PRTN3 and rheumatoid arthritis: The two citrullinated peptides significantly inhibited ACPA reactivity to citrullinated fibrinogen and, when tested together, almost completely abolished formation of macrophage-activating ICs, thereby diminishing the secreted TNF-α levels.15 Another in vitro study used ACPAs that were purified from pooled ACPA-positive RA sera by cyclic citrullinated peptide-conjugated affinity column.