Soluble interleukin-18 receptor complex is a novel biomarker in rheumatoid arthritis
© Takei et al.; licensee BioMed Central Ltd. 2011
Received: 24 September 2010
Accepted: 24 March 2011
Published: 24 March 2011
There has been no report in the literature of a soluble form of interleukin (IL)-18 receptor α (IL-18Rα). In this study, we evaluated the levels and characteristics of soluble IL-18Rα (sIL-18Rα) in the sera of patients with rheumatoid arthritis (RA) and compared these results to control populations.
The sIL-18Rα complex was isolated from pooled human blood serum using an anti-IL-18Rα monoclonal antibody affinity column. The purified sIL-18Rα was then examined using Western blot analysis and used in experiments to evaluate the effects on an IL-18-responsive natural killer (NK) human cell line, NK0. An enzyme-linked immunosorbent assay was developed, and sera from 145 patients with RA, 6 patients with adult-onset Still's disease, 31 patients with osteoarthritis (OA), 39 patients with systemic lupus erythematosus (SLE) and 67 controls were tested, along with levels of immunoglobulin M, rheumatoid factor, anticyclic citrullinated peptide antibody, IL-18, IL-13 and interferon (IFN)-γ. Area under the receiver operating characteristic curve (ROC-AUC) analysis was used to evaluate the diagnostic utility of the sIL-18Rα complex.
The isolated sIL-18Rα complex can be associated with IL-18 and the soluble form of the IL-18Rβ chain. The sIL-18Rα complex bound to the surface to the NK0 cell line, antagonized the stimulatory effects of IL-18 and IL-2 on the NK0 cell line and inhibited IFN-γ production by the cells. The serum levels of sIL-18Rα complex in RA (186.0 ± 33.5 ng/mL, n = 145) and adult-onset Still's disease (98.2 ± 8.9 ng/mL, n = 6) were significantly (P < 0.001) higher than those in the healthy controls (52.3 ± 8.5 ng/mL, n = 67), OA (38.6 ± 5.4 ng/mL, n = 31), SLE (44.6 ± 3.2 ng/mL, n = 39). The serum level of sIL-18Rα complex was not significantly different between RA and adult-onset Still's disease patients. The serum levels of IL-18, IL-13 and IFN-γ in the RA patients were significantly (P < 0.01) higher than in OA and SLE patients as well as healthy controls. ROC-AUC analysis of the serum concentration of sIL-18Rα indicated that it was significantly diagnostic of RA. Moreover, a tumor necrosis factor inhibitor, etanercept, significantly (P < 0.0001) decreased levels of sIL-18Rα in the sera of 29 RA patients 6 months after treatment.
The sIL-18Rα complex could be a potentially useful biomarker for the diagnosis of RA.
Interleukin (IL)-1α and IL-1β have two homologous receptors IL-1 receptor 1, type I (IL-1RI) and IL-1R, type II (IL-1RII). Functional IL-1R is a complex comprising IL-1RI and IL-1 receptor accessory protein (IL-1RAcP) (see reviews in [1–3]). Upon binding of IL-1α or IL-1β, IL-1RI forms a complex with IL-1RAcP and initiates a cytosolic signaling cascade (myeloid differentiation primary response protein (MyD88), IL1R-associated kinase (IRAK) and tumor necrosis factor-associated factor 6 protein (TRAF6), as well as activation of nuclear factor κB (NFκB), c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK)). In contrast, 60-kDa IL-1RII functions as a nonsignaling "decoy" receptor. It has been reported that IL-1RI has a soluble form (sIL-1RI) , which can be generated by metalloprotease . Soluble IL-1RII (sIL-1RII) is also generated primarily by proteolytic cleavage in response to a variety of stimuli and can attenuate excessive IL-1 bioactivity by preferentially binding IL-1β. In contrast, soluble IL-1RAcP (sIL-1RAcP) is generated by alternative splicing rather than by ectodomain cleavage. The soluble receptors sIL-1RI, sIL-1RII and sIL-RAcP may attenuate IL-1 signaling .
