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Anti-cyclic citrullinated peptide positivity in non-rheumatoid arthritis disease samples: citrulline-dependent or not?

Abstract

Background: Antibodies directed against citrullinated proteins (eg anti-cyclic citrullinated peptide (CCP)) have excellent diagnostic and good prognostic potential for rheumatoid arthritis. Type 1 autoimmune hepatitis (AIH-1) is a chronic liver disease characterised by a variety of serum autoantibodies. Recently, in a large group of patients with AIH-1 without clear rheumatoid arthritis overlap, a relatively high percentage (9%) of anti-CCP2 positivity was scored.

Objectives: To characterise the citrulline-dependence of the observed anti-CCP2 positivity in AIH-1 sera as well as in other groups of patients without rheumatoid arthritis (mainly rheumatic diseases).

Methods: Serum samples of 57 patients with AIH-1 and 66 patients without rheumatoid arthritis, most of them reported as anti-CCP positive, were tested for citrulline-specific reactivity with a second generation anti-CCP kit, with the citrullinated and the corresponding non-citrullinated (arginine-containing) antigen. A subset of AIH-1 sera was also tested with a CCP1 ELISA (and arginine control).

Results: The anti-CCP2 reactivity of most non-rheumatoid arthritis rheumatic diseases samples (87–93%) was citrulline-specific, whereas a relatively high percentage of AIH-1 samples (42–50%) turned out to be reactive in a citrulline-independent manner. The use of citrullinated and non-citrullinated CCP1 peptides confirmed a high occurrence of citrulline-independent reactivity in AIH-1 samples.

Conclusions: In rheumatoid arthritis and most non-rheumatoid arthritis rheumatologic disease sera, anti-CCP positivity is citrulline-dependent. However in some patients, particularly patients with AIH-1, citrulline-independent reactivity in the anti-CCP2 test can occur. A positive CCP test in a non-rheumatic disease (eg liver disease) should therefore be interpreted with care, and preferably followed by a control ELISA with a non-citrullinated antigen.

  • AIH-1, Type 1 autoimmune hepatitis
  • AIH, autoimmune hepatitis
  • ANA, antinuclear antibodies
  • Anti-CCP, anti-cyclic citrullinated peptide
  • IAHG, International Autoimmune Hepatitis Group
  • PsA, psoriatic arthritis
  • SjS, Sjögren’s syndrome
  • SLE, systemic lupus erythematosus
  • SMA, smooth muscle antibodies

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