THE ENDOTHELIAL-CELL ECTO-ADPASE RESPONSIBLE FOR INHIBITION OF PLATELET-FUNCTION IS CD39

We previously demonstrated that when platelets are in motion and in pr oximity to endothelial cells, they become unresponsive to agonists (Ma rcus, A.J., L.B. Safier, K.A. Hajjar, H.L. Ullman, N. Islam, M.J. Broe kman, and A.M. Eiroa. 1991. J. Clin. Invest. 88:1690-1696). This inhib ition is due to an ecto-ADPase on the surface of endothelial cells whi ch metabolizes ADP released from activated platelets, resulting in blo ckade of the aggregation response. Human umbilical vein endothelial ce lls (HUVEC) ADPase was biochemically classified as an E-type ATP-dipho sphohydrolase. The endothelial ecto-ADPase is herein identified as CD3 9, a molecule originally characterized as a lymphoid surface antigen. All HUVEC ecto-ADPase activity was immunoprecipitated by monoclonal an tibodies to CD39. Surface localization of HUVEC CD39 was established b y confocal microscopy and flow cytometric analyses. Transfection of CO S cells with human CD39 resulted in both ecto-ADPase activity as well as surface expression of CD39. PCR analyses of cDNA obtained from HUVE C mRNA and recombinant human CD39 revealed products of the same size, and of identical sequence. Northern blot analyses demonstrated that HU VEC express the same sized transcripts for CD39 as MP-1 cells (from wh ich CD39 was originally cloned). We established the role of CD39 as a prime endothelial thromboregulator by demonstrating that CD39-transfec ted COS cells acquired the ability to inhibit ADP-induced aggregation in platelet-rich plasma. The identification of HUVEC ADPase/CD39 as a constitutively expressed potent inhibitor of platelet reactivity offer s new prospects for antithrombotic therapeusis.