CD73 is a glycosyl phosphatidylinositol (GPI)-anchored purine salvage enzyme (ecto-5'-nucleotidase (ecto-5'-NT), E.C. whose expression on lymphocytes is dependent on their differentiation state and function. CD73 behaves as an agonistic molecule in signaling via the CD3/TCR and CD2 pathways and is associated with CTL generation, IgG production, and activation of resting naive CD8+ T cells. CD73 deficiency has been reported in a variety of patients with impaired T and/or B cell function. Thus, CD73 holds promise as a molecular target for intervention in the immune system, but the mechanisms regulating its expression are largely unknown. The aim of this study was to gain insight into the regulation of CD73 expression in human lymphocytes. CD38, another cell surface differentiation Ag with ecto-enzyme activities (NAD+ glycohydrolase, ADP-ribosyl cyclase, and cyclic ADP-ribose (cADPR) hydrolase), was found to specifically induce CD73 expression in T and B cell lines as well as in normal adult T and NK cells, cord blood T cells, and thymocytes. CD38 cross-linking induced a rapid export to the cell surface of pre-formed CD73 derived from an intracellular pool and not from de novo biosynthesis. This translocation was dependent on protein tyrosine kinase (PTK) activity and lasted approximately eight hours, after which CD73 was removed from the cell surface by enzymatic cleavage. CD73 was not induced by other agents that activate T cells and CD73 was the only GPI-anchored molecule up-regulated by CD38 ligation out of six analyzed. These results document a novel pathway in human lymphocytes leading from CD38 ligation to CD73 expression, which may result in the rapid acquisition of new functions, including increased purine salvage, increased sensitivity to Ag-induced activation, and the generation of adenosine (Ado) for Ado receptor signaling.

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