IL15 and rheumatoid arthritis: In addition, IL-15 expression has been observed in the intimal and subintimal layer of the RA synovial membrane [14], is transiently upregulated in the synovial fluid of early RA patients [15], and we have reported that constitutively expressed IL-15 on the surface of RASFib is biologically active on cocultured T lymphocytes through direct cell contact [16], [17].