Background The discovery of autoantibodies against citrullinated proteins (ACPA) and the consequent development of serological tests using cyclic citrullinated peptide (CCP) have improved diagnosis of rheumatoid arthritis (RA). ACPAs recognize specifically various citrullinated peptides that derive from different antigens. It has also been demonstrated that autoantibodies to different citrullinated peptides emerge at different stages of RA. However, by now the fine specificity against only a limited number of different peptides has been tested.
Objectives To increase knowledge about ACPAs and their fine specificities in RA patients we aimed to identify novel citrullinated antigens and to characterize the ACPA profile in early and late RA.
Methods 569 serum samples of CCP+ and CCP- RA patients with late RA, early RA or pre-RA and additional samples of volunteers and patients with early Arthritis (EA) but without RA at the time point tested have been analyzed in a multiplexed, bead-based, antigen array on the Luminex FlexMAP3D. Sera have been tested against a selection of 417 human proteins, including previously identified putative RA specific autoantigens and also known RA marker such as vimentin and fibrinogen. After coupling all proteins to individual color coded Luminex beads they were enzymatically citrullinated by peptidyl arginine deiminase and then tested against sera in the multiplex Luminex array. Complementarily all proteins were also measured in their native, unmodified form.
Results We found novel citrullinated antigens targeted by ACPAs. The citrullinated antigens are highly reactive in mature RA whereas the unmodified proteins show only low reactivity. Generally, we observe an increase of the number of ACPA fine specificities with disease duration. In CCP negative tested RA patients, more than 10% of the patients recognize specifically several citrullinated antigens suggesting their diagnostic potential.
Conclusions Novel citrullinated antigens targeted by ACPAs were identified. The identification of ACPA positive but CCP negative tested RA patients suggests that current CCP tests do not address the complete profile of ACPA fine specificities. Further studies are currently conducted to validate panels of citrullinated antigens in early RA patients.
Disclosure of Interest A. Lueking Employee of: Protagen AG, P. Schriek Employee of: Protagen AG, H. Goehler Employee of: Protagen AG, M. Gamer Employee of: Protagen AG, K. Marquart Employee of: Protagen AG, A. Telaar: None declared, D. Chamrad Employee of: Protagen AG, P. Schulz-Knappe Employee of: Protagen AG, J. Richter: None declared, S. Vordenbaeumen: None declared, G. Burmester: None declared, M. Schneider: None declared