Critical role of caspases in the regulation of apoptosis and proliferation of mucosal T cells

Background & Alms: Caspases are critical mediators of apoptosis and proliferation of peripheral blood T cells (PBT), but their role in lamina propria T cells (LPT), a cell population highly susceptible to apoptosis, has not been explored. Methods: RA(+), RO+ PBT, and LPT were activated with CD3, CD2, and CD28 antibodies, and caspase activity, apoptosis, and proliferation were measured by a fluorometric assay, DNA content, and thymidine incorporation, respectively. Levels of FLIP, an endogenous inhibitor of caspase 8, were measured by immunoblotting. Results: In RA(+) and RO+ PBT, activation leads to significant increase of caspase activity but not cell death, whereas in LPT a lower elevation of caspase activity was followed by a marked degree of apoptosis. Based on the results of its inhibition, caspase 8 seemed to be essential for LPT apoptosis but, in contrast to RA(+) PBT, had no effect on proliferation. In addition, compatible with their differential susceptibility to apoptosis, levels of FLIP were lower in LPT than PBT. Conclusions: The high susceptibility of LPT to apoptosis is associated with a distinct regulation of caspase 8 activity, which seems to reflect their mucosal origin rather than simply their memory status. This unique behavior may allow proper control of mucosal T-cell proliferation while still permitting elimination by apoptosis in the face of excessive antigenic pressure.