Interleukin-21 (IL21) is a secreted cytokine with immunoregulatory activity that belongs to the IL-15/IL-21 family. IL21 promotes the transition between innate and adaptive immunity and is critical for T follicular helper (Tfh) cell generation and maintenance, germinal center formation, and B cell differentiation. It induces immunoglobulin production (IgG1, IgG3) in B cells, regulates proliferation and maturation of natural killer (NK) cells in synergy with IL15, and stimulates interferon-gamma production in T cells and NK cells. IL21 signals through a receptor complex containing IL21R and the common gamma chain (IL2RG), activating the JAK-STAT pathway (primarily JAK1/JAK3 and STAT1/STAT3/STAT5). Expressed primarily by activated CD4+ T cells. Mutations cause common variable immunodeficiency 11 (CVID11).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0001819 positive regulation of cytokine production | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred annotation supported by IL21's demonstrated role in cytokine regulation. IL21 stimulates IFN-gamma production in synergy with IL15 and IL18, and enhances IL-17 production. Consistent with experimental evidence. |
| GO:0045954 positive regulation of natural killer cell mediated cytotoxicity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetically inferred annotation consistent with IL21's role in NK cell function. Supported by experimental data (PMID:18005035) showing IL21 activates NK cells and modulates their cytotoxic activity, but the focused OpenScientist review judged NK cytotoxicity enhancement to be a real pleiotropic effect rather than a signature IL21 core function. Reason: Retain the annotation as biologically real, but do not treat it as core. IL21's signature biology centers on cytokine activity driving the B cell/Tfh/humoral immunity axis; NK cytotoxicity is context-dependent and downstream of the same IL21R/JAK-STAT signaling program. |
| GO:0001819 positive regulation of cytokine production | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA-predicted annotation consistent with IL21's function. Duplicates IBA annotation with same term - maintain consistent action. |
| GO:0005125 cytokine activity | IEA GO_REF:0000043 | ACCEPT | Summary: Core molecular function. IL21 is a secreted cytokine that binds to its receptor complex (IL21R/IL2RG) and activates downstream signaling. This is the primary molecular function of IL21. |
| GO:0005126 cytokine receptor binding | IEA GO_REF:0000120 | MODIFY | Summary: IL21 binds the IL21R/IL2RG receptor complex, but under GO causal modeling for secreted signaling ligands, its molecular function is better represented by the active signaling term cytokine activity (GO:0005125) rather than a passive receptor-binding term. Reason: Per the signaling ligand curation convention, receptor-binding annotations on secreted cytokines are modified to GO:0005125 (cytokine activity) so that the causal molecular activity rather than passive binding is asserted on the ligand step. Proposed replacements: cytokine activity |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: Core localization. IL21 is a secreted cytokine released into the extracellular region where it binds cell-surface IL21R/IL2RG receptor complexes. Supporting Evidence: file:human/IL21/IL21-uniprot.txt CC -!- SUBCELLULAR LOCATION: Secreted {ECO:0000269|PubMed:11081504}. Reactome:R-HSA-9005980 IL21 binds Interleukin-21 receptor (IL21R, NILR) and Cytokine receptor common subunit gamma (IL2RG, GammaC). |
| GO:0005615 extracellular space | IEA GO_REF:0000043 | ACCEPT | Summary: Core localization. IL21 is a secreted cytokine localized to the extracellular space where it binds cell-surface IL21R/IL2RG receptor complexes. Supporting Evidence: file:human/IL21/IL21-uniprot.txt CC -!- SUBCELLULAR LOCATION: Secreted {ECO:0000269|PubMed:11081504}. Reactome:R-HSA-9005980 IL21 binds Interleukin-21 receptor (IL21R, NILR) and Cytokine receptor common subunit gamma (IL2RG, GammaC). |
