IL21 Core Function Hypothesis: Positive Regulation of T Cell Proliferation (GO:0042102)

Executive Judgment

Verdict: OVER-ANNOTATED as core function — recommend KEEP_AS_NON_CORE

Positive regulation of T cell proliferation (GO:0042102) is a real, experimentally documented effect of IL-21, but it does not represent a core (primary, signature) function of this cytokine. Eight independent lines of evidence converge on this conclusion: (1) both IDA-supporting papers demonstrate only costimulation-dependent proliferation; (2) human IL-21R deficiency phenotypes center on B cell and humoral immune defects, not T cell proliferative defects; (3) IL-21 is anti-proliferative for certain T cell subsets (Tfr cells); (4) IL-21's actual T cell biology is differentiation (Th17/Tfh lineage commitment), not proliferation; (5) cell-type-specific knockouts show B cells are the essential target; (6) even the Th17 autocrine role shows in vivo redundancy; (7) the strongest counter-evidence (CD8 T cell sustaining during chronic viral infection) is context-specific, mechanistically distinct from mitogenic proliferation, and maps to a different GO term; and (8) IL-21 is unique among gamma-chain cytokines in not having T cell proliferation as a primary function. The two IDA annotations (PMID:17673207, PMID:15207081) should be retained in the gene record but reclassified as non-core. The related annotation for positive regulation of NK cell mediated cytotoxicity (GO:0045954) should be treated equivalently — real but non-core — and removed from directly_involved_in.

The most important caveat is that "core" vs. "non-core" is a curation-level distinction, not a binary biological one. The T cell proliferative effect is genuine and reproducible, but it is secondary, context-dependent, and subordinate to IL-21's signature roles in germinal center B cell differentiation, Tfh differentiation, immunoglobulin production, and B cell proliferation.

Summary

IL-21 is a pleiotropic type I cytokine produced predominantly by CD4+ T cells (especially Tfh and Th17 cells) and NKT cells. It signals through the IL-21R/IL-2Rgamma (common gamma-chain) receptor complex, activating JAK1/JAK3 and primarily STAT3. Its signature, evolutionarily conserved functions center on the B cell/T follicular helper axis: germinal center B cell differentiation, T follicular helper cell differentiation, immunoglobulin class switching and production, and B cell proliferation. These functions are consistently revealed by loss-of-function studies in both mice (Il21-/-, Il21r-/-) and humans with IL-21R deficiency.

The question under review is whether positive regulation of T cell proliferation — currently annotated with two IDA evidence codes — qualifies as a core function or should be downgraded to a non-core annotation. After reviewing 87 primary research papers, key reviews, human genetic deficiency case series, and GO database evidence codes, we conclude that T cell proliferation is a downstream, context-dependent consequence of IL-21R signaling rather than a primary process the cytokine evolved to drive. The annotation should be retained (the effect is real) but classified as non-core using the KEEP_AS_NON_CORE action.

A secondary question concerns NK cell mediated cytotoxicity (GO:0045954). Our investigation revealed that this annotation has stronger experimental support than the source YAML indicated (IDA evidence from PMID:18005035, not just IBA/IEA), but the same logic applies: NK cytotoxicity enhancement is a real pleiotropic effect but not a signature function, and it should also be classified as non-core.

Key Findings

Finding 1: IL-21's T Cell Proliferative Effect Is Costimulation-Dependent

Both IDA-supporting papers demonstrate T cell proliferation only in the context of TCR costimulation, not as a standalone mitogenic activity. PMID:17673207 (Zhong et al.) showed that "both human IL-21 and IL-21iso showed comparable proliferative effect on anti-CD40 Ab-activated primary B cells, anti-CD3 Ab-activated primary T cells and human NK cell line, NK0" — critically, the T cell proliferation required anti-CD3 antibody activation. PMID:15207081 similarly showed that "the refolded rhIL-21 could stimulate the proliferation of mature human T-cells in the presence of anti-CD3" as part of a bioactivity validation assay for recombinant protein production, not a dedicated investigation of IL-21's T cell biology.

