Human small intestinal mucosa harbours a small population of cytolytically active CD8+ alphabeta T lymphocytes.

Intraepithelial lymphocytes (IEL) in normal human small intestine exhibit cytotoxicity. This study was undertaken to characterize the effector cells and their mode of action. Freshly isolated jejunal IEL and lamina propria lymphocytes (LPL), as well as IEL and LPL depleted of CD4+, CD8+ and T-cell receptor (TCR)-gammadelta+ cells were used as effector cells in anti-CD3-mediated redirected cytotoxicity against a murine FcgammaR-expressing cell line. Effector cell frequencies were estimated by effector to target cell titration and limiting dilution. The capacity of IEL and LPL to kill a Fas-expressing human T-cell line was also analysed. T-cell subsets were analysed for perforin, granzyme B, Fas-ligand (FasL), tumour necrosis factor-alpha (TNF-alpha) and TNF-related apoptosis inducing ligand (TRAIL) mRNA expression by reverse transcription-polymerase chain reaction (RT-PCR). Frequencies of IEL expressing the perforin and FasL proteins were determined by immunomorphometry. Both IEL and LPL exhibited significant Ca2+-dependent, anti-CD3-mediated cytotoxicity, approximately 30% specific lysis at the effector to target cell ratio 100. The cytotoxic cells constituted, however, only a small fraction of IEL and LPL ( approximately 0.01%). CD8+ TCR-alphabeta+ cells accounted for virtually all the cytotoxicity and expressed mRNA for all five cytotoxic proteins. The frequency of granzyme B-expressing samples was higher in CD8+ cells than in CD4+ cells (P<0.05 and <0.01 for IEL and LPL, respectively). In addition, both IEL and LPL exhibited significant spontaneous anti-CD3-independent cytotoxicity against Fas-expressing human T cells. This killing was mediated by Fas-FasL interaction. On average, 2-3% of the IEL expressed perforin and FasL. We speculate that CD8+ memory cells accumulate in the jejunal mucosa and that the CD8+ TCR-alphabeta+ lymphocytes executing TCR/CD3-mediated, Ca2+-dependent cytotoxicity are classical cytotoxic T lymphocytes 'caught in the act' of eliminating infected epithelial cells through perforin/granzyme exocytosis. The observed Fas/FasL-mediated cytotoxicity may be a reflection of ongoing down-regulation of local immune responses by 'activation-induced cell death'.