HLA-DRB3*01:01 is a predisposing factor for human platelet antigen 1a (HPA-1a) immunization, which is responsible for most cases of fetal and neonatal alloimmune thrombocytopenia. The aim of this study was to investigate if the HLA-DRB3*01:01 allele imposes a dose-dependent effect on anti-HPA-1a levels and neonatal platelet counts. One hundred and thirty HPA-1a-immunized women were divided into 3 groups: HLA-DRB3*01:01 negative, HLA-DRB3*01:01 hemizygous or heterozygous, and HLA-DRB3*01:01 homozygous. The dose of the HLA-DRB3*01:01 allele was determined by sequencing exon 2 of the HLA-DRB3 gene followed by HLA-DRB3 and HLA-DRB1 typing of selected samples. Anti-HPA-1a levels at time of delivery and neonatal platelet counts were compared among groups. There was a significant dose-dependent effect of the HLA-DRB3*01:01 allele on anti-HPA-1a levels (global P value [P _global] = .0032). Median (range) anti-HPA-1a levels were 1.5 IU/mL (0.0-19.0 IU/mL), 21.1 IU/mL (0.0-1967 IU/mL), and 43.7 IU/mL (1.0-980 IU/mL) in women with 0, 1, and 2 copies of the HLA-DRB3*01:01 allele, respectively. There was also a significant, but opposite, dose-dependent effect of the mother's HLA-DRB3*01:01 allele on the platelet count of the newborn (P _global = .0155). Median (range) neonatal platelet counts were 241 × 10⁹/L (59 × 10⁹/L to 393 × 10⁹/L), 107 × 10⁹/L (4 × 10⁹/L to 387 × 10⁹/L) and 32 × 10⁹/L (4 × 10⁹/L to 352 × 10⁹/L) for newborns of mothers with 0, 1, and 2 copies of the HLA-DRB3*01:01 allele, respectively. Thus, the HLA-DRB3*01:01 allele exhibits a dose-dependent impact on maternal anti-HPA-1a levels in HPA-1a-immunized women.

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