Fusion of IgG antibodies to albumin inhibits transport across the placenta.

Fusing IgG antibodies to albumin markedly reduced transplacental transport in mice and in an ex vivo human placental perfusion system, while preserving the FcRn-conferred prolonged plasma half-life. Fab fragments fused to engineered albumin with enhanced FcRn binding showed minimal fetal exposure. In a mouse model of fetal and neonatal alloimmune thrombocytopenia, albumin fusion of an anti-human platelet antigen IgG reduced fetal antibody transfer and attenuated thrombocytopenia in offspring.