Histone H3 is citrullinated by peptidylarginine deiminase 4 (PAD4) at residues R2, R8, and R17 during the processes of epigenetic chromatin remodeling and neutrophil extracellular trap (NET) formation. In vitro, citrullinated H3 (CitH3) is released from neutrophils that have been stimulated with calcium ionophore during the process of NETosis, and can be freed from NETs by treatment with nuclease. The measurement of CitH3, either in cell lysates or released during NETosis can be used as a functional readout of pharmacological PAD inhibition.
