Immunodeficiency 88 (IMD88): Human T-bet Deficiency — Comprehensive Disease Characteristics Report

Summary

Immunodeficiency 88 (IMD88; OMIM #619630; MONDO:0030483; MedGen C5562026) is an ultra-rare, autosomal recessive inborn error of IFN-γ immunity caused by complete loss-of-function of the T-box transcription factor T-bet, encoded by TBX21 (17q21.32; HGNC:11599; NCBI Gene 30009; UniProt O95936). It was first defined in a single consanguineous Moroccan boy who was homozygous for the in-frame indel TBX21 c.466_471delGAGATGinsAGTTTA (p.E156_M157delinsSerLeu), a two-residue substitution within the T-box DNA-binding domain that abolishes DNA binding, impairs protein production and nuclear translocation, and prevents induction of IFNG (PMID: 33296702).

The disease is mechanistically remarkable because one genetic lesion produces two distinct clinical arms. First, loss of T-bet abolishes the development and IFN-γ output of innate and innate-like lymphocytes — natural killer (NK) cells, invariant NKT (iNKT), mucosal-associated invariant T (MAIT), and Vδ2⁺ γδ T cells — placing IMD88 firmly within the Mendelian Susceptibility to Mycobacterial Disease (MSMD) spectrum and producing disseminated bacille Calmette-Guérin (BCG) disease. Second, loss of T-bet-mediated repression of the Th2 program derepresses IL-4/IL-5/IL-9/IL-13 production, driving blood eosinophilia and persistent upper-airway/asthma-like inflammation (PMID: 33296702; PMID: 34160550).

Because IMD88 has been reported in only a single patient, most disease-level characteristics (epidemiology, prognosis, treatment algorithms) are extrapolated from the broader MSMD framework and from the well-characterized Tbx21-knockout mouse, which spontaneously recapitulates the allergic-airway arm of the human disease (PMID: 11786643). This report organizes all available evidence across the 15 requested domains, flags where information is inferred rather than demonstrated, and supplies ontology term suggestions (HPO, GO, CL, UBERON, NCIT, MONDO) throughout to support knowledge-base curation.

Key Findings

Finding 1 — IMD88 is autosomal-recessive T-bet (TBX21) deficiency causing mycobacterial disease

IMD88 is catalogued as OMIM #619630, MONDO:0030483, and MedGen C5562026, and is caused by homozygous mutation in TBX21 (T-box transcription factor 21; T-bet; OMIM 604895) located at chromosome 17q21.32. The disorder was defined in a single index patient — a boy born to consanguineous Moroccan parents — reported by Yang et al. in Cell (2020). Whole-exome sequencing combined with genome-wide linkage identified a homozygous deletion/insertion in TBX21. In vitro reconstitution in HEK293T cells demonstrated that the mutant allele produces protein poorly, translocates to the nucleus poorly, fails to bind target regulatory DNA elements, and fails to induce IFNG transcription. The authors state directly: "We report a patient with mycobacterial disease due to inherited deficiency of the transcription factor T-bet" (PMID: 33296702). This establishes IMD88 as a monogenic, recessive loss-of-function disorder of a master immune transcription factor.

Finding 2 — T-bet deficiency abolishes innate/innate-like IFN-γ-producing lymphocytes

The cellular basis of the mycobacterial susceptibility is a selective failure of the IFN-γ-producing innate compartment. The patient had extremely low circulating counts of Mycobacterium-reactive NK, iNKT, MAIT, and Vδ2⁺ γδ T lymphocytes, along with reduced classical Th1 cells; the residual cells produced abnormally little IFN-γ. Notably, CD8⁺ αβ T cells and non-classical CD4⁺ αβ TH1 cells produced IFN-γ normally in response to mycobacterial antigens but could not compensate for the missing innate-like output. The authors summarize: "Human T-bet deficiency thus underlies mycobacterial disease by preventing the development of innate (NK) and innate-like adaptive lymphocytes (iNKT, MAIT, and Vδ2+ γδ T cells) and IFN-γ production by them"* (PMID: 33296702). This identifies the specific developmental and functional lesion (loss of a discrete IFN-γ-producing cellular compartment) as the proximate cause of disease.

