IMMUNOBIOLOGY Extracellular ligation-dependent CD45RB enzymatic activity negatively regulates lipid raft signal transduction

CD45 is the most prominent membrane protein on lymphocytes. The function and regulation of this protein tyrosine phos-phatase remain largely obscure, mainly because of the lack of a known ligand, and it still remains unknown whether such tyrosine phosphatases are subject to ex-tracellular control at all. We report that an anti-CD45RB antibody (Ab) that prevents rejection and induces tolerance activates CD45RB tyrosine phosphatase enzymatic activity in T lymphocytes, allowing us to directly monitor the effects of increased CD45RB activity on signal transduction. Using both kinase substrate peptide ar-rays as well as conventional biochemis-try, we also provide evidence of the vari-ous kinases involved in bringing about the inhibitory effect of this Ab on CD3-induced T-cell receptor signaling. Further-more, we report that activated CD45RB translocates to lipid rafts and interferes with lipid raft localization and activation state of CD45 substrate Lck. Thus, these findings indeed prove that CD45 is sub-ject to extracellular control and also de-fine a novel mechanism by which recep-tor tyrosine phosphatases control lymphocyte biology and provide further insight into the intracellular signaling pathways effected by anti-CD45RB mono-clonal Ab treatment. (Blood. 2009;113: 594-603