Structural Determinants of Unique Properties of Human IgG4-Fc

Human IgG4, normally the least abundant of the four subclasses of IgG in serum, displays a number of unique biological properties. It can undergo heavy-chain exchange, also known as Fab-arm exchange, leading to the formation of monovalent but bispecific antibodies, and it interacts poorly with Fc gamma RII and Fc gamma RIII, and complement. These properties render IgG4 relatively non-inflammatory and have made it a suitable format for therapeutic monoclonal antibody production. However, IgG4 is also known to undergo Fc-mediated aggregation and has been implicated in auto-immune disease pathology. We report here the high-resolution crystal structures, at 1.9 and 2.35 angstrom, respectively, of human recombinant and serum-derived IgG4-Fc. These structures reveal conformational variability at the C(H)3-C(H)3 interface that may promote Fab-arm exchange, and a unique conformation for the FG loop in the C(H)2 domain that would explain the poor Fc gamma RII, Fc gamma RIII and C1q binding properties of IgG4 compared with IgG1 and -3. In contrast to other IgG subclasses, this unique conformation folds the FG loop away from the C(H)2 domain, precluding any interaction with the lower hinge region, which may further facilitate Fab-arm exchange by destabilisation of the hinge. The crystals of IgG4-Fc also display Fc-Fc packing contacts with very extensive interaction surfaces, involving both a consensus binding site in IgG-Fc at the C(H)2-C(H)3 interface and known hydrophobic aggregation motifs. These Fc-Fc interactions are compatible with intact IgG4 molecules and may provide a model for the formation of aggregates of IgG4 that can cause disease pathology in the absence of antigen. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.