1B). This demonstrated that the enhanced fitness of F5 T cells transferred to Rag1−/− hosts was indeed IL-7 dependent. We wished to examine the molecular mechanisms that were responsible for the range of cellular fitness observed in F5 T cells receiving different strengths of IL-7 signalling in vivo. First, we asked whether IL-7R– F5 T cells PD-0332991 mouse had an increased susceptibility to apoptosis. We examined caspase activity in IL-7R– F5 T cells at the earliest stages of in vitro culture by assessing fluorescently-labelled caspase inhibitor peptide (FLICA) binding to active caspases. While little caspase activity was apparent

in control F5 T cells, caspase activation was readily detectable in a significant population of IL-7R− F5 T cells during the 1 h in vitro duration of the assay (Fig. 2A). We also assessed onset of apoptosis by measuring annexin V binding to phosphatidylserine, whose translocation from inner see more to outer membrane leaf is an early event during cell death. While few viable IL-7R+ or IL-7R– F5 T cells were annexin V+ ex vivo, 1 h culture of IL-7R– F5 T cells was sufficient to induce a substantial population of high forward scatter (FSChi) Annexin V+ cells not evident in control

IL-7R+ F5 T cells (Fig. 2B). Finally, we also assessed specific activation of caspase 3, one of the executioner caspases, in IL-7R– F5 T cells directly ex vivo and following culture in vitro. Ex vivo, neither IL-7R+ F5 control nor IL-7R– F5 T cells had elevated levels of activated caspase 3, suggesting that there were not high levels of detectable apoptosis in vivo. However, following culture

for 24 h, activated caspase 3 was readily detectable in both cell types but was particularly elevated in IL-7R– F5 T cells in which viability was also more reduced (Fig. 2C). Taken together, these data indicate that the reduced fitness of IL-7R– F5 T cells Interleukin-2 receptor is associated with a very substantial elevation in their susceptibility to induction of apoptosis. It has long been recognized that T cells cultured in vitro with IL-7 up-regulate Bcl2 and this is thought to be a key mechanism through which cell survival is promoted. We therefore investigated whether modulation of Bcl2 expression in vivo by IL-7 signalling could account for the differential survival of IL-7R– F5 T cells and IL-7R+ F5 T cells from lymphopenic hosts. Examination of F5 T cells transferred to Rag1−/− hosts revealed a robust increase in Bcl2 expression levels (Fig. 3A and C), consistent with the continued survival of these cells in vitro in the absence of exogenous growth factors (Fig. 1B). The increase in Bcl2 levels observed was similar to that previously reported in F5 T cells cultured in vitro with exogenous IL-7 2. Surprisingly, in IL-7R− F5 T cells that were incapable of receiving IL-7 signalling 2, Bcl2 levels were identical to those in control IL-7R+ F5 T cells (Fig. 3B and C).