title: Identification of a population of CD4+ CTL that utilizes a perforin- rather than a Fas ligand-dependent cytotoxic mechanism.
abstract: Although cytotoxic activity has generally been shown to reside in the CD8+ population of T lymphocytes, there are numerous examples of cytotoxic CD4+ T cells. In the present study, we found that after depletion of CD8+ T cells, CD4+ T cells cultured in short-term in vitro mixed lymphocyte cultures developed strong Ag-specific cytotoxic activity. Such CD4+ CTL lysed both Fas+ and Fas- target cells and developed in Fas ligand-deficient gld mice. Thus, in contrast to several recent reports involving long-term T cell clones or short-term mitogen-activated splenocytes, the cytotoxic activity of these CD4+ CTL is not dependent on Fas-Fas ligand interactions. However, CD4+ cells derived in a similar manner from perforin knockout mice showed greatly decreased target cell lysis, indicating that their cytotoxic activity is primarily dependent on a perforin-based mechanism. The Fas-Fas ligand pathway has been shown to be important in the maintenance of peripheral tolerance, but to  have a minimal role in protection against viral and bacterial pathogens. Thus, the existence of a population of CD4+ CTL that utilizes perforin re-emphasizes the likely importance of these cells in a classical defensive role against foreign pathogens, in addition to their already implied role as modulators of the immune response.
authors.affiliation: Beirne B. Carter Center for Immunology Research, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
authors.forename: N S
authors.initials: NS
authors.lastname: Williams
authors.forename: V H
authors.initials: VH
authors.lastname: Engelhard
mesh_terms: D000818:Animals; D015496:CD4-Positive T-Lymphocytes / Q000187:drug effects / Q000276:immunology*; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology; D003602:Cytotoxicity, Immunologic / Q000187:drug effects*; D053222:Fas Ligand Protein; D008024:Ligands; D008213:Lymphocyte Activation / Q000187:drug effects; D008212:Lymphocyte Depletion; D008562:Membrane Glycoproteins / Q000502:physiology*; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008969:Molecular Sequence Data; D054353:Perforin; D052899:Pore Forming Cytotoxic Proteins; D013602:T-Lymphocytes, Cytotoxic / Q000187:drug effects / Q000276:immunology*; D014407:Tumor Cells, Cultured; D019014:fas Receptor / Q000276:immunology
chemical_list: D053222:Fas Ligand Protein; C505658:Fasl protein, mouse; D008024:Ligands; D008562:Membrane Glycoproteins; D052899:Pore Forming Cytotoxic Proteins; D019014:fas Receptor; D054353:Perforin