Differential contribution of Fas- and perforin-mediated mechanisms to the cell-mediated cytotoxic activity of naive and in vivo-primed intestinal intraepithelial lymphocytes

Intestinal intraepithelial lymphocytes (IELs) are known to exert strong con stitutive cytotoxic activity, In the present study we compared the Ag-speci fic cytotoxic activity and the effector mechanisms involved in non-Ag-prime d, naive and in in vivo-primed IELs and splenic CD8 T cells. Ex vivo isolat ed naive CD8 alpha alpha TCR alpha beta IELs, CD8 alpha beta IELs, and sple nocytes from lymphocytic choriomeningitis virus (LCMV)-specific TCR transge nic mice exert Ag-specific cytotoxic activity. in a long-term, but not in a short-term, cytotoxicity assay. This cytotoxic activity is mainly Fas-Fas Ligand mediated and is significantly reduced in the presence of 20 mu g/ml Fas-Fc gamma(1) fusion protein. Both CD8 alpha beta IELs and CD8 alpha beta splenocytes isolated from LCMV-infected C57BL/6 mice exert potent perforin -dependent cell-mediated cytotoxicity. CD8 alpha alpha TCR alpha beta IELs from LCMV-infected animals, however, show only minimal Ag-specific cytotoxi city. The potent cytotoxic activity of in vivo activated CD8 alpha beta IEL s is not affected by the addition of Fas-Fc gamma(1). Nevertheless CD8 alph a beta IELs from LCMV-infected perforin-deficient mice exert Ag-specific cy totoxicity in a short-term cytotoxicity assay, and this cytotoxicity is alm ost completely blocked by the addition of Fas-Fc gamma(1). These results de monstrate that naive CD8 alpha beta IELs exert Ag-specific, Fas-Fas ligand- mediated, constitutive cytotoxic activity in a long-term cytotoxicity assay , whereas primed CD8 alpha beta IELs primarily use the perforin-dependent e xocytosis pathway to exert their potent cytotoxic activity, Furthermore, th ese results clearly illustrate the requirement for Ag-specific determinatio n of IEL-mediated cytotoxicity, because the elevated, but variable, frequen cies of memory-type T cells in this compartment may lead to ambiguous resul ts when polyclonal activation or redirected assays are used.