Background
Rheumatoid arthritis (RA) is an autoimmune disease characterized by the inflammation and destruction of joints and surrounding tissues [
1]. Most patients with RA are positive for anticitrullinated protein antibodies (ACPA), and these antibodies are highly specific for RA diagnosis [
2]. ACPA appear years before the onset of clinical RA and are the predictive factor of joint radiographic progression [
2]. In the mouse model, it was reported that ACPA appeared prior to the development of clinical disease in collagen-induced arthritis, and administration of anticitrullinated fibrinogen antibodies enhanced arthritis [
3]. Thus, ACPA is considered to be involved in the pathogenesis of RA.
ACPA recognize various citrullinated proteins, such as vimentin, α-enolase, fibrinogen, fibronectin, and glucose-6-phosphate isomerase (GPI) [
2,
4]. High levels of citrullinated proteins have been found in RA joints [
5,
6]. Also, citrulline-specific T cells were increased in peripheral blood of patients with RA and decreased by therapy [
7]. In murine models, immunization of citrullinated fibrinogen induced arthritis in HLA-DRB1*0401-transgenic mice, but unmodified fibrinogen did not [
8]. However, we do not know what citrullinated antigens specifically revealed in joints and sera in arthritis.
GPI is a ubiquitous glycolytic enzyme and was identified as a pathogenic autoantigen in K/BxN arthritic mice [
9]. Immunization with recombinant human GPI could induce arthritis in DBA/1 mice [
10]. A major epitope of GPI-specific CD4
+ T cells in glucose-6-phosphate isomerase-induced arthritis (GIA) was identified as peptide 325–339 glucose-6-phosphate isomerase (pGPI), and immunization with pGPI can also induce arthritis just as GIA does in DBA/1 mice (peptide 325–339 glucose-6-phosphate isomerase-induced arthritis [pGIA]) [
11]. CD4
+ T cells play a critical role in GIA [
10,
12], and the effect of biological agents in GIA is similar to RA [
12,
13]. However, the involvement of ACPA and citrullinated proteins have not investigated in this model.
In the present study, we explored the expression and commonality of citrullinated proteins in pGIA and patients with RA, and we further investigated its correlation with RA disease activity. We show the production of ACPA, and we detected citrullinated proteins in arthritic joints and sera in pGIA. Serum citrullinated protein in pGIA was identified as citrullinated inter-alpha-trypsin inhibitor heavy chain 4 (ITIH4). The expression of ITIH4 was increased in inflamed synovium at day 14 in pGIA, suggesting the association with arthritis. In addition, bands of the same size were also detected in sera of patients with RA by Western blotting, which were confirmed as citrullinated ITIH4 as well. The levels of citrullinated ITIH4 were specifically increased in sera of patients with RA and were clearly associated with RA disease activity. Moreover, antibody response to citrullinated epitope in ITIH4 was observed in patients with RA. Citrullinated ITIH4 could be a novel specific biomarker representing the disease activity of patients with RA.
Methods
Mice
Male DBA/1 mice were purchased from Charles River Japan (Tokyo, Japan), and used at 6–10 weeks of age. All mice were maintained under specific pathogen-free conditions at the University of Tsukuba. All experimental protocols were approved by the Institutional Animal Care and Use Committee of the University of Tsukuba, and all animal experiments were conducted in accordance with institutional ethics guidelines. All surgeries were performed under isoflurane anesthesia, and utmost care was taken to minimize suffering.
Serum samples
Serum samples were collected from Japanese patients with RA (
n = 60, mean age 52.2 years, range 20–73 years, females 80%) diagnosed by rheumatologists according to the 1987 American College of Rheumatology (ACR) classification criteria [
14] or the 2010 ACR/European League Against Rheumatism classification criteria [
15]. Serum samples of the disease control subjects were collected from Japanese patients with primary Sjögren’s syndrome (SS) (
n = 27, mean age 58.8 years, range 26–80 years, females 96%), systemic lupus erythematosus (SLE) (
n = 15, mean age 33.9 years, range 16–55 years, females 80%), or osteoarthritis (OA) (
n = 12, mean age 56.3 years, range 37–80 years, females 67%). All the patients with SS were diagnosed by rheumatologists according to the 1999 Japanese Ministry of Health criteria for diagnosis of SS [
16]. All the patients with SLE fulfilled the 1997 ACR classification criteria [
17]. None of the patients with SS or SLE had overlapping RA. Serum samples were collected from healthy subjects (HS) (
n = 30, mean age 49.0 years, range 34–65 years, females 80%). Serum samples were also collected from 17 patients with RA before and 24 weeks after treatment with biologic drugs (infliximab,
n = 9; abatacept,
n = 8). All samples were collected at the University of Tsukuba Hospital after informed consent was obtained. This study was reviewed and approved by the ethics committee of the University of Tsukuba.
