LCK IS NECESSARY AND SUFFICIENT FOR FAS-LIGAND EXPRESSION AND APOPTOTIC CELL-DEATH IN MATURE CYCLING T-CELLS

Apoptotic cell death is induced in mature cycling T cells upon ligatio n of the Ag-specific TCR. This process is essential for the maintenanc e of homeostasis in the immune system, as it is capable of down-regula ting ongoing immune responses. The analysis of the mechanism underlyin g TCR-induced programmed cell death has focused the attention of many scientists recently. In this regard, several recent reports have impli cated Fas/Fas-ligand molecules as the final mediators of this process. Several other gene products have been implicated in the control of ap optosis (as Bcl-2, p53, and c-Myc); however, no information was availa ble in the early signaling molecules that trigger this phenomena. The results presented in this work indicate that pp56(lck) src family kina se is actually required for the TCR to trigger cell death in mature cy cling T cells. In fact, while inhibition of pp56(lck) expression with antisense oligonucleotides blocked TCR-induced apoptosis, pharmacologi c inhibition of phosphatidylinositol 3-kinase activity had no effect. Accordingly, ligation of the Ag receptor in a cell line defective for pp56(lck) expression was unable to induce apoptosis, although it induc ed cellular stimulation, as measured by the expression of CD69. In add ition, we show in this work that expression of constitutively active p p56(lck) mutants, but not pp59(fyn) mutants, in the absence of any oth er TCR-derived signal, is sufficient to induce apoptosis not only in t ransformed, but also in normal cycling T cells. Finally, evidence is p resented indicating that a mechanism through which pp56(lck) regulates TCR-induced apoptosis in mature cycling T cells is by controlling Fas -ligand expression.