IL-21
Overview
Interleukin-21 (IL-21) is a type I cytokine produced by activated CD4⁺ T cells — both follicular helper T cells (Tfh) and peripheral helper T cells (Tph). IL-21 is a potent driver of B cell differentiation into antibody-secreting cells (ASCs/plasmablasts), promoting plasma cell gene expression (BLIMP-1/IRF4) and immunoglobulin secretion. In the extrafollicular B cell pathway, IL-21 is one of the three key signals (alongside TLR7 and IFN-γ) required for the aNAV → DN2 → plasmablast differentiation cascade.
Key Points from Literature
- IL-21 is required for EF differentiation in vitro: The Jenks2018 rNAV→aNAV→DN2→plasmablast differentiation protocol requires TLR7 (R848) + IFN-γ + IL-21. Removing IL-21 abolishes plasmablast generation. IL-21 cooperates with TLR7 signalling; BCR engagement is not required (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, in vitro differentiation).
- IL-21 is produced by Tph cells in acute dengue at levels comparable to cTfh: Intracellular cytokine staining shows CXCR5⁻PD-1⁺ (Tph) cells produce IL-21 at frequencies comparable to CXCR5⁺PD-1⁺ (cTfh) cells. This identifies Tph as a major IL-21 source in the extrafollicular context of dengue (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, n=170 cohort).
- Blocking IL-21 reduces Tph-driven plasmablast output by ~60%: In T-B coculture experiments, anti-IL-21 blocking antibody substantially reduces but does not abolish plasmablast generation from memory B cells cocultured with Tph. The residual ~40% output likely reflects other Tph-derived signals (CD40L, ICOS, other cytokines) (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
- IL-21 marks the helper Tph subcluster: scRNA-seq of activated CD4⁺ T cells in dengue identified an IL-21⁺ helper Tph cluster co-expressing ICOS, MAF, and TOX2, distinct from the GZMB⁺ cytotoxic Tph cluster. This separation suggests IL-21 production is a defining feature of the B cell help-competent Tph subset (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, scRNA-seq, 4,361 cells).
- IL-21 in the SLE EF programme: IL-21 is elevated in SLE and contributes to the sustained EF activation and plasmablast expansion that characterises disease flares. Genetic variants in the IL-21/IL-21R pathway are SLE risk alleles (see Sanz2025 - Human Atypical B Cells Overview, review).
Contradictions & Debates
None documented in current wiki sources.
Related Pages
Peripheral Helper T Cell, Extrafollicular Response, Plasmablast, DN2 B Cell, Activated Naive B Cell, TLR7, BLIMP-1, IRF4, Memory B Cell