Immature thymocytes become sensitive to calcium-mediated apoptosis with the onset of CD8, CD4, and the T cell receptor expression: A role for bcl-2?

During intrathymic negative selection by clonal deletion, crosslinking of the T cell receptor (TCR) induces cell death by delivering an apoptotic signal(s) to the nucleus along a calcium-dependent pathway. We investigated the reactivity of early precursor-containing thymocytes to Ca²⁺-induced signals, and discovered a breakpoint in their sensitivity to calcium-mediated cell death (CMCD). CD25⁺ CD8^-4^- TCR^- (triple negative [TN]) thymocytes stimulated with a calcium ionophore maintain their viability and precursor activity. By contrast, their immediate progeny, CD25^-CD8^lo4^loTCRαβ^lo (triple low [TL]) cells react to calcium elevation by abrogation of precursor activity and apoptotic cell death. This developmental difference is specific for CMCD, since both CD25⁺TN and CD25^-TL cells are susceptible to steroid-induced apoptosis. The presence of bcl-2 mRNA correlates directly to the resistance to CMCD-CD25⁺ TN cells express it and CD25^-TL cells do not. These experiments show that thymocytes become sensitive to Ca²⁺-induced apoptosis as soon as they begin to express molecules that mediate thymic selection, and suggest that a concomitant downregulation of bcl-2 may mediate this phenomenon.