Human IgG4 binds to IgG4 and conformationally altered IgG1 via Fc-Fc interactions.
The Fc fragment of IgG4 can interact with the Fc fragment of another IgG molecule. This interaction is a confounding factor when measuring IgG4 rheumatoid factor levels. Recently, we demonstrated that half-molecules of IgG4 can exchange to form a bispecific Ab. We expected these two phenomena to be related and investigated the physicochemical aspects of IgG4 Fc-Fc interactions. We found that IgG4 is >99% monomeric by size-exclusion chromatography; therefore, IgG4 Fc-Fc interactions in the fluid phase (if any) would be short-lived. However, (125)I-labeled IgG4 does bind to IgG1 and IgG4 coupled to a solid phase. By contrast, IgG1 does not bind to coupled IgG4. Furthermore, conditions that induce partial unfolding/dissociation of the CH3 domains enhance IgG4 Fc binding, suggesting that Fc binding is primarily CH3 mediated. IgG4 slowly associates with both IgG4 and IgG1 coupled to a biosensor chip. Remarkably, subsequent dissociation was much faster for IgG4 than for IgG1. Moreover, after binding of an IgG4 mAb to Sepharose-coupled Ag, we observed additional binding of IgG4 with irrelevant specificity, whereas similar binding was not observed with Ag-bound IgG1. We propose that the IgG4-IgG4 Fc interaction resembles an intermediate of the Fab-arm (half-molecule) exchange reaction that is stabilized because one of the IgG4 molecules is coupled to a solid phase. By contrast, IgG4 Fc recognizes IgG1 only after a conformational change that renders CH3(IgG1) accessible to an interaction with the CH3(IgG4). Such Fc interactions may enhance Ag binding of IgG4 in vivo.
Rispens
Theo
T
Sanquin Research, Amsterdam, The Netherlands. T.Rispens@sanquin.nl
Ooievaar-De Heer
Pleuni
P
Vermeulen
Ellen
E
Schuurman
Janine
J
van der Neut Kolfschoten
Marijn
M
Aalberse
Rob C
RC
D006801:Humans; D007141:Immunoglobulin Fc Fragments / Q000276:immunology / Q000378:metabolism*; D007074:Immunoglobulin G / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism
D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G