TCR stimulation protects CD8⁺ T cells from CD95 mediated apoptosis

Activation of T cells through the T-cell receptor (TCR) induces the expression of Fas Ligand (CD95L). In turn, CD95L binds to the Fas receptor (CD95) and rapidly induces apoptosis in cycling cells. This interaction is involved in the elimination of reactive lymphocytes during an immune response. However, TCR activation cannot always trigger apoptosis because an effective immune response would then be compromised. Here we show that a short (2 to 3 h) activation of T cells through the TCR simultaneously induces an increase in CD95L mRNA and a dramatic decrease in caspase-8 mRNA levels and proteolytic activity in human CD8⁺ T cells. In addition, there is a small reduction in CD95 mRNA and CD95 levels on the cell surface. We found that preactivation of T cells protected them from apoptosis induced by either religation of the TCR or direct exposure to CD95L. These results suggest a mechanism by which cycling CD95-sensitive peripheral T cells, become protected from CD95 mediated deletion when actively engaged in the specific recognition of target cells.