Identification of citrullinated rheumatoid arthritis-specific epitopes in natural filaggrin relevant for antifilaggrin autoantibody detection by line immunoassay

Arthritis Rheum. 2002 May;46(5):1185-95. doi: 10.1002/art.10229.


Objective: To identify immunodominant epitopes in natural filaggrin that are reactive with antifilaggrin autoantibodies (AFA) in the sera of patients with rheumatoid arthritis (RA) and to explore their use in a diagnostic assay format.

Methods: Based on the results of epitope mapping of human natural filaggrin as well as molecular modeling and computational chemistry, synthetic peptides together with recombinant citrullinated filaggrin were evaluated by a line immunoassay (LIA) for AFA detection. Diagnostic performance was assessed using 336 RA and 253 disease control sera and was compared with that of reference methods.

Results: Several immunoreactive epitopes were identified in natural filaggrin, all of which contained at least 1 citrulline residue. Three antigenic substrates, including 2 synthetic peptides and recombinant citrullinated filaggrin showing maximal reactivity on LIA, were finally selected. Using the 3-antigen LIA3, overall sensitivity, specificity, and positive predictive value for RA were 65.2%, 98.0%, and 89.1%, respectively, compared with 61.9%, 98.8%, and 92.8% using the 2-antigen LIA2 (without recombinant protein). Thirty-seven percent of the rheumatoid factor (RF)-negative RA samples (30 of 81) were AFA-positive by LIA2, and 52 of 54 RF-positive control samples had no AFA detected on LIA2. Higher specificity and sensitivity were obtained by LIA2 versus anti-RA33 immunoblot, whereas good agreement was observed with antikeratin antibody testing. LIA performed significantly better than AFA immunoblotting using natural filaggrin, at a specificity level of 99% (P = 0.0047).

Conclusion: Citrullinated residues are present in immunoreactive epitopes of natural human filaggrin. AFA can be readily detected by citrullinated peptides in an LIA-based test, resulting in high specificity and positive predictive value for RA. The LIA could serve as a user-friendly alternative to existing immunofluorescence tests and AFA immunoblot techniques. Given its complementarity to RF, this test can be a valuable tool in the differential diagnosis of arthritis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Arthritis, Rheumatoid / immunology*
  • Autoantibodies / analysis*
  • Autoantibodies / blood
  • Citrulline / chemistry
  • Cross Reactions
  • Epitope Mapping
  • Filaggrin Proteins
  • Humans
  • Immunoassay / methods*
  • Immunoassay / standards
  • Immunoblotting
  • Immunodominant Epitopes / chemistry
  • Immunodominant Epitopes / immunology*
  • Intermediate Filament Proteins / chemistry
  • Intermediate Filament Proteins / immunology*
  • Models, Molecular
  • Molecular Sequence Data
  • Peptide Fragments / chemical synthesis
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / immunology
  • Reproducibility of Results


  • Autoantibodies
  • FLG protein, human
  • Filaggrin Proteins
  • Immunodominant Epitopes
  • Intermediate Filament Proteins
  • Peptide Fragments
  • Recombinant Proteins
  • Citrulline