This contrasts sharply with bona fide T cell growth factors. IL-2, for example, "drives T-cell growth" as a standalone activity (PMID: 21889323) and is the defining T cell mitogen. IL-7 drives homeostatic T cell proliferation and is essential for T cell development. IL-15 drives CD8 memory T cell homeostatic proliferation via trans-presentation. IL-21 is unique among gamma-chain cytokines in requiring TCR costimulation for any T cell proliferative effect.

The context-dependency is further underscored by Spolski & Leonard (2008): "The regulatory activity of IL-21 is modulated by the differentiation state of its target cells as well as by other cytokines or costimulatory molecules" (PMID: 17953510).

Finding 2: Human IL-21R Deficiency Phenotype Confirms B Cell/GC/Humoral Immunity as Core

The most powerful test of a cytokine's core function is what happens when it is absent. Human IL-21R deficiency (13 patients, PMID:33929673) presents with: "Most patients exhibited hypogammaglobulinemia and reduced proportions of memory B cells, circulating T follicular helper cells, MAIT cells and terminally differentiated NK cells." The primary clinical manifestations were recurrent infections and cryptosporidiosis-associated cholangitis — hallmarks of humoral immune deficiency. No T cell proliferative defect was highlighted as a defining feature.

Mouse knockouts corroborate this. IL-21R-deficient mice showed "reduced numbers of germinal center and IgA" responses (PMID: 30087442), and comprehensive reviews conclude IL-21 has a "critical role in T cell-dependent B cell activation, germinal center reactions, and humoral immunity" (PMID: 31821441).

If T cell proliferation were a core function, its loss should be phenotypically prominent in IL-21R deficiency. It is not.

Finding 3: IL-21 Is Anti-Proliferative for T Cell Subsets

A core pro-proliferative function should be consistently directional. Instead, IL-21 actively suppresses proliferation of T follicular regulatory (Tfr) cells: "IL-21 restricts T follicular regulatory T cell proliferation through Bcl-6 mediated inhibition of responsiveness to IL-2" (PMID: 28303891). IL-21 also has "direct inhibitory effects on the antigen-presenting function of dendritic cells and can be proapoptotic for B cells and NK cells" (PMID: 17953510), and is described as "a double-edged sword" (PMID: 24751819).

This bidirectionality — promoting proliferation of some T cells while suppressing others — is inconsistent with a core pro-proliferative function and instead reflects downstream pleiotropic effects of STAT3 signaling in different cellular contexts.

Finding 4: IL-21's Actual T Cell Biology Is Differentiation, Not Proliferation

The dominant T cell biology of IL-21 is lineage commitment — Th17 and Tfh differentiation — not proliferative expansion. IL-21 is "an autocrine cytokine that is sufficient and necessary" for Th17 differentiation, mediating lineage commitment via STAT3/RORgammat (PMID: 17581589). It "serves as an autocrine factor secreted by Th17 cells that promotes or sustains Th17 lineage commitment" (PMID: 17884812). In chronic viral infection, "IL-21 from high-affinity CD4 T cells drives differentiation of brain-resident CD8 T cells" (PMID: 32948671) — again differentiation, not proliferation.

This means GO:0042102 (positive regulation of T cell proliferation) may be the wrong term to capture the actual T cell biology of IL-21, even setting aside the core vs. non-core question.

Finding 5: Cell-Specific Knockouts Show B Cells Are the Essential Target

Even in T cell-driven autoimmune models, IL-21's critical cellular target is B cells, not T cells. Block & Huang (2013) demonstrated: "T cells deficient in IL-21 did not induce GC formation or autoantibody production, but they went through normal TFH differentiation... IL-21 acts on B cells, because IL-21R expression on B cells was required to induce disease" (PMID: 23960240). This directly shows that when IL-21's function is dissected at the cellular level, the B cell arm — not the T cell arm — is where the essential biology resides.

