Ligation of CD44 leads to killing activity in human peripheral mononuclear cells via MAP kinase and tyrosine kinases.

CD44 is a widely distributed transmembrane glycoprotein associated with various lymphocyte functions, including lympho-hemopoiesis, adhesion to the extracellular matrix, and T cell activation. In this study, we examined the mechanisms of CD44 involvement in regulating the killing activity of human peripheral mononuclear cells (PMC). An anti-CD44 monoclonal antibody (mAb) J173 enhanced the killing activity of PMC against Daudi and K562 cells in a dose-dependent manner. The increased cytotoxicity peaked at mAb concentration of 1.25 microg/ml. Under this condition, triggering of CD44 enhanced the killing activity by 1.5- and 2.2-fold at an effector-to-target (E/T) ratio of 20 for Daudi and K562 cells, respectively. Cytotoxic activity was remarkably diminished by treatment of PMC with concanamycin A, suggesting that this PMC-mediated cytotoxicity is mainly exerted via the perforin pathway. Moreover, we found that ligation of CD44 transduced signals to PMC that led to the tyrosine phosphorylation of several intracellular proteins and activation of mitogen-activated protein (MAP) kinase. Genistein, an inhibitor of tyrosine phosphorylation, and PD98059, an inhibitor of MAP kinase, suppressed CD44-induced enhancement of cytotoxicity. These results suggest that the CD44 molecule, which is a main receptor for hyaluronan known to be expressed on the surface of tumor cells, plays an important role in PMC-mediated cytotoxicity, and that tyrosine kinases and MAP kinase are essential for CD44-mediated signaling in cytotoxicity.