| GO:0006952 defense response | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Very general term. IL21 does participate in immune defense but more specific terms (defense response to virus) are available. Term is too general for core function. |
| GO:0006955 immune response | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Very general term. All cytokine activity involves immune response. More specific terms are appropriate for IL21 function. |
| GO:0009615 response to virus | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: General term. IL21 plays a role in antiviral immunity but more specific term (defense response to virus, GO:0051607) is more appropriate. |
| GO:0045954 positive regulation of natural killer cell mediated cytotoxicity | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Duplicates IBA/IDA annotation for the same term. ARBA-predicted and consistent with experimental evidence, but non-core for IL21. Reason: Maintain consistency with the IBA and IDA GO:0045954 reviews: IL21 can enhance NK cytotoxicity, but this is not a signature core function of the cytokine. |
| GO:0048468 cell development | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Very general term. IL21 is involved in specific cell development processes (NK cell, B cell, Tfh cell development) but this term is too broad. More specific terms should be used. |
| GO:0051251 positive regulation of lymphocyte activation | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Consistent with IL21 function. IL21 promotes B cell and T cell activation. Relatively general but accurate for IL21's immunoregulatory role. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: NON-INFORMATIVE: protein binding is too generic. Does not describe what IL21 binds or the functional consequence. For a cytokine, cytokine receptor binding (GO:0005126) or interleukin-2 receptor binding (GO:0005134) are more informative. Supporting Evidence: PMID:32296183 Apr 8. A reference map of the human binary protein interactome. |
| GO:0005886 plasma membrane | IPI PMID:37339051 A structural blueprint for interleukin-21 signal modulation. | MARK AS OVER ANNOTATED | Summary: This annotation likely refers to IL21 associating with plasma membrane via receptor binding (IL21R is membrane-bound). However, IL21 itself is a secreted protein, not a membrane protein. This may be misleading - IL21 is transiently at the membrane when bound to receptor but its primary location is extracellular. Supporting Evidence: PMID:37339051 2023 Jun 19. A structural blueprint for interleukin-21 signal modulation. |
| GO:0002314 germinal center B cell differentiation | IEA GO_REF:0000107 | ACCEPT | Summary: Core function. IL21 is critical for germinal center formation and B cell differentiation. Supported by ISS annotation and mouse knockout data. |
| GO:0002639 positive regulation of immunoglobulin production | IEA GO_REF:0000107 | ACCEPT | Summary: Core function. IL21 induces IgG1 and IgG3 production in B cells. This is one of the primary biological roles of IL21. |
| GO:0061470 T follicular helper cell differentiation | IEA GO_REF:0000107 | ACCEPT | Summary: Core function. IL21 is implicated in the generation and maintenance of Tfh cells. Together with IL6, critical for effective antibody response to viral infection. |
| GO:0098586 cellular response to virus | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: IL21 plays a role in antiviral immunity through multiple mechanisms. Consistent with defense response to virus annotation. Keep as supporting. |