IL-18, a member of the IL-1 family, is well-known to play an important role in T helper 1 (Th1) cell polarization. IL-18 can also act as a cofactor for T helper 2 (Th2) cell development and immunoglobulin (Ig) E production [6–9]. Many lines of evidence indicate that IL-18 plays an important role in the pathogenesis of inflammatory diseases of the bowel, heart and lung [8–12]. The IL-18R belongs to the IL-1R family , and the IL-18R complex is composed of the IL-18Rα and IL-18Rβ chains. IL-18Rα (IL-1R5 or IL-1R-related protein 1 (IL-1Rrp1)) is the extracellular signaling domain [1, 13], whereas IL-18Rβ (IL-R7 and accessory protein-like (AcPL) or IL-18R accessory protein (IL-18RAP)) is an adapter molecule  in the complex. IL-18Rα alone binds IL-18 with low affinity (dissociation constant (Kd) 20 to 50 nM), and IL-18Rβ alone cannot bind IL-18. However, IL-18Rα can bind with high affinity (Kd 0.3 nM) by recruiting IL-18Rβ. Upon IL-18's binding to the IL-18R complex, IL-18 signaling uses the same adapter molecules (MyD88, TRAF6 and IRAK) as the IL-1 family cytokines and elicits similar responses (activation of NFκB, JNK and p38 MAPK) . Both the IL-18Rα and IL-18Rβ chains are thought to be essential for IL-18-mediated signaling [14, 15]. IL-18 binding protein (IL-18BP) is a soluble protein that binds to IL-18 with high affinity (Kd 0.4 nM) and exerts neutralizing activity against IL-18 [1, 8, 16]. Another member of the IL-1 family, IL-33, binds to the IL-1R family ST2 (IL-33Rα) and IL-1RAcP (IL-33Rβ) complex on cell surfaces and induces the production of Th2 cytokines such as IL-4, IL-5 and IL-13 . Soluble ST2 functions as a soluble decoy receptor for IL-33 and blocks IL-33 signaling . These characteristics suggest that the IL-18Rα or IL-18β chain may have a soluble form.
Recently, the cytokine milieu of the joint in RA patients has become well-understood, and data from human clinical trials are now available. Therapies designed to block the effects of inflammatory cytokine tumor necrosis factor (TNF)-α and the action of the IL-6 receptor (IL-6R) are well-known to be effective in many RA patients. Rheumatoid subcutaneous nodules have the features of Th1 granulomas, with abundant expression of inflammatory cytokines, including interferon (IFN)-γ and IL-18 . The level of IL-18 is reportedly increased in both the serum and rheumatoid synovial fluid, as well as in the bone marrow, of patients with RA, juvenile RA, adult-onset Still's disease and psoriatic arthritis [8, 18, 19]. Moreover, recombinant human IL-18 (rhIL-18) is being actively investigated for its potential efficacy and safety in the treatment of RA. Recent data illustrate the importance of IL-18 in the induction and perpetuation of chronic inflammation in RA patients [9, 19].
As yet, no reported study has focused on the sIL-18Rα chain. In the present study, we attempted to isolate and characterize the human sIL-18Rα complex from human serum. We also found that serum levels of the complex in RA patients were significantly higher than those in healthy controls. Our findings suggest that the sIL-18Rα complex could be a potentially useful biomarker for the diagnosis of RA.