Finding 3 — Clinical phenotype: disseminated BCG, asthma/reactive airway disease, eosinophilia

The HPO/MedGen clinical profile (MedGen C5562026) includes BCGosis / disseminated BCG infection (HP:0032262), Asthma (HP:0002099), Eosinophilia / increased eosinophil count (HP:0001880), and a general Abnormality of the immune system (HP:0002715). OMIM #619630 notes persistent reactive airway disease associated with increased Th2 cytokine production and decreased IFN-γ. Despite serologic evidence of exposure to numerous viruses and bacteria, the patient did not develop other clinical infectious diseases, indicating a relatively selective, mycobacteria-dominant infection phenotype layered on top of an allergic/atopic airway disease (PMID: 33296702).

Finding 4 — T-bet loss derepresses the Th2 program (the second mechanistic arm)

A follow-up study (Yang et al., J Exp Med 2021) established the mechanism of the allergic arm. The patient's mutant T-bet failed to inhibit Th2 cytokine production (IL-4, IL-5, IL-9, IL-13) when overexpressed in Th2 cells; Herpesvirus saimiri (HVS)-immortalized patient T cells overproduced Th2 cytokines; plasma IL-5 and IL-13 were markedly elevated; and patient CD4⁺ αβ T cells produced excess Th2 cytokines upon chronic stimulation regardless of antigen specificity, an effect reversed by wild-type T-bet. The result is blood eosinophilia and persistent upper airway inflammation (UAI). The authors state: "T-bet deficiency thus underlies the excessive production of Th2 cytokines, particularly IL-5 and IL-13, by CD4+ αβ T cells, causing blood eosinophilia and UAI" (PMID: 34160550). This confirms that the atopic manifestations are a direct consequence of the same TBX21 lesion, not a coincidental comorbidity.

Finding 5 — The Tbx21-knockout mouse recapitulates the asthma/airway arm

Finotto et al. (Science 2002) showed that mice with targeted deletion of the Tbx21 (T-bet) gene, and SCID mice reconstituted with CD4⁺ cells from T-bet-knockout mice, spontaneously developed multiple physiological and inflammatory features characteristic of asthma in the absence of allergen exposure. Human asthmatic airway T cells showed reduced T-bet expression. The authors report: "Mice with a targeted deletion of the T-bet gene and severe combined immunodeficient mice receiving CD4+ cells from T-bet knockout mice spontaneously demonstrated multiple physiological and inflammatory features characteristic of asthma" (PMID: 11786643). This provides a validated animal model for the allergic-airway component of IMD88, predating the human disease description by nearly two decades.

Finding 6 — IMD88 within the MSMD framework: diagnosis, treatment, prognosis

MSMD is defined by inborn errors of IFN-γ immunity, rendering patients "highly and selectively susceptible to weakly virulent mycobacteria, such as environmental mycobacteria and Bacillus Calmette-Guérin vaccines" (PMID: 32025907). A systematic review of 830 MSMD patients (PMID: 38341181) reported a mean age of ~10.4 years, 52.5% male, a positive family history in 45.5%, highest reported frequencies in Iran/Turkey/Saudi Arabia, and 299 unique mutations across 21 genes. Lymphadenopathy was the most common manifestation (45.5%), followed by fever (30.2%), organomegaly (24.8%), and sepsis (20.8%); "Lymphadenopathy was the most common clinical manifestation of MSMD, reported in 378 (45.5%) cases." MSMD carries substantial mortality, driven mostly by impaired control of infection. IMD88 (TBX21/T-bet deficiency) is one of these genetic etiologies, and its clinical management, diagnostic approach, and prognosis are reasonably extrapolated from this larger cohort.

Finding 7 — Exact TBX21 variant and gene/protein annotations

The index patient is homozygous for TBX21 c.466_471delGAGATGinsAGTTTA, an in-frame insertion/deletion in exon 1 that replaces two highly conserved amino acids, E156 and M157 (p.E156_M157delinsSerLeu), within the T-box DNA-binding domain. Parents were heterozygous carriers (WT/M); the patient was homozygous (M/M). The variant is private (not a recurrent/founder allele; absent as a benign homozygote in gnomAD). Gene/protein annotations: human TBX21 (HGNC:11599; NCBI Gene 30009; UniProt O95936; 17q21.32); mouse ortholog Tbx21 (NCBI Gene 57765; MGI:1888984; chromosome 11). The functional class is complete autosomal-recessive loss of function — the mutant protein shows impaired production, impaired nuclear translocation, and abolished DNA binding, failing to induce IFN-γ (PMID: 33296702).

