CD45 UP-REGULATION DURING LYMPHOCYTE MATURATION

CD4(+)CD8(+) double-positive thymocytes differentiate into CD4(+) and CD8(+) single-positive T cells during thymic positive selection. This process requires the interaction between the TCR and self MHC molecule s. In this context we have analyzed the expression of CD45, an abundan t transmembrane protein tyrosine phosphatase, and describe here its di fferential surface expression during T cell maturation. Using four-col or FAGS analysis of thymocytes from normal as well as TCR-transgenic m ice we demonstrate that CD45 is up-regulated only during positive sele ction concomitantly with the TCR-CD3 complex and the transient early a ctivation marker long, but that this up-regulation precedes heat stabl e antigen down-regulation. The tight linkage of the upregulation of th e TCR-CD3 complex and CD45 may be required because the CD45 tyrosine p hosphatase plays a role in modulating signal transduction by the TCR-C D3 complex during positive selection. In addition, our findings argue for a regulation mechanism that adapts the CD45 levels to increasing a ntigen receptor levels on mature T cells and B cells.