Rheumatoid arthritis–specific autoantibodies to peptidyl arginine deiminase type 4 inhibit citrullination of fibrinogen
Abstract
Objective
Autoantibodies to citrullinated proteins are specific for rheumatoid arthritis (RA) and recognize epitopes centered by citrulline, a posttranslationally modified form of arginine. Peptidyl arginine deiminase type 4 (PAD-4), the enzyme that converts arginine into citrulline, is in itself a target for RA-specific autoantibodies. This study was undertaken to assess whether anti–PAD-4 autoantibodies interfere with citrullination in vitro in patients with RA, and to identify peptide targets of anti–PAD-4 antibodies that can activate or inhibit citrullination.
Methods
To test whether autoantibodies to PAD-4 influence citrullination, an in-house citrullination assay was developed using purified autoantibodies to PAD-4. To map B cell epitopes on PAD-4, 65 overlapping 20-mer peptides encompassing the entire PAD-4 were analyzed for their reactivity in RA sera.
Results
Autoantibodies to PAD-4 inhibited PAD-4–mediated citrullination. Three linear peptides on PAD-4 were recognized almost uniquely by PAD-4 autoantibodies in the sera of patients with RA. One peptide was located in the N-terminal, calcium-binding domain of PAD-4, while 2 other peptides were located in the C-terminal, substrate-binding domain of PAD-4.
Conclusion
Autoantibodies to PAD-4 inhibit in vitro citrullination of fibrinogen by PAD-4. Most anti–PAD-4–positive sera recognize peptides located both in the N-terminal domain (211–290) and the C-terminal domain (601–650) of PAD-4.
