title: Structural determinants of unique properties of human IgG4-Fc.
abstract: 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 FcγRII and FcγRIII, 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 Å, respectively, of human recombinant and serum-derived IgG4-Fc. These structures reveal conformational variability at the CH3-CH3 interface that may promote Fab-arm exchange, and a unique conformation for the FG loop in the CH2 domain that would explain the poor FcγRII, FcγRIII 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 CH2 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 CH2-CH3 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.
authors.affiliation: Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, United Kingdom; Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma, London SE1 9RT, United Kingdom. Electronic address: anna.davies@kcl.ac.uk.
authors.forename: Anna M
authors.initials: AM
authors.lastname: Davies
authors.affiliation: Sanquin Research, Amsterdam 1066 CX, The Netherlands; Academic Medical Centre Landsteiner Laboratory, University of Amsterdam, Amsterdam 1066 CX, The Netherlands.
authors.forename: Theo
authors.initials: T
authors.lastname: Rispens
authors.affiliation: Sanquin Research, Amsterdam 1066 CX, The Netherlands; Academic Medical Centre Landsteiner Laboratory, University of Amsterdam, Amsterdam 1066 CX, The Netherlands.
authors.forename: Pleuni
authors.initials: P
authors.lastname: Ooijevaar-de Heer
authors.affiliation: Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, United Kingdom; Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma, London SE1 9RT, United Kingdom.
authors.forename: Hannah J
authors.initials: HJ
authors.lastname: Gould
authors.affiliation: College of Medical and Dental Sciences, School of Immunity and Infection, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
authors.forename: Roy
authors.initials: R
authors.lastname: Jefferis
authors.affiliation: Sanquin Research, Amsterdam 1066 CX, The Netherlands; Academic Medical Centre Landsteiner Laboratory, University of Amsterdam, Amsterdam 1066 CX, The Netherlands.
authors.forename: Rob C
authors.initials: RC
authors.lastname: Aalberse
authors.affiliation: Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, United Kingdom; Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma, London SE1 9RT, United Kingdom. Electronic address: brian.sutton@kcl.ac.uk.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Sutton
mesh_terms: D000911:Antibodies, Monoclonal / Q000378:metabolism*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000378:metabolism*; D007141:Immunoglobulin Fc Fragments / Q000737:chemistry* / Q000378:metabolism; D007074:Immunoglobulin G / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D017452:Receptors, IgG / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D000911:Antibodies, Monoclonal; D007140:Immunoglobulin Fab Fragments; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D017452:Receptors, IgG; D011994:Recombinant Proteins
keywords: 4-morpholineethanesulfonic acid; C1q; FAE; Fab-arm exchange; Fc receptor; IgG4-RD; IgG4-related disease; MES; antibody; immunoglobulin; rFc; recombinant IgG4-Fc; sdFc; serum-derived IgG4-Fc