Osteoarthritis (OA) is the most common degenerative disorder of the joint, which is characterized by articular cartilage destruction, subchondral sclerosis, and synovitis [
Biochemical markers show promise in the evaluation of the severity of the disease in addition to monitoring the efficacy and safety of disease-modifying OA drugs [
Interleukin-34 (IL-34) is a novel cytokine identified by Lin et al. in 2008 [
From August 2015 to May 2017, a total of 182 knee OA patients from Renji Hospital were invited to enroll in our study. The diagnosis of knee OA was determined according to the clinical and radiological criteria of the American College of Rheumatology [
Blood samples from all patients and controls were collected after overnight fast in plain tubes containing a separation gel. SF samples were obtained from the affected knee of OA patients before the treatment of hyaluronic acid injection or during the arthroscopy or surgery. No SF samples were collected from the controls for ethical concerns. Samples were collected into sodium heparin Vacutainer tubes (Becton Dickinson). Blood and SF samples were centrifuged and stored at −80°C until investigation. High-sensitivity CRP (hs-CRP) levels were measured in a Tecan Freedom EVOlyzer Automatic Biochemical Analyzer System (Tecan, Switzerland). IL-34 levels in serum and SF were determined using the commercial enzyme-linked immunosorbent assay (ELISA) kit (R&D Systems, Minneapolis, MN, USA) according to the manufacturer’s instructions. All the samples were synchronously and randomly detected by different ELISA kits. All the results of different kits were distributed similarly. According to the manufacturer, the intra-assay CV was 1.8% to 7.3% and the interassay CV was 4.1% to 6.0%. All measurements were taken in duplicate for each sample, and the results were averaged.
All patients underwent weight-bearing anteroposterior radiographs of the affected knee. The Kellgren-Lawrence (KL) grading system was used for classifying radiographic signs: grade 1, questionable narrowing of joint space and possible osteophytic lipping; grade 2, definite osteophytes and possible narrowing of joint space; grade3, moderate multiple osteophytes, definite narrowing of joint space, some sclerosis, and possible deformity of bone contour; and grade 4, large osteophytes, marked narrowing of joint space, severe sclerosis, and definite deformity of bone contour [
The symptomatic disease severity was evaluated according to Western Ontario McMaster University Osteoarthritis Index (WOMAC) [
Continuous data were presented as mean and standard deviation. Categorical variables were summarized as percentages. Normality assessment was performed with the Shapiro-Wilk test. A comparison of two independent groups was performed with the unpaired
The baseline clinical characteristics of the subjects were shown in Table
Baseline clinical characteristics and IL-34 levels.
Controls ( |
OA patients total ( |
KL grade 2 ( |
KL grade 3 ( |
KL grade 4 ( | |
---|---|---|---|---|---|
Age (years) | 64 (56–74) | 67 (59–73) | 66 (58–74) | 68 (61–72) | 66 (61–72) |
Female, |
44 (68.75%) | 122 (67.03%) | 47 (71.21%) | 42 (61.76%) | 33 (70.21%) |
BMI (kg/mm2) | 22.96 ± 2.29 | 23.30 ± 2.11 | 23.21 ± 2.08 | 23.25 ± 2.21 | 23.50 ± 2.07 |
WOMAC scores | 44 (33–57) | 34 (23–45) | 46 (36–57) | 57 (44–65) | |
Serum IL-34 levels (pg/ml) | 129.87 (88.66–214.49) | 121.35 (86.84–177.70) | 107.50 (80.19–195.32) | 128.33 (98.06–196.94) | 121.25 (91.01–172.32) |
SF IL-34 levels (pg/ml) | 222.55 (126.68–319.75) | 154.10 (117.71–237.07) | 226.44 (147.23–329.63) | 256.18 (212.09–432.08) |
All values are expressed as the percentages, median (interquartile range), or mean ± SD. IL-34: interleukin-34; OA: osteoarthritis; KL grade: Kellgren-Lawrence grade; BMI: body mass index; WOMAC: Western Ontario McMaster University Osteoarthritis Index; SF: synovial fluid.
As shown in Table
As shown in Table
Linear regression for the association between variables and serum IL-34 levels.
Variables | Univariant | Multivariant | ||
---|---|---|---|---|
Age | −0.004 (−0.011–0.003) | 0.295 | −0.013 (−0.026–0.001) | 0.064 |
Sex | −0.042 (−0.192–0.108) | 0.583 | −0.099 (−0.291–0.093) | 0.311 |
BMI | 0.011 (−0.022–0.044) | 0.505 | 0.052 (−0.004–0.108) | 0.066 |
SF IL-34 levels | <0.001 (−0.001–0.001) | 0.411 | <0.001 (−0.001–0.001) | 0.684 |
KL grade | 0.034 (−0.072–0.140) | 0.528 | 0.019 (−0.106–0.144) | 0.306 |
WOMAC scores | 0.002 (−0.003–0.007) | 0.425 | 0.002 (−0.004–0.008) | 0.529 |
CI = confidence interval. Other abbreviations are as in Table
Linear regression for the association between variables and SF IL-34 levels.