Peptide GPI-induced arthritis
DBA/1 mice were immunized with 25 μg of pGPI (Invitrogen/Thermo Fisher Scientific, Carlsbad, CA, USA) in complete Freund’s adjuvant (CFA) (BD Biosciences, San Jose, CA, USA). pGPI was emulsified with CFA at a 1:1 ratio (vol/vol), or PBS + CFA was prepared as a vehicle control. For induction of arthritis, 150 μl of the emulsion was injected intradermally at the base of the tails of the mice. Each mouse was also given an injection of 200 ng of pertussis toxin (Sigma-Aldrich, St. Louis, MO, USA) intraperitoneally on days 0 and 2 after immunization to induce arthritis. Arthritis was assessed every other day and evaluated using a scale of 0–3 for swelling and redness of each paw. The clinical score was the sum of the scores for four paws, as described previously [
11].
Measurement of anti-CCP antibodies in pGIA
Sera were obtained on days 0–28 every week from mice immunized with pGPI or control, and antibodies were measured by enzyme-linked immunosorbent assay (ELISA). Sera were diluted 1:25 in dilution buffer and added to the 96-well plate (Immunoscan CCPlus test kit; Euro Diagnostica, Malmö, Sweden) for 1 h at room temperature. After a washing step, horseradish peroxidase (HRP)-conjugated polyclonal rabbit antimouse immunoglobulin (Dako, Carpinteria, CA, USA) diluted 1:1000 was added for 30 minutes at room temperature. After another washing step, color was developed with 3,3′,5,5′-tetramethylbenzidine (TMB) microwell peroxidase substrate (KPL/SeraCare, Milford, MA, USA). The optical density (OD) was measured at 450 nm by using a microplate reader.
A standard pool was obtained by mixing sera obtained from several mice on day 28. The concentrations of antibodies in this pool were considered 100 U/ml. A standard curve was obtained using serum dilutions, and the Michaelis-Menten equation was used to convert OD values into units, as described previously [
18].
Measurement of anti-ITIH4 antibodies in patients with RA
Native peptide ITIH4428–447 and R438 citrullinated peptide ITIH4428–447 were synthesized (serum, purity 95%) and used in ELISA. Ninety-six-well plates (Nunc MaxiSorp; Thermo Fisher Scientific) were coated with 10 μg/ml peptides for 12 h at 4 °C. After washing and blocking steps, sera from patients with RA (n = 60) and HS (n = 30) were diluted 1:200 in 1% bovine serum albumin (BSA) in PBS, then added for 2 h at room temperature. After a washing step, HRP-conjugated goat antihuman immunoglobulin G (IgG) (heavy and light chains [H + L]) (Abcam, Cambridge, MA, USA) diluted 1:10,000 was added for 1 h at room temperature. After washing, color was developed with TMB microwell peroxidase substrate (KPL/SeraCare). The OD was measured at 450 nm by using a microplate reader. The cutoff value was determined as the mean + 2 SD of HS.
Histopathological analysis
Ankle joint tissue samples were harvested at days 0 and 14 from mice immunized with pGPI or control. They were fixed in neutralized 10% formalin, embedded in paraffin, and sectioned. To detect citrullinated proteins, we prepared modification buffer by mixing Reagent A (20% H
2SO
4, 25% H
3PO
4, and 0.025% FeCl
3) and Reagent B (1% diacetyl monoxime, 0.5% antipyrine, 1 M acetic acid) at a 2:1 ratio (vol/vol). The sections were covered with the modification buffer and incubated in a light-proof container at 37 °C for 2.5 h to modify citrulline residues. Then, the sections were incubated overnight at room temperature with rabbit anti-modified citrulline (AMC) polyclonal antibodies diluted 1:3200 in 2% BSA in PBS to detect modified citrulline residues [
19]. The sections were also incubated with HRP-conjugated goat antirabbit IgG (H + L) (Bio-Rad Laboratories, Hercules, CA, USA) for 30 minutes at room temperature. The sections were also stained with 3,3′-diaminobenzidine (DAB) (Nichirei Biosciences, Tokyo, Japan) and hematoxylin. To detect ITIH4 or macrophages, IHC analysis of the joint or liver (as a positive control of ITIH4 staining) sections was performed as described above, using rabbit antihuman ITIH4 antibodies (Abcam) diluted 1:1000 and rat antimouse F4/80 antibodies (BioLegend, San Diego, CA, USA) diluted 1:200 as primary antibodies and using rabbit-specific IHC polymer detection kit HRP/DAB (Abcam) and HRP-conjugated polyclonal rabbit antirat immunoglobulin (Dako) diluted 1:200 as secondary antibodies, respectively.
