<prompt> Answer this question truthfullyWhat is the role of peptidylarginine deaminase in rheumatoid arthritis RA and what is the consequence of its activity?  <response>: <reason> In RA, peptidylarginine deaminase is expressed by macrophages, pathogens, and synovial tissue. Its activity converts arginine residues to citrulline, which leads to antigen presentation and activation of CD4 helper T cells. 