Epitope Specificity of Anti-Citrullinated Protein Antibodies

Abstract

Anti-citrullinated protein antibodies are primarily associated with a progressive course in the autoimmune disease rheumatoid arthritis, a disease with a chronic and inflammatory nature. These antibodies do not appear to have any strict dependency for reactivity except from the presence of the non-genetically encoded amino acid citrulline, which is the result of a posttranslational modification, catalyzed by calcium-dependent peptidylarginine deiminase enzymes. Nevertheless, several amino acids surrounding the citrulline residue notably influence antibody reactivity, especially with a central-Cit-Gly-motif being essential for antibody reactivity. Most importantly, these antibodies have been proposed to be divided into two groups, based on their ability to recognize multiple citrullinated peptides. Thus, an “overlapping” antibody group, which appears to recognize several citrullinated peptides, and a “non-overlapping” antibody group, which only recognizes a limited number of citrullinated peptides, have been proposed. Based on these findings, we suggest that antibodies recognizing several citrullinated targets, also referred to as cross-reactive antibodies, primarily are backbone-dependent, whereas less cross-reactive antibodies primarily depend on the side chains of the amino acids comprising the epitopes for stable antibody-antigen interactions, which reduces the degree of cross-reactivity significantly. Clarifying the reactivity pattern of anti-citrullinated protein antibodies may contribute to determining their true nature of origin.

Specificity of Anti-Citrullinated Protein Antibodies to Citrullinated α-Enolase Peptides as a Function of Epitope Structure and Composition

“…Increased sensitivities were obtained for the longest (C-19-Cit) and the shortest (C-10-Cit) cyclic α-enolase peptides compared to the C-14-Cit and C-12-Cit peptides, indicating that peptide length and conformation are crucial for peptides containing 12-19 amino acids. These findings are supported by the literature, where it has been reported that the reduced number of amino acids in the cyclic structure may constrain the peptide in a more locked conformation, reducing the flexibility of the peptides and hence negatively influencing the ACPA reactivity [16]. The fact that the smaller peptide (C-10-Cit) was as sensitive as the longest peptide (C-19-Cit), and thus more sensitive than the mid-length peptides (C-12-Cit and C-14-Cit), is intriguing.…”

“…Citrullination is the result of a post-translational modification, where the positively charged guanidino group of Arg is substituted by the neutral ureido group. Ultimately, this modification may lead to structural unfolding of the citrullinated protein [15,16]. Citrullination is catalyzed by Peptidyl Arginine Deiminase (PAD) enzymes, which are calcium-dependent metalloenzymes [17].…”

“…It has been proposed that ACPAs can be divided into two groups, based on their ability to interact with citrullinated peptides [16,17], one group that appears to recognize a large variety of citrullinated targets and another group that recognizes a very limited number of citrullinated peptides [29]. The first group of ACPAs, also referred to as "overlapping" or "cross-reactive" antibodies, is primarily backbone-dependent, whereas the second group, also referred to as "nonoverlapping" or "epitope-specific", depends on the specific amino acid side-chains of the epitope to establish a stable antibody-antigen interaction [16].…”

Antibodies to a strain-specific citrullinated Epstein-Barr virus peptide diagnoses rheumatoid arthritis

“…Highly significant reactivity was found to peptide 17 (P < 0.0001), which yielded a sensitivity of 93%. Interestingly, the two peptides, which were not recognised by the RA sera, contained no charged amino acids C-terminal to Cit, which previously has been reported to be essential for ACPA reactivity 13 , 28 , 29 . None of the HD sera reacted with the citrullinated peptides, which is illustrated by specificities of 100%.…”

“…ACPA has been found to recognise a number of different citrullinated substrates 5 – 12 . The fact that several non-homologous citrullinated proteins have been recognised by ACPA confirms that these antibodies are cross-reactive 6 , 10 , 13 , 14 .…”