Intravenous immunoglobulin suppresses IL-10 production by activated B cells in vitro

A therapeutic preparation of polyclonal human IgG, i.e., intravenous immunoglobulin (IVIg), has been employed to treat several inflammatory and autoimmune disorders. B cells are supposed to be a target of IVIg, but the molecular mechanism is elusive because of the lack of a suitable experimental system. To gain an insight into the beneficial effect of IVIg on B cells, we first established an experimental setting in which IVIg modulates a murine B cell function in vitro, and then aimed at identifying the mechanistic features at the molecular level. Here we show that IVIg down-regulates IL-10 production by CpG-activated B cells in vitro. The responsible component of IVIg was identified as the F(ab’)2 portion, whose polyclonality is mandatory for the suppressive effect. IVIg, bound to the surface of activated B cells, was found to be co-localized with intracellular SHP-1 on confocal laser microscopy, suggesting that B cell-surface immunoreceptor tyrosine-based inhibitory motif-harboring receptors that recruit SHP-1 are target molecules for IVIg in our experimental setting. Overall, we postulate a scenario in which IVIg attenuates B cells by suppressing IL-10 production, a B cell growth factor, and thus down-regulates the production of pathogenic antibodies.