Intravenous Immunoglobulins Regulation of Human B Cells Is Mediated through Extracellular Signal-Regulated Kinase 1 and 2.

Intravenous immunoglobulin (IVIg) has become increasingly important as therapy in autoimmune and inflammatory diseases, but its immunomodulatory action is still not fully understood. In the treatment of autoimmune diseases, the large repertoire of human immunoglobulin G (IgG), which constitutes IVIg, is assumed to modulate several immune cells including B cells. As a matter of fact, IVIg can act directly upon B cells in vitro by reducing their expansion and enhancing the differentiation of small populations of both naïve and memory B cells. In the present work, we used a panel of human cell lines representative of B cell major maturation stages and treated them with IVIg in order to identify model cell lines representative of normal B cells that could be used to further investigate IVIg effects on B cell physiology and identify the signal transduction pathways involved. Consistent with the observed effects on B cells, one of these cell lines, the DB cells, which express both FcγRIIB receptor (CD32) and surface IgG (B cell receptor [BCR]), showed reduced proliferation and upregulated secretion in presence of IVIg. As revealed by immunobloting and flow cytometry analysis, DB cells response to IVIg was paralleled by increased phosphorylation of ERK1/2, Gab1 as well as Akt, and addition of ERK1/2 activity inhibitors such as PD98059 to cultures of DB cells abolished the negative IVIg effect on proliferation of this cell line. Furthermore, in contrast to the above described IVIg consequence on ERK1/2 phosphorylation, incubation of DB cells in presence of various amounts of aggregated IgG within intramuscular immunoglobulin (IMIg) or IVIg all prevented ERK1/2 up-regulated phosphorylation. Comparable results were obtained using Pfeiffer cells, which are surface IgG⁺ as DB cells. Considering that CD32 can induce negative signaling when stimulated by aggregates of monomeric IgG present in IVIg, our results with the DB and Pfeiffer model cell lines thus suggest that the direct effect of IVIg on B cells could involve interaction with their BCR.