Design, expression and characterization of a soluble single-chain functional human neonatal Fc receptor

The neonatal Fc receptor (FcRn) is responsible for transporting maternal IgGs to fetus/newborns and maintaining the homeostasis of IgGs in adults. FcRn resembles class I major histocompatibility complex in structure, and is composed of a transmembrane heavy chain and an invariant beta 2 microglobulin. Changes in the affinity of IgGs to FcRn lead to changes in the half-life of engineered IgGs and Fc fusion proteins. Longer half-life of therapeutic antibodies means lower dose and longer interval between administering. For some diagnostic agents including imaging or radio-labeled agents a shorter half life in circulation results in lower non-specific binding and decreased side effects. Therefore, studying the interaction of FcRn and therapeutic antibodies has direct clinical implications. A reliable method to prepare soluble and functional FcRn protein is essential for such studies. In this study, we describe a new method to express in mammalian cells soluble human FcRn (sFcRn) as a single-chain soluble fusion protein. The highly hydrophilic beta 2 microglobulin was joined with the hydrophobic heavy chain via a 15 amino acid linker. The single-chain fusion protein format not only improved the expression level of the heavy chain but also simplified the purification process. The sFcRn maintained its pH-dependent binding to IgG. This method typically yielded ～1 mg/100ml culture without optimization, and is easy to scale up for production of large quantities.