Real-time qPCR analysis
We extracted total RNA from the ankle joints of pGIA or control mice using the ISOGEN (Wako Pure Chemical Industries, Tokyo, Japan) extraction method according to the instructions provided by the manufacturer. The extracted RNA was reverse-transcribed to complementary DNA with random primers. We performed real-time qPCR using a TaqMan gene expression assay (Applied Biosystems/Thermo Fisher Scientific, Foster City, CA, USA), and the Padi4 (Mm01341658_m1) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) (NM_002046) primers. Real-time qPCR was carried out using an ABI 7500 analyzer (Applied Biosystems). The expression of GAPDH was used as the control.
Western blot analysis
Serum samples were obtained at days 0, 7, 14, and 28 from mice immunized with pGPI or control, as well as from patients with RA, SS, SLE, or OA and from HS. For analysis of citrullinated protein expression, serum samples were loaded into each well (50 μg for SDS-PAGE or 100 μg for two-dimensional PAGE [2D-PAGE]), separated by SDS-PAGE or 2D-PAGE, and transferred to polyvinylidene difluoride membranes. The modification buffer was added to the blots before incubation in a light-proof container at 37 °C for 2.5 h to modify citrulline residues, as described above. Blots were washed with 0.05% Tween 20 in Tris-buffered saline (TBST) and blocked with 5% milk in TBST, then incubated overnight at 4 °C with AMC antibodies diluted 1:3200 in 5% milk in TBST [
19]. After a washing step, the blots were incubated with secondary antibody HRP-conjugated goat antirabbit IgG (H + L) (Bio-Rad Laboratories) diluted 1:5000 in 5% milk in TBST for 1 h at room temperature. Densitometric analysis was carried out using an ImageQuant LAS 4000 densitometer (GE Healthcare Life Sciences, Marlborough, MA, USA). The band intensity was determined with ImageQuant TL software (GE Healthcare Life Sciences) and normalized using the value of one sample (pGIA or RA) as 10. The difference of citrullinated ITIH4 levels from baseline to 24 weeks after treatment was normalized using each value at baseline. For analysis of ITIH4 expression, serum samples were loaded 10 μg/well and Western blot analysis was performed as described above, using rabbit antihuman ITIH4 antibodies (Abcam) diluted 1:3000.
Measurement of citrulline content
Sera were obtained at days 0, 7, 14, and 28 from mice immunized with pGPI or control, and citrulline contents were measured by using the color development reagent (COLDER) assay, as described previously [
20]. Sera were diluted 1:100 in COLDER buffer (50 mM NaCl, 10 mM CaCl
2, 2 mM dithiothreitol, and 100 mM Tris, pH 7.4), and 60 μl of samples were added to the 96-well plates (Nunc MaxiSorp). Then, 200 μl of COLDER were added to detect citrullinated proteins. The samples were incubated for 30 minutes at 95 °C. The OD was measured at 570 nm by using a microplate reader and compared with a standard curve of known citrulline concentrations. Data were normalized relative to the protein concentration in each sample.
LC-MS
Sera from pGIA mice, control mice, patients with RA, and HS were separated by 2D-PAGE, and gels were stained with GelCode Blue Stain Reagent (Thermo Fisher Scientific) for Coomassie brilliant blue staining. Gel slices were incubated overnight with MS-grade modified trypsin (6.7 ng/μl; Promega, Madison, WI, USA) at 37 °C. The resultant peptides were analyzed with the nanoACQUITY ultrahigh-performance LC (UPLC) system (Waters, Milford, MA, USA). Data were collected in centroid mode from mass-to-charge ratios (m/z) 50 to 1990. All analyses were acquired with an independent reference. BiopharmLynx version 1.2 software (Waters) was used for baseline subtraction and smoothing, de-isotoping, de novo peptide sequence identification, and database searches.
Statistical analysis
All data were expressed as mean ± SEM. Differences between groups were evaluated for statistical significance using Student’s t test. The Kruskal-Wallis test was used for evaluation of band intensity among the five subject groups. P values less than 0.05 were considered significant. Statistical analyses were performed using IBM SPSS Statistics software (IBM, Armonk, NY, USA).
Discussion
To our knowledge, this is the first report on high levels of citrullinated ITIH4 in arthritis. ITIH4 is a heavy-chain protein, and ITIHs form inter-alpha-trypsin inhibitors (ITIs) by binding to a common light chain [
22,
23]. Members of the ITI family are recruited to extravascular sites, where ITIHs are converted to hyaluronan (HA), to form serum-derived HA-associated protein (SHAP)-HA complexes in HA-rich tissues such as synovial fluid [
22,
23]. Researchers in previous studies reported high levels of SHAP-HA complexes in RA serum and synovial fluid, suggesting their association with inflammation through the regulation of adhesion of infiltrating leukocytes [
24]. In particular, ITIH4 was identified as one of various citrullinated proteins in RA synovium [
6]. In addition, human ITIHs are reported to inhibit the complement system, suggesting a link to the immunocomplex cascades [
25]. More recently, high serum levels of anticitrullinated ITIH3 antibodies were found in patients with RA [
26].
