Background Current perception of RA pathogenesis suggests that posttranslational modification (PTM) of either arginine (into citrulline) or lysine (into homocitrulline) residues in antigens are crucial for the generation of specific autoantibodies in RA.
Objectives To examine how anti–citrullinated protein antibody (ACPA) and anti-homocitrulline protein antibody (AHPA) arise in the immune response of clinically overt rheumatoid arthritis (RA), and to analyze the influence of B cell directed therapy on antibody reactivity.
Methods Homocitrullin residues were inserted into human mutated vimentin by non-enzymatic carbamoylation, in contrast to the enzymatic citrullination. Peptide librariesfrom the identified major RA epitope of mutated vimentin weregenerated with citrulline and homocitrulline residues, respectively. Reactivity to 5 distinct modified peptides was measured by enzymelinked immunosorbent assay and validatedwith sera from human healthy individuals and disease controls (systemic autoimmune diseases such as SLE and Sjögren’s syndrome). The study included 142 patients (mean age at inclusion 54.3±12.6 years) with RA (fulfilled the ACR 1987 RA criteria), who started on treatment with TNF-inhibitor followed by B-cell directed therapy. Multiple consecutive serum samples from RA patients were included for analysis of antibody titers in the follow-up.
Results Sensitivities of 86,8% and 69,2% were calculated for the ACPA and AHPA assays, respectively. Sera from healthy persons and disease control groups showed a comparable specificity for both assays (ACPA 97% and AHPA 91%) and none had anti-antibodies against the unmodifed vimentin. With peptide assay distinctive only in citrulline and homocitrulline, more than 45% of RA patients were analyzed to have both ACPA and AHPA, and singular AHPA response was not observed. The lowest concentration of ACPA and AHPA were estimated to be 60 μg/ml and 25 μg/ml, respectively. Remarkably, after one cycle of RTX treatment a bisection of AHPA levels and a 25% reduction of ACPA levels were observed. Patients under TNF blockers without DAS28 remission after a period of 6 months showed a lesser reduction of ACPA and AHPA levels of only 15 – 28%.
Conclusions Our findings indicate that the significant serological variation in RA patients may be an indication that RA may encompass a number of distinct serologic entities, of which particularly ACPA and AHPA may play a role in different phases of RA pathogenesis.
Disclosure of Interest H. Bang Employee of: Orgentec Diagnostika GmbH, K. Egerer: None Declared, B. Lehmann: None Declared, A. Krämer Employee of: Orgentec Diagnostika GmbH, E. Feist: None Declared, G. R. Burmester: None Declared