Materials and methods
Characteristics of rheumatoid arthritis (RA) patients and control participantsa
Control (n= 67)
RA (n= 145)
AOSD (n= 6)
OA (n= 31)
SLE (n= 39)
Mean age, yr (± SEM)
60.7 ± 2.1
56.7 ± 1.2
54.3 ± 9.9
76.3 ± 1.5b
38.8 ± 2.1b
Smoking status, n
Joint damagec, n
Functional classificationd, n
Mean DAS28 (CRP) (± SEM)
4.3 ± 0.1
Mean DAS28 (ESR) (± SEM)
4.9 ± 0.1
Mean HAQ (± SEM)
5.8 ± 0.7
Mean WBC (cell count/μL) (± SEM)
7,307.3 ± 211.0
8,950.0 ± 533.1
5,134.7 ± 304.8
Mean CRP level, mg/dL (± SEM)
2.27 ± 0.22
2.15 ± 1.14
0.08 ± 0.02
Mean ESR, mm/hour (± SEM)
45.0 ± 2.7
40.3 ± 18.6
RF-positived, n (%)
Mean RF, U/mL (± SEM)
3.7 ± 1.3
158.0 ± 20.9 b
4.8 ± 2.7
5.4 ± 1.3b
3.8 ± 1.0
CCP-positivee, n (%)
Mean CCP, U/mL (± SEM)
0.3 ± 0.1
261.5 ± 27.2b
22.5 ± 22.3
0.5 ± 0.1b
PSL, n (%)
Mean dose, mg/day (± SEM)
5.91 ± 0.32
MTX, n (%)
Mean dose, mg/week (± SEM)
6.87 ± 0.34
DMARDsf, n (%)
rhIL-18 (catalog no. B003-5) was purchased from MBL (Nagoya, Japan). rhIL-18Rα (IL-1 R5) or Fc chimera (catalog no. 816-LR), mouse antihuman IL-18Rα monoclonal antibody (mAb) (clone 70625; catalog no. MAB840) and mouse antihuman IL-18Rβ mAb (clone 132016, catalog no. MAB1181; and clone 132029, catalog no. MAB118) were purchased from R&D Systems, Inc. (Minneapolis, MN, USA). Rabbit antihuman IL-18Rα polyclonal antibody (pAb)  was kindly provided by Dr Tsukasa Seya (Hokkaido University, Sapporo, Japan). Antihuman-IL-18 mAb (clone 8 (IgG2a)) was kindly provided by Dr Do-Young Yoon (Laboratory of Cellular Biology, Korea Research Institute of Bioscience and Biotechnology, Taejon, Korea ). Antihuman-IL-18Rα mAb (H44 (IgG1)) was established in our laboratory  and is commercially available from BD Pharmingen (San Diego, CA, USA), eBioscience (San Diego, CA, USA), BioLegend (San Diego, CA, USA) and Serotec (Oxford, UK).
Purification of soluble human interleukin-18 receptor α complex from human blood serum
Mouse antihuman IL-18Rα mAb H44 was used on an anti-IL-18Rα mAb affinity column to isolate the complex. The H44 hybridoma cell line was cultured in serum-free medium (GIT; Wako Pure Chemical Industries, Ltd., Osaka, Japan). Proteins in cell-free culture supernatants were precipitated with ammonium sulfate and then further purified using a protein G column (GE Healthcare, Tokyo, Japan) as reported previously . This purified H44 mAb was coupled to a HiTrap NHS-activated HP column (GE Healthcare) in accordance with the manufacturer's protocol. Samples of pooled human blood serum were then applied to this affinity column. Phosphate buffer (10 mM, pH 6.8) was used as the binding buffer, 10 mM phosphate buffer plus 50 mM NaCl (pH 6.8) was used as the washing buffer, 100 mM glycine buffer (pH 2.5) was used as the elution buffer and 1 M phosphate buffer (pH 8.0) was used as the neutralization buffer. All buffers were filtered through a 0.45-μm filter (Millipore, Tokyo, Japan) before the experiments. Serum samples were diluted twofold with the binding buffer, filtered through a 0.45-μm filter (Millipore) and applied to an antihuman IL-18Rα mAb affinity column that had been equilibrated beforehand with the binding buffer. The affinity column was washed with the washing buffer. The elution buffer was then allowed to flow through the column, and every 1-mL sample was collected into a test tube containing 50 μL of neutralization buffer (fractions collected were denoted in numerical order as fractions 1, 2, 3 and so on), with the fractions being monitored with UV radiation at 280 nm. The collected fractions were dialyzed against distilled H2O (Otsuka Pharmaceutical Co., Ltd., Tokushima, Japan) in the presence of 1 mM phenylmethanesulfonyl fluoride (Sigma) at 4°C. Purified hIL-18Rα complex was concentrated using Centricon centrifugal filters (Millipore), and all of the samples and buffers were then kept at 4°C.
