Therefore, although in both T1D and RA the pathways of Arg and Trp metabolism do not seem to be properly interlinked (and this may require cautions when attempting immunotherapies potentiating both ARG1 and IDO1), in MS, the pieces of evidence, when put together, would suggest that the induction of the immunosuppressive interplay between ARG1 and IDO1 would represent a valid therapeutic objective. This evidence concerns the gene IDO1 and rheumatoid arthritis.