%, and 5% cases, respectively. The median time from the first transfusion to the development of thrombocytopenia was 8 days, reaching a platelet nadir in 9 days. The platelet nadir ranged between 0 - 50,000 cells/ µL. Clinical presentation included post-transfusion fever (15%), petechial rash, epistaxis, and oral bleeding (65%, 17%, and 12, respectively), and bleeding from the genitourinary system, gastrointestinal tract, surgical incisions, lower respiratory tract, and central nervous system (38%, 28%, 6%, 6%, and 2% of cases, respectively). Death was reported in 9% of cases. The median time from PTP diagnosis to initiation of first treatment was 1 day. 83% of cases achieved an adequate response, defined as a platelet count > 100,000 cells/µL, after a mean/median time of 14.2/11.5 days (available in 68% of cases). Corticosteroids, intravenous immunoglobulin, and plasmapheresis were administered in 68%, 63%, and 27%, of cases, respectively. 6 patients received HPA 1a negative platelet transfusions. Conclusions: The majority of reported PTP cases were females with a history of pregnancy or blood transfusions consistent with existing literature. Over 60% of cases were linked to a surgical procedure, with cardiovascular cases constituting the largest portion. Anti-HPA-1a represented the most commonly detected antibody, followed by HPA-1b, HPA-5b, and HPA-3a. The most common sites of bleeding were the gastrointestinal and genitourinary tracts. 9% died from PTP. The majority of patients (83%) achieved an adequate response, with the most common treatment modalities being corticosteroids, IV immunoglobulins, and plasmapheresis.

Fetal and neonatal alloimmune thrombocytopenia: No evidence of systemic inflammation as a modulator of disease severity. Couldplacental inflammation be key?

In fetal/neonatal alloimmune thrombocytopenia (FNAIT), maternal alloantibodies against paternal human platelet antigens (HPA) cross the placenta and lead to platelet destruction. The extent of thrombocytopenia varies among neonates, and inflammation may constitute an important trigger. A set of stable inflammatory markers was measured in serum samples from neonates with low platelet counts, of which n = 50 were diagnosed with FNAIT due to anti-HPA-1a antibodies and n = 50 were thrombocytopenic without detectable maternal HPA antibodies. Concentrations of C-reactive protein, soluble CD14, procalcitonin, and sFlt-1 did not differ between the two cohorts. There was no correlation between C-reactive protein or soluble CD14 and the platelet count, but a negative correlation between procalcitonin concentrations and the neonatal platelet count in both cohorts. sFlt-1 concentration and the platelet count were correlated in FNAIT cases exclusively. None of the inflammatory markers was statistically different between cases with and without intracranial haemorrhage. We were unable to identify systemic inflammation as a relevant factor for thrombocytopenia in FNAIT. The antiangiogenic enzyme sFlt-1, released by the placenta, did correlate with the platelet count in FNAIT cases. Our findings may give rise to the hypothesis that placental inflammation rather than systemic inflammation modulates disease severity in FNAIT.

HLA antibodies in fetal and neonatal alloimmune thrombocytopenia.

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by antibodies against human platelet antigens (HPA). However, in many cases that meet clinical criteria for the condition, maternal sera do not have HPA antibodies. In studies examining whether human leukocyte antigen (HLA) antibodies cause FNAIT, the results are limited and inconclusive. This study sought to examine whether clinically suspected FNAIT cases with absent maternal HPA antibodies had different HLA antibody strength and specificity compared to controls. A retrospective case-control study assessed class I HLA antibody strength and specificity in cases submitted for testing to Versiti, Wisconsin. There were 813 cases that met initial screening criteria, but written consent could only be obtained for 50. After review of medical records and expert panel review, 31 cases with clinical criteria of FNAIT and maternal HLA but not HPA antibodies were included. Each case was matched for maternal age, gestational age at delivery, parity, and race/ethnicity to two controls from unaffected pregnancies that had maternal serum HLA antibodies. FNAIT cases were found to have both significantly higher HLA antibody strength, measured by mean fluorescence index (MFI), and broader HLA antibody specificity at antigen epitope level, compared to matched controls (p < .001). p-values remained significant after controlling for parity and gestational age at delivery. Additional studies are needed to further examine whether the strong HLA antibodies identified in HPA-antibody-negative cases directly cause neonatal thrombocytopenia and whether prenatal treatment may be warranted in select cases to prevent recurrence.

