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A2.11 Antibodies to carbamylated α-enolase epitopes in rheumatoid arthritis also bind citrullinated epitopes and are largely indistinct from anti-citrullinated protein antibodies
  1. E Reed1,
  2. X Jiang2,
  3. N Kharlamova1,
  4. AJ Ytterberg1,3,
  5. A Catrina1,
  6. L Israelsson1,
  7. L Mathsson-Alm4,
  8. M Hansson1,
  9. L Alfredsson2,5,
  10. J Rönnelid6,
  11. K Lundberg1
  1. 1Rheumatology Unit, Department of Medicine, Karolinska Institutet, Stockholm, Sweden
  2. 2Cardiovascular Epidemiology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden
  3. 3Division of Physiological Chemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden
  4. 4ThermoFisher Scientific, Uppsala, Sweden
  5. 5Centre for Occupational and Environmental Medicine, Stockholm County Council, Stockholm, Sweden
  6. 6Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden

Abstract

Objectives In addition to anti-citrullinated protein antibodies (ACPAs), antibodies targeting carbamylated (i.e. homocitrullinated) proteins (anti-CarP antibodies) have been described in rheumatoid arthritis (RA). However, the extent to which  anti-CarP antibodies are truly distinct from ACPA remains unclear, and few studies have focused on specific autoantigens. Here, we examine cross-reactivity between ACPA and anti-CarP antibodies, in the context of the candidate autoantigen α-enolase.

Methods Cross-reactivity was examined by immunoblotting of citrullinated and carbamylated proteins using purified ACPA; and by peptide absorption experiments, using the citrullinated α-enolase peptide CEP-1 and a homocitrulline-containing version (carb-CEP-1) in ELISA. The population-based case-control cohort EIRA (n = 2,836 RA; 373 controls) was screened for reactivity with CEP-1 and carb-CEP-1, using the ISAC multiplex array. Associations of RA subsets with smoking and genetic risk alleles were calculated using unconditional logistic regression models. Differences in antibody levels were examined using Mann-Whitney U test.

Results Affinity-purified ACPA was found to bind carbamylated proteins and homocitrulline-containing peptides, demonstrating definitive cross-reactivity between ACPA and anti-CarP antibodies. Anti-carb-CEP-1 reactivity in EIRA was almost exclusively confined to the CEP-1-positive subset, and this group (21%) displayed a particularly strong ACPA response with marked epitope-spreading. The small RA subset (3%) with homocitrulline-reactivity in the absence of citrulline-reactivity did not associate with smoking or risk genes, and had significantly lower anti-carb-CEP-1 antibody levels.

Conclusion Our data presented herein cast doubt on the specificity of anti-CarP antibodies in RA, which we posit may be a subset of cross-reactive ACPA.

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