CD4 and juvenile idiopathic arthritis: However, CD27 is expressed to higher levels on Tregs than conventional T cells in naïve mice [17] and CD27 expression on human CD4+CD25+ T cells from peripheral blood and from the synovia of idiopathic juvenile arthritis patients has been shown to correlate with suppressive activity [43, 44], suggesting a role of CD27 for Treg function.