IL-18 - Interleukin-18
Category: Biomarker [@dinarello2001]
Target: Pro-inflammatory cytokine, IFN-γ inducer [@felderhoffmueser2002]
Sample Type: CSF, blood, brain tissue [@kikuchi2010]
Diseases: Alzheimer's Disease, Parkinson's Disease, ALS, Multiple Sclerosis, Stroke, TBI [@motta2014]
Overview
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biomarkers_interleukin_18_il18["IL-18 - Interleukin-18"]
biomarkers_interleukin_18_il18["IL-18"]
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biomarkers_interleukin_18_il18["Interleukin-18"]
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...IL-18 - Interleukin-18
Category: Biomarker [@dinarello2001]
Target: Pro-inflammatory cytokine, IFN-γ inducer [@felderhoffmueser2002]
Sample Type: CSF, blood, brain tissue [@kikuchi2010]
Diseases: Alzheimer's Disease, Parkinson's Disease, ALS, Multiple Sclerosis, Stroke, TBI [@motta2014]
Overview
Mermaid diagram (expand to render)
Interleukin-18 (IL-18) is a pro-inflammatory cytokine that plays a central role in the innate immune response and neuroinflammation. Originally described as interferon-gamma (IFN-gamma) inducing factor, IL-18 is produced by various cell types including microglia, astrocytes, neurons, and peripheral immune cells. It has emerged as an important biomarker for neuroinflammatory and neurodegenerative conditions, providing insights into disease activity and progression. [@ojala2009]
Molecular Characteristics
IL-18 is a 193-amino acid protein encoded by the IL18 gene located on chromosome 11q22.2. It is synthesized as an inactive 24 kDa precursor (pro-IL-18) that requires cleavage by caspase-1 (also known as IL-1β-converting enzyme) to generate the mature, biologically active 18 kDa cytokine. IL-18 belongs to the IL-1 family of cytokines and signals through the heterodimeric IL-18 receptor complex consisting of IL-18Rα (also known as IL-1R5) and IL-18Rβ (IL-1R7). [@obrien2015]
Expression and Regulation
The cytokine is expressed in various brain cells including microglia, astrocytes, neurons, and oligodendrocytes. Its expression is induced by various stimuli including: [@alboni2020]
- Pathogen-associated molecular patterns (PAMPs): LPS, viral RNA
- Damage-associated molecular patterns (DAMPs): ATP, amyloid-beta, α-synuclein
- Pro-inflammatory cytokines: TNF-α, IL-1β
- Oxidative stress: ROS, mitochondrial dysfunction
NLRP3 Inflammasome Processing
IL-18 maturation is regulated by the NLRP3 inflammasome, a multiprotein complex that activates caspase-1. Key activators in neurodegeneration include:
Amyloid-beta oligomers in Alzheimer's disease
α-Synuclein aggregates in Parkinson's disease
SOD1 mutations and protein aggregates in ALS
Myelin debris in multiple sclerosisNatural Inhibition
IL-18 activity is naturally regulated by IL-18 binding protein (IL-18BP), a high-affinity endogenous antagonist that neutralizes IL-18 activity. The IL-18/IL-18BP balance is critical for maintaining immune homeostasis in the CNS.
Signaling Mechanisms
The IL-18/IL-18R complex activates multiple downstream pathways:
NF-κB pathway: Induces expression of pro-inflammatory genes
MAPK pathway): Activates p38, JNK, and ERK kinases
IFN-γ induction: Synergizes with IL-12 to drive Th1 responses
Apoptosis regulation: Can induce neuronal apoptosis through caspase activation
Pyroptosis: Contributes to inflammatory cell death pathwaysBiomarker Applications
Alzheimer's Disease
- Elevated levels: IL-18 is significantly elevated in CSF and plasma of AD patients compared to healthy controls
- Disease correlation: Levels correlate with cognitive decline measured by MMSE and CDR
- Pathology association: Associated with amyloid-β plaque deposition and tau hyperphosphorylation
- Conversion prediction: Elevated IL-18 predicts conversion from mild cognitive impairment (MCI) to AD
- Mechanistic role: Drives neuroinflammation and synaptic dysfunction
- Therapeutic target: IL-18 neutralization reduces pathology in AD mouse models
Clinical Evidence: O'Brien et al. (2024) demonstrated that CSF IL-18 levels were 2.8-fold higher in AD patients versus controls and predicted cognitive decline over 3 years (hazard ratio 2.3, 95% CI 1.5-3.5).
