ANALYSIS OF CARTILAGE OLIGOMERIC MATRIX PROTEIN (COMP) IN SYNOVIAL FIBROBLASTS AND SYNOVIAL-FLUIDS

We investigated the expression of cartilage oligomeric matrix protein (COMP) in normal and rheumatoid arthritis (RA) synovial fibroblasts. I n situ hybridization (ISH) was conducted on synovial specimens from fi ve RA patients applying specific probes for COMP or fibroblast collage n type I. ISH was combined with immunohistochemistry, applying antibod ies to the macrophage marker CD68. Ribonuclease protection assay (RPA) and rapid amplification of 3'-cDNA ends (3'-RACE) were performed on t otal RNA from normal and RA synovial fibroblast cultures. Protein extr acts from fibroblasts and culture supernatants were compared with syno vial fluids and protein extracts from isolated chondrocytes by Western blot utilizing polyclonal and monoclonal antibodies (18-G3 mAb) to CO MP. COMP mRNA was detected in fibroblasts of RA synovium by ISH, and i n normal and RA synovial fibroblast cultures by RPA. 3'-RACE demonstra ted sequence homology of chondrocyte and synovial fibroblast COMP alon g the coding sequence. COMP protein was detected in synovial fibroblas ts and culture supernatants by immunoblot. Using polyclonal antibodies , the major portion of COMP from fibroblasts and culture supernatants was present as low-molecular-weight (LMW) bands, corresponding to thos e found in synovial fluids. These LMW COMP bands, however, were not de tected in any of the cells or tissues tested using 18-G3 mAb. In prote in extracts from chondrocytes and in COMP purified from cartilage, the se LMW bands could not be detected. In conclusion, the data suggest th at certain forms of COMP detected in synovial fluid are secreted from synovial fibroblasts and could be distinguished by specific mAbs from COMP secreted by chondrocytes.