The Role Of RANKL/OPG In The Pathogenesis Of Peripheral Joint Bone Destruction Of Ankylosing Spondylitis

OBJECTIVESTo detect the RANKL, OPG and CD68 protein levels in synovial tissues from ankylosing spondylitis(AS ) patients and compare the expression levels of RANKL, OPG and CD68 protein in AS , rheumatoid arthritis(RA), and osteoarthritis(OA) and normal synovial tissues. RANKL/OPG expressions were correlated with pathological changes of inflammatory joints to explore the role of RANKL / OPG in the pathogenesis of bone destruction in AS. METHODSImmunohistochemical analysis was performed using monoclonal antibodies to detect RANKL, OPG and CD68 protein expression in 13 AS, 16 RA,17 OA patients and 6 healthy controls. The presence of TRAP positive cells in the synovial tissues of patients with AS and RA was examined by enzyme histochemistry. The labeled synovial tissue sections were quantified by digital image analysis and semiquantitative analysis to compare the expression of RANKL, OPG, CD68 and TRAP positve cells in different patient groups and normal subjects. In addition, RANKL expression and OPG expression were correlated with certain inflammatory indices (including ESR, CRP, blood platelet count) and radiological stage of involved joints respectively. RESULTS(1) Positive staining of OPG was seen in all 13 AS patients. OPG expression was predominantly seen in the synovial lining layer and subling areas. Positive staining of OPG was also found in the synovial tissues of 2 normal subjects, but the OPG levels were significantly lower. No positive staining of OPG was found in synovial tissues from all patients with RA and OA.(2) Positive staining of RANKL was seen respectively in all 13 AS patients and 16 patients with RA, and positive expression was distributed predominantly in the synovial lining layer and at synovium-cartilage junctions. There was no significant difference between levels of RANKL expression in tissues from patients with AS and in tissues from RA. No positive staining of RANKL was observed in 6 normal subjects and all OA patients.(3) Positive correlation was found between RANKL protein expression and X-ray stage of involved joints destruction of patients with AS and RA (r=0.73,0.41, P=0.003,0.021 respectively).(4) Positive staining of CD68 was seen in synovial tissues from all the patients with AS, RA, OA and normal subjects, and the expression levels of CD68 from patients with AS and RA were higher than those from OA patients and healthy subjects. The CD68 positive cells were abundant mainly in lining layer.(5) In areas where elevated RANKL expression levels were present, the number of TRAP positive staining cells were found significantly increased in AS and RA synovium. The percentage of TRAP positive cells in synovium from AS patients was significantly lower than that from RA patients. The intensity of TRAP staining at synovium-cartilage junctions was notably increased. TRAP positive cells were rarely observed in synovium from OA patients and normal controls.(6) There was positive correlation between the number of TRAP positive cells and the RANKL expression (r=0.442, P-0.043) in RA patients. CONCLUSIONS(1) Higher levels of OPG were expressed in synovial tissues from AS patients than in tissues from normal subjects (P<0.001). No OPG was found in synovial tissues from patients with RA and OA, suggesting that OPG expression may be the consequenceof synoviocytes' reaction to inflammation and that OPG may have a protective effect on bone integrity. This serves a possible explanation for the better prognosis of the peripheral joint involvement in AS patients in comparison with RA patients.(2) Decrease in OPG levels at synovium-cartilage junctions is likely to be the main cause of bone destruction in AS patients. OPG may well have a therapeutic role in preventing bone destruction in AS.(3) Higher levels of RANKL were expressed in synovial tissues from AS patients with bone destruction than in tissues from subjects with healthy joints. RANKL protein expression was closely related with the degree of bone destruction, suggesting that it play a role in...