The B cell response to citrullinated antigens in the development of rheumatoid arthritis

The immune response to citrullinated antigens is found almost exclusively in patients with rheumatoid arthritis (RA). It is a dynamic response that expands before the onset of disease and generates antibodies (anti-citrullinated protein antibodies (ACPAs)) that are extensively glycosylated in the variable domain. This feature of ACPAs is remarkable and warrants detailed investigation, as it can offer insights into the earliest immunologic mechanisms that lead up to the development of RA. The acquisition of variable domain glycans, in fact, could enable ACPA-expressing B cells to breach tolerance. Although the underlying mechanisms are still elusive, data to support this concept are emerging, owing to the reliable identification and isolation of citrullinated antigen-directed B cells from patients with RA. This technological proficiency also allows for the generation of an increasing number of well-defined monoclonal ACPAs, and provides the opportunity to test and define the mechanisms by which the citrullinated antigen-directed B cell response contributes to the onset and persistence of inflammation. Together with a revised perception of the HLA-risk effect and novel insights into how T cells can govern antibody effector functions, these developments shape an increasingly clear picture of the B cell response to citrullinated antigens in the development of RA.