the antigenic peptides by the mec class 1 to mouse cytotoxic t lymphocytes 1 effect of ritonavir on the cycototoxic t lymphocyte mediated lysis of the target cells infected with lcmv we and treated in vitro with ritonavir cytotoxic t lymphocytes were prepared from splenocytes of mice infected with lcmv and restimulated in vitro by sensitized cells either with the peptide gp33 41 fig2 a or with the peptide np396 404 fig2 b their cytolytic activity was tested for against mc57 fibroblasts h 2 b infected with lcmv at a multiplicity of infection of 0 04 and incubated or otherwise in the presence of ritonavir at 5 u03bcg ml for 36 hours the direct effect of ritonavir on the cytotoxic t lymphocytes in vitro was excluded by extensively washing the target cells before the lysis test these in vitro studies on viral antigen presenting cells after infection with a virus indicate that the inhibition of the cytotoxic t lymphocyte responses occurs at the level of the presentation of the antigen the incubation of the mc57 cells infected with the lcmv virus with ritonavir strongly inhibits lysis by the anti lcmv cytotoxic t lymphocytes fig2 a a similar inhibitory effect was also observed on mc57 cells treated with ritonavir and expressing upon transfection the lcmv glycoprotein fig2 b effect of ritonavir on the presentation.