Widespread Fab-arm exchange affects all endogenous serum IgG4.

Out of all human antibodies, IgG4 stands out by its ability to dissociate in half-molecules and reassemble forming novel bivalent antibodies. Although this so-called Fab-arm exchange mechanism is acknowledged, detailed analysis of whether all IgG4 clones in serum are involved in such processes is not known. Here, by introducing a liquid chromatography-mass spectrometry-based approach enabling the analysis of serum IgG4 clonal repertoires, we show that widespread Fab-arm exchange occurs in serum of healthy donors and leads to a massive explosion in the molecular diversity of the IgG4 clonal repertoire. These findings provide new insight into IgG4, which plays a critical role in allergy, autoimmunity and vaccination settings, and may also impact the use of IgG4 as scaffold for therapeutics.