Co-stimulatory signal delivered by CD73 molecule to human CD45RAhiCD45ROlo (naive) CD8+ T lymphocytes

CD73 is a molecule expressed by a subset of CD8+ human T lymphocytes and is involved in T cell activation. CD73 expression and function were analyzed in peripheral blood CD45RAhiCD45ROlo (naive) and CD45RAloCD45ROhi (memory) CD8+ cells. We found that CD73 was expressed by a majority of naive cells (74 +/- 12%), whereas fewer memory cells were CD73+ (29 +/- 10%). Moreover, CD73 was selectively expressed by the CD11b- subset of naive CD8+ cells, which were almost all CD73+. The same result was found on CD8+ cord blood lymphocytes, which prevalently display the naive phenotype. Naive CD8+ CD11b- cells were almost unresponsive to CD3 engagement, but this apparent anergy was completely overcome when CD3 and CD73 were simultaneously cross-linked by plastic-immobilized CD73 and CD3 mAb, showing that CD73 delivers an accessory signal that allows their activation via the CD3/TCR. This costimulatory signal was tenfold more potent than that induced by CD28 ligation. A phosphotyrosine analysis by Western blotting showed that cross-linking of CD73 induced the phosphorylation of two proteins with a molecular mass of approximately 28 and 100 kDa respectively, whereas ligation of CD3 induced phosphorylation of many substrates. When CD3 and CD73 were simultaneously triggered these substrates were hypophosphorylated. Because CD73 is linked to the cell surface by a GPI anchor, the transduction of this signal is probably mediated by a lateral interaction with transmembrane molecules. This hypothesis was assessed by cocapping, which showed that CD73 associates strongly with CD45RC, moderately with CD8, and weakly with CD3. These data suggest that CD73 signaling is coupled to both tyrosine kinase and phosphatase activities.