| GO:0007259 cell surface receptor signaling pathway via JAK-STAT | NAS PMID:17938255 The opposite effects of IL-15 and IL-21 on CLL B cells corre... | ACCEPT | Summary: Core signaling mechanism. IL21 signals through the heterodimeric IL21R/IL2RG receptor complex to activate JAK1/JAK3 and downstream STAT1/STAT3/STAT5 transcription factors. Retained alongside the ligand-specific pathway term GO:0038114 (interleukin-21-mediated signaling pathway), which sits in the parallel cytokine-mediated signaling branch under GO:0007166. Reason: In GO, GO:0038114 (interleukin-21-mediated signaling pathway, is_a GO:0019221 cytokine-mediated signaling pathway) and GO:0007259 (cell surface receptor signaling pathway via JAK-STAT) are parallel branches under GO:0007166 (cell surface receptor signaling pathway) that capture complementary aspects of IL21 signaling: the specific cytokine-initiated pathway and the intracellular JAK-STAT cascade. Supporting Evidence: PMID:17938255 Oct 15. The opposite effects of IL-15 and IL-21 on CLL B cells correlate with differential activation of the JAK/STAT and ERK1/2 pathways. file:human/IL21/IL21-deep-research-falcon.md IL-21 signals via a heterodimeric receptor composed of IL-21R and the common Ξ³ chain (CD132), activating JAK1/JAK3 to predominantly induce STAT3 and STAT1 phosphorylation (with weaker STAT5), and also engages MAPK and PI3K/AKT pathways. |
| GO:0016064 immunoglobulin mediated immune response | NAS PMID:37339051 A structural blueprint for interleukin-21 signal modulation. | MARK AS OVER ANNOTATED | Summary: IL21 promotes immunoglobulin production by B cells, thus indirectly supporting immunoglobulin-mediated immunity. However, IL21 itself does not directly participate in immunoglobulin-mediated immune responses - it's an upstream regulator. Supporting Evidence: PMID:37339051 2023 Jun 19. A structural blueprint for interleukin-21 signal modulation. |
| GO:0030101 natural killer cell activation | NAS PMID:37339051 A structural blueprint for interleukin-21 signal modulation. | ACCEPT | Summary: Supported by experimental evidence. IL21 activates NK cells and modulates their surface receptor expression. Core function in NK cell biology. Supporting Evidence: PMID:37339051 2023 Jun 19. A structural blueprint for interleukin-21 signal modulation. |
| GO:0002314 germinal center B cell differentiation | ISS GO_REF:0000024 | ACCEPT | Summary: Duplicates IEA annotation. Based on manual transfer from mouse ortholog. Core function supported by multiple evidence types. Maintain consistent action. |
| GO:0002639 positive regulation of immunoglobulin production | ISS GO_REF:0000024 | ACCEPT | Summary: Duplicates IEA annotation. Based on manual transfer from mouse ortholog. Core function supported by multiple evidence types. Maintain consistent action. |
| GO:0061470 T follicular helper cell differentiation | ISS GO_REF:0000024 | ACCEPT | Summary: Duplicates IEA annotation. Based on manual transfer from mouse ortholog. Core function supported by multiple evidence types. Maintain consistent action. |
| GO:0098586 cellular response to virus | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Duplicates IEA annotation. Based on manual transfer from mouse ortholog. Consistent with IL21 antiviral role. |
| GO:0051607 defense response to virus | TAS PMID:28798470 Identification of the novel 3' UTR sequences of human IL-21 ... | ACCEPT | Summary: IL21 contributes to antiviral immunity through multiple mechanisms including enhancing NK cell cytotoxicity and T cell responses. Supported by literature. Supporting Evidence: PMID:28798470 Identification of the novel 3' UTR sequences of human IL-21 mRNA as potential targets of miRNAs. |
| GO:0042531 positive regulation of tyrosine phosphorylation of STAT protein | IDA PMID:17673207 Cloning and characterization of an isoform of interleukin-21... | ACCEPT | Summary: Correct annotation. IL21 activates JAK1/JAK3 which then phosphorylate STAT proteins. IL21 POSITIVELY REGULATES this process but does not catalyze it directly. This is the correct term to use. Supporting Evidence: PMID:17673207 Epub 2007 Jul 25. Cloning and characterization of an isoform of interleukin-21. |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-9005980 | ACCEPT | Summary: Reactome pathway reference for IL21 binding to IL21R in the extracellular region. Consistent with core secreted localization. Supporting Evidence: Reactome:R-HSA-9005980 Interleukin-21 (IL21) is a pleiotropic cytokine with four alpha-helical bundles. It is produced primarily