Identification and function of neonatal Fc receptor in mammary gland of lactating mice

In addition to its proposed function in regulating serum IgG levels, the MH C class I-related neonatal Fc receptor (FcRn) is known to play a role in Ig G transfer across rodent yolk sac and neonatal intestine. In contrast to hu mans, for which transplacental transfer of IgG appears to be the only mecha nism of maternal IgG delivery, the transmission of IgG in mice occurs both antenatally (yolk sac) and neonatally (transport from mother's milk across intestinal epithelial cells). In the current study, a possible role for FcR n in regulating IgG transfer into milk has been investigated. FcRn has been shown to be present in functional form in the mammary gland of lactating m ice, and is localized to the epithelial cells of the acini. Analysis of the transfer of Pc fragments and IgG which have different affinities for FcRn indicate that, unexpectedly, these proteins are transferred in inverse corr elation with their binding affinity for FcRn. Thus, in the lactating mammar y gland FcRn appears to play a role in recycling IgG in a mode that may hav e relevance to FcRn trafficking during the maintenance of constant serum Ig G levels.