IL21R Deficiency (Immunodeficiency 56) — Comprehensive Disease Characterization

Autonomous literature-based discovery report. Evidence types are labeled: [human clinical], [model organism], [in vitro], [computational/database]. Primary citations are given as PMIDs.

Summary (Answer to the Research Question)

IL21R Deficiency is an ultra-rare, autosomal-recessive combined immunodeficiency (Immunodeficiency 56; OMIM #615207) caused by biallelic loss-of-function mutations in IL21R (chromosome 16p12.1). Loss of a functional IL-21 receptor abolishes IL-21 → common-gamma-chain/JAK1–JAK3 → STAT3 (and STAT1/STAT5) signaling, crippling B-cell class switching/plasma-cell differentiation, T-follicular-helper help, CD8 memory, and NK/MAIT/NKT cytotoxicity. Clinically this produces recurrent bacterial/fungal/viral infections, hypogammaglobulinemia with paradoxically elevated IgE/atopy, and—most characteristically—chronic Cryptosporidium infection driving sclerosing cholangitis and progressive liver disease. Hematopoietic stem cell transplantation (HSCT) is the only curative therapy but must be performed before irreversible biliary/liver damage; overall prognosis without early transplant is poor.

First described: Kotlarz et al., J Exp Med 2013 (23440042). Largest cohort: Cagdas et al. 2021, n=13 (33929673).

1. Disease Information

Overview. A monogenic inborn error of immunity in the "combined immunodeficiency (CID)" category, defined by absent/non-functional IL-21 receptor signaling. It affects both adaptive (B and T cell) and innate (NK) immunity and characteristically predisposes to opportunistic Cryptosporidium infection of the biliary tree. [human clinical] (23440042 33929673)

Key identifiers (computational/database): - OMIM: #615207 — "Immunodeficiency 56" (IMD56) - Gene: IL21R, OMIM 605383; HGNC:6006; NCBI Gene 50615; Ensembl ENSG00000159374; UniProt Q9HBE5 (IL21R_HUMAN); cytoband 16p12.1 - Orphanet: Listed under combined immunodeficiencies / IL-21R-related; no widely used dedicated ORPHAcode—commonly grouped with "Combined immunodeficiency." - MONDO: Suggested MONDO:0014219 ("immunodeficiency 56") — to be verified against current MONDO release. - ICD-10: D81.8 / D81.9 (other/unspecified combined immunodeficiencies). ICD-11: 4A01.3Y (combined immunodeficiencies, other). - MeSH:* No dedicated descriptor; indexed under "Severe Combined Immunodeficiency"/"Primary Immunodeficiency Diseases" + "Receptors, Interleukin-21."

Synonyms / alternative names: IL-21R deficiency; Interleukin-21 receptor deficiency; Immunodeficiency 56 (IMD56); IL21R-related combined immunodeficiency.

Information source. Aggregated disease-level knowledge derived from individual patient case series (≈20+ patients worldwide) synthesized in cohort papers and reviews—not from large registries/EHR. [human clinical]

2. Etiology

Disease causal factors. Monogenic/genetic. Biallelic (homozygous or compound-heterozygous) loss-of-function variants in IL21R are necessary and sufficient. An infectious trigger (chronic Cryptosporidium parvum/hominis biliary infection) is the principal driver of the signature organ pathology (cholangitis/liver disease) in the setting of the genetic immune defect—an obligate gene × pathogen interaction. [human clinical] (23440042: "cryptosporidial infections associated with chronic cholangitis and liver disease")

Genetic risk factors. - Causal variants: e.g., c.G602T (p.Arg201Leu, missense causing receptor mistrafficking/loss of ligand binding); c.240_245delCTGCCA (p.C81_H82del, in-frame deletion). 8 unique mutations reported across 8 families by 2021 (missense, in-frame indels, and other LOF classes). [human clinical/in vitro] (23440042 33929673) - Modifier genes: None established (cohort too small).