Blockage of CD72 reduces B cell proliferation in immune thrombocytopenic purpura, involving interleukin 1 and macrophage migration inhibitory factor secretion

ABSTRACT Aim This study aims to explore the expression and role of CD72 in B lymphocytes in immune thrombocytopenic purpura (ITP). Methods The expression level of CD72 in B lymphocytes was detected by flow cytometry in 18 ITP patients and 19 controls of healthy donor or iron-deficiency anemia patients. B cell proliferation was determined by 5-bromo-2′-deoxyuridine incorporation (BrdU) in the culture of 17 ITP patients’ and 11 controls’ peripheral mononuclear cells (PMNCs). The secretion levels of antibodies against human platelet antigens (HPA), as well as B cell proliferation-related cytokine interleukin 1(IL-1) and macrophage migration inhibitory factor (MIF) in culture supernatants were measured by ELISA. Results CD72 was significantly increased in B cells of newly diagnosed or persistent ITP compared with ITP in remission. B cell proliferation in culture with CD72 antibody addition was significantly decreased both in ITP patients and in controls compared with isotype antibody addition. CD72 antibody did not significantly alter HPA antibody level in ITP patients. CD72 antibody increased IL-1 and MIF levels in ITP patients’ cell culture supernatant but not in controls. Conclusion CD72 expression elevation accompanies the active status of ITP. In vitro addition of CD72 antibody has a negative impact on B cell proliferation. The function of CD72 in B cell proliferation in ITP may be related to IL-1 and MIF secretion.

Fc galactosylation of anti-platelet human IgG1 alloantibodies enhances complement activation on platelets.

Approximately 20% of patients receiving multiple platelet transfusions develop platelet alloantibodies, which can be directed against human leukocyte antigens (HLA) and, to a lesser extent, against human platelet antigens (HPA). These antibodies can lead to the rapid clearance of donor platelets, presumably through IgG-Fc receptor (FcγR)-mediated phagocytosis or via complement activation, resulting in platelet refractoriness. Strikingly, not all patients with anti-HLA or -HPA antibodies develop platelet refractoriness upon unmatched platelet transfusions. Previously, we found that IgG Fc glycosylation of anti-HLA antibodies was highly variable between patients with platelet refractoriness, especially with respect to galactosylation and sialylation of the Fc-bound sugar moiety. Here, we produced recombinant glycoengineered anti-HLA and anti-HPA-1a monoclonal antibodies with varying Fc galactosylation and sialylation levels and studied their ability to activate the classical complement pathway. We observed that anti-HLA monoclonal antibodies with different specificities, binding simultaneously to the same HLA-molecules, or anti-HLA in combination with anti-HPA-1a monoclonal antibodies interacted synergistically with C1q, the first component of the classical pathway. Elevated Fc galactosylation and, to a lesser extent, sialylation significantly increased the complement-activating properties of anti-HLA and anti-HPA-1a monoclonal antibodies. We propose that both the breadth of the polyclonal immune response, with recognition of different HLA epitopes and in some cases HPA antigens, and the type of Fc glycosylation can provide an optimal stoichiometry for C1q binding and subsequent complement activation. These factors can shift the effect of a platelet alloimmune response to a clinically relevant response, leading to complement-mediated clearance of donor platelets, as observed in platelet refractoriness.

Frequency, reactivity and evolution of human leukocyte antigen and human platelet antigen antibodies in the setting of hematopoietic cell transplantation

