Suppression of in vitro megakaryopoiesis by maternal sera containing anti-HPA-1a antibodies.

Incompatibility of the human platelet antigen-1 (HPA-1) system is the most common cause of fetal/neonatal alloimmune thrombocytopenia (F/NAIT) and is thought to be mediated by accelerated clearance of antibody-opsonized fetal platelets. We evaluated the effect of maternal sera containing anti-HPA-1a antibodies (F/NAIT sera) on in vitro megakaryopoiesis. Compared with control maternal sera, 14 out of 17 F/NAIT sera significantly reduced megakaryocyte (MK) number. This finding was associated with increased apoptosis and cell death of early MKs/MK progenitors, but normal maturation and differentiation of surviving MKs. An analysis of platelet counts in infants born to mothers following antenatal intravenous immunoglobulin (IVIG) ± prednisone therapy demonstrated a significant and moderately strong correlation between the MK growth in cultures and the infants' platelet counts at birth. These findings suggest that maternal anti-HPA-1a antibodies can suppress fetal megakaryopoiesis by inducing early cell death and that this influences the neonatal platelet count. Thus, the ability of maternal antibodies to suppress MK growth is a potential predictive factor for the fetal response to maternal IVIG therapy.