Environmental risk factors. Consanguinity (parental relatedness) is the dominant epidemiologic risk factor, increasing homozygosity for rare recessive alleles. Exposure to Cryptosporidium (contaminated water/food) converts the immune defect into life-threatening cholangiopathy. No sex, occupational, or toxin risk factors. [human clinical] (23440042)

Protective factors. No genetic protective/modifier alleles identified. Environmentally, avoidance of Cryptosporidium exposure (water precautions/filtration) and early curative HSCT are protective against the worst outcomes. [human clinical] (inferred)

Gene–environment interactions. The core GxE interaction: IL21R-null immune state + Cryptosporidium exposure → sclerosing cholangitis. Restoration of immune competence (HSCT) enables clearance of the parasite, halting the environmental driver. [human clinical] (30850087: cholangiopathy improvement "following establishment of immune competence")

3. Phenotypes (with HPO suggestions & frequencies)

Frequencies from the 13-patient cohort (33929673) unless noted. [human clinical]

Characteristics. Onset pediatric — median 2.5 years (range 0.5–7). Severity variable but the hepatobiliary component is typically progressive and life-limiting. Infections are recurrent/chronic; atopy (IgE, asthma, anaphylaxis) is episodic.

Quality-of-life impact. No formal EQ-5D/SF-36/PROMIS data. Qualitatively severe: chronic diarrhea/malabsorption → growth failure; progressive cholangiopathy → cholestasis, portal hypertension, end-stage liver disease; recurrent infections and hospitalizations; transplant-related morbidity. [human clinical]

Key quote (33929673): "The main clinical manifestations were recurrent bacterial (84.6%), fungal (46.2%), and viral (38.5%) infections; cryptosporidiosis-associated cholangitis (46.2%); and asthma (23.1%). Inflammatory skin diseases (15.3%) and recurrent anaphylaxis (7.9%) constitute novel phenotypes."

4. Genetic / Molecular Information

• Causal gene: IL21R (HGNC:6006; 16p12.1; NCBI 50615; UniProt Q9HBE5). Type I cytokine receptor; heterodimerizes with the common gamma chain (γc / IL2RG / CD132). [computational/database]
• Pathogenic variants (ACMG): classified pathogenic/likely pathogenic. Reported types: missense (p.Arg201Leu), in-frame deletion (p.C81_H82del), and additional LOF alleles (8 unique across 8 families). Loss-of-function is the uniform functional consequence—no gain-of-function or dominant-negative disease described; heterozygous carriers are healthy (recessive). [human clinical/in vitro] (23440042 33929673)
• Allele frequency: Biallelic IL21R LOF is essentially absent in gnomAD; individual pathogenic alleles are ultra-rare/private. [computational/database]
• Somatic vs germline: Germline. No somatic/oncogenic IL21R role in this disease.
• Functional consequences: p.Arg201Leu → aberrant receptor trafficking to the plasma membrane, loss of IL-21 ligand binding, and abrogated STAT1/STAT3/STAT5 phosphorylation (23440042: "aberrant trafficking of the IL-21R to the plasma membrane, abrogates IL-21 ligand binding, and leads to defective phosphorylation of ... STAT1, STAT3, and STAT5"). [in vitro]
• Modifier genes / epigenetics / chromosomal abnormalities: None reported; disease is a point-mutation/small-indel monogenic disorder, not a structural/aneuploidy syndrome.

5. Environmental Information

• Environmental factors: No toxin/radiation/pollution etiology. The critical environmental element is exposure to the waterborne protozoan Cryptosporidium.
• Lifestyle factors: Not applicable as risk drivers; hygiene/water safety are relevant for preventing opportunistic infection.
• Infectious agents (NCBI Taxonomy): Cryptosporidium parvum (NCBI:txid5807) / C. hominis (NCBI:txid237895) — biliary/intestinal; Cytomegalovirus (HHV-5, NCBI:txid10359); plus recurrent pyogenic bacteria and fungi (incl. Pneumocystis). These are opportunistic consequences of the immunodeficiency, not primary causes. [human clinical] (23440042 33966600)

