IL18BP Human

What is IL18BP and what is its primary function in human immunology?

IL-18BP is a secreted glycoprotein that contains an Ig-like C2-type domain and functions as a potent inhibitor of the proinflammatory cytokine IL-18 . It binds to IL-18 with high affinity (approximately 400 pM), preventing IL-18 from binding to its receptor and thus blocking IL-18-induced IFN-gamma production . This interaction is critical for regulating immune responses, particularly Th1-type immunity . IL-18BP is considered a natural regulatory molecule that helps maintain immune homeostasis by modulating IL-18 activity through a high-affinity binding mechanism .

How many isoforms of human IL18BP exist and how do they differ functionally?

Human IL18BP exists in at least four isoforms (A, B, C, and D) resulting from alternative splicing, with differences primarily in their carboxyl termini while sharing identical N-terminal sequences . Isoforms A and C each contain a complete immunoglobulin (Ig)-like C2-type domain, which is essential for binding and neutralizing IL-18 . Human IL-18BP A has the highest affinity for IL-18 with a dissociation constant of 399 nM, while IL-18BP C binds with a dissociation constant of 2.94 nM . In contrast, isoforms B and D lack a complete Ig domain and consequently cannot bind to IL-18 or inhibit its activity . IL-18BP A is the most abundant isoform found in cDNA libraries, while the other isoforms show more tissue-specific distribution patterns .

Where is IL18BP primarily expressed in the human body?

IL-18BP mRNA is highly expressed in specific human tissues including the heart, lung, placenta, spleen, and colon . Research shows that IL-18BP is also strongly expressed in the hypothalamus of rats, suggesting potential neuroimmune functions . Expression patterns vary among tissues, with only low levels of IL-18BP mRNA found in unstimulated human keratinocytes, colon cancer cells, and glomerular mesangial cells . IL-18BP and IL-18 are both secreted primarily by hepatocytes and macrophages in the liver, highlighting the liver as a key site for IL-18/IL-18BP regulation .

What cellular mechanisms regulate IL18BP expression?

The expression of IL-18BP is markedly upregulated by IFN-gamma, creating a negative feedback loop that modulates IL-18 activity . This feedback mechanism is critical for controlling IL-18-mediated inflammatory responses . When IL-18 stimulates Th1 cells and other immune cells to produce IFN-gamma, the increased IFN-gamma subsequently enhances IL-18BP production, which then binds to and neutralizes IL-18, preventing excessive inflammation . This regulatory circuit demonstrates how IL-18BP serves as a natural "brake" on the IL-18/IFN-gamma axis . Additionally, IL-18BP can regulate the activation of TLR3 through microRNA mechanisms, specifically involving miR-134 .

What is the molecular basis for the high-affinity interaction between IL18BP and IL-18?

The high-affinity binding between IL-18BP and IL-18 is primarily mediated by the immunoglobulin (Ig)-like C2-type domain of IL-18BP . This domain shares structural similarities with the extracellular Ig structure of cytokine receptors but differs from the IL-1 and IL-18 receptor families that contain three IgG domains . The mature peptide of human IL-18BP consists of 134 amino acids (after cleavage of the 30-amino acid signal peptide) and contains four N-glycosylation sites that influence its functional properties . Despite having no amino acid sequence homology to membrane-associated IL-18 and IL-1 receptor proteins, IL-18BP achieves remarkable binding specificity and affinity through its unique structural configuration . The binding affinity of IL-18BP for IL-18 (400 pM) is significantly higher than that of IL-18 for its receptor IL-1Rrp (39 nM), explaining why IL-18BP effectively competes with cellular receptors for IL-18 binding .

How does IL18BP deficiency contribute to fulminant viral hepatitis?

