In pursuit of enigmatic platelet antibodies--anti-HPA-2b and anti-HPA-3a

Collaborative studies to establish the first World Health Organization International Standard for detection of human antibody against human platelet antigen-3a

BACKGROUND AND OBJECTIVES

The platelet-specific antibody anti-human platelet antigen-3a (anti-HPA-3a) is involved in neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet refractoriness. However, HPA-3a antibodies are often difficult to detect, probably because the antigen is labile. This report describes the production of a freeze-dried preparation of pooled human plasma, coded 03/190, containing IgG antibodies against the HPA-3a. The material is intended for use as a minimum sensitivity reagent in glycoprotein-specific assays currently used for anti-HPA-3a detection. Laboratories can use it to assess the sensitivity of their 'in-house' assays for anti-HPA-3a and to calibrate local controls for routine use in each batch of tests.

MATERIALS AND METHODS

Plasma containing anti-HPA-3a was obtained from a mother of two babies both born with severe thrombocytopenia, and following dilution it was freeze dried in glass ampoules.

RESULTS

Two collaborative studies demonstrated that the candidate material contained anti-HPA-3a and human leucocyte antigen (HLA) class I antibodies, but no other HPA antibodies that might confuse the detection of the anti-HPA-3a. The minimum dilution that should give a positive result was determined to be 1 : 8 by two further international collaborative studies involving a total of 49 laboratories in 23 countries.

CONCLUSION

The material also contains HLA antibodies and is suitable for use only in techniques that are glycoprotein specific (i.e. monoclonal antibody immobilization of platelet antigens and enzyme-linked immunosorbent assay) where only HPA antibodies will be detected. This standard will allow laboratories to measure their sensitivity of detection of anti-HPA-3a and will also allow those laboratories with relatively insensitive techniques to monitor their performance as they improve their methodology.

Detection of MNS hybrid molecules in the Thai population using PCR-SSP technique

We developed a polymerase chain reaction-sequence-specific primer (PCR-SSP) technique to screen for hybrid molecules in the MNS blood group in the Thai population using two sets of newly designed primers specific for four GYP(B-A-B) hybrids, GP.Mur, GP.Hop, GP.Bun and GP.HF, and two GYP(A-B-A) hybrids, GP.Vw and GP.Hut. One thousand and forty-one blood samples were tested with human anti-Mi(a) by conventional tube technique, and 598 samples of these were tested by the PCR-SSP technique. Ninety-four samples (9.03%) were strongly positive with human antisera by conventional tube technique. For PCR-SSP test results, the GP.Hut, GP.Mur, GP.Hop, GP.Bun and GP.HF genotypes were amplified with the first set of primers, whereas GP.Vw genotype was amplified with a second set of primers. The GYP(A-B) hybrids (GP. Hil and GP.JL), GYP(A-B-A) hybrids (GP.Nob, GP.Joh and GP.Dane), GYPA, GYPB and GYPE were not amplified by either set of primers. Results of testing 94 Mi(a+) and 504 Mi(a-) by conventional tube technique and PCR-SSP were concordant. This study shows that analysis by PCR-SSP is simple and convenient; therefore, it can be used as an alternative to conventional tube technique for mass screening for MNS hybrids, especially when specific antisera are not available.

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.

Immunoglobulin G subclasses of anti-human platelet antigen 1a in maternal sera: relation to the severity of neonatal alloimmune thrombocytopenia

The monoclonal antibody immobilization of platelet antigen (MAIPA) technique was employed to detect and semiquantitatively assess total IgG anti-HPA-1a and its subclasses in sera of mothers who gave birth to severely thrombocytopenic (< 50 x 10(9) platelets/L)(n = 14) or mildly thrombocytopenic/unaffected (> 50 x 10(9) platelets/L)(n = 13) neonates. There was no statistically significant difference between the IgG anti-HPA-1a subclass composition of the 2 groups of sera. The majority of sera (26/27, 96%) showed IgG1 + IgG3 while 17/27 (63%) had all 4 subclasses of the antibody. No significant differences between the severely thrombocytopenic and mildly thrombocytopenic/unaffected groups were detected in the levels of IgG, IgG1, IgG2 or IgG4 of the antibody. However, the values of IgG3 anti-HPA-1a were significantly higher in the severely thrombocytopenic than in the mildly thrombocytopenic/unaffected group of sera with only little overlap (median 2.94 vs. 1.68, range 1.36-9.71 vs. 1.50-2.84, respectively; p < 0.01). The results suggest that maternal IgG3 anti-HPA-1a has predictive value for severe thrombocytopenia of the neonate. However, a prospective study of IgG HPA-1a subclasses in a greater number of maternal sera at different times of pregnancy is needed to test if IgG3 anti-HPA-1a is predictive of the degree of fetal/neonatal thrombocytopenia.