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Correspondence to: Jan Paul Medema, Dept. of Immunohematology and Bloodbank, LUMC, Albinusdreef 2a, 2333AA Leiden, The Netherlands. Tel:31-71-52-63013 Fax:31-71-52-16751 E-mail:medema{at}mail.medfac.leidenuniv.nl.
The antiapoptotic protein cellular FLICE (Fas-associated death domainlike IL-1ßconverting enzyme) inhibitory protein (cFLIP) protects cells from CD95(APO-1/Fas)-induced apoptosis in vitro and was found to be overexpressed in human melanomas. However, cytotoxic T cellinduced apoptosis, which is critically involved in tumor control in vivo, is not inhibited by cFLIP in vitro, as only CD95- and not perforin-dependent lysis is affected. This calls into question whether cFLIP is sufficient to allow escape from T celldependent immunity. Using two murine tumors, we directly demonstrate that cFLIP does result in escape from T cell immunity in vivo. Moreover, tumor cells are selected in vivo for elevated cFLIP expression. Therefore, our data indicate that CD95-dependent apoptosis constitutes a more prominent mechanism for tumor clearance than has so far been anticipated and that blockade of this pathway can result in tumor escape even when the perforin pathway is operational.
Key Words: CD95, apoptosis, CTL, perforin, cytotoxicity
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