Anti-HLA Antibodies in Neonatal Alloimmune Thrombocytopenia - Is There Any Clinical Significance?

Background: Neonatal alloimmune thrombocytopenia (NAIT) is caused by maternal alloantibodies raised against paternally inherited alloantigens carried on fetal platelets. Platelets express both HLA class I and specific human platelet antigens (HPA). Although anti-HLA class I antibodies are often detectable in pregnant women, NAIT is considered to be mainly associated with antibodies against HPA. Cases where NAIT has been caused by antibodies against HLA class I are relatively rare and the role of these antibodies in NAIT remains debatable.

We hereby describe a sample case of NAIT proved to be caused solely by anti-HLA antibodies and discuss laboratory measures aimed at identification of pregnancies at risk of NAIT related to anti-HLA class I antibodies based on a series of similar cases.

Methods: This sample case presents laboratory work-up on a young mother who delivered her first son with a platelet count of 20x10⁹/L, minor petechiae and normal WBC count. Thrombocytopenia in the newborn resolved spontaneously two weeks after birth. Laboratory investigation included platelet immunofluorescence test (PIFT), monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay, genotyping of both parents and the newborn for platelet antigens, including rare antigens, and HLA antibody identification using the panel reactive antibodies (PRA) assay (Luminex, USA). A serum sample of this mother, drawn during her second pregnancy, and those of ten other women referred to our laboratory with a similar obstetric history of neonatal thrombocytopenia, were evaluated for the anti-HLA antibody titer using the MAIPA assay.

Results: The Rambam Platelet & Neutrophil Immunology Laboratory, as well as 32 other laboratories worldwide, that participated in the 2014 International Workshop organized by the ISBT Platelet Immunobiology Working Party failed to detect anti-HPA antibodies in the mother's serum during her second pregnancy, despite using the most sensitive serological analysis and molecular methods. Only strong anti-HLA antibodies with no single specificity were found in the analyzed samples by all the laboratories. Her second child was born by caesarean section with a platelet count of 50x10⁹/L and maternal anti-HLA antibodies were found in his serum and on his platelets. The anti-HLA antibody titer of the mother, determined by the MAIPA assay, was greater than 1:1024, with antibodies being multi-specific, as demonstrated by PRA. The anti-HLA antibody titer ≥1:16 was found to correlate with low platelet counts in the additional ten cases tested, as opposed to the titer of ≤1:4 in cases with mild and not clinically significant neonatal thrombocytopenia.

Conclusions: The presence of anti-HLA class I antibodies should be considered as a potential cause of NAIT, especially in cases with a very high titer of antibodies. The mechanism underlying the effect of these antibodies on fetal platelets needs to be further investigated.