Background and objectivesAntibodies (Ab) against HLA and HPA antigens play an important role in HCT. In this prospective study we evaluated prevalence and kinetics of HLA- and HPA-Ab after HCT, including a possible donor-recipient transfer and their clinical relevance in respect to platelet transfusion refractoriness (PTR). Materials and methodsPatients were consecutively recruited. Ab were determined by microbead assay technique and a mean fluorescence intensity cut-off of 1,000. ResultsAt baseline, 21 donors (42 %) and 27 patients (54 %) had HLA-Ab with a mean panel reactivity (cPRA) of 34.9 ± 29.4 % and 46.1 ± 36.5 %, respectively. We observed a significant higher number of HLA-Ab specificities in female donors and patients and a predominance of HLA-class I Ab. At day 0 we detected an increase of HLA-Ab (from 526 to 673) and cPRA (55.2 ± 31.9 %). Thirty-six patients (72 %) developed new HLA-Ab, mainly 3 weeks after HCT. In 7 patients an HLA-Ab with the same specificity as detected in the corresponding donor emerged, suggesting a possible transfer from the donor to the recipient. Overall, MFI showed a high variation. Type and number of transfusions were not associated with number and intensity of HLA-Ab (ρ: -0.05 – 0.02). Number of HLA-Ab, cPRA and intensity were not associated with PTR, which occurred in 9 patients (18 %) and none had bleeding WHO > 2. ConclusionsAlthough a considerable number of patients have and develop HLA-Ab before and early after HCT, we found no association with PTR and bleeding and management should be individualized.

Are HPA-5b antibodies a significant cause of FNAIT and associated bleeding or merely an incidental finding?

Are HPA-5b antibodies a significant cause of FNAIT and associated bleeding or merely an incidental finding?

Clinical characteristics of human platelet antigen (HPA)-1a and HPA-5b alloimmunised pregnancies and the association between platelet HPA-5b antibodies and symptomatic fetal neonatal alloimmune thrombocytopenia.

SummaryFetal neonatal alloimmune thrombocytopenia (FNAIT) is caused by maternal alloantibodies directed against the human platelet antigens (mostly HPA‐1a or HPA‐5b) of the (unborn) child and can lead to severe bleeding. Anti‐HPA‐1a‐mediated FNAIT shows a severe clinical outcome more often than anti‐HPA‐5b‐mediated FNAIT. Given the relatively high prevalence of anti‐HPA‐5b in pregnant women, the detection of anti‐HPA‐5b in FNAIT‐suspected cases may in some cases be an incidental finding. Therefore we investigated the frequency of anti‐HPA‐5b‐associated severe bleeding in FNAIT. We performed a retrospective nationwide cohort study in cases with clinical suspicion of FNAIT. HPA antibody screening was performed using monoclonal antibody‐specific immobilisation of platelet antigens. Parents and neonates were typed for the cognate antigen. Clinical data were collected by a structured questionnaire. In 1 864 suspected FNAIT cases, 161 cases (8·6%) had anti‐HPA‐1a and 60 (3·2%) had anti‐HPA‐5b. The proportion of cases with severe bleeding did not differ between the cases with anti‐HPA‐1a (14/129; 11%) and anti‐HPA‐5b (4/40; 10%). In multigravida pregnant women with a FNAIT‐suspected child, 100% (81/81) of anti‐HPA‐1a cases and 79% (38/48) of anti‐HPA‐5b cases were HPA‐incompatible, whereas 86% and 52% respectively were expected, based on the HPA allele distribution. We conclude that anti‐HPA‐5b can be associated with severe neonatal bleeding symptoms. A prospective study is needed for true assessment of the natural history of anti‐HPA‐5b mediated FNAIT.

Slot blotting and flow cytometry: two efficient assays for platelet antibody screening among patients with platelet refractoriness.

Frequent platelet transfusion may lead to the formation of alloantibodies and immune-mediated platelet destruction. Currently, identifying economic and effective screening methods is necessary for the management of platelet transfusion while different tests were recommended. The present study aims to challenge the performance of slot blotting (SB) and flow cytometry (FC) assays in detecting immune platelet refractoriness. Sera from 118 patients who received blood components and were clinically suspected of platelet refractoriness were enrolled. Platelet-reactive antibodies were explored in parallel by SB, FC and monoclonal antibody-specific immobilization of platelet antigens (MAIPA) techniques. In a further study, chloroquine-treated platelets were incubated with MAIPA-positive serum, and then, the results of the SB and FC techniques were compared. Using MAIPA as a reference, antibodies were detected in 51 sera, with specificity for human leucocyte antigens (HLA), human platelet antigens (HPA) or both HLA/HPA, in 27, 18 and 6 patients, respectively. The sensitivity and specificity of SB and FC were 86·3%, 88·1%, 82·4% and 95·5%, respectively. The Spearman correlation revealed significant (P<0·001) correlations between FC (r=0·763) and SB (r=0·738) with MAIPA. In respect to HPA antibody detection, SB had 83·3% sensitivity and 92·6% specificity compared to 91·7% and 96·3% for FC while both approaches are acceptable (P<0·001, r=0·69; P<0·001, r=0·773) and can be recommended. The present study acknowledges that among the used methods, the flow cytometry's performance is the most appropriate, but slot blotting, with acceptable sensitivity, can be used as an acceptable and convenient procedure for platelet antibody screening.

