CD4 and juvenile idiopathic arthritis: Introduction: CD161 (KLRB1)-expressing T cells have been shown to be highly enriched in the synovial fluid of children with Juvenile Idiopathic Arthritis (JIA), with CD161 expressed on several T cell populations including Th17, some Th1 cells (‘ex-Th17’ cells), Th17/1 cells, as well as FOXP3+ regulatory CD4 T cells (Treg).