Background/Purpose: Anti-carbamylated protein autoantibodies (anti-CarP Abs) of IgG and/or IgA isotype have potential diagnostic and prognostic value Carbamylated Fetal Calf Serum (FCS) is the substrate for the ELISA tests being currently developed (gold standard) for their detection. However, the precise target(s) of these Abs remained undetermined in vivo, even though some works suggest that carbamylated fibrinogen would be the most recognized protein by them, particularly the β chain. The objectives were (i) to identify the targets recognized by anti-CarP in situ, (ii) to determine whether these anti-CarP Abs recognizing one or several in situ targets have a cross-reactivity with ACPA and (iii) to evaluate their diagnostic and prognostic value in a well-documented cohort.

Methods: All the sera used are from the VeRA cohort of 310 early inflammatory rheumatic diseases (median inclusion at 4 months) including 185 RA (meeting ACR 2010 criteria). To identify new carbamylated targets in situ, we performed mass spectrometry on sera from RA patients. An epitopic mapping was performed on the γ-chain of the carbamylated fibrinogen (target identified in the previous step) using 15 27-mer peptides overlapping on 3 amino acids, carbamylated in vitro (i). Two immuno-dominant epitopes were identified. The specificity of these targets was asked by performing inhibition assays with the major antigens of the citrulline response (ii). An anti-CarP FCS IgG ELISA test on the one hand and anti-CarP Fib IgG ELISA test on the other hand were developed. An analysis with clinical, biological (CRP) and radiological data (van der Heijde score) at inclusion, 6 months 2 years was performed to determine the diagnostic and prognostic value of these antibodies (iii).

Results: Using a label free proteomic approach, we have identified in RA sera the γ chain of fibrinogen as a potential new target epitope mapping of this chain led to the identification of 2 immuno-dominant epitopes (peptides 5 and 13). Inhibition tests by immunodominant epitopes targeted by ACPA revealed that only peptide 13 has a specific reactivity distinct from that of anti-citrullinated fibrinogen.

The prevalence of Anti-CarP-Fib in VeRA cohort, regardless the patient status (RA or not) is 37% at baseline, which is similar to that of anti-CarP-FCS. Anti-CarP-Fib IgG has diagnostic value since 10.9% of ACPA negative RA are immuno-positive for anti-CarP-Fib IgG at early stage of the disease.

In ACPA-negative patients, anti-CarP-Fib positivity is associated with more inflammatory (higher CRP levels) and erosive disease at baseline (p< 0.05). However, this autoantibody population is not associated with radiological progression, which remains strongly related to the presence of ACPA.

Conclusion: One of the primary antigen targeted by the anti-CarP response in RA is carbamylated fibrinogen, particularly 2 epitopes of the γ chain whose one of them does not overlap with the ACPA response. This specificity seems to be associated to a distinct clinical phenotype since anti-CarP-Fib IgG are linked to systemic inflammation, notably in the early stage of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-anti-carbamylated-fibrinogen-response-in-rheumatoid-arthritis-targets-a-specific-epitope-on-the-%ce%b3-chain-and-is-associated-with-a-more-active-disease/