Cross-Strain Platelet Immunization in Selected Mouse Strains Recapitulates Clinical and Serologic Findings Typical of Post-Transfusion Purpura (PTP)

Post-transfusion purpura (PTP) is an uncommon but life-threatening condition characterized by profound thrombocytopenia (TP) occurring one week after transfusion of blood products. The hallmark of PTP is a potent IgG antibody specific for a transfused human platelet antigen (HPA), usually HPA-1a located on αIIb/β3 integrin (GPIIb/IIIa). It is widely thought that, in PTP, the alloantibody somehow causes destruction of the recipient's platelets even though they lack the antigen for which the alloantibody is specific. Several reports have suggested that the underlying cause of PTP is a platelet-specific autoantibody that can be difficult to detect because it is absorbed in the process of destroying autologous platelets and is overshadowed by the accompanying, very potent alloantibody but experimental support for this concept is minimal. Platelet alloantigens comparable to HPAs have not been defined in animals. Using a public database, we identified four mouse strains (C57BL/6J (C57), 129S1/Svlmj (129), PWK/PhJ (PWK), AND SPRET/EIJ) differing from each other at amino acid residues in extracellular domains of GPIIb/IIIa that could comprise potential alloantigens. Cross-strain platelet immunizations (intraperitoneal with adjuvant) were performed weekly for 5 weeks while monitoring platelet counts and platelet associated IgG (PAIgG) and saving plasma samples for serologic studies. After 2-4 immunizations, each of 39 cross-strain but none of 9 strain-identical immunizations induced "alloantibodies" that recognized donor but not recipient platelets (flow cytometry). Thrombocytopenia (&lt;50% of maximum platelet count) developed in 28 of 39 mice (71%) given strain-disparate platelets but not in mice given strain-identical platelets; 12 of these mice (30%) developed profound TP (&lt;15%). The most consistent and severe declines in platelet counts occurred in PWK mice immunized with 129 platelets and vice versa (N=13) in which the mean platelet count decline was 88% (range 59-96%, median 92%). Autoantibodies recognizing syngeneic platelets were identified in all animals that developed profound TP and their potency (measured by flow cytometry) correlated closely with the severity of TP (p&lt;0.001) (Fig 1). Alloantibodies were shown by immunoprecipitation to be mainly specific for GPIIb/IIIa (N=13) and GPIb/IX (N=1) on donor platelets. Two monoclonal antibodies (mAbs MBC417.1 and MBC425.1) specific for a single polymorphic amino acid at positions 111(Gly) and 37(Val), respectively, on GPIIb of C57 and PWK mice were generated using spleen cells of two immunized mice. To our knowledge, these are the first alloantibodies in mice that are specific for single amino acid polymorphisms in a platelet membrane glycoprotein and are thus comparable to HPA antibodies found in humans. The findings define a model (platelet immunization between PWK and 129 mice) in which a routine alloantibody response recognizing GPIIb/IIIa on donor platelets regularly transitions to an autoimmune response capable of causing profound thrombocytopenia, thus mimicking the course of PTP in human patients and supporting the hypothesis that PTP is an autoimmune disorder. Successful development of the model could be related to use of the more recently developed, wild-caught PWK strain as one of the partners for immunization. The inherent utility of a mouse model is expected to facilitate further work to define the molecular basis for a transition from allo- to auto-immunity in the human condition, post-transfusion purpura. Disclosures No relevant conflicts of interest to declare.

