Stimulated human lamina propria T cells manifest enhanced Fas-mediated apoptosis.

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Abstract
Lamina propria (LP) T cells respond poorly to a proliferative stimulus delivered via TCR/CD3 pathway, but retain considerable ability to respond to a stimulus delivered via CD2 costimulatory or accessory pathway. In the present study, we showed first that unstimulated LP T cells, as compared to unstimulated peripheral blood (PB) T cells, exhibit an increased level of apoptosis which is further increased following CD2 pathway stimulation, but not following via TCR/CD3 pathway stimulation. We next showed that IL-2 had a sparing effect on apoptosis of unstimulated LP T cells in that IL-2 decreased and anti-IL-2 increased apoptosis of these cells; in contrast, IL-2 had no effect on apoptosis of CD2-pathway stimulated cells. Finally, we showed that increased apoptosis of LP T cells induced by CD2-pathway stimulation is inhibited when Fas antigen is blocked by a nonstimulatory anti-Fas antibody. These studies suggest that LP T cells are characterized by increased susceptibility to Fas-mediated apoptosis most due to a downstream change in the Fas signaling pathway. Given that IFN-gamma secretion is significantly increased in LP T cells in which apoptosis is inhibited, this feature of LP T cells may represent a mechanism of regulating detrimental immune responses in the mucosal environment.