Finding 6: Even the Th17 Autocrine Role Shows In Vivo Redundancy

IL-21's strongest T cell role is in Th17 differentiation, but even this is redundant in vivo: "While IL-21 is an essential autocrine amplification factor for differentiation of Th17 cells, the loss of IL-21 or IL-21 receptor (IL-21R) does not protect mice from actively induced EAE" (PMID: 26413871). If IL-21's most established T cell function is dispensable for disease outcomes, its weaker T cell proliferative effect is even less likely to represent a core function.

Finding 7: Strongest Counter-Evidence Still Supports Non-Core Classification

The strongest argument for IL-21 having a core T cell function comes from chronic viral infection studies. Elsaesser et al. (2009, Science) showed "CD8+ T cells directly require IL-21 to avoid deletion, maintain immunity, and resolve persistent infection" (PMID: 19423777). Frohlich et al. (2009) showed "Cell-autonomous IL-21R-dependent signaling by CD8+ T cells was required for sustained cell proliferation and cytokine production during chronic infection."

However, three critical qualifications limit this counter-evidence:

1. Context-specificity: "Il21r-/- mice showed normal CD8+ T cell expansion, effector function, memory homeostasis, and recall responses during acute and after resolved infection with several other nonpersistent viruses" (PMID: 19478140).
2. Mechanism is not mitogenic: The mechanism is BATF-IRF4-Blimp-1 transcriptional reprogramming to prevent exhaustion/deletion (PMID: 26527008), not classical cell cycle entry.
3. Memory independence: "The generation of memory CD8 T cells, which are capable of mounting protective recall responses, proceeds independently of IL-21" (PMID: 20844201).

The chronic infection CD8 sustaining effect is better captured by terms related to T cell survival/maintenance during chronic infection than by GO:0042102 (positive regulation of T cell proliferation).

Finding 8: GO Database Evidence Code Asymmetry

A QuickGO query revealed an important paradox: IL-21's true core functions (GC B cell differentiation, Ig production, Tfh differentiation) only have ISS and IEA evidence codes (no IDA), while the non-core T cell proliferation term has IDA annotations. This asymmetry means that the presence of IDA evidence codes should not be conflated with "core function" status. It also reveals a curation gap: the signature functions of IL-21 lack the strongest evidence codes despite having the strongest biological support.

Additionally, GO:0045954 (NK cell cytotoxicity) was found to have IDA evidence from PMID:18005035 (Skak et al. 2008: "IL-21 increased the cytotoxicity of NK cells against K562 target cells"), contradicting the source YAML which listed only IBA/IEA evidence.

Evidence Matrix

GO Curation Implications

Primary Recommendation: KEEP_AS_NON_CORE for GO:0042102

The IDA annotations from PMID:17673207 and PMID:15207081 should be retained in the gene record — the experimental evidence for T cell proliferative activity is valid. However, the term should be reclassified from core to non-core status. Specifically:

• Action: KEEP_AS_NON_CORE — retain annotations but remove from directly_involved_in in the core function model
• Rationale: The effect is real but costimulation-dependent, bidirectional across T cell subsets, contradicted by deficiency phenotypes, and secondary to the signature B cell/GC/humoral immunity axis
• Relation type: If the GO framework supports it, model as acts_upstream_of_or_within rather than directly_involved_in, reflecting that IL-21 can influence T cell proliferation as a downstream consequence of its cytokine activity but does not directly drive it as a primary process

Secondary Recommendation: KEEP_AS_NON_CORE for GO:0045954

NK cell mediated cytotoxicity enhancement (GO:0045954) should also be classified as non-core and removed from directly_involved_in:

• The source YAML listed this as IBA/IEA only, but IDA evidence exists (PMID:18005035)
• The effect is well-documented but parallels T cell proliferation as a pleiotropic downstream consequence
• Human IL-21R deficiency shows reduced NK terminal differentiation but NK cytotoxicity is not the defining clinical phenotype
• Action: Remove from directly_involved_in in core functions; retain annotation as non-core

Curation Gap Alert: Core Functions Lack IDA

The true core functions of IL-21 — GO:0002314 (GC B cell differentiation), GO:0002639 (Ig production), GO:0061470 (Tfh differentiation) — currently only have ISS/IEA evidence codes. This creates a paradox where non-core terms have stronger evidence codes than core terms. Curators should prioritize adding IDA annotations to these core processes from the extensive primary literature supporting them (e.g., PMID:31821441, PMID:30087442, PMID:33929673).

Mechanistic Scope

Direct Gene Product Activity

IL-21 is a secreted cytokine (extracellular region, GO:0005576) that binds the IL-21R/IL-2Rgamma heterodimeric receptor complex, activating JAK1/JAK3 kinases and primarily STAT3 (with lesser STAT1 and STAT5 activation). This is the core molecular function: cytokine activity (GO:0005125).

Signature Downstream Processes (Core)

The primary biological processes driven by IL-21 signaling are:

IL-21 --> IL-21R/gamma-c --> JAK1/JAK3 --> STAT3
    |
    |---> Germinal center B cell differentiation (GO:0002314)  <-- CORE
    |---> Positive regulation of Ig production (GO:0002639)    <-- CORE
    |---> Tfh cell differentiation (GO:0061470)                <-- CORE
    |---> Positive regulation of B cell proliferation (GO:0030890) <-- CORE

These are consistently revealed by loss-of-function studies across species and represent the primary selective pressure maintaining IL-21 in the genome.

Secondary/Pleiotropic Effects (Non-Core)

T cell proliferation and NK cytotoxicity are downstream of the same JAK/STAT3 signaling cascade but represent context-dependent, cell-type-specific pleiotropic effects:

IL-21 --> IL-21R/gamma-c --> JAK1/JAK3 --> STAT3
    |
    |---> T cell proliferation (GO:0042102)              <-- NON-CORE
    |       - Requires TCR costimulation (anti-CD3)
    |       - Anti-proliferative for Tfr cells
    |       - Dispensable in IL-21R deficiency
    |
    |---> Th17 differentiation                           <-- NON-CORE (autocrine, redundant in vivo)
    |       - Via STAT3/RORgammat, not mitogenic signaling
    |
    |---> CD8 T cell sustaining in chronic infection      <-- NON-CORE
    |       - Via BATF-IRF4-Blimp-1, not cell cycle entry
    |       - Absent in acute infection
    |
    |---> NK cell cytotoxicity (GO:0045954)              <-- NON-CORE
            - Real but not defining clinical phenotype
            - Enhanced perforin/granzyme expression

Key Distinction: Differentiation vs. Proliferation

IL-21's actual T cell biology is differentiation (Th17 lineage commitment, Tfh specification, CD8 tissue-resident memory formation), not proliferation (cell cycle entry and clonal expansion). GO:0042102 (positive regulation of T cell proliferation) may therefore be the wrong term entirely for IL-21's T cell effects, even as a non-core annotation. The more precise biology would be captured by terms related to T cell differentiation (GO:0030217) or T helper cell lineage commitment.

