The effector function of immunocompetent cells in the gut mucosa has not yet been defined. The cytotoxic function of these cells might be important in the normal immune response and could be relevant to the mucosal damage seen in inflammatory conditions. The cytotoxic function of isolated intraepithelial and lamina propria mononuclear cells in six and 18 hour assays after the addition of various stimuli that interact with the human leukocyte antigens CD2 and CD3 on the mucosal effector cells was investigated. T cell phenotypes were determined using CD4, CD8, and HML1 to characterise cells of the appropriate compartments. Anti-CD3 and phytohaemagglutinin can induce toxic activity of lamina propria lymphocytes in most individuals after six hours and in all individuals after 18 hours. Anti-CD2, anti-CD3, and phytohaemagglutinin are similarly effective at triggering lamina propria lymphocytes. Intraepithelial lymphocytes contain predominantly CD8 and HML1 positive T cells, differentiating phenotypically intraepithelial lymphocytes from lamina propria lymphocytes. Intraepithelial lymphocytes are not cytotoxic at six hours, but have a toxic function comparable with lamina propria lymphocytes after 18 hours with all three triggers. Intraepithelial lymphocytes from inflamed mucosa (Crohn's disease and diverticulitis) mediate significantly reduced cytotoxicity in vitro compared with normal mucosa, whereas lamina propria lymphocyte toxicity is not different. Reduced numbers of cytotoxic cells and reduced reactivity to the trigger substances used after in vivo activation or cold target inhibition could explain the observed differences between intraepithelial lymphocytes from inflamed and uninflamed mucosa. Changes in cell mediated cytotoxicity of intraepithelial lymphocytes and lamina propria lymphocytes may be involved in the mucosal damage in these inflammatory conditions.
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