Establishment of enzyme-linked immunosorbent assay system for measurement of serum soluble human interleukin-18 receptor α complex
H44 mouse antihuman IL-18Rα primary mAb dissolved at 4 μg/mL in phosphate-buffered saline (PBS) was dispensed into enzyme-linked immunosorbent assay (ELISA) plates (Nunc, Tokyo, Japan) in aliquots of 100 μL/well and left undisturbed overnight at 4°C to allow it to become immobilized. The plates were then washed twice with 200 μL of PBS buffer containing 0.5% Tween 20, and 200 μL/well of 10% Block Ace blocking solution (Dainippon Sumitomo Pharma Co., Ltd., Osaka, Japan) were added and left for at least 2 hours at room temperature to prevent nonspecific adhesion of the secondary antibody to the plates. The plates were then washed again twice with 200 μL of PBS buffer containing 0.5% Tween 20. Human blood serum samples were then aliquoted at 100 μL/well. rhIL-18Rα protein (R&D Systems, Inc.) diluted to 400, 200, 100, 50, 25, 12.5 and 6.25 ng/mL was used as the standard for the human sIL-18Rα complex ELISA system. After 2 hours of incubation at room temperature, each well was washed three times with 200 μl of PBS buffer containing 0.5% Tween 20. Next, 2 μg/mL biotin-labeled rabbit antihuman IL-18Rα secondary pAb was dispensed at 100 μL/well, followed by incubation for 90 minutes at room temperature, and then each well was washed four times with 200 μl of PBS buffer containing 0.5% Tween 20. This was followed by addition of 100 μL of 0.5 μg/mL streptavidin-bound horseradish peroxidase (Upstate, Tokyo, Japan) to each well, and the plates were left undisturbed for 30 minutes at room temperature. Each well was then washed five times with 200 μL of PBS buffer containing 0.5% Tween 20. 2,2'-Azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium salt (ELISA POD Substrate A.B.T.S. kit; Nakarai, Kyoto, Japan) was then added at 100 μl/well, and the plates were left undisturbed for 30 minutes at room temperature, followed by addition of a stop solution at 100 μL/well to stop the enzyme reaction. The amounts of the sIL-18Rα complex protein were determined by measuring the absorbance at 450 nm in comparison with the standard rhIL-18Rα protein sample. The limit of sensitivity of this ELISA system was <5 ng/mL.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis
SDS-polyacrylamide gel electrophoresis (PAGE) was performed using premade 10% to 20% or 15% to 25% polyacrylamide gel (Multigel II Mini; Dai-ichi Kagaku Yakuhin, Tokyo, Japan). The gel was stained with Coomassie Brilliant Blue (CBB) or assessed using Western blot analysis as described in our previous reports [30, 32]. MagicMark™ Western Standard (Invitrogen, Carlsbad, CA, USA) was used for protein band standardization.
Cytokine enzyme-linked immunosorbent assays
Levels of mature IL-18, IL-13 and IFN-γ in serum were measured using ELISA kits (IL-18: MBL, Nagoya, Japan; IL-13 and IFN-γ: R&D Systems, Inc.). The limits of sensitivity of these ELISA kits were 12.5 pg/mL, 32 pg/mL and 8 pg/mL, respectively.
Flow cytometric analysis
The purified sIL-18Rα complex protein and H44 mAb were labeled with biotin as reported previously . Cells were incubated with 2 μg of biotinylated control mouse IgG1 (Caltag, Burlingame, CA, USA), 2 μg of biotinylated anti-IL-18Rα mAb H44 (mouse IgG1) or 10 μg of biotinylated anti-IL-18Rα complex protein at 4°C, then washed with PBS and incubated with streptavidin-PE (BD Pharmingen), followed by flow cytometric analysis as reported previously .
