Perforin-independent CD8(+) T-cell-mediated cytotoxicity of alveolar epithelial cells is preferentially mediated by tumor necrosis factor-alpha: relative insensitivity to Fas ligand.
CD8(+) T cells appear to play an important pathophysiologic role in many inflammatory lung diseases. The primary effector function of this T-cell subset is cytolysis of virus-infected cells, and it is widely believed that there are two primary molecular mechanisms by which this occurs: the perforin/granzyme-mediated pathway of cytolysis, and the Fas ligand (FasL)-Fas (CD95/APO-1) pathway of induction of target-cell apoptosis. This conclusion is based primarily on data obtained with hematopoetic cell lines as target cells. There is also a growing body of evidence that Fas is involved in the transduction of apoptotic signals in a variety of inflammatory disease states, particularly involving the liver and the lung. In the study reported here we took advantage of a novel in vitro assay to directly assess the effector mechanisms employed in CD8(+) T-cell-mediated cytolysis of alveolar epithelial cells. We present evidence that FasL-induced, Fas-mediated apoptosis does not directly contribute to T-cell-mediated cytolysis of alveolar epithelial-derived cells, even though Fas is expressed and functional on these cells. We also demonstrated that the perforin-independent cytolytic activity of CD8(+) T cells against alveolar epithelial-derived cells is explained entirely by tumor necrosis factor-alpha (TNF-alpha), which is expressed on CD8(+) T cells. Furthermore, we show that bystander cytolysis of alveolar epithelial-derived cells by antiviral CD8(+) T cells is entirely perforin-independent. This activity is mediated exclusively by TNF-alpha. Both alveolar epithelial-derived cells and primary murine type II cells show susceptibility to apoptosis triggered by soluble TNF-alpha, without the need for transcriptional or translational inhibition. We also confirmed the resistance of alveolar type II cells to FasL in vivo by performing adoptive transfer of perforin-deficient antiviral CD8(+) T cells into transgenic mice expressing a target antigen in type II epithelial cells. Significant lung injury developed in the transgenic CD8(+) T-cell recipients, whether or not Fas was expressed in these animals. Furthermore, preincubation of the T cells with antibody to TNF-alpha completely abolished the injury. These results suggest that alveolar epithelial cells are relatively sensitive to T cell-triggered, TNF-alpha-mediated apoptosis, and resistant to apoptosis triggered by FasL. These observations may have important ramifications for understanding of the pathophysiology of interstitial and inflammatory lung diseases.
Liu
A N
AN
Beirne B. Carter Center for Immunology Research, Department of Medicine, University of Virginia School of Medicine, Charlottesville, Virginia, USA.
Mohammed
A Z
AZ
Rice
W R
WR
Fiedeldey
D T
DT
Liebermann
J S
JS
Whitsett
J A
JA
Braciale
T J
TJ
Enelow
R I
RI
D000818:Animals; D017209:Apoptosis; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology*; D002460:Cell Line; D003602:Cytotoxicity, Immunologic; D004847:Epithelial Cells / Q000276:immunology; D053222:Fas Ligand Protein; D008562:Membrane Glycoproteins / Q000502:physiology*; D051379:Mice; D008807:Mice, Inbred BALB C; D008822:Mice, Transgenic; D054353:Perforin; D052899:Pore Forming Cytotoxic Proteins; D011650:Pulmonary Alveoli / Q000166:cytology / Q000276:immunology*; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*
D053222:Fas Ligand Protein; C505658:Fasl protein, mouse; D008562:Membrane Glycoproteins; D052899:Pore Forming Cytotoxic Proteins; D014409:Tumor Necrosis Factor-alpha; D054353:Perforin