IL-21 promotes plasmablast differentiation independently of proliferation in vitro

Antibodies of the IgE and IgG1 isotypes are relevant for type 2 immunity. In vivo, the production of both is elevated by IL-4, but differentially affected by IL-21, with IgE suppressed and IgG1 production enhanced by IL-21. However, whether the cytokines drive these outcomes primarily by impacting antibody-secreting, proliferating plasmablasts (PB), or their germinal center B cell precursors, is challenging to unravel. In vitro analyses using Nojima cultures, wherein naïve B cells are activated on fibroblasts co-expressing CD40L and BAFF, allows for evaluation of this problem. Here, we explore how IL-4 and IL-21 alone and in combination affect Nojima-cultured B cell proliferation and fate, asking what is unique or redundant about exposure to each. In secondary culture, as expected, IL-21 amplified replicative expansion. IL-21 also selectively promoted the differentiation of IgG1⁺ B cells into PB. The effect was countermanded by synchronous exposure to IL-4, suggesting competing signaling pathways are triggered by the two cytokines independently. Secondary culture with IL-4 alone promoted IgE⁺ B cell genesis without increasing replicative expansion. Combined exposure to IL-21 and IL-4 led to increased IgE class-switching and proliferative expansion, suggesting that once B cells are switched to IgE, IL-21 can promote IgE+ B cell proliferation. Thus, in culture IL-21 operates to promote proliferation and also drives differentiation of IgG1+ B cells into PB whereas IL-4 has an ongoing role in IgE B cell genesis. The balance of IL-4 and IL-21 thus impacts the fate of in vitro-generated germinal center B cells and highlights how the notable IgE-suppressing effects of IL-21 in vivo likely precede the class-switch step, after which IL-21 may amplify IgE production by virtue of its pro-proliferative effects.