An Unusual Case of Fetal/Neonatal Alloimmune Thrombocytopenia

Abstract Objectives Fetal/neonatal alloimmune thrombocytopenia (FNAIT) occurs when maternal IgG alloantibodies against paternal human platelet antigens (HPA) cross the placenta and cause the destruction of fetal platelets. The vast majority (up to 95%) of FNAIT cases are caused by antibodies against HPA-1a or HPA-5b antigens, while the remaining cases are usually due to antibodies against a variety of other HPA antigens. Cases of FNAIT due to anti-HLA antibodies are extremely uncommon and have only rarely been reported. We present a case of FNAIT suspected to be caused by anti-HLA class I alloantibodies. Methods The patient is a term infant boy born to a 32-year-old G2T2L2 mother. The mother had a previous diagnosis of Still disease (an adult form of systemic juvenile rheumatoid arthritis) but experienced complete resolution of symptoms and was off all treatment during the pregnancy. At birth, laboratory testing revealed isolated severe thrombocytopenia (platelet count 38,000/mcL) in an otherwise healthy-appearing infant. Results The infant had no evidence of bleeding, and testing for TORCH infection, sepsis, and DIC was negative. The maternal blood type was O positive. The maternal platelet count was normal. FNAIT was suspected and the infant was given two platelet transfusions from the same HPA 1a and 5b antigen-negative donor with no significant or sustained improvement in platelet count. Maternal platelet antibody testing subsequently revealed an absence of HPA antibodies, but anti-HLA class I alloantibodies were present. The infant was treated with three subsequent doses of IVIg with improvement in platelet count. No significant hemorrhage occurred. Conclusion HLA class I antibodies are commonly found in multiparous women but are not generally thought to cause significant fetal complications during subsequent pregnancies. This case suggests that, although rarely reported, HLA class I alloantibodies may be capable of causing FNAIT.

Novel method for simultaneously detecting HPA and HLA antibodies using Luminex microbeads

BackgroundAlloantibodies against human platelet antigens (HPAs) and human leukocyte antigen (HLA) are implicated in several immune-mediated platelet disorders. Detection of these antibodies is crucial in the diagnosis and management of these disorders. The aim of this study was to establish a novel method to simultaneously detect HPA-1, HPA-2, HPA-3, HPA-5 and HLA antibodies with Luminex microbeads technology.MethodsMonoclonal antibodies specific for platelet glycoproteins and HLA class I molecules were separately coupled to the Luminex microbeads. We validated specificity of the Luminex platform using the following antibodies: anti-HPA-1a, anti-HPA-2b, anti-HPA-3a, anti-HPA-5a, and anti-HLA positive samples. Sensitivity was evaluated by a serial dilution (from neat to 1/1024) using the following antibodies: anti-HPA-1a, anti-HPA-3a standard sera, and anti-HPA-5a positive serum. Serum samples were collected from 36 neonatal alloimmune thrombocytopenia (NAIT) patients suspected of having HPA or HLA antibodies and 8 samples from ISBT platelet workshop were tested using the Luminex assay.ResultsThe Luminex assay detected all antibodies tested from the known samples. The sensitivities of the Luminex assay detecting anti-HPA-1a, anti-HPA-3a, and anti-HPA-5a were 1:512, 1:64, and 1:128, respectively. The sensitivity of Luminex assay was higher than monoclonal antibody immobilization of platelet antigen method (MAIPA). No cross-reactivity was observed in the samples containing multi-platelet antibodies or mixture antibodies against HPA and HLA. The results of 44 samples with platelet disorders were consistent with those of the same samples processed with the MAIPA assay.ConclusionLuminex microbeads coupled with monoclonal antibodies could be successfully used to detect HPA and HLA antibodies simultaneously, especially with high sensitivity in detecting HPA antibodies.

P008 Case report: Platelet refractory due to human platelet antigen-antibody, rather than human leukocyte antigen-antibodies

P008 Case report: Platelet refractory due to human platelet antigen-antibody, rather than human leukocyte antigen-antibodies

P095 Antibodies to human platelet antigens: when do they cause an incompatible crossmatch