Report by Requested Domain

1. Disease Information

2. Etiology

3. Phenotypes

Phenotype Type HPO term Onset Severity Frequency (n=1 + MSMD context)
Disseminated BCG infection (BCGosis) Clinical sign / infection HP:0032262 Childhood (post-vaccination) Severe Present in index patient; BCG complications in ~55% of BCG-vaccinated MSMD (PMID: 36630059)
Asthma / reactive airway disease Clinical sign HP:0002099 Childhood Moderate, persistent Present in index patient
Eosinophilia Laboratory abnormality HP:0001880 Childhood Moderate–marked Present in index patient
Elevated Th2 cytokines (IL-5, IL-13) Laboratory abnormality (no direct HPO) Childhood Marked Present (PMID: 34160550)
Upper airway inflammation Clinical sign HP:0012384 (airway) Childhood Persistent Present
Abnormality of the immune system General HP:0002715 Childhood — Present
Lymphadenopathy (MSMD-context) Clinical sign HP:0002716 Childhood Variable Most common MSMD feature (45.5%) (PMID: 38341181)

4. Genetic / Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

  1. Homozygous TBX21 c.466_471delinsAGTTTA (p.E156_M157delinsSerLeu) leads to a T-bet protein with two altered residues in the T-box DNA-binding domain (demonstrated).
  2. This results in impaired protein production, impaired nuclear translocation, and abolished binding to target DNA regulatory elements (demonstrated in HEK293T reconstitution) (PMID: 33296702).
  3. Loss of T-bet transcriptional activity branches into two arms:

Arm A (immunodeficiency / mycobacterial disease): - 4a. Failure to activate the T-bet-dependent Th1/innate transcriptional program leads to failed development and maturation of NK, iNKT, MAIT, and Vδ2⁺ γδ T lymphocytes (demonstrated: extremely low counts). - 5a. This results in loss of early, innate IFN-γ production against mycobacteria (demonstrated). - 6a. Deficient IFN-γ leads to impaired macrophage activation and failure to control weakly virulent mycobacteria (inferred from the MSMD paradigm) (PMID: 32025907). - 7a. This results in disseminated BCG disease and MSMD-spectrum susceptibility (demonstrated clinically).

Arm B (allergic airway disease / eosinophilia): - 4b. Loss of T-bet-mediated repression of the Th2 program leads to derepressed IL-4/IL-5/IL-9/IL-13 output by CD4⁺ αβ T cells, independent of antigen specificity (demonstrated) (PMID: 34160550). - 5b. Elevated IL-5 results in blood eosinophilia; elevated IL-13 results in airway mucus/inflammation (demonstrated: elevated plasma IL-5/IL-13). - 6b. Sustained type-2 inflammation leads to persistent upper-airway inflammation and asthma/reactive airway disease (demonstrated clinically; recapitulated in Tbx21-KO mice, PMID: 11786643).

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

(All treatment is extrapolated from MSMD and Th2-directed asthma care; no IMD88-specific trials exist.)

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

          Homozygous TBX21 c.466_471delinsAGTTTA (p.E156_M157delinsSerLeu)
                                  |
                (T-box DNA-binding domain: 2 conserved residues altered)
                                  |
        Impaired protein production + nuclear import + ABOLISHED DNA binding
                                  |
                     Loss of T-bet transcriptional function
                    /                                        \
     ARM A: Loss of ACTIVATION                     ARM B: Loss of REPRESSION
     of Th1/innate program                          of Th2 program
              |                                              |
   Failed development/IFN-γ of                    Derepressed IL-4/IL-5/
   NK, iNKT, MAIT, Vδ2+ γδ T                       IL-9/IL-13 (antigen-independent)
              |                                              |
   Deficient innate IFN-γ                          ↑ IL-5 → eosinophilia
              |                                     ↑ IL-13 → airway inflammation
   Impaired macrophage control                              |
   of weakly virulent mycobacteria                 Persistent asthma / upper
              |                                     airway inflammation
   DISSEMINATED BCG / MSMD                          (recapitulated in Tbx21-KO mouse)

The unifying insight is that T-bet is simultaneously an activator of type-1 immunity and a repressor of type-2 immunity. A single loss-of-function lesion therefore removes both functions, yielding the paradoxical combination of an immunodeficiency (susceptibility to mycobacteria) and an immune-dysregulation/allergy phenotype (eosinophilic airway disease) in the same patient. Arm A is the demonstrated cause of the MSMD phenotype and is developmental (missing cell lineages); Arm B is cell-intrinsic, antigen-independent, and directly reversible by wild-type T-bet in vitro. The mouse model validates Arm B decisively and supports Arm A through the conserved T-bet dependence of Th1/IFN-γ immunity.