6. Mechanism / Pathophysiology

Ordered causal chain

1. Biallelic IL21R loss-of-function mutation (e.g., p.Arg201Leu) → leads to absent or mistrafficked IL-21R protein that fails to reach/function at the plasma membrane and cannot bind IL-21. [in vitro, demonstrated] (23440042)
2. Loss of surface IL-21R → prevents assembly of the IL-21R/γc receptor complex and abrogates JAK1/JAK3 activation → results in failed phosphorylation of STAT3 (and STAT1/STAT5). [in vitro, demonstrated] (23440042; pathway review 24126614)
3. Absent STAT3 signaling → fails to up-regulate the plasma-cell master regulator BLIMP1/PRDM1 (and to down-regulate BCL6) → impairs IL-21-driven B-cell activation, immunoglobulin class-switch recombination, and differentiation into plasmablasts/plasma cells → causes hypogammaglobulinemia, poor specific-antibody responses, and reduced memory B cells. [in vitro + human clinical] (23440042 24126614 18354204 — the latter: "stimulation with IL-21 ... induced robust and prolonged STAT3 activation in primary human B cells" and STAT3 "triggered BLIMP1 mRNA and protein up-regulation, plasma cell phenotypic features, and Ig secretion")
4. Branch A (humoral/atopy): Loss of IL-21's normal restraint on IgE, unopposed IL-4/Th2 activity → leads to elevated IgE, asthma, eczema, anaphylaxis. [model organism + human clinical] (12446913: high IgE/low IgG1 in Il21r-/- mice; 33929673: IgE elevated in 50%)
5. Branch B (cellular cytotoxicity): Absent STAT3 downstream of IL-21R → reduces CD8+ T-cell memory and lytic-machinery induction, NK-cell cytotoxicity, and MAIT/NKT-cell numbers → impairs control of intracellular/opportunistic pathogens. [human clinical/in vitro] (23830147 25941256 23440042)
6. Combined humoral + cytotoxic failure + reduced Tfh help → permits chronic Cryptosporidium infection of intestinal and biliary epithelium. [human clinical] (23440042)
7. Persistent biliary Cryptosporidium → drives chronic inflammation of bile ducts → causes sclerosing cholangitis, fibrosis, cirrhosis, portal hypertension, end-stage liver disease (and rare cholangiocarcinoma risk by analogy to other cryptosporidial-cholangitis PIDs). [human clinical] (23440042 30850087)
8. Chronic immune dysregulation/impaired tumor surveillance → contributes to lymphoproliferation/lymphoma (e.g., marginal zone lymphoma). [human clinical, inferred] (33966600)

Detail by category

• Molecular pathways: IL-21 → IL-21R/γc → JAK1/JAK3 → STAT3 (canonical), with STAT1/STAT5 and PI3K–AKT and MAPK as secondary arms (Reactome "Interleukin-21 signaling"; KEGG "Jak-STAT signaling pathway hsa04630"). GO:0038114 IL-21-mediated signaling pathway; GO:0007259 JAK-STAT signaling.
• Cellular processes: B-cell differentiation (GO:0030183), isotype/class switching (GO:0045190), plasma-cell differentiation (GO:0002317), germinal-center/Tfh help, NK-mediated cytotoxicity (GO:0042267), CD8 memory formation.
• Protein dysfunction: Receptor misfolding/mistrafficking and loss of ligand binding (not aggregation); pure loss of function.
• Immune system involvement: Combined immunodeficiency + immune dysregulation (atopy, lymphoproliferation).
• Tissue damage: Immune-mediated + infection-driven biliary fibrosis/sclerosis and hepatocellular injury.
• Cell types (CL): B cells CL:0000236 (memory B, plasmablast CL:0000980), Tfh CL:0002038, CD8 memory T CL:0000909, NK CL:0000623, MAIT CL:0000940, NKT cells; biliary epithelial (cholangiocyte) as the injured target.
• Omics: No dedicated transcriptomic/proteomic/metabolomic disease signatures published (ultra-rare). Mechanistic data are from targeted immunophenotyping and STAT-phosphorylation flow assays. [human clinical/in vitro]

