Expression of PAD enzymes and occurrence of citrulline-containing proteins in human blood and synovial fluid cells

Antibodies directed against citrulline-containing antigens are extremely specific for RA. The amino acid citrulline is not incorporated into proteins during protein synthesis. It is generated by posttranslational modification of arginine residues by PAD (peptidylarginine deiminase) enzymes. We investigated the expression of PAD enzymes and the occurrence of citrullinated proteins in peripheral blood (PB) and synovial fluid (SF) cells. PAD types 1 and 3 were absent from the investigated cells, while PAD types 2 and 4 (also known as type 5) were present. In monocyte-derived macrophages PAD type 2 mRNA expression was at a similiar level as in monocytes, while PAD type 2 protein was increased. PAD type 4 mRNA expression was significant in monocytes and almost absent in monocyte-derived macrophages, while PAD type 4 protein levels were similar. In monocytes no citrullinated protein could be detected, while in monocyte-derived macrophages citrullinated vimentin, which is (part of) the Sa-antigen, was present. A similar pattern of mRNA and protein expression was observed in mononuclear cells in paired PB and SF samples of RA patients. These results suggest that PAD type 2 is involved in the citrullination of SF proteins during inflammation.