The TY/23 monoclonal antibody specifically binds to CD73 or Ecto-5'-nucleotidase (5'-NT), a 69 kDa GPI-anchored cell-surface protein with enzymatic and signal transduction activities. 5'-NT catalyzes the dephosphorylation of extracellular nucleoside 5' monophosphates into a form which can enter cells to meet their metabolic needs. It also regulates the concentration of extracellular adenosine, which initiates a variety of physiological responses through the adenosine receptors in many tissues. CD73 expression appears to be developmentally regulated on leukocytes. In the bone marrow, it is found on most CD11b+ myeloid cells and very few CD19+ cells of the B-lymphocyte lineage. It is not found on CD11b+ cells in the periphery nor on marrow-derived GMCSF-stimulated dendritic cells. Some peripheral B lymphocytes express CD73, with higher levels on isotype-switched B cells. The few thymocytes which have detectable surface CD73 are of the CD4-CD8- (double negative) and the single-positive populations. In the peripheral lymphoid organs, significant subpopulations of the CD4+ and CD8+ T lymphocytes express CD73, with variation in the percentages of CD73-bearing T cells observed among inbred mouse strains. In the thymus and peripheral lymphoid organs, CD73 is found on endothelia and stromal cells. CD73 has also been detected on bone marrow and thymic epithelial cell lines, kidney glomeruli and proximal-tubule epithelial cells, and liver endothelial cells and hepatocytes. The TY/23 mAb inhibits the enzymatic activity of CD73. Although soluble TY/23 mAb by itself does not affect T lymphocyte proliferation, it is an effective co-stimulator with PMA, but not with Concanavalin-A or plate-bound anti-CD3ε mAb 145-2C11.