Interleukin 18 Protein is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
The Interleukin-18 Protein (IL-18) is a pro-inflammatory cytokine belonging to the IL-1 family that plays a critical role in innate and adaptive immune responses. Originally described as "interferon-gamma inducing factor," IL-18 is a potent inducer of interferon-gamma (IFN-γ) and promotes Th1 cell differentiation. In the central nervous system, IL-18 is produced by microglia, astrocytes, and neurons, where it contributes to neuroinflammatory processes implicated in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis [1].
| Interleukin-18 Protein | |
|---|---|
| Gene | IL18 |
| UniProt ID | Q14116 |
| PDB IDs | 1J0S, 3F62 |
| Molecular Weight | 24 kDa (pro-IL-18), 18 kDa (mature) |
| Subcellular Localization | Cytoplasm, Secreted |
| Protein Family | IL-1 cytokine family |
IL-18 is constitutively expressed at low levels in various tissues but is dramatically upregulated during inflammation. Unlike many cytokines, IL-18 lacks a signal peptide and is secreted through an unconventional protein secretion pathway involving gasdermin D pores formed during inflammasome activation [2].
The IL-18 precursor consists of:
The propeptide maintains IL-18 in an inactive state until proteolytic cleavage by caspase-1 [1].
The mature IL-18 adopts a distinctive β-trefoil fold, characterized by:
The structure shares significant similarity to IL-1β, though with distinct surface properties conferring receptor specificity [3].
IL-18 initiates signaling by binding to a heterodimeric receptor complex:
IL-18Rα (IL1RL1): The ligand-binding chain with high affinity for mature IL-18. Contains an extracellular IL-1 receptor domain.
IL-18Rβ (IL1RAP): The signaling chain that lacks ligand-binding capacity but is required for downstream signal transduction. Associates with MyD88 adaptor protein.
The IL-18BP (IL-18 binding protein) is a natural antagonist that can neutralize IL-18 activity [1].
IL-18 is synthesized as an inactive propeptide in various cell types:
IL-18 binding triggers a cascade of intracellular events:
IL-18 → IL-18Rα/IL-18Rβ → MyD88 → IRAK4/1 → TRAF6
↓
NF-κB / AP-1 / MAPK
↓
Gene transcription (IFN-γ, cytokines)
Key downstream effects:
IFN-γ induction: IL-18 is one of the most potent IFN-γ inducers known, acting on NK cells and T cells [1].
Th1 differentiation: Promotes differentiation of naïve CD4+ T cells toward Th1 phenotype.
NK cell activation: Enhances cytotoxic activity and cytokine production.
Synergy with IL-12: IL-12 dramatically potentiates IL-18-induced IFN-γ production.
IL-18 is significantly elevated in AD brain tissue and cerebrospinal fluid, with levels correlating with disease severity [4].
Sources in AD brain:
Pathogenic mechanisms:
Therapeutic implications: IL-18 neutralizing strategies may reduce neuroinflammation in AD [4].
Elevated IL-18 levels are found in the substantia nigra and striatum of PD patients and in animal models of dopaminergic degeneration [5].
Pathogenic mechanisms:
Therapeutic targeting: IL-18R antagonists and IL-18BP have shown protective effects in PD models [5].
Elevated in ALS:
Pathogenic mechanisms:
Biomarker potential: IL-18 CSF levels may serve as a prognostic biomarker [6].
IL-18 is elevated in MS lesions and CSF, particularly during active disease [7].
Pathogenic mechanisms:
IL-18 Binding Protein (IL-18BP):
Anti-IL-18 Antibodies:
IL-18R Antagonists:
| Agent | Condition | Status | Notes |
|---|---|---|---|
| Tadekinig alfa (IL-18BP) | Rheumatoid arthritis | Phase II | Some efficacy |
| Tadekinig alfa | Psoriasis | Phase II | Positive results |
| Tadekinig alfa | ALS | Planned | Neuroprotective potential |
The study of Interleukin 18 Protein 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.