FcRn, but not FcgRs, drives maternal-fetal transplacental transport of human IgGantibodies
The IgG Fc domain has the capacity to interact with diverse types of receptors, including FcRn and Fc{gamma}Rs, which confer pleiotropic biological activities. Whereas FcRn regulates IgG epithelial transport and recycling, Fc effector activities, such as ADCC and phagocytosis are mediated by Fc{gamma}Rs, which upon crosslinking transduce signals that modulate the function of effector leukocytes. Despite the well-defined and non-overlapping functional properties of FcRn and Fc{gamma}Rs, recent studies have suggested that Fc{gamma}Rs mediate transplacental IgG transport, as certain Fc glycoforms were reported to be enriched in fetal circulation. To determine the contribution of Fc{gamma}Rs and FcRn to the maternal-fetal transport of IgG, we characterized the IgG Fc glycosylation in paired maternal-fetal samples from patient cohorts from Uganda and Nicaragua. No differences in IgG1 Fc glycan profiles and minimal differences in IgG2 Fc glycans were noted, whereas the presence or absence of galactose on the Fc glycan of IgG1 did not alter Fc{gamma}RIIIA or FcRn binding, half-life, or their ability to deplete target cells in Fc{gamma}R/FcRn humanized mice. Modeling maternal/fetal transport in Fc{gamma}R/FcRn humanized mice confirmed that only FcRn contributed to transplacental transport of IgG; IgG selectively enhanced for FcRn binding resulted in enhanced accumulation of maternal antibody in the fetus. In contrast, enhancing Fc{gamma}RIIIA binding did not result in enhanced maternal/fetal transport. These results argue against a role for Fc{gamma}Rs in IgG transplacental transport, suggesting Fc engineering of maternally administered antibody to only enhance FcRn binding as a means to improve maternal/fetal transport of IgG. Significance StatementTransport of IgG antibodies from the maternal to the fetal circulation is a key process for neonatal immunity, as neonates cannot sufficiently generate IgG antibodies to reach protective levels during the first months after birth. In humans and other primates, maternal to fetal transport of IgG antibodies is largely mediated through the placental tissue. FcRn has been previously identified as the major driver of IgG transplacental transport. Here we examined whether other receptors, such as Fc{gamma}Rs, also contribute to the maternal-fetal IgG transfer. By characterizing the Fc domain structure of paired maternal-fetal IgG samples and modeling transplacental IgG transport in genetically engineered mouse strains, we determined that FcRn, but not Fc{gamma}Rs, is the major receptor that mediates transplacental IgG transport.