Interferon-γ production inhibition assay using a human interleukin-18-responsive natural killer cell line
We used the hIL-18-responsive human natural killer (NK) cell line NK0, a subclone of the original NK92 cell line. Using NK0 cells, we performed IFN-γ production inhibition assay in vitro as reported previously [29, 30]. Briefly, NK0 cells were washed three times and cultured in RPMI 1640 medium with 10% fetal calf serum (FCS) for 18 hours. The cells were then washed once, suspended at 5 × 105 cells/mL in RPMI 1640 medium with 10% FCS and pretreated with sIL-18Rα complex (0.01 to 30 μg/mL) for 10 minutes at room temperature. The cells were then treated with rhIL-2 (50 IU/mL) plus rhIL-18 (50 ng/mL) for 18 hours. Production of human IFN-γ in the supernatants was analyzed using ELISA kits (R&D Systems, Inc.).
Data are presented as means ± standard errors of the mean (SEM). Differences between the RA patients and the controls were analyzed by using the Wilcoxon rank-sum test. The sensitivity and specificity for the diagnosis of RA were analyzed using area under the receiver operating characteristic curve (ROC-AUC) analysis generated by logistic regression as reported previously . Statistical analysis was performed using the JMP 7.0.1 software package (SAS Institute, Japan, Tokyo, Japan). Differences were considered significant at P < 0.05.
Isolation and characterization of soluble human interleukin-18 receptor α complex from human sera
We performed Western blot analysis to confirm that the IL-18Rα protein was present in fraction 3. This step showed that anti-IL-18Rα mAb clone 70625 detected a band of approximately 60 kDa among the rhIL-18Rα/Fc chimera proteins (lane 1 in Figure 1B). The antibody detected bands of approximately 50 and 30 kDa in the isolated sIL-18Rα complex (lane 2 in Figure 1B), suggesting that the complex had at least two different sIL-18Rα proteins. Upon IL-18 binding, the IL-18Rα chain forms a complex with the IL-18Rβ chain on cell surfaces . Therefore, we hypothesized that the sIL-18Rα complex associates with IL-18 and sIL-18Rβ proteins, and we investigated this possibility using Western blot analysis. The anti-IL-18 mAb (clone 8) detected a strong band of approximately 30 kDa and a weak band of 50 kDa (Figure 1C). As the IL-18 monomer has a calculated molecular mass of 14 kDa , IL-18 associated with the IL-18Rα complex could be dimeric. The anti-IL-18Rβ mAb (clone 132016) detected bands of approximately 60, 40 and 18 kDa in the isolated sIL-18Rα complex (Figure 1D). Furthermore, a hIL-18 ELISA kit detected IL-18 protein in the isolated sIL-18Rα complex (data not shown). These results suggest that the human sIL-18Rα complex might be associated with dimeric IL-18 and the sIL-18Rβ chain.
Ability of the soluble interleukin-18 receptor α complex to bind to the surface of a natural killer cell line
Soluble interleukin-18 receptor α complex prevents interferon-γ production by a human natural killer cell line
We examined whether the isolated IL-18Rα complex protein was able to prevent or increase the production of IFN-γ by NK0 cells stimulated in vitro with IL-2 or IL-18. We reported previously that rhIL-18 proteins synergistically increased the production rhIL-2-induced IFN-γ by NK0 cells . Here we found that the sIL-18Rα complex protein alone did not induce IFN-γ production by NK0 cells and did not synergistically induce IFN-γ production by NK0 cells stimulated with IL-2 alone or IL-18 alone (data not shown). Therefore, we examined whether the sIL-18Rα complex would be able to prevent IFN-γ production by NK0 cells stimulated in vitro with IL-2 or IL-18. Interestingly, IFN-γ production by NK0 cells stimulated with IL-2 or IL-18 was dose-dependently inhibited by the sIL-18Rα complex (0.003 to 10 μg/mL; approximately 70% to 80% inhibition at 10 μg/mL) (Figure 2B). Thus, the sIL-18Rα complex exhibited antagonistic, but not agonistic, activity. In addition, treatment with rhIL-18Rα protein (0.1, 1 and 10 μg/mL) inhibited the production of IFN-γ by NK0 cells stimulated with the combination of rhIL-2 (50 IU/mL) and rhIL-18 (50 ng/mL) by 0%, 2% and 34%, respectively (data not shown). These results show that the inhibitory effect of the sIL-18Rα complex on IFN-γ production by NK0 cells stimulated in vitro with IL-18 or IL-2 is much stronger than that of rhIL-18Rα protein.