by natural killer T cells, T follicular helper cells and TH17 cells, with lower levels of production by numerous other populations of lymphohaematopoietic cells (Spolski & Leonard 2014). IL21 binds Interleukin-21 receptor (IL21R, NILR) and Cytokine receptor common subunit gamma (IL2RG, GammaC). |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-9006844 | ACCEPT | Summary: Reactome pathway reference for extracellular IL21 receptor complex assembly. Supporting Evidence: Reactome:R-HSA-9006844 Interleukin-21 receptor (IL21R, NILR) can bind Janus kinase 1 (JAK1) (Ozaki et al. 2000) but little or no signaling occurs (Asao et al. 2001) unless IL21R is combined with IL2RG, which is required for a fully signaling capable IL21 receptor complex (Ozaki et al. 2000, Asao et al. 2001, Habib et al. 2002). The heteromeric IL21 receptor complex can activate JAK1 and JAK3. |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-9006850 | ACCEPT | Summary: Reactome pathway reference for IL21 signaling complex in the extracellular region. Supporting Evidence: Reactome:R-HSA-9006850 The IL21 heteromeric receptor complex can activate JAK1 and JAK3 in response to Interleukin-21 (IL21), leading to JAK tyrosine phosphorylation |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-9006870 | ACCEPT | Summary: Reactome pathway reference for IL21 signaling complex in the extracellular region. Supporting Evidence: Reactome:R-HSA-9006870 The IL21R:IL2RG complex can activate Signal transducer and activator of transcription 1 (STAT1), STAT3, STAT4 and STAT5, depending on the cell type. In cultured T-cells IL21 induced phosphorylation of JAK1, JAK3, STAT1, STAT3 and weakly STAT5 (Asao et al. 2001). |
| GO:0005576 extracellular region | TAS Reactome:R-HSA-9006873 | ACCEPT | Summary: Reactome pathway reference for IL21 signaling complex in the extracellular region. Supporting Evidence: Reactome:R-HSA-9006873 The IL21 receptor complex can activate Signal transducer and activator of transcription 1 (STAT1), STAT3, STAT4 and STAT5, depending on the cell type. In cultured T-cells IL21 induced phosphorylation of JAK1, JAK3, STAT1, STAT3 and weakly STAT5 (Asao et al. 2001). |
| GO:0008284 positive regulation of cell population proliferation | IDA PMID:17673207 Cloning and characterization of an isoform of interleukin-21... | KEEP AS NON CORE | Summary: General term. IL21 promotes proliferation of specific cell types (B cells, T cells, NK cells). More specific terms are available. Keep as supporting annotation. Supporting Evidence: PMID:17673207 Epub 2007 Jul 25. Cloning and characterization of an isoform of interleukin-21. |
| GO:0030890 positive regulation of B cell proliferation | IDA PMID:17673207 Cloning and characterization of an isoform of interleukin-21... | ACCEPT | Summary: Specific and correct annotation. IL21 promotes B cell proliferation as part of its role in humoral immunity. Core function. Supporting Evidence: PMID:17673207 Epub 2007 Jul 25. Cloning and characterization of an isoform of interleukin-21. |
| GO:0042102 positive regulation of T cell proliferation | IDA PMID:17673207 Cloning and characterization of an isoform of interleukin-21... | KEEP AS NON CORE | Summary: IL21 has a real but weak, context-dependent proliferative effect on anti-CD3-activated primary T cells (PMID:17673207). Whether this rises to a core function of IL21 (vs its signature B-cell/Tfh axis) is borderline. Reason: The focused OpenScientist review resolved the core-function question by recommending KEEP_AS_NON_CORE. The effect is directly evidenced (PMID:17673207: "comparable proliferative effect on ... anti-CD3 Ab-activated primary T cells"), but it is costimulation-dependent, context-sensitive, and subordinate to IL21's signature B-cell/Tfh/humoral immunity functions. GO:0042102 remains out of core_functions. Supporting Evidence: PMID:17673207 Epub 2007 Jul 25. Cloning and characterization of an isoform of interleukin-21. file:human/IL21/IL21-hypotheses/core-function-1-go-0042102/openscientist.md The annotation should be retained (the effect is real) but classified as non-core using the `KEEP_AS_NON_CORE` action. |
