Homozygosity for rare or common hypomorphic IL23R variants confers a predisposition to tuberculosis in humans

This study demonstrates that homozygosity for both rare and common hypomorphic IL23R variants impairs IL-23-dependent IFN-gamma production in innate immune cells, thereby conferring a specific genetic predisposition to tuberculosis while preserving immunity against less virulent mycobacteria.

The Gist

Imagine your body is a fortress defending itself against invaders. One of the most dangerous invaders is the bacterium that causes Tuberculosis (TB). To fight this, your fortress relies on a specialized alarm system and a rapid-response team.

This new research discovers a hidden flaw in the "alarm wiring" of some people's immune systems that makes them much more likely to get sick from TB, even though they seem healthy otherwise.

Here is the story of that discovery, broken down into simple concepts:

1. The Alarm System: IL-23 and the "General"

Think of your immune system as a military base. When a threat is detected, a messenger molecule called IL-23 acts like a radio signal sent out to the troops.

• The Signal: IL-23 tells the soldiers, "We have an intruder! Prepare to fight!"
• The Receiver: On the surface of the soldiers (specifically certain immune cells like NK cells and MAIT cells), there is a receiver called IL-23R. This is the antenna that catches the radio signal.
• The Action: Once the signal is caught, the soldiers release a powerful weapon called Interferon-gamma (IFN-γ). This weapon is the heavy artillery needed to destroy TB bacteria.

2. The Broken Antennas: The "Hypomorphic" Variants

The researchers found that some people have a genetic glitch in their IL-23R antenna. They call these "hypomorphic" variants, which is a fancy way of saying "partially broken."

Imagine four different types of broken antennas:

• The Rare, Broken Antennas (G300V, G149R, L372F): These are like antennas that are very rare in the population. They are so damaged that they barely pick up the signal.
• The Common, Wobbly Antenna (R381Q): This is the most surprising find. This specific broken antenna is actually very common. About 1 in 10 people in some populations (like Ashkenazi Jewish or Amish communities) carry two copies of this "wobbly" antenna.

The Crucial Difference:

• Complete Breakage: If someone has no working antenna at all, they get sick from very weak bacteria (like the vaccine strain or environmental bugs). This is a known, rare disease.
• Partial Breakage (The Discovery): The people in this study have some working signal, but it's weak. It's strong enough to fight off the weak bacteria (so they don't get sick from the vaccine), but too weak to fight off the real, dangerous TB bacteria.

3. The "Goldilocks" Problem

The study explains a "Goldilocks" scenario for immunity:

• Too Strong: If the immune system is overactive, it causes autoimmune diseases (like Crohn's disease). Interestingly, having these broken antennas actually protects people from those inflammatory diseases.
• Too Weak: If the antenna is totally broken, you get sick from weak bugs.
• Just Right (But Not Quite): These people have a "just right" level of immunity for most things, but when it comes to the super-virulent TB bacteria, their "just right" level isn't enough. It's like having a fire extinguisher that works great on a candle but fails on a forest fire.

4. The Evidence: Finding the "Smoking Gun"

The researchers looked at thousands of patients. They found that people who had two copies of these broken antennas (one from mom, one from dad) were much more likely to have Tuberculosis than people with working antennas.

• The "Common" Culprit: They were shocked to find that the common R381Q variant (the wobbly antenna) was found in many TB patients. It turns out that while this variant protects against inflammatory bowel disease, it leaves a door open for TB.
• The "Rare" Culprit: They also found rare variants that caused the same problem in specific families.

5. Why Did This Happen? (Evolutionary Trade-off)

Why would a broken antenna exist in so many people? The paper suggests an evolutionary trade-off.
Thousands of years ago, TB was a massive killer in Europe. However, inflammatory diseases were also a problem. The "wobbly" antenna (R381Q) might have helped people survive by calming down their immune system (preventing inflammation), but as a side effect, it made them slightly more vulnerable to TB. Because TB was so deadly, nature tried to weed out these genes, but they haven't disappeared completely yet.

The Bottom Line

This paper tells us that Tuberculosis isn't just about the bacteria; it's also about the host.
Some people are genetically predisposed to get TB not because their immune system is "weak" in a general sense, but because their specific "TB alarm" is slightly tuned down. They can fight off most infections, but TB slips through the cracks.

Why does this matter?

1. Diagnosis: If a patient gets TB, doctors might now check if they have these specific genetic variants.
2. Treatment: If a patient has this "broken antenna," they might respond better to treatments that boost their immune system directly (like giving them extra Interferon-gamma), rather than just using standard antibiotics.
3. Understanding Disease: It shows that "partial" genetic defects can cause serious diseases, even if the person looks perfectly healthy otherwise.

In short: Your immune system is a complex machine. Sometimes, a small, common glitch in one specific part of the machine can leave you vulnerable to a specific, deadly enemy.

Technical Summary

1. Problem Statement

Mendelian Susceptibility to Mycobacterial Diseases (MSMD) is a group of genetic disorders characterized by severe infections with weakly virulent mycobacteria (e.g., BCG vaccine strains, environmental mycobacteria). While complete loss-of-function (LOF) mutations in the IL-23 receptor (IL23R) are known to cause MSMD, the genetic basis for susceptibility to highly virulent Mycobacterium tuberculosis (TB) in the general population remains less defined.

The authors hypothesized that partial (hypomorphic) deficiencies in the IL-23R pathway, caused by either rare or common genetic variants, might specifically predispose individuals to tuberculosis while sparing them from MSMD. This challenges the classical dichotomy where IL-12 drives Th1 (IFN-γ) immunity and IL-23 drives Th17 (IL-17) immunity, suggesting instead that IL-23 is also critical for IFN-γ production in human mycobacterial defense.

