Immunology and AllergyTier 2Topic

Laboratory investigations - cyclic citrullinated peptide (CCP) antibodies

What it is

Anti-CCP (anti-citrullinated protein antibody) - targets citrullinated proteins (post-translationally modified via peptidylarginine deiminase) - a key serological marker for rheumatoid arthritis
Present in ~60-70% of RA patients at diagnosis; can predate clinical RA onset by years

Mechanism

Citrullination converts arginine residues to citrulline in various proteins; in genetically susceptible individuals (particularly HLA-DRB1 "shared epitope" carriers) this generates neoantigens that break tolerance and drive an autoimmune response specifically targeting joint tissue

Interpretation

More specific than rheumatoid factor for RA (~95% specificity vs ~70-80% for RF) - a positive anti-CCP in the correct clinical context is more confidently attributed to RA
Anti-CCP positivity, particularly combined with RF positivity, predicts more erosive/aggressive disease and is incorporated into the ACR/EULAR 2010 classification criteria for RA

Acting on the result

Anti-CCP positivity supports earlier, more aggressive treatment given its association with erosive disease, though treatment decisions are made on the overall clinical picture, not serology alone

Associations

Rheumatoid arthritis, smoking (a specific environmental trigger for citrullination in genetically susceptible individuals), interstitial lung disease in RA (more common with high anti-CCP titres)

Preclinical positivity

Anti-CCP can be positive years before clinical arthritis develops - the "pre-RA" phase - an area of active research interest for early intervention, though testing asymptomatic individuals is not current standard practice

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