Monoclonal anti-citrullinated protein antibodies selected on citrullinated fibrinogen have distinct targets with different cross-reactivity patterns

OBJECTIVE: ACPAs are thought to play a pathogenic role in RA. Because of their polyclonal nature it is difficult to study characteristics of ACPAs such as cross-reactivity or affinity. This study aimed to analyse the ACPA response at clonal level.

METHODS: Citrullinated fibrinogen-specific B cells were isolated from blood derived from an RA patient by fluorescent automated cell sorting (FACS). Antigen specificity was verified by ELISA of culture supernatant. RNA of antigen-specific B cells was isolated and VH and VL chains were cloned and subsequently expressed as IgG1 antibodies.

RESULTS: Two human recombinant antibodies were obtained that bind to citrullinated fibrinogen peptide (cFib). Both monoclonal antibodies originate from different naive B cells, undergo extensive somatic hypermutation and bind to cFib (but not to Fib) with moderate avidity. Furthermore, they show distinct cross-reactivity patterns towards other citrullinated peptides, suggesting that both antibodies have different primary targets.

CONCLUSION: Together these data suggest that ACPAs are formed by antigen-driven maturation, and that multiple citrullinated antigens are involved in activating the B-cell response.