ISOLATION AND CHARACTERIZATION OF A CARTILAGE-SPECIFIC MEMBRANE ANTIGEN (CH65) - COMPARISON WITH CYTOKERATINS AND HEAT-SHOCK PROTEINS

We report the isolation and characterization of a 65,000 MW chondrocyt e autoantigen (CH65) which may be involved in rheumatoid arthritis. Th is chondrocyte-specific antigen reacted with sera from patients with r heumatoid arthritis (RA). CH65 did not cross-react with a polyclonal a ntibody raised against microbial heat-shock protein (hsp) 65, anti-hum an hsp 65 monoclonal antibodies (mAb) (LK1 and LK2), anti-microbial hs p 65 mAb (IA10, IIC8 and WTB-78H1) and anti-cytokeratin 8, 18, 19 mAb (NCL5D3MAb). CH65 could be purified from chicken chondrocyte membranes by ammonium sulphate precipitation and a novel electro-gelfiltration method. The amino acid analysis yielded an unusually high degree of gl ycine, serine and asparagin residues. The internal amino acid sequence obtained by tryptic digestion revealed homologies with the cytokerati n family. Despite these homologies, CH65 lacked immunological cross-re activity with commercial anti-cytokeratin antibodies. Mice mAb generat ed against the purified CH65 (C6) were used to identify the protein as a tissue-specific constitutive protein membrane from chondrocytes. Se ra from patients with RA crossreacted with purified CH65. The stress o r heat-shock protein (hsp 65), implicated in the development of experi mental and clinical arthritis, showed no immunological cross-reactivit y with CH65 in Western blots. These findings suggest that CH65 may rep resent an interesting cartilage-specific new antigen in RA. The availa bility of this antigen in purified form and specific mAb may offer use ful tools in arthritis research.