| GO:0007260 tyrosine phosphorylation of STAT protein | IDA PMID:12244150 IL-21 up-regulates the expression of genes associated with i... | MODIFY | Summary: OVER-ANNOTATION: IL21 is a cytokine that INDUCES STAT phosphorylation via the JAK-STAT signaling pathway, but does NOT catalyze the phosphorylation reaction itself. PMID:12244150 states IL-21 "induced STAT3 activation" - the cytokine triggers downstream JAK kinases (JAK1/JAK3) to phosphorylate STAT proteins. IL21 has no kinase activity. The appropriate term is GO:0042531 "positive regulation of tyrosine phosphorylation of STAT protein" which correctly describes the regulatory role. Note IL21 already has the correct annotation at GO:0042531 (IDA, PMID:17673207). Proposed replacements: positive regulation of tyrosine phosphorylation of STAT protein Supporting Evidence: PMID:12244150 IL-21 up-regulates the expression of genes associated with innate immunity and Th1 response. |
| GO:0045954 positive regulation of natural killer cell mediated cytotoxicity | IDA PMID:18005035 Interleukin-21 activates human natural killer cells and modu... | KEEP AS NON CORE | Summary: Correct annotation with experimental evidence. IL21 activates NK cells and modulates their cytotoxic activity, but this should be treated as a real non-core pleiotropic effect rather than a signature IL21 function. Reason: The focused OpenScientist review found that NK cytotoxicity enhancement has IDA support from PMID:18005035, but parallels T cell proliferation as a downstream, context-dependent effect of IL21R/JAK-STAT signaling rather than a defining core process. Supporting Evidence: PMID:18005035 Epub 2007 Nov 14. Interleukin-21 activates human natural killer cells and modulates their surface receptor expression. file:human/IL21/IL21-hypotheses/core-function-1-go-0042102/openscientist.md NK cytotoxicity enhancement is a real pleiotropic effect but not a signature function, and it should also be classified as non-core. |
| GO:0001819 positive regulation of cytokine production | IDA PMID:19322899 Il-21 enhances NK cell activation and cytolytic activity and... | ACCEPT | Summary: Correct annotation. IL21 enhances cytokine production including IL-17 and IFN-gamma. Core immunoregulatory function. Supporting Evidence: PMID:19322899 Il-21 enhances NK cell activation and cytolytic activity and induces Th17 cell differentiation in inflammatory bowel disease. |
| GO:0032740 positive regulation of interleukin-17 production | IDA PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL... | ACCEPT | Summary: Specific and correct annotation. IL21 promotes IL-17 production by Th17 cells. Supported by experimental evidence. Core function in Th17 biology. Supporting Evidence: PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL-17 by human T cells. |
| GO:0034105 positive regulation of tissue remodeling | IC PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL... | KEEP AS NON CORE | Summary: Inferred from context. IL21 promotes IL-17 production, which is associated with tissue remodeling in inflammatory conditions. Indirect effect, not core function. Supporting Evidence: PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL-17 by human T cells. |
| GO:0050729 positive regulation of inflammatory response | IC PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL... | KEEP AS NON CORE | Summary: Inferred from context. IL21 through IL-17 induction can promote inflammation. Context-dependent effect rather than core function. Supporting Evidence: PMID:16482511 Divergent effects of IL-12 and IL-23 on the production of IL-17 by human T cells. |
| GO:0005126 cytokine receptor binding | TAS PMID:15546387 Biology of IL-21 and the IL-21 receptor. | MODIFY | Summary: Review article citing IL21 binding to the IL21R/IL2RG receptor complex. Modify to GO:0005125 (cytokine activity) for consistency with the IEA row and signaling ligand curation convention. Reason: For secreted cytokines, receptor-binding annotations are replaced by the active signaling molecular function GO:0005125 (cytokine activity). Proposed replacements: cytokine activity Supporting Evidence: PMID:15546387 Biology of IL-21 and the IL-21 receptor. |