A groundbreaking case study reported an autosomal recessive IL-18BP deficiency in a child who died of fulminant hepatitis following hepatitis A virus (HAV) infection . The patient was homozygous for a private 40-nucleotide deletion in the IL18BP gene, resulting in complete loss of function . In the absence of IL-18BP, excessive NK cell activation by unopposed IL-18 led to uncontrolled killing of hepatocytes . This case provides direct evidence that human IL-18 can be highly toxic to the liver and that IL-18BP functions as its natural antidote . The findings suggest that inherited IL-18BP deficiency can underlie fulminant HAV hepatitis by unleashing IL-18-mediated cytotoxicity, establishing proof-of-principle that fulminant viral hepatitis can be caused by single-gene inborn errors that selectively disrupt liver-specific immunity .

How do viral homologs of IL18BP influence host immune responses?

Several poxviruses encode proteins with sequence similarity to human and mouse IL-18BP, representing a fascinating example of viral mimicry of host immune regulators . These viral IL-18BPs have been shown to bind and inhibit IL-18 responses and may play crucial roles in modulating host immune responses during infection . By neutralizing IL-18 activity, viral IL-18BPs can potentially suppress antiviral Th1 responses and NK cell activation, thus facilitating viral persistence and replication . The convergent evolution of viral IL-18BP homologs suggests the critical importance of IL-18 in antiviral defense and highlights how pathogens have evolved sophisticated mechanisms to counteract host immunity . Understanding the structural and functional properties of viral IL-18BPs may provide insights into the design of novel immunomodulatory therapies .

What are optimal methods for measuring IL18BP levels in clinical samples?

For accurate quantification of IL-18BP in clinical samples, enzyme-linked immunosorbent assays (ELISAs) using monoclonal antibodies such as clone #136033 have been developed . When measuring IL-18BP in serum samples, researchers should consider potential interference from endogenous IL-18 and use methods that can distinguish free IL-18BP from IL-18-bound forms . In healthy individuals, serum IL-18BP levels are typically 20-fold higher than IL-18 levels, while in conditions like sepsis, IL-18BP A can be elevated to approximately 21.9±1.44 ng/ml with total IL-18 levels around 1.5±0.4 ng/ml . Researchers should employ standardized protocols that account for the glycosylation of IL-18BP, as this post-translational modification affects its activity . For comprehensive analysis, measuring both IL-18BP and IL-18 simultaneously is recommended to determine the ratio of free to bound IL-18, which better reflects the biological activity of this cytokine system .

What expression systems are recommended for producing recombinant IL18BP for research purposes?

Mammalian cell lines are strongly recommended for producing recombinant IL-18BP due to the glycosylation-dependent activity of this protein, which weighs approximately 20 kDa . Various forms of recombinant IL-18BP have been successfully produced, including intact protein, C-terminal his-tagged versions, and Fc fusion proteins, each offering different advantages for in vitro and in vivo experiments . The Fc chimera form (such as Recombinant Human IL-18 BPa Fc Chimera Protein) provides enhanced stability and half-life, making it particularly useful for in vivo studies . When designing expression constructs, researchers should focus on the isoforms with complete Ig domains (isoforms A and C in humans) to ensure binding activity . The effective concentration range for inhibitory activity in vitro is approximately 0.02-0.12 μg/mL when testing against 40 ng/mL of recombinant human IL-18 . For optimal results, expression should include the mature peptide starting from Thr29 to maintain proper protein folding and function .

How can researchers effectively study the IL18BP-IL-18 interaction in experimental models?

To study IL-18BP-IL-18 interactions, researchers can employ several approaches, including surface plasmon resonance for direct binding kinetics, cell-based bioassays measuring IFN-γ inhibition, and co-immunoprecipitation experiments . When designing in vitro experiments, it's crucial to use physiologically relevant ratios of IL-18 to IL-18BP, considering that in healthy individuals, IL-18BP levels are approximately 20-fold higher than IL-18 . For cellular models, researchers should consider using human keratinocytes, hepatocytes, or specific immune cells like NK cells, which are key targets of IL-18 activity . Animal models with IL-18BP gene deletions can provide insights into systemic effects, though researchers should note the significant differences between human and mouse IL-18BP isoforms (sharing approximately 61% amino acid sequence identity) . The hepatitis A virus infection model in IL-18BP-deficient animals has proven particularly valuable for understanding the role of IL-18BP in liver protection, as excessive NK cell activation by uninhibited IL-18 results in hepatocyte killing .

