Interlaboratory variation in the detection of HPA-specific alloantibodies and in molecular HPA typing

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.

Identification of anti-HPA-1a allo-antibodies using IgG platelet antibody detection and crossmatch system assay with Galileo Echo

Fetal/neonatal allo-immune thrombocytopenia is the most frequent and the most dangerous clinical condition involving anti-human platelet antigens (HPA)-1a allo-antibodies. Anti-HPA-1a allo-immunization requires rapid and accurate diagnosis to determine appropriate treatment. The Capture-P Ready-Screen assay (C-PRS) is a new qualitative immunoassay to detect IgG anti-human leukocyte antigen (HLA) and anti-HPA allo-antibodies. The aim of this study is to assess the identification of anti-HPA-1a allo-antibodies using the C-PRS assay, associated with HLA class I stripping reagents, on the automated benchtop analyzer Galileo Echo. Forty-nine sera were analyzed: without anti-HLA class I or anti-HPA allo-antibodies, with anti-HLA class I allo-antibodies, with anti-HPA-1a allo-antibodies, among which with anti-HLA class I allo-antibodies. None of the samples without allo-antibodies were reactive. Only anti-HLA antibodies, detected by cytotoxicity-dependent complement and not by Luminex, remained positive before and after stripping reagents. Of the 13 samples, anti-HPA-1a allo-antibodies that were correctly identified before and after incubation with HLA assassin reagent were 70% and 85%, respectively. Anti-glycoprotein auto-antibodies and anti-HLA allo-antibodies do not interfere with the detection of anti-HPA-1a antibodies. This preliminary study indicates that further improvement of the test will be helpful in developing a clinically useful assay in the future.

Platelet antibody analysis by three different tests

BACKGROUND

Platelet-reactive antibodies lead to thrombocytopenia and bleeding disorders, and diverse assays are used for their detection. In this retrospective analysis, the applicability of three different test systems was compared and antibody specificities were assessed.

METHODS

Sera of 1,234 patients were tested with an enzyme-linked immunosorbent assay (ELISA; Lifecodes PAKPLUS(®) or PAK 12(®), Gen-Probe) and a solid-phase assay (Capture-P Ready Screen(®), Immucor Inc.). In cases of suspected anti-HLA class I antibodies, a specific lymphocytotoxicity test (LCT, Bio-Rad(®)) was performed.

RESULTS

Platelet antibodies were detected in 366 of 1,234 samples (29.7%). In 70.3% concordant negative but only in 8.4% concordant positive results were obtained with both the methods; 185 of 1,053 in the solid-phase assay negative samples were positive in the ELISA (15.0%). In samples positive in both methods, most antibodies reacted against HLA class I antigens. Glycoprotein (GP) specific platelet antibodies, mainly against GPIIb/IIIa and GPIa/IIa, were more frequently detectable in the ELISA than in the solid-phase assay, whereas weakly positive results have to be interpreted cautiously.

CONCLUSION

ELISA, solid-phase assay, and LCT showed highly divergent results. Due to several limitations, the additional analysis by the "monoclonal antibody-specific immobilization of platelet antigen" (MAIPA)-assay is highly recommended.

Laboratory testing for platelet antibodies

Laboratory testing for immune-mediated thrombocytopenias involves identification and classification of antibodies present in patient sera or attached to patient platelets. This article summarizes the available types of platelet antibody testing and applications in disorders such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, multiple platelet transfusion refractoriness, immune thrombocytopenia, and drug-induced thrombocytopenia.

Sensitivity of assays for the detection of HPA-1a antibodies: results of an international workshop demonstrating the impact of cation chelation from integrin αIIbβ3 on three widely used assays

BACKGROUND AND OBJECTIVES

HPA-1a antibodies account for 70-80% of cases of fetal-neonatal alloimmune thrombocytopenia (FNAIT) in Caucasians. However, numerous workshops have demonstrated variability in their detection. We recently showed that exposure of αIIbβ3 to ethylene diamine tetraacetic acid (EDTA) affected binding of many anti-αIIbβ3 monoclonal, and HPA-1a allo-, antibodies; this adversely affected sensitivity of the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay and indirect platelet immunofluorescence test (PIFT). This study presents results from an international workshop studying the impact of cation chelation on HPA-1a antibody detection in routine diagnostic laboratories.