Conflicts and Alternatives

Counter-Evidence: Chronic Viral Infection CD8 Maintenance

The strongest evidence that could support T cell proliferation as a core IL-21 function comes from chronic viral infection models. Three landmark papers (PMID:19423777, 19478140, 20844201 — all published in 2009-2010) demonstrated that IL-21 is essential for maintaining CD8 T cell responses during chronic LCMV infection. However, this counter-evidence is limited by:

1. Context restriction: The effect is absent in acute infections with LCMV, VSV, VV, and Listeria (PMID: 19478140)
2. Mechanistic distinction: The underlying mechanism is BATF/IRF4-mediated transcriptional reprogramming to prevent exhaustion, not mitogenic signaling (PMID: 26527008)
3. Wrong GO term: Sustaining an existing pool (anti-apoptotic/anti-exhaustion) is mechanistically and ontologically distinct from "positive regulation of T cell proliferation"
4. Memory independence: Memory CD8 T cell generation proceeds independently of IL-21 (PMID: 20844201)

Potential Paralog Confusion

IL-21 shares the gamma-chain with IL-2, IL-4, IL-7, IL-9, and IL-15. Some T cell proliferative effects attributed to IL-21 could reflect experimental systems where other gamma-chain cytokines are present (e.g., IL-2 in culture media). The IDA papers used recombinant IL-21 in defined conditions, mitigating but not eliminating this concern.

Organism-Specific Considerations

Most mechanistic data come from mouse models. Human IL-21R deficiency phenotypes are concordant (B cell/humoral defects dominate), but the relatively small number of identified patients (13 as of 2021) limits statistical power for detecting subtle T cell proliferative defects. However, the fact that T cell proliferation defects are not even mentioned as secondary features in these case series is informative.

Alternative Interpretation: IL-21 as a T Cell Quality Factor

An alternative framing is that IL-21 does not regulate T cell quantity (proliferation) but rather T cell quality (differentiation state, effector function, exhaustion resistance). This interpretation reconciles all the data: IL-21 promotes Th17/Tfh differentiation, sustains CD8 effector function during chronic infection, and restricts inappropriate Tfr expansion — all effects on cell fate and function rather than cell number.

Knowledge Gaps

Gap 1: Dose-Response for T Cell Proliferation vs. B Cell Proliferation

What was checked: Both IDA papers used single concentrations of IL-21 in costimulatory assays. Why it matters: If IL-21 drives B cell proliferation at physiological concentrations but requires supraphysiological levels for T cell effects, this would further support non-core status. Resolution: Comparative dose-response curves for IL-21-driven proliferation of purified B cells vs. T cells (with matched costimulation) under identical conditions.

Gap 2: In Vivo T Cell Proliferation Kinetics in IL-21 KO

What was checked: IL-21R KO phenotype studies focus on B cell and humoral endpoints. Why it matters: Formal measurement of T cell proliferation rates (e.g., BrdU incorporation, Ki-67) in lymphoid organs of IL-21-/- mice during normal immune responses would definitively test whether T cell proliferation is affected. Resolution: BrdU pulse-chase experiments in IL-21-/- vs. WT mice after immunization, measuring T cell and B cell proliferation in parallel.

Gap 3: IDA Annotations for Core Functions

What was checked: QuickGO database query. Why it matters: The true core functions lack IDA evidence codes, creating an artificial asymmetry that could mislead automated curation pipelines. Resolution: Curators should mine existing primary literature (especially from the germinal center and Tfh differentiation fields) to add IDA annotations to GO:0002314, GO:0002639, and GO:0061470.

Gap 4: Human-Specific T Cell Proliferative Effects

What was checked: Both IDA papers used human cells; chronic infection studies are mouse-based. Why it matters: Species-specific differences in gamma-chain cytokine biology exist. Resolution: IL-21 proliferation assays with purified human T cell subsets in defined conditions, with head-to-head comparison to IL-2 and IL-15.

Gap 5: Single-Cell Resolution of IL-21 Effects

What was checked: Bulk proliferation assays from the IDA papers. Why it matters: Bulk assays cannot distinguish whether IL-21 drives proliferation of all T cells or only a subset that responds preferentially. Resolution: Single-cell RNA-seq or CITE-seq of T cells +/- IL-21 (with anti-CD3) to identify responding subpopulations.

