Fas-Independent and Nonapoptotic Cytotoxicity Mediated by a Human CD4+ T-Cell Clone Directed Against an Acute Myelogenous Leukemia–Associated DEK-CAN Fusion Peptide

The mechanism underlying the cytotoxicity mediated by a human CD4+ cytotoxic T-lymphocyte (CTL) clone directed against a peptide derived from the acute myelogenous leukemia-associated fusion protein, DEK-CAN, was investigated. A DEK-CAN fusion peptide-specific CD4+ Th0 CTL clone, designated HO-1, was established from the peripheral blood lymphocytes of a healthy individual. HO-1 exerted direct but not “innocent bystander” cytotoxicity within 2 hours. The cytotoxicity mediated by HO-1 was completely Ca2+-dependent. Because HO-1 lysed peptide-loaded Fas-deficient target cells derived from a patient with a homozygous Fas gene mutation, its cytotoxicity appeared to be mediated by a Fas-independent pathway. In addition, its cytotoxicity was only partially inhibited by treatment with concanamycin A and strontium ions, which are inhibitors of the perforin-based cytotoxic pathway. Although membrane-bound type of tumor necrosis factor- (TNF-) was expressed on HO-1, an anti–TNF- antibody had no effect on HO-1–mediated cytotoxicity. HO-1 expressed mRNA for apoptosis-inducing mediators, including perforin, granzyme B, Fas ligand, TNF-, and lymphotoxin; however, no DNA fragmentation was detected in target cells incubated with HO-1 by 5-[125I]Iodo-2′-deoxyuridine release assay and agarose gel electrophoresis of DNA. Although it has been suggested that the Fas/Fas ligand system is the main pathway by which CD4+ CTL-mediated cytotoxicity is exerted in murine systems, HO-1 produced peptide-specific and HLA-restricted cytotoxicity via a Fas-independent and nonapoptotic pathway. The present study thus describes a novel mechanism of cytotoxicity mediated by CD4+ CTL.