Cytokine-Mediated Regulation of Plasma Cell Generation: IL-21 Takes Center Stage

Abstract

During our life, we are surrounded by continuous threats from a diverse range of invading pathogens. Our immune system has evolved multiple mechanisms to efficiently deal with these threats so as to prevent them from causing disease. Terminal differentiation of mature B cells into plasma cells (PC) - the antibody (Ab) secreting cells of the immune system - is critical for the generation of protective and long-lived humoral immune responses. Indeed, efficient production of antigen (Ag)-specific Ab by activated B cells underlies the success of most currently available vaccines. The mature B-cell pool is composed of several subsets, distinguished from one according to size, surface marker expression, location, and Ag exposure, and they all have the capacity to differentiate into PCs. For a B-cell to acquire the capacity to produce Abs, it must undergo an extensive differentiation process driven by changes in gene expression. Two broad categories of Ags exist that cause B-cell activation and differentiation: T cell dependent (TD) or T cell independent (TI). In addition to the B-cell subset and nature of the Ag, it is important to consider the cytokine environment that can also influence how B-cell differentiation is achieved. Thus, while many cytokines can induce Ab-secretion by B cells after activation with mimics of TD and TI stimuli in vitro, they can have different efficacies and specificities, and can often preferentially induce production of one particular Ig isotype over another. Here, we will provide an overview of in vitro studies (mouse and human origin) that evaluated the role of different cytokines in inducing the differentiation of distinct B-cell subsets to the PC lineage. We will place particular emphasis on IL-21, which has emerged as the most potent inducer of terminal B-cell differentiation in humans. We will also focus on the role of IL-21 and defects in B-cell function and how these contribute to human immunopathologies such as primary immunodeficiencies and B-cell mediated autoimmune conditions.

Keywords: IL-21; autoimmune diseases; cytokines; differentiation; human B cells; immunodeficiency; plasma cells.

Figure 1

T cell dependent B-cell differentiation. Following the receipt of signals provided by the microenvironment [e.g., Ag, CD4⁺ T (Tfh) cells, DC], naïve B cells undergo activation and can initially differentiate into either extrafollicular short-lived Ab-secreting plasma cells (secreting predominantly IgM), or can seed a germinal center (GC). Within GCs, B cells undergo somatic hypermutation of their Ig V region genes and only those B cells with the highest affinity are selected to then differentiate into long-lived memory B cells or plasma cells that are capable of secreting a variety of Ig isotypes, including the switched isotypes IgG, IgA, and IgE. The outcome of the GC reaction is heavily influenced by Tfh cells, especially those within the GC itself. These cells are not depicted on the figure but they contribute greatly at this stage of B-cell differentiation. Following re-encounter with the initiating Ag, memory B cells rapidly differentiate into plasma cells. The differentiation of naïve B cells to these distinct effector fates is controlled by the balanced expression and regulated function of various transcription factors, including (but not exclusively) PAX5, BCL-6, BLIMP-1, XBP-1, and IRF4.

Figure 2

Cytokine-induced differentiation of human B cells in vitro: requirement for IL-21 signaling in vivo. In vitro studies demonstrated that human B cells could undergo events such as Ig class switching and differentiation to become Ig-secreting cells following stimulation with a diverse range of cytokines. However, analysis of individuals with hypomorphic mutations in genes encoding STAT3, γc (IL2RG), JAK3, or IL-21R have revealed that this pathway – activated by IL-21 – is critical for the generation of memory B cells and the establishment of Ag-specific Abs in vivo. Thus, although cytokines such as IL-4, IL-13, IL-10, and BAFF/APRIL are strong B-cell growth and differentiation factors, their function is insufficient to compensate for impaired IL-21/IL-21R signaling in vivo in the setting of generating robust, long-lived Ag-specific Ab, and memory responses. Consequently, IL-21-mediated B-cell activation is a necessary and sufficient step in the generation of protective long-lived humoral immune responses in humans.

Figure 3

Effects of IL-21 on human naïve B cells in vitro. When human naïve B cells are stimulated with CD40L together with IL-21, they were found to undergo intense proliferation. This was followed by induction of Ig class switching – predominantly to IgG₃ and IgG₁; a lesser extent to IgA₁ – as determined by acquisition of expression of switched isotypes or differentiation to plasma-like cells capable of secreting all major Ig isotypes. The B cells that had undergone switching to become IgG⁺ or IgA⁺ were distinct from those that committed to a plasma cell fate – thus, the secreted Ig detected in these cultures was derived from the in vitro-derived plasma cells rather than the surface IgG⁺/IgA⁺ class switched cells. Both class switching and plasma cell formation induced by IL-21 could be modulated by additional cytokines, such as IL-4 (promoted switching to IgG; inhibited switching to IgA; increased secretion of IgG and IgE; suppressed secretion of IgM, IgA), IL-10 (increased IgA secretion), and IL-2 (increased secretion of IgM, IgG). These differentiation events coincided with the induction in expression of BCL-6, AICDA (required for class switching), and BLIMP-1/XBP-1 (required for plasma cell formation). The ability of IL-21 to induce naïve B cells to differentiate into plasmablasts/plasma cells in vitro was abolished by hypomorphic mutations in STAT3, as well as null mutations in IL21R or IL2RG.