| GO:0005134 interleukin-2 receptor binding | IPI PMID:12504082 Human IL-21 and IL-4 bind to partially overlapping epitopes ... | MODIFY | Summary: IL21 binds the shared common gamma chain (IL2RG, a subunit shared with the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors) to assemble the heterodimeric IL21R/IL2RG signaling complex. Modify passive receptor-binding term to GO:0005125 (cytokine activity). Reason: IL21 does not bind the IL-2-specific receptor alpha/beta subunits; it engages the shared common gamma chain (IL2RG) as part of its cytokine activity (GO:0005125). Under the ligand curation convention, receptor-binding terms on secreted cytokines are modified to GO:0005125. Proposed replacements: cytokine activity Supporting Evidence: PMID:12504082 Human IL-21 and IL-4 bind to partially overlapping epitopes of common gamma-chain. |
| GO:0005615 extracellular space | NAS PMID:11081504 Interleukin 21 and its receptor are involved in NK cell expa... | ACCEPT | Summary: Core localization. IL21 is a secreted cytokine localized to the extracellular space. Supporting Evidence: PMID:11081504 Interleukin 21 and its receptor are involved in NK cell expansion and regulation of lymphocyte function. |
| GO:0032729 positive regulation of type II interferon production | NAS PMID:15765404 Interleukin-21 enhances T-helper cell type I signaling and i... | ACCEPT | Summary: IL21 enhances IFN-gamma production. IFN-gamma is a type II interferon. Correct annotation supported by synergy with IL15 and IL18 in IFN-gamma induction. Supporting Evidence: PMID:15765404 Interleukin-21 enhances T-helper cell type I signaling and interferon-gamma production in Crohn's disease. |
| GO:0042102 positive regulation of T cell proliferation | IDA PMID:15207081 [Cloning and expression of human interleukin-21 cDNA in E.co... | KEEP AS NON CORE | Summary: Duplicate of the GO:0042102 annotation from PMID:17673207; IL21 shows a proliferative effect on activated T cells. Reason: Resolved consistently with the GO:0042102 annotation from PMID:17673207 and the focused OpenScientist review. The effect is real, but should be retained as non-core because it is costimulation-dependent and not part of IL21's signature B-cell/Tfh/humoral immunity axis. Supporting Evidence: PMID:15207081 AIM: To clone full length cDNA of human interleukin-21 (IL-21) and express it in E.coli. file:human/IL21/IL21-hypotheses/core-function-1-go-0042102/openscientist.md The annotation should be retained (the effect is real) but classified as non-core using the `KEEP_AS_NON_CORE` action. |
| GO:0048469 cell maturation | IDA PMID:14635054 IL-21 induces both rapid maturation of human CD34+ cell prec... | KEEP AS NON CORE | Summary: IL21 induces maturation of CD34+ precursors towards NK cells. Specific to NK cell development. Keep as supporting annotation. Supporting Evidence: PMID:14635054 IL-21 induces both rapid maturation of human CD34+ cell precursors towards NK cells and acquisition of surface killer Ig-like receptors. |
| GO:0007165 signal transduction | NAS PMID:11081504 Interleukin 21 and its receptor are involved in NK cell expa... | MODIFY | Summary: Overly general pathway term. IL21 specifically initiates the interleukin-21-mediated signaling pathway (GO:0038114) via the heterodimeric IL21R/IL2RG receptor complex and JAK1/JAK3-STAT3/STAT1/STAT5 cascade. Reason: Refine the broad signal transduction term to the specific descendant pathway term GO:0038114 (interleukin-21-mediated signaling pathway). Proposed replacements: interleukin-21-mediated signaling pathway Supporting Evidence: PMID:11081504 Interleukin 21 and its receptor are involved in NK cell expansion and regulation of lymphocyte function. |
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