The intestinal intraepithelial lymphocytes (iIEL) may play a critical role in preventing overwhelming sensitization to foreign luminal antigens. The purpose of this experiment was to identify the subpopulation of the iIEL responsible for this inhibitory action. One-way mixed lymphocyte cultures (MLC) were performed with rat splenocytes [Brown Norway (BN) as responder; irradiated Lewis as stimulator]. BN iIEL (comprising 5% of cells per well) were added to assess inhibitory function. In the control group, irradiated BN splenocytes were added to maintain identical cell numbers. Proliferation assays were expressed as mean counts per minute (CPM) ± SD. Subpopulations of the iIEL were created by biomagnetically extracting iIELs labeled with monoclonal antibodies. The addition of iIELs to the MLC resulted in a 59% reduction in proliferation (P < 0.05). When the CD45+ population was removed from the iIEL this inhibitory activity was lost. Removal of the CD8+ population, but not the CD4+ population, also caused a loss of inhibitory activity. Separate analysis of either CD8αα+ or CD8αβ+ subpopulations identified the CD8αα+ population as having the majority of the inhibitory effect. In conclusion: 1) The iIEL has an inhibitory action on proliferation. 2) The involved population is of lymphoid origin, as a loss of CD45+ cells resulted in a loss of inhibition. 3) Loss of CD8+ iIEL cells resulted in a loss of inhibition demonstrating that these cells are responsible for this action. This inhibitory activity appears to be restricted to the CD8αα+ subpopulation.
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