AGBL1 and rheumatoid arthritis: For our studies, we used a cyclic citrullinated peptide, CCP4, asour model antigen (Figure 1A).16 The CCP4 sequence is derivedfrom CCP2, a diagnostic peptide marker used for the detection of ACPAsin patients with RA, and binds with comparable affinity as determinedby ELISA (Figure S1A).29 The peptide was obtained by solid-phase peptide synthesisusing standard synthetic protocols.