MATERIALS AND METHODS

Serum and EDTA-anticoagulated plasma samples containing anti-HPA-1a were distributed to 39 laboratories. Participants were asked to detect and identify any HPA antibodies present.

RESULTS

2/39 (5.1%) participants were able to detect and identify anti-HPA-1a in the serum, but not in the plasma sample. EDTA plasma reduced MAIPA assay sensitivity by ≥ 20% in 17/24 (70.8%) laboratories and by ≥ 50% in 9/24 (37.5%) when using HPA-1a1a platelets (mean: 27.7%, range 0-85.1%); when using HPA-1a1b platelets 3/4 (75%), participants reported ≥ 50% loss of sensitivity (mean 65.6%, range 0-96.6%). A small but significant increase in optical densities was observed in antigen capture ELISA assays when using plasma (mean difference: 0.081, P < 0.01). Insufficient PIFT data were returned to draw firm conclusions.

CONCLUSION

Use of EDTA plasma significantly affects the sensitivity of the MAIPA assay and can affect detection of even potent, FNAIT-causing examples of anti-HPA-1a. These data highlight the importance of use of αIIbβ3 in an appropriate conformation for the sensitive detection of anti-HPA-1a.

Human platelet antigen 1a epitopes are dependent on the cation-regulated conformation of integrin α(IIb)β(3) (GPIIb/IIIa)

BACKGROUND

The HPA-1a (Leu(33)) polymorphism of platelet integrin αIIbβ3 is the target of alloantibodies in 70-80% cases of neonatal alloimmune thrombocytopenia (NAIT) in Caucasians and reliable detection of these antibodies is essential for appropriate clinical management. However, the ability to detect such antibodies is highly variable between laboratories and, in a number of clinical cases where there is a HPA-1 genotype mismatch between mother and neonate, HPA-1a antibodies are undetectable. Furthermore, some studies have not shown a consistent relationship between maternal anti-HPA-1a level and neonatal platelet count. Since the integrity and conformation of the αIIbβ3 complex are dependent on divalent cations, we investigated whether HPA-1a epitope integrity and/or conformation might be affected by the presence of the cation chelator EDTA in patient samples or in assay buffers, thus providing a possible explanation for the variable sensitivity of current assays.

PRINCIPLE FINDINGS

Exposure of the αIIbβ3 complex to EDTA resulted in reduced reactivity of three anti-HPA-1a mAbs (B2, 19-7 and 23-15). More significantly, cation chelation adversely affected detection of polyclonal anti-HPA-1a, not only in the platelet immunofluorescence assay, where alloantibody binding was reduced compared to control platelets (mean MFI reduction 44.5%, range 17.3-69.7%, n=4), but also in the commonly used monoclonal antibody specific immobilisation of platelet antigens assay (MAIPA) where both alloantibody and monoclonal capture antibody binding were reduced (mean OD reduction 82.8%, range 68.3-96.6%, n=9).

CONCLUSIONS

These data show that HPA-1a antibodies recognise epitopes on αIIbβ3 that are sensitive to EDTA treatment and that cation chelation grossly reduces the sensitivity of the MAIPA assay by diminishing not only HPA-1a alloantibody binding but also 'capture' monoclonal antibody binding. These findings may, in part, explain the current variability in antibody measurement and will guide the development of more sensitive tests for anti-integrin antibodies in NAIT and other conditions.

The importance of using multiple techniques for detection of platelet antibodies

In red cell immunology, it has long been known that no one technique will detect all clinically significant antibodies. The same appears to be true for platelet immunology, and we highlight this fact by showing four examples of anti-human platelet antigen-1a that were not detected by the monoclonal antibody-specific immobilization of platelet antigen test, the most commonly used technique. Each antibody was found in a case of fetomaternal alloimmune thrombocytopenia in which the fetus or neonate was severely affected.