Further observations on the clinical significance and inheritance of the low-frequency platelet antigen HPA-28bw.

BACKGROUND: Most recently described human platelet antigens (HPAs) have been low-frequency polymorphisms identified in cases of fetomaternal alloimmune thrombocytopenia (FMAIT). There is limited opportunity to study the clinical significance or different antenatal management strategies in cases involving low-frequency HPA antibodies because many are single pregnancies. We have previously described a low-frequency platelet (PLT) antigen, HPA-28bw, implicated in FMAIT in two of the three infants in the same family. This report describes the outcome of an additional two pregnancies in this family.

STUDY DESIGN AND METHODS: The fourth and fifth pregnancies in a HPA-28bw-alloimmunized mother with a heterozygous partner were investigated to determine the risk of FMAIT. The presence of anti-HPA-28bw was assessed by paternal crossmatch studies. Prenatal HPA genotyping of amniocytes was performed to inform antenatal management.

RESULTS: GPIIb/IIIa antibodies reactive only with paternal PLTs were detected. These antibodies had been previously identified as HPA-28bw specific using recombinant GPIIb glycoprotein mutated to contain the HPA-28bw (V740L) mutation. The fetus in the fourth pregnancy did not inherit the HPA-28bw mutation, no antenatal management was required, and the baby had a normal PLT count. The fetus in the fifth pregnancy did inherit the HPA-28bw mutation. The mother received IVIG (2 g/kg/week) and prednisolone during pregnancy, and the baby was born with a normal PLT count.

CONCLUSION: Study of this family has provided a unique opportunity to assess the clinical significance of antibodies against the low-frequency PLT antigen (HPA-28bw) during five pregnancies and to compare the outcomes of different antenatal treatments.