Apoptosis inhibition by anti-M(r) 23,000 (Thy-1) monoclonal antibodies without inducing bcl-2 expression

Mouse malignant T-lymphoma CS-21 cells grow in vitro in the presence of CA-12 stromal cells, but they undergo apoptotic cell death with DNA fragmentation when cultured alone. Because apoptosis of CS-21 cells was not inhibited by soluble factors secreted from CA-12 stromal cells, cell-cell interactions between the two seemed to be important to inhibit apoptosis. We found that CS-21 cell adhesion was mediated by M(r) 168,000 and M(r) 23,000 proteins and that apoptosis-inhibitory signals were transmitted through these proteins. In this study, we identified the M(r) 23,000 cell adhesion molecule as a glycosylphosphatidylinositol-anchored Thy-1 (CD90) glycoprotein. Cross- linking of M(r) 23,000 protein with anti-M(r) 23,000 mAb and a second antibody transiently raised the [Ca²⁺](i) and activated calcineurin in CS- 21 cells, as has been observed in normal T lymphocytes stimulated by cross- linking anti-Thy-1 mAbs. However, differing from normal T lymphocytes, CS-21 cells could grow either by the transient increase in [Ca²⁺](i) or by the activation of protein kinase C. Furthermore, M(r) 23,000 protein-mediated cell survival of CS-21 cells was not accompanied by expression of the apoptosis-inhibiting protein bcl-2, although protein kinase C-activated cell survival was attended by bcl-2 expression. These results indicate that the M(r) 23,000 protein (Thy-1) of CS-21 lymphoma cells functions as a cell adhesion molecule capable of transducing signals of cell survival and growth that are not followed by bcl-2 expression.