Identification of Cellular Intermediates and Molecular Pathways Induced by IL-21 in Human B Cells

Abstract

The complex process of B cell development is controlled by multiple factors from the surrounding microenvironment including cytokines. IL-21 is a recently identified type I cytokine, mainly produced by activated CD4+ T cells. It has been shown to promote differentiation of human primary B cells into Ig-secreting plasma cells. The objective of our study was to describe cellular intermediates that exist during IL-21-induced transition from an activated B cell to an Ig-secreting cell and to identify molecular mechanisms involved in this process. Novel Epstein-Barr Virus-positive human B cell lines with phenotypes characteristic of Ag-activated IgG+ B cell blasts were used as a model system to study IL-21 effects in vitro. We show that IL-21 increased both proliferation and survival of B cell lines during the first 3 days of in vitro culture. This process was associated with CD38low/intCD23intHLA-DRhighCD19highCD20int cell surface phenotype. Continued culture with IL-21 resulted in accumulation of cells in G0/G1 stage of the cell cycle and increased apoptosis. This coincided with differentiation into small, CD38highCD23low/−HLA-DRintCD19intCD20low late plasmablasts/early plasma cells that expressed lower levels of c-Myc protein, and secreted greater amounts of Ig than the control cells. Partial inhibition of IL-21-induced JAK/STAT signaling by the low-dose pharmacological agent, JAK inhibitor I, did not prevent the initial increase in proliferation. However, decrease in c-Myc protein expression and subsequent differentiation to late plasmablasts/early plasma cells were strongly inhibited. Our study is the first to show the link between IL-21-induced JAK/STAT signaling, c-Myc regulation, and differentiation of human B cells.