Increased levels of soluble interleukin-18 receptor α and inflammatory cytokines in sera of rheumatoid arthritis patients
Tumor necrosis factor inhibitors decreased levels of soluble interleukin-18 receptor α in sera of rheumatoid arthritis patients
In this study, we have demonstrated the presence of the sIL-18Rα chain in human serum. The sIL-18Rα complex is composed of the soluble forms of the IL-18Rα and IL-18Rβ chains and IL-18. The complex was shown to bind to the surfaces of the human NK cell line NK0 and prevented the production of IFN-γ by NK0 stimulated with IL-18 and IL-2 in vitro. Thus our results suggest that the sIL-18Rα complex binds to the cell surface and attenuates IL-18 signaling.
A previous study showed that the molecular mass of IL-18Rα protein isolated from the Hodgkin's disease cell line L428 was 64 to 100 kDa. The DNA sequence of hIL-18Rα encodes the signal peptide (Met 1 to Ala 19) domains 1 through 3, to which IL-18 can bind (Glu 20 to Arg 329), and the transmembrane domain (Gly 330 to Tyr 351) . The predicted molecular mass of the extracellular domain of the hIL-18Rα chain is approximately 36 kDa. These results suggest that hIL-18Rα protein on cell surfaces is highly glycosylated and shows sugar chain heterogeneity . Our SDS-PAGE analysis using CBB staining showed that the sIL-18Rα complex comprised a 50-kDa major band and other bands of approximately 60 to 80, 30 and 15 kDa. Western blot analysis showed that the IL-18Rα protein associated with the IL-18Rα complex had at least two components of approximately 50 and 30 kDa. As our anti-IL-18Rα mAb (H44) recognizes the extracellular domain of hIL-18Rα protein , the IL-18Rα protein associated with the IL-18Rα complex would have been derived from the extracellular domain of IL-18Rα. However, the molecular mass of the sIL-18Rα protein associated with the IL-18Rα complex was much smaller than that of the extracellular domain of hIL-18Rα as reported previously on the cell surface [13, 30]. Therefore, the sIL-18Rα protein appears to be glycosylated or shows heterogeneity of its sugar chains.
Soluble cytokine receptors of the IL-1R/Toll-like receptor superfamily are thought to be generated by intramembrane proteolysis and/or alternative splicing. sIL-1RI is generated by the action of metalloprotease . sIL-1RII can be generated by both proteolytic cleavage of receptor ectodomains and alternative splicing events. β-secretase 1 and β-secretase 2 can function as IL-1RII sheddases that cleave the IL-1RII ectodomain at a site adjacent to the α-secretase site [2, 36]. TNF-α-converting enzyme (ADAM17), a transmembrane metalloprotease, is also responsible for the proteolytic release or shedding of IL-1RII . It has been reported that sIL-1RAcP is generated by alternative splicing rather than by ectodomain cleavage . Therefore, the sIL-18Rα chain seems to be generated by intramembrane proteolysis and/or alternative splicing events.
IL-18Rβ shares structural similarities with IL-1RAcP . Like sIL-1RAcP, a short form of the IL-18Rβ (sIL-18Rβ mRNA transcript has been described in the rat , as well as in human and mouse . A recent study showed that intravenous administration of adenoviruses encoding sIL-18Rβ promoted collagen-induced arthritis in DBA/1 mice . However, the biological function of sIL-18Rβ is largely unknown. In the present study, we have shown that the sIL-18Rα complex comprises IL-18 and sIL-18Rα and sIL-18Rβ. Two different anti-IL-18Rβ mAb clones, 132016 and 132029 (R&D Systems, Inc.), detected bands of approximately 60, 40 and 18 kDa in the isolated sIL-18Rα complex (data not shown). These two anti-IL-18Rβ mAbs recognize the extracellular domain of the hIL-18Rβ chain. Therefore, sIL-18Rβ in human serum is likely derived from the extracellular domain of the hIL-18Rβ chain generated by differential messenger RNA splicing.
