Genetic predisposition (HLA-SE) is associated with ACPA-IgG variable domain glycosylation in the predisease phase of RA.

In addition to Fc glycans, IgG can carry N -linked glycans in the variable domain. The abundant presence of disialylated variable domain glycans (VDGs) is a special feature of anti-citrullinated protein antibody (ACPA)-IgG and possibly other autoantibodies. The introduction of glycosylation sites is mediated by somatic hypermutation (SHM), a T-cell dependent process.1 The high frequency of glycosylation sites does not correlate with the number of SHM, pointing towards a selective advantage of B cells expressing variable domain glycosylated ACPA.2 Previously, we observed that ACPA-IgG VDGs are already present in the phase preceding rheumatoid arthritis (RA) onset and predictive for disease development.3 In addition, we provided first evidence that the human leucocyte antigen (HLA) ‘shared epitope’ (SE) alleles, the most prominent genetic risk factor for ACPA-positive RA, are associated with the presence of VDG on ACPA-IgG predisease.4 Hence, VDG could possibly explain the contribution of HLA-SE restricted T cells in the maturation of the ACPA response. Building on these results, we now hypothesised that HLA-SE alleles may not be associated with ACPA positivity as such, but with the specific presence of variable domain glycosylated ACPA-IgG, a favourable factor for the development of this multifactorial disease. To substantiate our hypothesis, we expanded the set of presymptomatic individuals (n=228) and RA-patients (n=126) from Sweden and analysed two additional cohorts comprising ACPA-positive Dutch subjects with arthralgia (n=239) and ACPA-positive healthy Japanese individuals (n=58) (online supplemental table S1). We determined the presence/percentage of ACPA-IgG VDG using liquid chromatography5 and assessed associations with HLA-SE (online supplemental materials and methods). In particular, we focused on the most prominent glycan peak (GP24) found on top of the variable domain,1 which carries a bisecting N -acetylglucosamine and two terminal sialic acids (G2FBS2) (figure 1A). ACPA-IgG VDG were, with a median of 58%, already …