PECAM1 and systemic lupus erythematosus: To determine the differentiation pathways by which ICOS+ RTE Tresps differentiate into ICOS+ CD31− memory Tresps, we determined the percentages of Ki67+ T cells within RTE Tresps, MN Tresps, CD31+ memory Tresps, and CD31− memory Tresps in healthy controls of different ages compared with SLE patients with low disease activity.