A major histocompatibility complex class I-like Fc receptor cloned from human placenta: possible role in transfer of immunoglobulin G from mother to fetus.

TLDR

Maternal antibodies are essential for newborn immunity, and IgG is actively transported across the human placenta, yet the receptor mediating this transfer has not been definitively identified, although similar receptors transport IgG from milk to the bloodstream of suckling rodents. This study reports the isolation of placental cDNA clones encoding a class‑I MHC‑like IgG‑Fc receptor (hFcRn). The authors identified hFcRn as a receptor that binds IgG with high affinity at low pH, implying that IgG associates with hFcRn in acidic intracellular compartments during placental transport. The hFcRn sequence is highly similar to rat and mouse Fc receptors, and its low‑pH IgG binding confirms a conserved mechanism for maternal IgG transfer.

Abstract

The acquisition of maternal antibodies is critical for immunologic defense of the newborn. In humans, maternal IgG is actively transported across the placenta. The receptor responsible for this transport has not been identified definitively. We report the isolation from a placental cDNA library of clones encoding the alpha-chain of an immunoglobulin G (IgG)-Fc receptor (hFcRn) that resembles a class I major histocompatibility complex antigen. The DNA and predicted amino acid sequences are very similar to those of the neonatal rat and mouse intestinal Fc receptors, rFcRn and mFcRn. These receptors mediate transport of maternal IgG from milk to the blood-stream of the suckling rat or mouse. Like rat and mouse FcRn, hFcRn binds IgG preferentially at low pH, which may imply that IgG binds hFcRn in an acidic intracellular compartment during transport across the placenta.