Variables | Univariant | Multivariant | ||
---|---|---|---|---|
Age | −0.008 (−0.018–0.002) | 0.117 | −0.010 (−0.023–0.002) | 0.112 |
Sex | −0.050 (−0.256–0.155) | 0.631 | −0.080 (−0.279–0.119) | 0.427 |
BMI | −0.022 (−0.068–0.024) | 0.341 | −0.003 (−0.062–0.055) | 0.910 |
Serum IL-34 levels | <0.001 (−0.001–0.001) | 0.667 | <0.001 (−0.001–0.001) | 0.480 |
KL grade | 0.322 (0.212–0.432) | <0.001 | 0.273 (0.150–0.395) | <0.001 |
WOMAC scores | 0.010 (0.005–0.016) | <0.001 | 0.005 (0.001–0.011) | 0.034 |
All abbreviations are as in Tables
In OA patients, SF IL-34 levels were significantly associated with WOMAC scores (
Correlations of SF IL-34 levels with WOMAC scores. SF = synovial fluid; IL = interleukin; WOMAC scores = Western Ontario and McMaster Universities Arthritis Index scores.
Linear regression for the association between variables and WOMAC scores.
Variables | Univariant | Multivariant | ||
---|---|---|---|---|
Age | 0.004 (−0.003–0.010) | 0.305 | 0.002 (−0.007–0.011) | 0.672 |
Sex | −0.024 (−0.165–0.118) | 0.742 | 0.045 (−0.009–0.180) | 0.510 |
BMI | 0.023 (−0.009–0.054) | 0.157 | 0.016 (−0.024–0.056) | 0.429 |
Serum IL-34 levels | <0.001 (−0.001–0.001) | 0.683 | <0.001 (−0.001–0.001) | 0.761 |
SF IL-34 levels | 0.038 (0.020–0.056) | <0.001 | 0.020 (0.001–0.038) | 0.035 |
KL grade | 0.244 (0.170–0.318) | <0.001 | 0.212 (0.130–0.293) | <0.001 |
All abbreviations are as in Tables
Considerable attention has been paid recently to the identification of biomarkers for OA. In the present study, we found that IL-34 levels in SF were significantly higher than those in paired serum samples in OA patients. Moreover, SF IL-34 levels were independently associated with the symptomatic and radiographic severity of the disease. These results indicated that IL-34 might play a significant role in the synovial inflammation of OA.
Biochemical biomarkers with sensitivity and reliability can facilitate early diagnosis of joint destruction, disease prognosis, and progression monitoring. Although historically OA was considered a degenerative disorder, recent studies have found that inflammation might be the primary trigger of the OA process, which results in cartilage damage [
Close monitoring of radiographic progression may facilitate the design of proper therapies. We found that the KL grade was associated with SF levels but not with serum SF levels. After adjusting for confounding factors, the KL grade was still independently associated with SF IL-34 levels. These results revealed the association between SF IL-34 levels and the radiographic severity of OA. As a macrophage colony-stimulating factor, IL-34 can stimulate the viability of monocytes and colony formation of macrophages [
We also found that SF IL-34 levels were significantly associated with self-reported knee pain after adjustment for confounding factors. Inflammatory cytokines can activate innervating nociceptors and increase excitability of sensory neurons [
Our study has several limitations. First, we were limited by the trial design. This cross-sectional study showed association but not causation. In addition, our sample size remains small. These findings will require further replication, validation, and qualification in large, longitudinal population cohorts. Second, although SF biomarkers provide a more proximal indicator of the disease state than serum biomarkers, they do not provide definitive indications of their tissue of origin. Third, as patients with synovial effusion can be a different pattern of knee OA, analyzing these patients would be interesting. However, we did not have data concerning these patients. Fourth, the control enrolled in this study underwent a clinical examination but no X-ray explanation was performed. Therefore, we cannot evaluate if control patients had radiographic knee OA, which might induce some bias.
In conclusion, we showed for the first time that IL-34 levels in SF were significantly associated with the radiographic and symptomatic severity of knee OA.
The SPSS Statistics Data Document.sav data used to support the findings of this study are available from the corresponding author (Email: shlizhanchun@21cn.com) upon request.
The authors declare that they have no conflict of interest.
This study was supported by the National Natural Science Foundation of China (Grant 81370976) and Medical-Engineering Joint Fund of Shanghai Jiaotong University (no. YG2014MS41).