What approaches can differentiate between free and IL-18-bound IL18BP in biological samples?

Distinguishing between free and IL-18-bound IL-18BP in biological samples remains technically challenging but crucial for accurate assessment of IL-18 biological activity . One approach involves using a combination of total IL-18BP ELISA and a functional assay that detects only free IL-18BP through its ability to neutralize exogenous IL-18 . Alternatively, researchers can employ sandwich ELISAs with antibodies that selectively recognize epitopes accessible only in the free form of IL-18BP . Mathematical modeling based on known binding affinities can also be used to calculate the concentration of free IL-18 in a mixture containing both IL-18 and IL-18BP . In sepsis patients, for example, although total IL-18 (1.5±0.4 ng/ml) and IL-18BP A (21.9±1.44 ng/ml) are both elevated, the remaining free IL-18 levels (64±17 pg/ml) are higher than in healthy individuals, demonstrating the importance of measuring the free, biologically active fraction . Researchers should also consider that different isoforms of IL-18BP have varying affinities for IL-18, with IL-18BP A showing the highest affinity (dissociation constant of 399 nM) .

What therapeutic potential does recombinant IL18BP hold for inflammatory and autoimmune diseases?

Recombinant IL-18BP (rIL-18BP) shows significant therapeutic potential for inflammatory and autoimmune diseases characterized by IL-18 dysregulation . Early clinical trials with rIL-18BP demonstrated promising results in neutralizing IL-18 activity, although the journey from discovery to widespread clinical use has been lengthy since IL-18BP was first identified in 1999 . Future research should focus on optimizing delivery methods, dosing regimens, and potentially combining IL-18BP therapy with other immunomodulatory approaches for synergistic effects . Particular attention should be given to adult-onset Still's disease, rheumatoid arthritis, and lupus erythematosus, where IL-18 plays a documented pathogenic role . Additionally, the discovery of IL-18BP deficiency in fulminant viral hepatitis suggests a potential therapeutic application for rIL-18BP in acute liver inflammation scenarios . As our understanding of the IL-18/IL-18BP axis evolves, development of isoform-specific recombinant proteins or modified versions with enhanced stability and efficacy could further expand therapeutic options .

How might genetic variations in IL18BP contribute to disease susceptibility?

The discovery of a loss-of-function mutation in IL18BP causing fulminant viral hepatitis opens a new research avenue into how genetic variations in this gene might influence disease susceptibility . Future studies should conduct comprehensive genetic analyses in populations with unusual susceptibility to viral hepatitis or inflammatory disorders to identify additional IL18BP variants . Research should also explore how common polymorphisms in the IL18BP gene might subtly alter protein function or expression levels, potentially predisposing individuals to inflammatory conditions or modifying disease severity . Association studies between IL18BP genetic variants and autoimmune or inflammatory diseases could reveal new insights into pathogenesis and identify at-risk populations . Additionally, investigating potential interactions between IL18BP variants and environmental factors, such as viral infections, could help explain variable disease presentations and outcomes . This genetic approach might ultimately lead to personalized medicine strategies based on an individual's IL18BP genetic profile.

What novel interactions between IL18BP and other immune pathways remain to be discovered?

While the interaction between IL-18BP and IL-18 is well-characterized, future research should investigate potential crosstalk between IL-18BP and other immune signaling pathways . Recent findings suggest that IL-18BP can regulate TLR3 activation through miR-134, indicating potentially broader immunomodulatory functions . Research should explore how IL-18BP might influence other cytokine networks beyond the IL-18/IFN-γ axis, particularly those involved in innate immunity and inflammation resolution . The relationship between IL-18BP and complement components, pattern recognition receptors, or other soluble immune mediators remains largely unexplored . Additionally, investigating how IL-18BP expression and function are affected by metabolic conditions, aging, or environmental factors could reveal new regulatory mechanisms . Understanding these complex interactions could potentially lead to novel therapeutic approaches targeting the IL-18BP pathway in various inflammatory and autoimmune conditions .