Commensal microbiota and protective maternal antibodies active in neonates

Although maternal antibodies protect newborns from infection, little is known about how protective antibodies are induced without prior pathogen exposure. Neonatal mice lacking the capacity to produce IgG are protected by maternal IgG antibodies to the enteric pathogen enterotoxigenic Escherichia coli (ETEC) when antibodies are delivered either trans-placentally or through milk. By challenging pups fostered on either maternal antibody–sufficient or –deficient dams, we found that milk-derived IgG was critical for protection against ETEC-induced disease. Pups utilize the neonatal Fc receptor (FcRn) totransfer IgG from milk into serum, and this IgG provides protection against systemic and mucosal E. coli infection. The maternal commensal microbiota can induce antibodies that recognize antigens expressed by ETEC and other Enterobacteriaceae species. Induction of maternal antibodies against a commensal strain confers ETEC protection in pups. The surprising role of the microbiota in eliciting protective antibodies to a specific bacterial pathogen of neonates may represent an important mechanism of hostdefense against neonatal infection.