2. Methodology

The study employed a multi-disciplinary approach combining human genetics, functional immunology, and evolutionary biology:

• Genetic Screening: The authors screened a cohort of 24,046 patients with various infectious diseases (including 1,874 with TB and 901 with MSMD) and public databases (gnomAD v4.1, ATAVDB, etc.) for biallelic IL23R coding variants.
• Functional Characterization (In Vitro):
  • Luciferase Reporter Assays: HEK293T cells were transfected with wild-type (WT) or mutant IL23R cDNAs to measure IL-23-induced STAT3 signaling activity.
  • Surface Expression & Binding: NanoLuciferase-based assays (NanoBRET) and luminescence imaging were used to assess cell-surface expression levels and IL-23 binding affinity of the variants.
  • Patient-Derived Cells: Epstein-Barr Virus (EBV)-immortalized B cells and T-cell blasts from patients homozygous for specific variants were stimulated with IL-23 to measure STAT3 phosphorylation and IFN-γ production.
• Single-Cell Analysis: scRNA-seq was performed on PBMCs from patients and controls to analyze IFNG mRNA induction upon IL-23 stimulation across specific immune subsets (MAIT, NK, $\gamma\delta$ T cells).
• Ex Vivo Cytokine Assays: PBMCs were stimulated with IL-23, IL-18, and BCG to quantify IFN-γ secretion via ELISA and intracellular flow cytometry.
• Statistical & Evolutionary Analysis: Enrichment analysis (Regressing) compared variant frequencies in TB patients vs. controls. Founder effect analysis estimated the age of specific mutations.

3. Key Contributions & Results

A. Identification of Hypomorphic Variants

The study identified four specific biallelic IL23R variants that are hypomorphic (partial loss of function) rather than complete LOF:

1. G300V: Rare (MAF < 0.01%), found in Turkish and European cohorts.
2. G149R: Rare (MAF ~0.6%), enriched in East Asian populations.
3. L372F: Rare (MAF < 0.01%), enriched in East Asian populations.
4. R381Q: Common (MAF ~5.4% globally, up to 10.2% in Amish/Ashkenazi Jewish populations).

Note: 15 other missense variants found in homozygous states were determined to be isomorphic (functionally normal).

B. Mechanism of Impairment

Functional assays revealed that these variants impair IL-23 signaling through two distinct mechanisms, without abolishing receptor-ligand affinity:

• Reduced Surface Expression: Variants G300V and R381Q showed significantly lower cell-surface expression levels compared to WT.
• Conformational Changes: Variants G149R and L372F expressed normally at the surface but exhibited altered receptor conformation, leading to reduced agonist efficacy (lower STAT3 phosphorylation) despite normal binding affinity.
• Downstream Effect: In patient cells, IL-23-induced STAT3 phosphorylation was impaired but not abolished. Crucially, IFN-γ production by innate-like T cells (MAIT, V$\delta$2+ $\gamma\delta$ T) and NK cells was significantly reduced upon IL-23 stimulation.

C. Clinical Phenotype: TB vs. MSMD

• Tuberculosis Predisposition: There was a significant enrichment of homozygosity for these hypomorphic variants in TB patients compared to controls.
  • G300V: Odds Ratio (OR) for TB = 26.5.
  • Combined Burden: An activity-weighted burden test confirmed significant enrichment of hypomorphic homozygotes in the TB cohort ($P=3.8 \times 10^{-2}$), with even stronger significance in Middle Eastern ancestry groups ($P=9.7 \times 10^{-5}$).
• Preserved Immunity to Weak Mycobacteria: Unlike patients with complete IL23R deficiency (who suffer from MSMD and Chronic Mucocutaneous Candidiasis), patients with these hypomorphic variants did not show an enrichment in MSMD or CMC. Their residual IL-23 signaling was sufficient to control weakly virulent mycobacteria (BCG, environmental) and fungi but insufficient to control M. tuberculosis.

D. Evolutionary Context

The R381Q variant (and the similar TYK2 P1104A variant) has undergone negative selection in European populations over the last 4,500 years, with frequencies dropping from ~10-15% in the Neolithic/Bronze Age to ~5.5% today. This suggests a historical trade-off: these alleles likely provided protection against inflammatory disorders (e.g., IBD) but increased susceptibility to TB, leading to their gradual removal from the gene pool as TB became a major selective pressure.

4. Significance

• Redefining IL-23 Function: The study overturns the strict Th1/Th17 dichotomy in humans, demonstrating that IL-23 is essential for IFN-γ-mediated immunity against mycobacteria, particularly in innate-like lymphocytes.
• Genetic Architecture of TB: It establishes that recessive partial deficiencies in immune pathways can be a major cause of common infectious diseases like TB, not just rare monogenic disorders.
• Common vs. Rare Variants: It highlights that common variants (like R381Q) can have significant clinical impacts when homozygous, contributing to the "missing heritability" of complex infectious diseases.
• Therapeutic Implications: Patients with these genotypes may benefit from recombinant IFN-γ therapy, similar to those with complete deficiencies, as their endogenous production is impaired but their signaling machinery is intact.
• Evolutionary Trade-offs: The findings illustrate how alleles selected for protection against inflammatory diseases (like IBD) may inadvertently increase susceptibility to infectious diseases (like TB), shaping human genetic diversity.

In conclusion, this paper provides robust evidence that homozygosity for rare or common hypomorphic IL23R variants creates a specific immunodeficiency that predisposes humans to tuberculosis while largely sparing them from other mycobacterial diseases, driven by a partial defect in IL-23-dependent IFN-γ production.