7. Anatomical Structures Affected

• Organ level (primary): Bile ducts (UBERON:0002394; intra- and extrahepatic — sclerosing cholangitis, bilateral/diffuse), liver (UBERON:0002107), intestine/GI tract (UBERON:0000160), immune/lymphoid organs (bone marrow UBERON:0002371, lymph node UBERON:0000029, spleen UBERON:0002106, thymus). [human clinical]
• Secondary/complications: lung/respiratory tract (UBERON:0002048; recurrent pneumonia incl. CMV/Pneumocystis), skin (UBERON:0002097; eczema), portal hypertension complications.
• Body systems: immune/hematolymphoid, hepatobiliary/digestive, respiratory, integumentary.
• Tissue/cell level: biliary/intestinal epithelium (infection target); lymphoid cell populations (B/T/NK as above).
• Subcellular (GO CC): plasma membrane (GO:0005886) — site of the receptor-localization defect; secretory pathway/ER involved in receptor mistrafficking.
• Localization/lateralization: Cholangitis is typically diffuse/bilateral intrahepatic ± extrahepatic; systemic immune involvement.

8. Temporal Development

• Onset: Pediatric/early childhood; median 2.5 y (0.5–7 y); onset often insidious (chronic diarrhea, recurrent infections) (33929673). [human clinical]
• Progression: Hepatobiliary disease is chronic and progressive (cholangitis → fibrosis → cirrhosis → end-stage liver disease). Infections are recurrent; atopy episodic. Rate variable between patients.
• Course/duration: Chronic, lifelong without curative HSCT; frequently fatal in childhood if untreated or if transplant occurs after advanced organ damage.
• Critical period: A key therapeutic window exists — HSCT before advanced cholangiopathy/liver damage markedly improves outcome (30850087). [human clinical]

9. Inheritance and Population

• Epidemiology: Ultra-rare; ~20+ patients reported worldwide by 2021; no reliable prevalence/incidence figures. [human clinical] (33929673)
• Inheritance: Autosomal recessive; biallelic IL21R mutations. [human clinical] (23440042)
• Penetrance/expressivity: Immunodeficiency penetrance appears complete in biallelic-null individuals; expressivity variable (onset age, presence of cholangitis/atopy/lymphoma).
• Anticipation / germline mosaicism: Not applicable/none reported.
• Founder effects: None established (8 distinct mutations/8 families).
• Consanguinity: Strong association — most families consanguineous.
• Carrier frequency: Not defined; alleles are private/ultra-rare in gnomAD → cascade (family-specific) rather than population carrier screening.
• Demographics: Reported across multiple ethnic groups (Middle Eastern, Turkish, European and others via 7 centers); no sex predilection (autosomal); age distribution skewed pediatric.

10. Diagnostics

• Genetic testing (definitive): Whole-exome sequencing or NGS inborn-errors-of-immunity gene panels identify biallelic IL21R variants; targeted single-gene/Sanger for cascade testing. WGS useful for non-coding/structural variants. CMA/karyotype/FISH not indicated. [human clinical] (33929673: 5 novel patients found by "exome or NGS panel sequencing")
• Functional confirmation (in vitro): absent surface IL-21R expression; abrogated IL-21-induced STAT3 phosphorylation by flow cytometry; impaired IL-21-driven B-cell class switch/Ig secretion (23440042). [in vitro]
• Laboratory (LOINC-type): serum immunoglobulins (hypogammaglobulinemia; high IgE), specific antibody titers, lymphocyte subsets (low memory B, low Tfh/MAIT, altered NK), liver function/cholestatic panel.
• Microbiology: Cryptosporidium stool/bile PCR (more sensitive than microscopy) — critical and often missed (12690272: "Cryptosporidium could be detected by PCR but not by microscopy").
• Imaging: MRI/MRCP or cholangiography for sclerosing cholangitis (ductal strictures/beading, dilatation); ultrasound.
• Biopsy/pathology: liver biopsy showing sclerosing cholangiopathy/fibrosis.
• Clinical criteria / differential: No disease-specific criteria; diagnosed under IUIS CID framework. Differential: IL-21 (ligand) deficiency (phenocopy; 24746753), CD40L/CD40 (hyper-IgM), MHC class II deficiency, DOCK8 deficiency, STAT3-LOF hyper-IgE syndrome, CVID — distinguished by genetics + the IL-21-signaling assay.
• Screening: Not on newborn-screening TREC panels (patients usually have T cells). Carrier/cascade testing and prenatal/PGD available for known familial variants. [human clinical]