Discriminating Tests

1. Head-to-head cytokine comparison: Measure T cell and B cell proliferation (CFSE dilution) in response to IL-21, IL-2, IL-7, IL-15, and IL-4 at matched concentrations, +/- costimulation. If IL-21's T cell effect is consistently weaker and more costimulation-dependent than other gamma-chain cytokines, this would confirm non-core status.

2. Conditional IL-21R deletion: T cell-specific (CD4-Cre) vs. B cell-specific (CD19-Cre) IL-21R deletion, measuring immune responses after immunization. If B cell-specific deletion recapitulates the full IL-21R KO phenotype while T cell-specific deletion does not, this definitively identifies B cells as the core target.

3. CRISPR screen for IL-21 dependencies: Genome-wide CRISPR screen in T cells vs. B cells comparing IL-21-dependent gene programs. This would identify whether the transcriptional programs activated by IL-21 in T cells are primarily differentiation-related or proliferation-related.

4. Evolutionary analysis: Compare IL-21's T cell proliferative activity across vertebrate species. If this activity is gained late in evolution or is poorly conserved relative to B cell effects, it further supports non-core status.

5. Dose-titration with phospho-STAT profiling: Map the signaling dose-response curve for IL-21 in T cells vs. B cells. If B cells show higher IL-21R expression and/or more sensitive STAT3 activation, this provides a molecular basis for the B cell-centric biology.

Curation Leads

Lead 1: Change GO:0042102 Action to KEEP_AS_NON_CORE

• Current action: UNDECIDED
• Recommended action: KEEP_AS_NON_CORE
• Confidence: High (8 independent evidence lines)
• Key references: PMID:17673207 (costimulation-dependent), PMID:33929673 (deficiency phenotype), PMID:28303891 (anti-proliferative for Tfr), PMID:23960240 (B cells are essential target)
• Snippet to verify (PMID:33929673): "Most patients exhibited hypogammaglobulinemia and reduced proportions of memory B cells, circulating T follicular helper cells, MAIT cells and terminally differentiated NK cells"

Lead 2: Remove GO:0045954 from directly_involved_in

• Current status: Listed in directly_involved_in in core functions YAML
• Recommended action: Remove from core, classify as KEEP_AS_NON_CORE
• Rationale: Same logic as GO:0042102; NK cytotoxicity is a real pleiotropic effect but not a signature function
• Note: Source YAML incorrectly states IBA/IEA only; IDA evidence exists from PMID:18005035
• Snippet to verify (PMID:18005035): "IL-21 increased the cytotoxicity of NK cells against K562 target cells"

Lead 3: Add IDA Annotations for Core Functions

• Gap identified: GO:0002314, GO:0002639, GO:0061470 lack IDA evidence codes
• Action: Curators should mine primary literature for IDA-quality evidence for these core terms
• Candidate references: PMID:31821441 (review), PMID:30087442 (mouse KO), PMID:25586558 (IgA/B cell), PMID:21708925 (Bcl-6/Tfh/GC)

Lead 4: Consider Adding T Cell Differentiation Terms as Non-Core

• Rationale: IL-21's actual T cell biology is Th17/Tfh differentiation, not proliferation
• Candidate terms: GO:0030217 (T cell differentiation), GO:0072539 (T-helper 17 cell differentiation)
• Evidence: PMID:17581589 (Th17 differentiation), PMID:17884812 (Th17 commitment), PMID:32948671 (CD8 tissue-resident differentiation)
• Status: Non-core, as these are secondary to the B cell axis

Lead 5: Correct Source YAML Evidence Codes for GO:0045954

• Issue: Source YAML states GO:0045954 has only IBA/IEA evidence
• Correction: IDA evidence exists from PMID:18005035 (Skak et al. 2008)
• Action: Update YAML to reflect accurate evidence codes

Lead 6: Resolve Issue #1418

• Recommendation: Close issue #1418 with decision KEEP_AS_NON_CORE for GO:0042102
• Summary for issue: The effect is real and well-documented but does not meet the threshold for core function based on costimulation dependency, deficiency phenotype analysis, bidirectional effects, and comparative cytokine biology

GO Decision Table

Evidence Base: Key Literature

Defining Papers for the Core Function Question

• Spolski & Leonard (2008) (PMID: 17953510): Foundational review establishing IL-21's pleiotropic biology and context-dependency. Key quote: "The regulatory activity of IL-21 is modulated by the differentiation state of its target cells as well as by other cytokines or costimulatory molecules."