Aim Our center evaluates approximately 100 platelet refractory patients each year. Previously, we have correlated single antigen bead MFI with platelet crossmatch results in an effort to optimize our process for selection of HLA-compatible platelets. However, we continue to struggle to appreciate when antibodies to human platelet antigens (HPA) are also contributing to the platelet refractory state. In this study, our aim is to understand the relationship between HPA antibody testing (PakPlus, Immucor) and platelet crossmatch (CaptureP, Immucor). Methods One patient sample with a singular & strong specificity, HPA-1a, was identified based on PakPlus testing. This sample was also negative for HLA antibodies based on PakPlus testing. The plasma was then diluted and the PakPlus result, optical density (OD), was determined at each dilution. Ten antigen-positive platelet units were crossmatched with the battery of plasma dilutions. The strength of the platelet crossmatch was graded by one technologist, blinded to the dilution, with 4+ being strong positive to negative. Results The patient sample demonstrated a fairly linear dilution of HPA-1a, beginning with a neat result of OD 1.33. All 10 antigen-positive platelet units were positive at OD 0.42 or greater, yet negative at OD 0.04. There was platelet-specific variability between OD 0.3 and OD 0.06 (see Table 1). Conclusions These results demonstrate that HPA antibodies cause crossmatch positivity at low OD. The variability seen may be due to homozygosity versus heterozygosity of antigen expression. These initial results suggest that a PakPlus result as low as OD 0.06 may cause incompatibility in the platelet crossmatch and could be considered when selecting platelets for the platelet refractory patient. Download : Download high-res image (413KB) Download : Download full-size image

How do I … manage the platelet transfusion-refractory patient?

Platelet transfusion-refractoriness is a challenging and expensive clinical scenario seen most often in patients with hematologic malignancies. Although the majority of platelet transfusion-refractory cases are due to nonimmune causes, a significant minority are caused by alloimmunization against Class I human leukocyte antigens (HLAs) or human platelet antigens (HPAs). Such platelet transfusion-refractory patients can be effectively managed with appropriate antigen-negative products. Our institution has developed a diagnostic and management algorithm for the platelet transfusion-refractory patient with an early focus on identifying those cases caused by immune-mediated factors. Using physical platelet cross-matches to initially classify platelet transfusion-refractory patients as immune-mediated or not, cross-match-compatible inventory is then provided to immune-mediated patients, whereas subsequent HLA (with or without HPA) testing is performed. Our blood donor program performs Class I HLA typing of all repeat platelet donors to facilitate the identification of antigen-negative platelet units (virtual cross-matching) as well as the recruitment of HLA-matched donors. The platelet transfusion-refractoriness algorithm realizes an initial net cost savings once two apheresis platelets are saved from use for each newly identified, immune-mediated platelet transfusion-refractory patient. An algorithm utilizing physical platelet cross-matches, Class I HLA and HPA antibody testing, and upfront Class I HLA typing of platelet donors leads to overall resource savings and improved clinical management for platelet transfusion-refractory patients.

Development of quantitative monoclonal antibody-specific immobilization of platelet antigens assay for antibodies against human platelet antigen-1a, 3a, and 5b

Human platelet antigens (HPAs) are platelet-specific alloantigens that cause alloimmunization. Alloantibodies induced via pregnancy and transfusion may result in various clinical syndromes. The monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay is the gold standard for the detection and identification of HPA antibodies. The present study aimed to develop standardized quantitative MAIPA assays for antibodies against HPA-1a, 3a, and 5b. MAIPA protocol was applied on serial dilutions of standard or reference reagents containing antibodies against HPA-1a, 3a, and 5b. Optimal experimental parameters were determined, and standard curves were constructed with a linear regression. The sensitivity, specificity, and reproducibility of the assays were also evaluated. The optimum reagents and conditions for the MAIPA assays were decided through repeated testing and comparison. The intra-assay coefficients of variation (CVs) are 1.3~3.6%, 2.7~5.2%, and 4.7~5.3%; the inter-assay CVs are 4.0~5.6%, 4.4~5.9%, and 3.2~6.3%, respectively, for HPA-1a, HPA-3a, and HPA-5b antibodies. In specificity tests, some monoclonal antibodies and sera with other antibodies were tested and no obvious positive result was observed. The standardized quantitative MAIPA assays for antibodies against HPA-1a, 3a, and 5b have good sensitivity, reproducibility, and specificity. They have important application value in clinical diagnosis of alloimmunization disease caused by HPA antibodies and safe transfusion of platelets.

Human platelet antigen antibody induction in uncomplicated pregnancy is associated with HLA sensitization.

