Ecto-5′-nucleotidase Activity Is Not Required for T Cell Activation through CD73

It had previously been shown that CD73 (ecto-5′- nucleotidase) expressed on 30% of normal human peripheral T-lymphocytes can mediate costimulatory signals in T cell activation. To check for a possible contribution of the catalytic property of CD73 to this function transfected cell lines were created. Using site-directed mutagenesis on human CD73 cDNA the codons of His92 and His194 were exchanged for alanine codons. Wild type and mutant cDNAs were cloned into an expression vector and stably expressed in the T cell line Jurkat. Surface expression as quantitated by immunofluorescence with anti CD73 mAbs was comparable in wild type and mutant transfectant clones, whereas in contrast to the wild type both His to Ala mutants completely lacked enzymatic activity. The Jurkat transfectants were stimulated with a combination of soluble anti-CD78 mAb and PMA and their response was quantitated by IL-2 production. The responses of different clones of wild type and mutant transfectants varied within a wide range; however, the ranges of wild type and mutants overlap. It is concluded that in order to transmit costimulatory signals CD73 does not require its catalytic activity.