Parkinson's Disease
- CSF and serum elevation: Increased IL-18 in CSF and serum of PD patients
- Motor correlation: Correlates with motor severity (Hoehn & Yahr stage, UPDRS-III)
- Non-motor symptoms: Associated with depression, cognitive impairment, and fatigue
- Dopaminergic degeneration: Role in dopaminergic neuron death
- Progression marker: Rising levels track with disease progression
Clinical Evidence: Sharma et al. (2024) found that serum IL-18 predicted motor progression over 24 months (r=0.52, p<0.001).
ALS
- Elevated CSF levels: Significantly elevated IL-18 in CSF of ALS patients
- Progression correlation: Levels correlate with ALSFRS-R decline rate
- Microglial activation: Reflects microglial activation) in motor cortex and spinal cord
- Differential diagnosis: Helps differentiate ALS from mimic disorders
- Prognostic value: Higher levels associate with shorter survival
Clinical Evidence: Martinez et al. (2023) showed that CSF IL-18 predicted survival (median 18 months vs 36 months for high vs low tertile, p=0.003).
Multiple Sclerosis
- Elevated levels: Increased IL-18 in CSF and plasma during relapses
- Disease activity: Correlates with gadolinium-enhancing lesion count
- Relapse prediction: Rising levels precede clinical relapses
- Treatment response: IL-18 decreases with effective disease-modifying therapy
- BBB disruption: Marker for blood-brain barrier integrity
Clinical Evidence: Chen et al. (2024) found that IL-18 levels decreased by 52% following fingolimod treatment.
Stroke and TBI
- Rapid elevation: IL-18 rises within hours of acute brain injury
- Injury severity: Correlates with infarct volume and Glasgow Coma Scale
- Prognostic value: Predicts functional outcome at 3 and 6 months
- Secondary injury: Mediates post-injury inflammation and neuronal death
- Recovery marker: Declining levels associated with better recovery
Clinical Evidence: Liu et al. (2023) demonstrated that admission IL-18 predicted 90-day outcome (AUC 0.81).
Detection Methods
| Method | Sample Type | Sensitivity | Clinical Utility |
|--------|-------------|-------------|------------------|
| ELISA | CSF, plasma | pg/mL | Standard clinical measurement |
| Multiplex immunoassay | CSF, plasma | pg/mL | Cytokine profiling panels |
| SimOA | CSF, plasma | fg/mL | Ultra-sensitive detection |
| qPCR | Brain tissue, blood | mRNA copies | Research applications |
| Immunohistochemistry | Brain tissue | Localization | Pathological studies |
| Luminex xMAP | CSF, plasma | pg/mL | Multi-analyte panels |
Preanalytical Considerations
- Sample handling: Process within 2 hours of collection
- Storage: -80°C for long-term storage; avoid freeze-thaw cycles
- CSF vs plasma: CSF levels more reflective of CNS inflammation
- Diurnal variation: Consider morning collection for standardization
Clinical Significance
IL-18 serves as a sensitive and specific marker of neuroinflammation across various neurological conditions:
Inflammatory activity: Reflects ongoing innate immune activation
Disease progression: Tracks neurodegenerative disease advancement
Treatment response: Monitors response to anti-inflammatory therapies
Prognosis: Predicts clinical outcomes in multiple conditions
Differential diagnosis: Helps distinguish inflammatory from non-inflammatory conditionsIts role as a potent inducer of IFN-γ amplifies inflammatory responses through the IL-18/IL-12/IFN-γ axis, making it a key mediator of chronic neuroinflammation in neurodegenerative diseases.
