Antigenic epitopes recognized by autoantibodies to calpastatin in patients with rheumatoid arthritis and their clinical significance

We have previously described that novel autoantibodies to calpastatin (endogenous inhibitor for calcium-dependent neutral protease, calpain) were detected in patients with rheumatoid arthritis (RA) and other disorders. Since calpain is thought to mediate inflammatory process and cartilage destruction, autoantibodies to its inhibitor protein, calpastatin, may be involved in the pathogenic mechanism of rheumatoid arthritis. In the present study, we analyzed antigenic epitopes reactive with autoantibodies to calpastatin and their clinical correlation. cDNA encoding the C-terminal 178 amino acids of human calpastatin (RA-6) was digested by restriction enzymes and ligated in to pEX expression vectors. These recombinant plasmids were tranfected into E. coli POP2136 and screened by colony blots using RA sera containing anticalpastatin antibodies and a mouse monoclonal antibody. RA patient sera recognized the C-terminus of domain IV (epitope C1 ; aa. 647-673) and C-terminus of domain III (epitope C2 ; aa. 496-571), whereas the mouse monoclonal antibody recognized an entirely different region containing the calpain-binding site (epitope B2 ; aa. 572-625). To evaluate epitope reactivity of patient autoantibodies, 15 RA sera containing anti-calpastatin were reacted with epitope fusion proteins. In immunoblotting, most RA sera recognized either C1 or C2 epitopes (67% and 40%, respectively), and only one patient recognized both epitopes. B2 epitope a more progressed and sever state of arthritis than those not reacting with C1. These results suggests that anti-calpastatin antibodies may play a role in the pathogenic mechanisms of RA and their epitope reactivity may be important for disease progression.