• Tangye & Ma (2020) (PMID: 31821441): Comprehensive review concluding IL-21's "critical role in T cell-dependent B cell activation, germinal center reactions, and humoral immunity."

• Spolski & Leonard (2014) (PMID: 24751819): "Double-edged sword" framing that captures IL-21's bidirectional, context-dependent effects.

Human Genetic Evidence

• Cagdas et al. (2021) (PMID: 33929673): Largest human IL-21R deficiency cohort (13 patients). Phenotype dominated by hypogammaglobulinemia and B cell defects, not T cell proliferative defects.

• Kotlarz et al. (2014) (PMID: 30903457): IL2RG mutation with selective IL-21 signaling defect phenocopied IL-21R deficiency with B cell differentiation defects.

T Cell Differentiation (Not Proliferation)

• Nurieva et al. (2007) (PMID: 17581589): IL-21 as essential autocrine factor for Th17 differentiation via STAT3/RORgammat.

• Wei et al. (2007) (PMID: 17884812): IL-21 sustains Th17 lineage commitment.

Counter-Evidence (Chronic Infection)

• Elsaesser et al. (2009) (PMID: 19423777): CD8 T cells require IL-21 during chronic viral infection — strongest counter-evidence but context-limited.

• Frohlich et al. (2009) (PMID: 19478140): Critically shows normal CD8 T cell expansion in acute infection, limiting the chronic infection effect.

• Yi et al. (2010) (PMID: 20844201): Memory CD8 T cell generation is IL-21-independent.

Limitations

1. Subjectivity of "core" definition: The distinction between core and non-core functions involves judgment about evolutionary purpose and functional hierarchy. Different curators may reasonably draw the line differently.

2. Bias toward mouse data: Most mechanistic studies use mouse models. While human genetic data corroborate the conclusions, the small number of identified patients (13 as of 2021) limits power to detect subtle T cell effects.

3. In vitro vs. in vivo gap: The IDA evidence for T cell proliferation comes from in vitro assays. It is possible that in vivo T cell proliferative effects are more robust but undetectable in the available knockout studies due to redundancy.

4. Publication bias: Studies emphasizing IL-21's B cell and GC functions may be more numerous because these are the phenotypically dominant effects, creating circular reinforcement of the "core function" narrative.

5. Temporal dynamics: IL-21 may have different roles at different stages of immune responses (early proliferative vs. late differentiative), and bulk endpoint assays may miss transient proliferative contributions.

Proposed Follow-up Actions

1. Immediate curation action: Change GO:0042102 from UNDECIDED to KEEP_AS_NON_CORE and close issue #1418 with this report as supporting documentation.

2. Remove GO:0045954 from directly_involved_in: Reclassify as non-core with notation that IDA evidence exists (PMID:18005035) but the effect is non-signature.

3. Prioritize IDA annotation for core functions: Mine existing primary literature to add IDA evidence codes to GO:0002314, GO:0002639, and GO:0061470, resolving the evidence code asymmetry.

4. Consider new non-core annotations: Add GO:0072539 (T-helper 17 cell differentiation) and/or GO:0030217 (T cell differentiation) as non-core annotations based on PMID:17581589 and PMID:17884812, which more accurately capture IL-21's T cell biology than GO:0042102.

5. Flag for computational pipeline review: Ensure that automated annotation transfer pipelines do not re-promote GO:0042102 to core status based on IDA evidence codes alone, as this case demonstrates that IDA evidence does not equal core function.