We have previously shown that CTL and NK cells rapidly down regulate perforin mRNA and become functionally inactive within 4-6 hr after exposure to sensitive target cells (TC). We report here for the first time that CTL also down regulate perforin mRNA upon exposure to resistant, but binding, TC. When three separate human MHC-restricted CTL lines were exposed to resistant TC, perforin mRNA was rapidly degraded. Removal of both extracellular Ca++ and Mg++ prevented perforin message down regulation, whereas removal of Ca++ alone did not, indicating that CTL: TC binding was required. Unlike the response of CTL exposed to sensitive TC, resistant TC did not trigger serine esterase (SE) release, suggesting distinct signalling pathways for perforin mRNA down regulation and granule exocytosis. Moreover, using western analysis, we showed that there was limited (< 10%) perforin protein release after CTL: TC interaction, suggesting that CTL loss of lytic activity after exposure to sensitive TC is not due to massive depletion of perforin. Treatment of CTL with mAb to CD2, CD3, CD2 + CD3, CD8, Class I and LFA-1 did not induce perforin mRNA down regulation. Furthermore, mAb to CD2, CD3, CD8, Class I, Class II, CD54 and LFA-1 did not block TC-mediated perforin mRNA down regulation although lysis of TC was inhibited.