11. Outcome / Prognosis

• Survival/mortality: Poor without early curative therapy. In the largest cohort, post-HSCT overall survival was only 33.3% (2/6), and mortality among non-transplanted patients was high (33929673). In a broader PID sclerosing-cholangitis cohort, 7/13 (53.8%) died a median 4 months post-HSCT, while 6/13 with milder cholangiopathy survived and improved (30850087). [human clinical]
• Prognostic factors: Pre-existing organ (liver/biliary) damage is the key negative prognostic factor (33929673: "pre-existing organ damage constituting a negative prognostic factor"); earlier diagnosis/transplant improves outcome.
• Morbidity/QoL: growth failure, chronic liver disease/portal hypertension, transplant-related complications; substantial disability.
• Complications: end-stage liver disease, portal hypertension, disseminated/opportunistic infection, lymphoma/lymphoproliferation, post-transplant Cryptosporidium recrudescence.
• Recovery potential: Immune reconstitution and cholangiopathy reversal possible if HSCT precedes advanced fibrosis.
• Prognostic biomarkers: severity/stage of cholangiopathy on imaging/histology; persistent Cryptosporidium positivity.

12. Treatment

• Definitive (curative): Allogeneic hematopoietic stem cell transplantation (HSCT) — NCIT:C15431; ideally with reduced-intensity conditioning, performed before advanced liver disease (23440042 recommends "early diagnosis and allogeneic hematopoietic stem cell transplantation"; 30850087). [human clinical]
• Sequential liver + HSCT (NCIT:C15393 liver transplantation): for end-stage liver disease with combined immunodeficiency; curative in analogous PID cryptosporidial cirrhosis (29377874). [human clinical]
• Supportive pharmacotherapy:
• Immunoglobulin replacement therapy (IVIG/SCIG) — NCIT:C603 — for hypogammaglobulinemia.
• Anti-Cryptosporidium agents: nitazoxanide (CHEBI:189102/NCIT), paromomycin; often only partially effective without immune reconstitution.
• Antimicrobial prophylaxis: e.g., trimethoprim-sulfamethoxazole for Pneumocystis; antivirals/antifungals as indicated.
• Atopy management: asthma controllers, anaphylaxis precautions.
• Advanced/experimental: No approved gene therapy or targeted molecular therapy; IL21R is a plausible future gene-addition/gene-editing target by analogy to other γc-family SCID gene therapies (20660403). No IL21R-specific clinical trials (NCT) identified. [human clinical/inferred]
• Pharmacogenomics: none specific.
• Strategy: Diagnose early → prevent/treat Cryptosporidium + IgG replacement + prophylaxis → proceed to HSCT before liver damage; combined liver+HSCT if cirrhotic.

13. Prevention

• Primary prevention: Not preventable at the genetic level; genetic counseling for consanguineous/at-risk families; preimplantation/prenatal genetic diagnosis for known familial variants.
• Secondary prevention: Early molecular diagnosis (NGS) in infants with recurrent infection/chronic diarrhea; surveillance for Cryptosporidium (PCR) and hepatobiliary monitoring (MRCP/LFTs).
• Tertiary prevention: IgG replacement, antimicrobial prophylaxis, timely HSCT to prevent progression; post-transplant vigilance against Cryptosporidium recrudescence.
• Environmental/public-health: Water safety/filtration and hygiene to avoid Cryptosporidium exposure (boil/filter water, avoid recreational water risk).
• Immunization: standard non-live vaccines as able; avoid live vaccines in combined immunodeficiency.
• Counseling: autosomal-recessive 25% recurrence risk per pregnancy; cascade carrier testing. [human clinical/guideline-inferred]