In the present study, we show that citrullinated ITIH4 was recognized with high specificity in patients with RA as compared with patients with other autoimmune and arthritic diseases or in HS, indicating a potential role for citrullinated ITIH4 in RA pathogenesis. Additionally, in the investigation of autoantigenicity of citrullinated ITIH4, higher levels of anticitrullinated pITIH4 antibody were observed in patients with RA than in HS, whereas there was no significant difference in the levels of antinative pITIH4 antibody. These results suggest that citrullinated ITIH4 may be one of the candidate targets of ACPA and may be involved in the inflammatory response by forming immune complexes, which play an important role in the pathophysiology of RA [
27]. ITIH4 has been reported to have an anti-inflammatory role, such as inhibition of the phagocytosis of polymorphonuclear cells [
28]. Regarding the pathogenetic role of citrullinated ITIH4 in RA, another possibility is that citrullination of ITIH4 might suppress the anti-inflammatory effect of ITIH4 in situ, because the biologic activity of many proteins will be affected by their citrullination. Further studies are needed to evaluate the precise role of citrullinated ITIH4 in RA pathogenesis.
Which cells express ITIH4, and how and where is ITIH4 citrullinated? In this study, ITIH4 and macrophages colocalized in some areas in inflamed synovium of pGIA. These results suggest that synovial macrophages may express ITIH4 in pGIA, but there is a limitation of comparing serial sections by IHC. In addition, ITIH4 was also found in synovial or cartilage surface layers in pGIA, suggesting the presence of ITIH4 in the synovial fluid. It is possible that circulating ITIH4 was recruited into synovial fluid, because the ITI family was synthesized by hepatocytes and recruited to extravascular sites such as the synovial fluid [
22,
23], and ITIH4 and citrullinated ITIH4 were also detected in RA synovial fluid (our preliminary observations), supporting the possibility of this scenario. In previous reports, peptidyl arginine deiminase 2 (PAD2) and PAD4 enzymes were increased in RA synovium or synovial fluid, and they contributed to protein citrullination in arthritic joints [
5,
29,
30]. In this study, we show higher levels of Padi4 in the arthritic joints of pGIA, as well as the expression of ITIH4 and citrullinated ITIH4 in inflamed synovium of pGIA. Taking these findings together, we speculate that citrullination of ITIH4 was induced by aberrant PAD activity in arthritic joints and leaked into the blood circulation.
In this study, on one hand, serum citrullinated ITIH4 levels varied in relation to the clinical findings of pGIA, increasing at day 14 then falling at day 28. On the other hand, ACPA titers were significantly higher after day 21, despite the gradual resolution of arthritis. Furthermore, our preliminary studies showed that treatment with Cl-amidine (PAD inhibitor) resulted in suppression of arthritis coupled with a significant decrease in serum citrullinated ITIH4 levels, although there was no significant decrease in ACPA titers in pGIA. In patients with RA, the levels of CRP, RF, and DAS28-CRP were significantly higher in patients with RA positive for serum citrullinated ITIH4. In addition, basal serum citrullinated ITIH4 levels were significantly correlated with disease activity markers in patients with active RA and decreased after effective treatment in correlation with the disease activity. These results suggest that citrullinated ITIH4, not ACPA, seems to fluctuate in parallel with the severity of arthritis and could be a disease-specific biomarker representing the disease activity of patients with RA.
We observed that there were several citrullinated sites in ITIH4, and those sites were not identical between pGIA and patients with RA, and they were different between individuals. However, the R438 site was commonly citrullinated in our analyzed samples. Thus, this position is considered as a dominant citrullinated site. We show that the antibody reactivities against ITIH4428–447 peptides containing citrullinated R438 were significantly higher in patients with RA than in HS, whereas there was no significant difference in the reactivities against native ITIH4428–447 peptides containing arginine. Furthermore, the levels of anticitrullinated pITIH4 antibody tended to be higher than those of native pITIH4 in patients with RA, although there was no significant difference. There is a possibility that this citrullinated epitope in ITIH4 may be one of the targets of the immune system in RA. Nonetheless, the reactivities against the synthesized linear citrullinated peptides do not necessarily completely reflect the reactivities against the corresponding citrullinated proteins in vivo, and there is also a possibility of antibodies cross-reacting. It is necessary to verify in future studies the antibody reactivities against whole citrullinated ITIH4 or circulating citrullinated epitopes at several other sites.