Human platelet antigen (HPA)‐specific immunoglobulin M antibodies in neonatal alloimmune thrombocytopenia can inhibit the binding of HPA‐specific immunoglobulin G antibodies

• Published in: Transfusion (Philadelphia, Pa.) Pa.), 2017-05, Vol.57 (5), p.1267-1271
• Main Authors: Hopkins, Matthew, Lucas, Geoff, Calvert, Anthony, Bendukidze, Nina, Green, Frances, Kotecha, Krishna, Poles, Anthony
• Format: Article
• Language: English
• Subjects: Antigens, Human Platelet - immunology; Binding Sites, Antibody - immunology; Humans; Immunoglobulin G - immunology; Immunoglobulin M - immunology; Immunologic Tests; Infant, Newborn; Isoantibodies - immunology; Thrombocytopenia, Neonatal Alloimmune - immunology
• ISSN: 0041-1132; 1537-2995
• DOI: 10.1111/trf.14047

BACKGROUND A term baby with unexplained thrombocytopenia and a platelet (PLT) count of 14 × 109/L (maternal PLT count was 200 × 109/L) was investigated for neonatal alloimmune thrombocytopenia. STUDY DESIGN AND METHODS Serologic investigations were performed using the PLT immunofluorescence test (PIFT), monoclonal antibody immobilization of PLT antigens (MAIPA), and a bead‐based assay (BBA) with maternal sera taken up to 56 days postdelivery. One serum sample was also separated into “immunoglobulin (Ig)M‐rich” and “IgM‐depleted” fractions and tested for PLT‐specific antibodies. The family was genotyped for HPA. RESULTS HPA‐3a–specific IgM antibodies were detected in the PIFT and confirmed in the BBA. PLT‐specific IgG HPA‐3a antibodies were not detected in the MAIPA assay and BBA in the initial sample but were detected in both techniques in subsequent serum samples. Testing of IgM‐rich and IgM‐depleted fractions in the MAIPA assay revealed that IgG antibody binding of the IgM‐depleted fraction was inhibited by approximately 50% when it was reconstituted with the IgM‐rich fraction suggesting that the IgM antibodies blocked the binding of the IgG antibodies. This effect was not observed when the IgM‐depleted fraction or untreated serum was diluted with elution buffer. Incompatibility for HPA‐3 was identified between the mother and the infant. The infant received one HPA‐1a, −5b negative neonatal PLT transfusion, and one random PLT transfusion, with satisfactory outcomes. Both units were later found to be HPA‐3b3b. CONCLUSION HPA‐3a IgM antibodies can inhibit PLT‐specific HPA‐3a IgG antibodies in the MAIPA assay.