14. Other Species / Natural Disease

• Taxonomy/orthologs: IL21R is conserved in mammals. Mouse Il21r (NCBI Gene 60504; chr 7; NCBI:txid10090); rat, zebrafish orthologs exist.
• Natural disease: No well-characterized naturally occurring IL21R-deficiency disease in companion animals/wildlife (OMIA — none prominent). Veterinary relevance is chiefly as engineered research models.
• Comparative biology: IL-21/IL-21R humoral-regulatory function is evolutionarily conserved; the γc-cytokine receptor family (IL-2/4/7/9/15/21) shares CD132 across species (structural conservation, 37535730). [computational/model organism]
• Transmission/zoonosis: Not applicable to the genetic disease (though Cryptosporidium itself is zoonotic).

15. Model Organisms

• Mouse (mammalian) — Il21r-knockout (Ozaki et al. 2002, Science, 12446913): normal lymphoid development but, after immunization, higher IgE and lower IgG1; Il4/Il21r double-KO → dysgammaglobulinemia with severely impaired IgG. [model organism]
• Phenotype recapitulation: reproduces human antibody dysregulation (IgE-high/IgG-low, impaired specific antibody) and demonstrates IL-21/IL-4 cooperation.
• Limitations: mice have grossly normal lymphoid development and do not spontaneously develop cryptosporidial sclerosing cholangitis (redundancy + differing pathogen exposure), so the hepatobiliary phenotype is not captured.
• Applications: dissecting IL-21 control of B-cell class switching, Tfh/germinal-center biology, CD8/NK function.
• Other models: γc-family biology also modeled in zebrafish/other systems (e.g., IL-2Rγc SCID models, 35216498), informing the shared-receptor mechanism though not IL21R-specific.
• Genetic model types available: knockout (constitutive); conditional/humanized IL21R models feasible but not disease-defining. Resources: MGI (mouse), Alliance of Genome Resources.

Supported vs. Refuted Hypotheses

Supported (evidence-backed): - Biallelic IL21R LOF causes an autosomal-recessive combined immunodeficiency (23440042 33929673). - Mechanism is failed IL-21R→JAK/STAT3 signaling affecting B, Tfh, CD8-memory, NK, MAIT/NKT compartments (23440042 24126614 23830147 25941256). - Cryptosporidium-driven sclerosing cholangitis/liver disease is the prognosis-defining complication (23440042 30850087). - HSCT is curative but timing-dependent; organ damage worsens outcome (33929673 30850087). - Il21r-/- mice model the antibody dysregulation (12446913).

Refuted / not applicable: - Not X-linked (distinct from γc/IL2RG SCID); heterozygotes unaffected → not dominant/dominant-negative. - No environmental/toxin primary etiology; no founder mutation; no somatic/oncogenic driver role.

Limitations & Future Directions

• Evidence rests on <25 patients; frequencies/prognosis have wide uncertainty. No prevalence, QoL (EQ-5D/SF-36), or omics datasets exist.
• MONDO/Orphanet identifiers should be verified against current releases.
• Future needs: international registry/natural-history study; standardized Cryptosporidium surveillance; evaluation of gene-editing/gene-addition therapy; optimized reduced-intensity conditioning and combined liver+HSCT protocols.

Key References (PMIDs)

23440042 (Kotlarz 2013, original description) · 33929673 (Cagdas 2021, cohort n=13) · 33966600 (Edeer Karaca 2021, marginal zone lymphoma) · 24746753 (Salzer 2014, IL-21 ligand deficiency) · 12446913 (Ozaki 2002, Il21r-/- mouse) · 24126614 (Desjardins 2013, IL-21/B-cell memory review) · 23830147 (Ives 2013, CD8 memory) · 25941256 (Wilson 2015, MAIT/NKT via STAT3) · 30850087 (Hadžić 2019, HSCT for PID cholangitis) · 29377874 (sequential liver+HSCT) · 12690272 (Cryptosporidium PCR diagnosis) · 18354204 (Diehl 2008, IL-21/STAT3→BLIMP1 plasma-cell differentiation) · 37535730 (γc receptor structure) · 20660403 (γc-family gene therapy).