Reduced FcRn-mediated transcytosis of IgG2 due to a missing Glycine in its lower hinge

Nigel M Stapleton,Mike R Clark,Kathryn L Armour,Juk Yee Mok,A Robin Temming,Arthur E H Bentlage,Gestur Vidarsson,Wim J E Van Esch,C Ellen Van Der Schoot,Marielle Maas,Lorna M Williamson,Maximilian Brinkhaus,Steven W De Taeye,Giso Brasser

doi:10.1038/s41598-019-40731-2

Abstract

Neonatal Fc-receptor (FcRn), the major histocompatibility complex (MHC) class I-like Fc-receptor, transports immunoglobuline G (IgG) across cell layers, extending IgG half-life in circulation and providing newborns with humoral immunity. IgG1 and IgG2 have similar half-lives, yet IgG2 displays lower foetal than maternal concentration at term, despite all known FcRn binding residues being preserved between IgG1 and IgG2. We investigated FcRn mediated transcytosis of VH-matched IgG1 and IgG2 and mutated variants thereof lacking Fc-gamma receptor (FcγR) binding in human cells expressing FcRn. We observed that FcγR binding was not required for transport and that FcRn transported less IgG2 than IgG1. Transport of IgG1 with a shortened lower hinge (ΔGly236, absent in germline IgG2), was reduced to levels equivalent to IgG2. Conversely, transport of IgG2 + Gly236 was increased to IgG1 levels. Gly236 is not a contact residue between IgG and FcRn, suggesting that its absence leads to an altered conformation of IgG, possibly due to a less flexible Fab, positioned closer to the Fc portion. This may sterically hinder FcRn binding and transport. We conclude that the lack of Gly236 is sufficient to explain the reduced FcRn-mediated IgG2 transcytosis and accounts for the low maternal/fetal IgG2 ratio at term.

Highlights

During the first months after birth, before infants acquire their own immunity, they are protected by maternal immunoglobuline G (IgG) class antibodies, transferred by the neonatal Fc-receptor (FcRn)-mediated transplacental transcytosis[1,2,3,4,5]
Similar to previous studies[21,23,24,25,26], we found that only IgG1 had a higher concentration in cord blood than in maternal serum, IgG2 and IgG3 were present in lower amounts in cord blood than in maternal serum, and IgG4 concentrations were equal (Fig. 1)
The long half-life of IgG and its transplacental transport are both mediated by the neonatal Fc receptor[5,19,23]

Summary

Introduction

During the first months after birth, before infants acquire their own immunity, they are protected by maternal IgG class antibodies, transferred by the neonatal Fc-receptor (FcRn)-mediated transplacental transcytosis[1,2,3,4,5] For this transport, FcRn binds IgG with nanomolar affinities at low pH (≤6.5) (found in intracellular vacuoles), while at physiological pH (7.4) this affinity is low[6,7,8,9]. We recently demonstrated that the short half-life and lowered FcRn-meditated transcytosis of IgG3 were due to competition between different subclasses for FcRn-mediated rescue and that IgG3 was less successful due to a single amino acid difference within its FcRn-binding site At this position (435) IgG3 has an arginine compared to histidine in other subclasses[28,29]. Individuals with the histidine-containing allotype of IgG3 (G3m(s,t) allotype which is uncommon in Europe, but relatively common in Asia and Africa), have half-life and transport across the placenta comparable to IgG125,28,30

Methods

Results

Conclusion