The exceptionally high ACPA titers in FS patients with the most severe neutropenia and the fact that of some of the citrullinated proteins we detect in perforin-treated neutrophils are present in their citrullinated form in the synovial fluid from RA patients34 support a connection between intracellular neutrophil citrullination, ACPA formation, and RA disease activity. Here, PRF1 is linked to neutropenia.