IL-21R
Overview
The IL-21 receptor is where the IFN-gamma priming signal and the IL-21 differentiation signal meet. It is arguably the most mechanistically important receptor on the DN2 pathway that the wiki had no page for until 2026-08-18, because it is the node at which IFN-γ’s epigenetic reprogramming is cashed out as an increased ability to respond to T cell help.
It is also the only receptor in this wiki that has been blocked in a human dengue B cell response.
Key Points from Literature
IFN-γ upregulates IL-21R — the licensing mechanism
- IFN-γ exposure during the first 3 days of culture raised IL-21R protein 5.5–6-fold by day 6 (day-6 MFI: 163 without IFN-γ vs 1047 with) (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, human in vitro, ≥2 experiments)
- ATAC-seq identified 2 differentially accessible regions at the IL21R locus. One contained two putative T-bet binding motifs and aligned with a published T-bet ChIP-seq peak; it appeared only in IFN-γ-exposed cells and was most enriched when IL-2 was also present (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, n=2–3/group)
- The same IL21R differentially accessible region was identified in T-bet^hi^ DN2 cells purified from SLE patients, which are reported to be highly responsive to IL-21 (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation) — linking the in vitro programme to the in vivo cell
- Functionally, IL-21-induced phospho-STAT3 was significantly increased in B cells that had seen IFN-γ during priming, while basal phospho-STAT3 was similar and low across all conditions (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation). See STAT3.
- → The receptor, not the ligand, is the regulated variable.
Blockade evidence
- IL-21R-Fc reduced plasmablast output by ~60% in an acute dengue system — against anti-IL-10 at ~25% and anti-IL-4 at no effect (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, human, n=170 acute). This is the wiki’s largest single blockade effect in a human infection.
- IL-21R-Fc blockade reduced B cell output by 60% in the SLE/ABC literature as well (see Cancro2020 - Age-Associated B Cells, review, murine)
Downstream requirement
- No ASCs formed at all in cultures lacking IL-21, and IL-21 was required specifically during the later differentiation phase (days 3–6), not during priming (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, human in vitro). See Extrafollicular T Cell Help.
Contradictions & Debates
★ The plasma IL-21 paradox. IL-21 is functionally required for ASC formation in vitro and IL-21R blockade removes 60% of the dengue plasmablast response — yet plasma IL-21 showed no correlation whatsoever with DN2 frequency in SLE patients (Spearman r=0.087, not significant), in the same study that found strong correlations for TNFα, CXCL10, IL-6 and IFN-γ (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, n=26 SLE).
The most economical reading is that IL-21 is delivered by cell contact at short range and its serum concentration is therefore uninformative — which would make surface IL-21R on the B cell, not plasma IL-21, the meaningful measurement. The same logic applies to the BAFF serum null in GarciaBates2013 - Plasmablast Response and Dengue Severity. This is a hypothesis the wiki holds, not a demonstrated result.
Related Pages
IL-21, IFN-gamma, STAT3, Peripheral Helper T Cell, Extrafollicular T Cell Help, DN2 B Cell, Plasmablast, Atypical B Cell Effector Output, ATAC-seq