To analyze the genetic impact of allelic variants of the protein tyrosine phosphatase N22 (PTPN22) and HLA-DRB1 alleles on IgG autoantibody formation directed toward an immunodominant conformational epitope (C1(III); amino acid residues 359-369) of type II collagen (CII) in early rheumatoid arthritis (RA).
Sera obtained at study inclusion from an inception cohort of RA patients (n = 221; mean symptom duration 6 months) were analyzed for circulating anti-C1(III) IgG autoantibodies. An enzyme-linked immunosorbent assay based on solid-phase-coupled synthetic triple-helical collagen peptides was used to quantify humoral autoimmune responses. HLA-DRB1 genotypes were determined by allele-specific polymerase chain reaction amplification of genomic DNA and sequence-specific hybridization. PTPN22*620W genotyping was performed using an allelic discrimination TaqMan assay.
Anti-C1(III) IgG autoantibody titers were significantly elevated in patients with early RA as compared with those in healthy controls (n = 70). The increased titers were more pronounced in RA patients harboring alleles of the RA-associated HLA-DRB1 shared epitope (SE) consensus sequence than in those lacking the SE. In addition, the PTPN22*620W variant was strongly associated with a vigorous humoral autoimmune response to the cartilage-specific CII determinant C1(III).
Allelic variants encoding the binding pocket for peptide presentation (SE) to T cells and a functional domain of a negative regulator of T cell receptor signaling (PTPN22*620W), respectively, synergize in early RA to break self tolerance toward C1(III), an evolutionarily conserved cartilage determinant that is also frequently targeted in arthritogenic humoral autoimmunity in mice.