A sensitive assay for the measurement of serum chondroitin sulfate 3B3(-) epitope levels in human rheumatic diseases.

Objective: To develop a sensitive assay to quantitate serum 3B3(-) levels in patients with rheumatoid arthritis (RA) and osteoarthritis (OA) as well as levels in control sera.

Methods: An enzyme-linked immunosorbent assay (ELISA) was developed using the monoclonal antibody (MAb) 3B3 to detect a chondroitin sulfate (CS) epitope in the sera and synovial fluid (SF) of RA and OA patients. Keratan sulfate levels were measured in the same biological fluids using the 5D4 monoclonal antibody.

Results: The detection limit for our 3B3(-) assay was 2 micrograms/L. Most OA sera sample curves run on the 3B3 assay were parallel (87.5%) to the standard curve and detectable (90.0%). RA sera sample curves were 87.1% detectable and 85.2% parallel. The 3B3(-) epitope was detectable in 60% of control sera and of these 66.7% of sample curves were parallel to the standard curve. All RA and OA SF had detectable quantities of 3B3(-). For the 3B3 assay, the OA and RA sera levels were significantly higher than for control sera (P = 0.03, P = 0.04 respectively). We found a significant correlation in a subset of paired OA sera and SF 3B3(-) concentrations. No correlation was found between age, joint activity scores, HAQ and CRP in RA patients and sera 3B3(-) and 5D4 levels.

Conclusion: We have validated this assay for the quantification of 3B3(-) epitope in RA and OA serum. Levels of this epitope are significantly higher in sera from RA and OA patients than controls. 3B3(-) levels in RA sera were found to correlate with disease duration.