Introduction Receptor Activator of NfκB-Ligand (RANKL) and Osteoprotegerin (OPG) play a key role in the pathogenesis of osteoporosis in rheumatic diseases. We have recently shown that autoantibodies against OPG (OPG ab) are detectable in patients with rheumatic diseases and are associated with increased bone resorption in RA and reduced bone mineral density (BMD) in AS patients. Our aim was to examine the relation between OPG ab concentration and RANKL and OPG levels andto investigate if OPG abs interfere with free RANKL ELISA, which uses recombinant OPG as capture antigen.
Methods Circulating free soluble RANKL (Biomedica), total soluble RANKL (BioVendor) and OPG (BioVendor)were measured in sera from 26 RA, 15 Spondyloarthropathy and 14 SLE patients, using commercial available ELISA kits. OPG ab concentrations were measured by an in-house made ELISA, using recombinant human OPG as the capture antigen with detection of antigen-antibody complex through HRP conjugated anti-human IgG antibodies. In order to investigate the effect of OPG antibodies on free RANKL detection, purified IgG from 6 patients serawas added to different RANKL concentrations (200 and 800pg/ml) and analysed by free RANKL ELISA.
Results Free RANKL concentrations were undetectably low in 23 (41.8%) patients however this was not associated with the presence of OPG ab. In contrastthere was a significant positive correlation between free RANKL and OPG Ab concentrations (r = 0.273, p = 0.001) found. Mean OPG concentrations were significantly higher in patients above 67 years (6.31 ± 2.50 pmol/L) than in patients under 67 years of age (4.75 ± 1.70pmol/L) (p = 0.022). There was no association between OPG ab concentrations and OPG or total RANKL measurements. The addition of purified IgG reduced free RANKL detection through ELISA significantly (p < 0.001) however there was no difference on RANKL suppression between IgG addition with high and low OPG ab levels.
In conclusion there is no correlation between circulating OPG and OPG ab concentrations. The presence of circulating OPG ab may increase free RANKL concentrations by stopping OPG binding to RANKL. Purified Immunoglobulins interfere with the measurement of free soluble RANKL independent of OPG ab presence.