It has been reported that the levels of various inflammatory cytokines and chemokines, such as IL-2, IL-4, IL-5, IL-6, IL-7, IL-10, IL-13, IFN-γ, granulocyte colony-stimulating factor, granulocyte macrophage colony-stimulating factor, monocyte chemotactic protein-1 and macrophage inflammatory protein-1β, are increased in the sera of RA patients . Inflammatory cytokines and chemokines, including IL-18, IL-13 and IFN-γ, may play an important role in the pathogenesis and development of RA [8, 9]. In this study, we have shown that the serum levels of the IL-18Rα complex in patients with RA or adult-onset Still's disease were significantly higher than those in healthy controls as well as in patients with OA or SLE. However, the origin of the IL-18Rα complex is still unclear. Therefore, we analyzed the levels of the sIL-18Rα complex in both the synovial fluid and sera in three RA patients (26-, 39- and 81-year-old females). The levels of sIL-18Rα complex in the synovial fluid in these RA patients were 22.1, 30.8 and 6.9 ng/mL, respectively. The levels of sIL-18Rα complex in their sera were 39.8, 23.6 and 16.2 ng/mL, respectively. Thus, the levels of sIL-18Rα complex were not greatly increased in the synovial fluid in these three RA patients. Further analysis is needed to address the origin of the IL-18Rα complex in RA patients.
ROC-AUC analysis revealed that RA patients could be discriminated by the serum level of the sIL-18Rα complex as determined by ELISA. Moreover, the TNF inhibitor etanercept significantly decreased levels of sIL-18Rα in the sera of 29 RA patients 6 months after the treatment. Thus, the sIL-18Rα complex may play an important role in the inflammatory process of RA and adult-onset Still's disease. Our present results suggest that the level of the sIL-18Rα complex in serum has potential clinical application as a biomarker for the diagnosis or differential diagnosis of RA or for the evaluation of disease activity in RA patients treated with TNF inhibitors.
In this study, we evaluated the levels and characteristics of the sIL-18Rα complex in the sera of patients with RA and compared these to control populations. Our study indicates that sIL-18Rα is present in the sera of RA patients and is complexed with IL-18 and IL-18Rβ. The levels detected by ELISA were substantially higher in the RA patients and patients with adult-onset Still's disease than in controls, and modeling using ROC-AUC analysis suggested that this assay might be of diagnostic value.
Coomassie Brilliant Blue
IL-1 receptor accessory protein
interleukin 1-associated kinase
polyacrylamide gel electrophoresis
area under the receiver operating characteristic curve
systemic lupus erythematosus
tumor necrosis factor.
The authors thank Dr Howard A Young (National Cancer Institute, Frederick, MD, USA) for editing; Drs J Itoh (Chikugogawa-onsen Hospital, Ukiha, Japan), M Kawahara, H Koga (Keishinkai Hospital, Tosu, Japan) and Y Ohkubo (Chiwata Hospital, Nagasaki, Japan) for providing samples; Dr Tsukasa Seya (Hokkaido University, Sapporo, Japan) for providing rabbit antihuman IL-18Rα polyclonal antibody; and Dr Do-Young Yoon (Laboratory of Cellular Biology, Korea Research Institute of Bioscience and Biotechnology, Taejon, Korea) for providing antihuman IL-18 monoclonal antibody (clone 8). We also thank Ms Emiko Kuma, Ms Chie Ohki and Ms Kyoko Yamaguchi (Kurume University, Fukuoka Japan) for their technical assistance. This work was supported by a Grant-in-Aid for Scientific Research (C) (no. 21590977) (to TH) from the Ministry of Education, Science, Sports, and Culture of Japan; a grant to the Respiratory Failure Research Group from the Ministry of Health, Labor and Welfare, Japan (to HA); and by a grant from the Okamoto Satoshi Memorial Fund for Pulmonary Fibrosis Research (Tokyo, Japan) (to TH).
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