Alloimmunization against human platelet antigens (HPAs) during pregnancy is rare but can lead to severe bleeding disorders, such as fetal and neonatal alloimmune thrombocytopenia. In a cohort of 241 uncomplicated pregnancies, we investigated the immunogenicity of HPA mismatches and correlated HLA sensitization with HPA antibody formation. HPA antibodies were measured with a Luminex-based multiplex assay. HPA mismatches were observed in 109 of 241 pregnancies (45%), but child-specific HPA antibodies were only found in two of 109 cases (2%), indicating a low immunogenicity. Only nine of 241 women (4%) had detectable HPA antibodies. HLA sensitization was identified as a strong and independent predictor for HPA antibody formation (hazard ratio, 10.2; 95% confidence interval, 1.8-193; p = 0.006), whereas the number of pregnancies was not. Our observational data indicated a low immunogenicity of HPA and suggest that a broader immune response-inferred by HLA sensitization-is probably associated with HPA antibody induction.

Contemporary management of neonatal alloimmune thrombocytopenia: good outcomes in the intravenous immunoglobulin era: results from the Australian neonatal alloimmune thrombocytopenia registry

Objective: To describe the natural history, antenatal and postnatal therapy, and clinical outcomes of Australian patients with fetomaternal/neonatal alloimmune thrombocytopenia (NAIT) recorded in the Australian NAIT registry.Methods: Analysis of registry data of Australian mothers treated antenatally for NAIT and any fetus/newborn with thrombocytopenia (TCP) and maternal human platelet antigen (HPA) antibodies.Results: Ninety four potential cases (91 pregnancies; three twin pregnancies) were registered between December 2004 and September 2015 with 76 confirmed or treated as NAIT. NAIT was frequently unanticipated (44 cases, 58%), whilst 32 cases (42%) were anticipated due to personal or family history. In 70/76 cases, the diagnosis of NAIT was made based on HPA antibody results; anti-HPA-1a was most commonly detected (58/70, 82%), followed by anti-HPA-5b (5/70, 7%). Intracranial haemorrhage (ICH) was detected in seven cases (9%). Maternal antenatal therapy resulted in improved clinical outcomes. For antenatally treated cases, whilst 10/29 (34%) neonates had severe TCP, only one ICH was detected.Conclusions: This study provides data on contemporary “real world” management of Australian mothers and babies with NAIT. Antenatal IVIG therapy was associated with better neonatal outcomes. Maternal side-effects and treatment costs were substantial.

Guidelines for the diagnosis and management of adult aplastic anaemia.

Sally B. Killick (Writing Group Chair), Nick Bown, Jamie Cavenagh, Inderjeet Dokal, Theodora Foukaneli, Peter Hillmen, Robin Ireland, Austin Kulasekararaj, Ghulam Mufti, John A Snowden, Sujith Samarasinghe, Anna Wood (BCSH Task Force Member), Judith C.W. Marsh on behalf of the British Society for Standards in Haematology. The Royal Bournemouth and Christchurch Hospitals NHS Foundation Trust, Bournemouth, Northern Genetics Service, Newcastle upon Tyne, St Bartholomew’s Hospital, Barts Health NHS Trust, London, Barts and The London School of Medicine and Dentistry, Queen Mary University of London and Barts Health NHS Trust, London, Addenbrooks Hospital, University of Cambridge, Cambridge, Leeds Teaching Hospitals, Leeds, Kings College Hospital NHS Foundation Trust, London, Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield Great Ormond Street Hospital for Children NHS Foundation Trust, London, West Hertfordshire NHS Trust, Watford.

Advances in alloimmune thrombocytopenia: perspectives on current concepts of human platelet antigens, antibody detection strategies, and genotyping.

Alloimmunisation to platelets leads to the production of antibodies against platelet antigens and consequently to thrombocytopenia. Numerous molecules located on the platelet surface are antigenic and induce immune-mediated platelet destruction with symptoms that can be serious. Human platelet antigens (HPA) cause thrombocytopenias, such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet transfusion refractoriness. Thirty-four HPA are classified into 28 systems. Assays to identify HPA and anti-HPA antibodies are critically important for preventing and treating thrombocytopenia caused by anti-HPA antibodies. Significant progress in furthering our understanding of HPA has been made in the last decade: new HPA have been discovered, antibody-detection methods have improved, and new genotyping methods have been developed. We review these advances and discuss issues that remain to be resolved as well as future prospects for preventing and treating immune thrombocytopenia.