Evidence Base

PMID Title (abbrev.) Evidence type Role in this report
33296702 Human T-bet Governs Innate and Innate-like Adaptive IFN-γ Immunity against Mycobacteria Human clinical + in vitro Defines IMD88; identifies variant; proves DNA-binding/IFN-γ defect; establishes Arm A cellular mechanism
34160550 High Th2 cytokine levels and upper airway inflammation in human inherited T-bet deficiency Human clinical + in vitro Proves Arm B: Th2 derepression → IL-5/IL-13 → eosinophilia/UAI
11786643 T-bet KO mice spontaneously develop asthma features Mouse model Validates the allergic-airway arm; provides model organism
32025907 MSMD: recent discoveries Review Defines MSMD disease class to which IMD88 belongs
38341181 830 MSMD patients: systematic review Human cohort Epidemiologic/clinical context, prognosis
36630059 MSMD in 22 Moroccan patients (incl. 1 TBX21) Human cohort Population context; BCG-complication rate; TBX21 in Moroccan cohort
41209815 Innate/intrinsic immunity in Morocco Human registry MSMD prevalence in Moroccan IEI registry
30264912 MSMD: 2014–2018 update Review Differential diagnosis / MSMD gene list
34623322 TREM-2 promotes Th1 via CD3ζ-ZAP70 Mouse/human mechanistic Upstream signaling context for T-bet induction
39381601 Rapid identification of primary atopic disorders by genomic sequencing Review Diagnostic strategy (upfront WGS)
35163778 / 33126494 ILCs / CD4 Th subsets reviews Review T-bet/GATA3 master-regulator biology; conservation
29275836 Th17 plasticity / IFNG epigenetics Review Epigenetic context of T-bet/IFNG regulation

Limitations and Knowledge Gaps

  1. Single-patient basis (n=1). Every disease-specific characteristic — penetrance, expressivity, full phenotype spectrum, natural history, treatment response, prognosis — rests on one individual. Population-level parameters are borrowed from the broader MSMD class and must be interpreted as inference, not established fact for IMD88.
  2. No treatment evidence. No therapy has been tested specifically in T-bet deficiency. IFN-γ1b efficacy is uncertain because the defect lies downstream at a transcription factor that impairs both IFN-γ production and responsiveness. Type-2 biologics (anti-IL-5/IL-4Rα) are mechanistically rational but untested in this disorder.
  3. Incomplete model concordance. The mouse Tbx21-KO robustly models the airway/Th2 arm but only partially models the human innate/innate-like lymphocyte deficiency, because murine iNKT/MAIT/γδ compartments differ from human.
  4. No epidemiology, QoL, imaging, or omics datasets exist for IMD88 as a distinct entity; ICD coding and Orphanet classification are not specifically assigned.
  5. Modifier genetics and penetrance are entirely unknown; whether heterozygous carriers have subtle immune/atopic phenotypes has not been examined.

Proposed Follow-up Experiments / Actions

  1. Case-finding / cohort expansion: Systematically screen MSMD and unexplained eosinophilia/severe-asthma cohorts (especially consanguineous, BCG-endemic populations) for biallelic TBX21 variants via WES/WGS and GeneMatcher, to move IMD88 beyond n=1 and define its true phenotype spectrum and penetrance.
  2. Therapeutic proof-of-concept: Evaluate type-2-targeting biologics (mepolizumab/benralizumab/dupilumab) for the eosinophilic-airway arm and formally assess HSCT outcomes and IFN-γ1b response for the mycobacterial arm in any newly identified patients.
  3. Single-cell/transcriptomic profiling: Perform scRNA-seq/ATAC-seq on patient PBMCs to map, at cellular resolution, the loss of innate-like lymphocytes and the derepressed Th2 program, and to identify T-bet direct target loci that fail activation vs fail repression.
  4. Structure-function studies: Model p.E156_M157delinsSerLeu in the T-box domain (crystallography/AlphaFold + EMSA) to define precisely how the two-residue substitution abolishes DNA binding, informing potential correction strategies.
  5. Refined animal models: Generate humanized or knock-in mice carrying the patient allele, and models that better reconstitute human innate-like lymphocyte compartments, to test the mycobacterial arm and candidate therapies.
  6. Carrier / heterozygote studies: Assess immune and atopic phenotypes in obligate heterozygous relatives to determine whether TBX21 haploinsufficiency contributes to common asthma/atopy risk.

Report compiled from 5 investigation iterations, 7 confirmed findings, and 22 reviewed papers. Evidence types are distinguished throughout as human clinical, model organism, in vitro, or review/inference. Where information is unavailable for IMD88 specifically, this is stated explicitly and MSMD-class data are used as the nearest available proxy.