The first case of alloantibody against human platelet antigen-15b in Japan: possible alloimmunization by a hydatidiform mole.

Background: The involvement of the human platelet antigen (HPA)-15 system in neonatal alloimmune thrombocytopenia (NAIT) has been reported in various populations, but not in the Japanese population. In Japan, the mixed passive hemagglutination assay (MPHA) is used for detection of HPA alloantibodies. However, most of the reported cases of HPA-15 incompatibility are based on the monoclonal antibody immobilization of platelet antigen (MAIPA) assay or immunoprecipitation; thus there is a possibility that HPA-15 alloantibodies are not efficiently detected by the MPHA, and currently, the causative antibody is not detectable in approximately half of the suspected NAIT cases in Japan.

Study Design And Methods: We examined the sera of mothers from NAIT cases, previously with undetected HPA antibodies by MPHA, using the MAIPA technique. Sera from 90 mothers of suspected NAIT were tested by MAIPA for the presence of anti-HPA-15 alloantibodies.

Results: Anti-HPA-15b was detected in one case. This case was a mother in the first pregnancy diagnosed as hydatid mole-coexisting fetus, and the baby was born with suspected NAIT. The familial analysis revealed compatibility of HPA-15 genotype between the mother and the baby (both HPA-15a/a), but incompatibility with the paternal one (HPA-15a/b). The hydatid mole's tissue was genotyped as HPA-15b positive. Besides anti-HPA-15b, maternal sera contain strong HLA Class I antibody

Conclusions: Here we reported the first case of anti-HPA-15 in Japan. Alloimmunization against the hydatid mole seems to be responsible for the production of HPA-15b alloantibody. This antibody, however, did not apparently involve in the development of NAIT of the newborn, the coexisting anti-HLA Class I being the possible cause.