Regulation of T-cell apoptosis: a mixed lymphocyte reaction model

Despite the capacity for antigen-specific activation and rapid clonal expan sion, homeostatic mechanisms ensure that the mature immune system contains a relatively stable number of T cells. In recent years, it has become appar ent that this stability is a consequence of apoptotic death of most of the specific T cells generated during an immune response. Clearly this process must be tightly regulated in order to retain sufficient T-cell progeny to m ediate an effective response, whilst allowing the rapid deletion of these c ells at the end of the response to prevent lymphadenopathy and cross-reacti ve autoimmunity. In this study, the factors that regulate the sensitivity o f T cells to apoptosis were investigated in vitro after the induction of pr imary T-cell activation within a mixed lymphocyte reaction (MLR). It was fo und that activated T cells rapidly acquire the expression of both Fas and F as ligand (FasL) on their surface and contain high levels of the precursor form of the pro-apoptotic enzyme, caspase 8 (FLICE). However, these T cells were resistant for up to 5 days to apoptosis following the stimulation of Fas; a maximal apoptotic response was observed after 7 days. This time poin t coincided with a marked reduction in expression of the FLICE inhibitory p rotein (FLIP) and maximal activity of caspase 8. At time points beyond day 7, the number of viable cells in the MLR decreased further despite a reduct ion in the expression of FasL. However, the expression of interleukin-2 (IL -2) at these late time points was low, resulting in a decrease in expressio n of the anti-apoptotic protein Bcl-2. This can produce apoptosis by allowi ng leakage of cytochrome-c from mitochondria resulting in direct activation of the caspase cascade. In this study, it is shown that T cells are resist ant to apoptosis for the first 5 days after activation as a consequence of insensitivity of the Fas pathway and the presence of intracellular Bcl-2. A fter between 5 and 7 days, the cells become sensitive to Fas-mediated apopt osis while retaining Bcl-2 expression. At later time points, Fas ligation i s reduced but the cells respond to a decreased availability of IL-2 by redu cing Bcl-2 expression; this encourages further apoptosis by allowing the di rect activation of caspase enzymes.