COSTIMULATORY SIGNAL DELIVERED BY CD-73 MOLECULE TO HUMAN CD45RA(HI)CD45RO(LO) (NAIVE) CD8-LYMPHOCYTES( T)

CD73 is a molecule expressed by a subset of CD8+ human T lymphocytes a nd is involved in T cell activation. CD73 expression and function were analyzed in peripheral blood CD45RA(hi)CD45RO(lo) (naive) and CD45RA( lo)CD45RO(hi)(memory) CD8+ cells. We found that CD73 was expressed by a majority of naive cells (74 +/- 12%), whereas fewer memory cells wer e CD73+(29 +/- 10%). Moreover, CD73 was selectively expressed by the C D11b- subset of naive CD8+ cells, which were almost all CD73+. The sam e result was found on CD8+ cord blood lymphocytes, which prevalently d isplay the naive phenotype. Naive CD8+CD11b- cells were almost unrespo nsive to CD3 engagement, but this apparent anergy was completely overc ome when CD3 and CD73 were simultaneously cross-linked by plastic-immo bilized CD73 and CD3 mAb, showing that CD73 delivers an accessory sign al that allows their activation via the CD3/TCR. This costimulatory si gnal was tenfold more potent than that induced by CD28 ligation. A pho sphotyrosine analysis by Western blotting showed that cross-linking of CD73 induced the phosphorylation of two proteins with a molecular mas s of approximately 28 and 100 kDa respectively, whereas ligation of CD 3 induced phosphorylation of many subtrates. When CD3 and CD73 were si multaneously triggered these substrates were hypophosphorylated. Becau se CD73 is linked to the cell surface by a GPI anchor, the transductio n of this signal is probably mediated by a lateral interaction with tr ansmembrane molecules. This hypothesis was assessed by cocapping, whic h showed that CD73 associates strongly with CD45RC, moderately with CD 8, and weakly with CD3. These data suggest that CD73 signaling is coup led to both tyrosine kinase and phosphatase activities.