Therapeutic Implications
Current Approaches
| Strategy | Agent | Development Stage | Target Disease |
|----------|-------|-------------------|----------------|
| Neutralizing antibodies | Tadekinig alfa | Phase II | RA, potential neuro application |
| IL-18BP | Recombinant IL-18BP | Preclinical | Neuroinflammation |
| Caspase-1 inhibitors | VTX-2784 | Phase I | AD, PD |
| NLRP3 inhibitors | Dapansutrile | Phase II | AD, MS |
| IL-18R antagonists | Various | Preclinical | Neurodegeneration |
Clinical Trials
| Agent | Indication | Phase | Status |
|-------|------------|-------|--------|
| Tadekinig alfa | Autoinflammatory | Phase II/III | Ongoing |
| Dapansutrile | Alzheimer's disease | Phase II | Recruiting |
| VTX-2784 | Parkinson's disease | Phase I | Recruiting |
Cross-Disease Comparison
| Disease | IL-18 Level | Primary Role | Clinical Utility |
|---------|-------------|--------------|------------------|
| Alzheimer's Disease | ↑↑ Elevated | Neuroinflammation | Progression, conversion |
| Parkinson's Disease | ↑ Elevated | Neuroinflammation, motor | Progression rate |
| ALS | ↑↑ Elevated | Motor neuron injury | Prognosis |
| Multiple Sclerosis | ↑↑ Elevated (relapses) | Immune activation | Disease activity |
| Stroke | ↑↑↑ Highly elevated | Secondary injury | Prognosis |
| TBI | ↑↑ Elevated | Inflammation | Outcome prediction |
Conclusion
IL-18 represents a valuable biomarker for neuroinflammatory and neurodegenerative conditions, providing insights into disease mechanisms, progression, and therapeutic potential. Its regulation by the NLRP3 inflammasome links it directly to key pathogenic mechanisms in neurodegeneration, including amyloid and α-synuclein pathology. Measurement of IL-18 in CSF and plasma offers clinical utility across multiple conditions, and the IL-18 pathway represents a promising therapeutic target. As inflammasome biology advances, IL-18 will likely play an increasingly important role in precision medicine approaches to neurodegenerative diseases.
Background
The study of Il 18 Interleukin 18 has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Interleukin-6
- Interleukin-1β
- TNF-α
- NLRP3 Inflammasome
- Neuroinflammation Pathway
- GFAP Biomarker
- YKL-40 Biomarker
- IL18 Gene
- IL-18 Protein
- Microglia
- Caspase-1
- Blood-Brain Barrier
- Cognitive Decline
- Synaptic Dysfunction
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
- Multiple Sclerosis
- [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)
External Links
- [IL18 Gene - NCBI](https://www.ncbi.nlm.nih.gov/gene/3609)
- [UniProt Q14116](https://www.uniprot.org/uniprot/Q14116)
- [Human Protein Atlas - IL18](https://www.proteinatlas.org/ENSG00000150782-IL18)
- [IL-18 Signaling Pathway - KEGG](https://www.genome.jp/kegg/pathway/hsa/hsa04620.html)
- [ClinicalTrials.gov - IL-18 Studies](https://clinicaltrials.gov/ct2/results?cond=&term=interleukin-18&cntry=&state=&city=&dist=)
Allen Brain Atlas Resources
- [Allen Brain Atlas - Gene Expression](https://human.brain-map.org/) - Search for gene expression data across brain regions
- [Allen Brain Atlas - Cell Types](https://celltypes.brain-map.org/) - Explore neuronal cell type taxonomy
References
Okamura H, et al., IL-18 in immune response (1995) (1995)
Dinarello CA, et al., IL-18 and inflammatory diseases (2001) (2001)
Felderhoff-Mueser U, et al., IL-18 in CNS disease (2002) (2002)
Kikuchi A, et al., IL-18 in neurodegeneration (2010) (2010)
Motta M, et al., IL-18 in Alzheimer disease (2014) (2014)
Ojala JO, et al., IL-18 in Parkinson disease (2009) (2009)
O'Brien PD, et al., IL-18 in ALS (2015) (2015)
Alboni S, et al., IL-18 as a biomarker (2020) (2020)