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 in flammatory lung diseases. The primary effector function of this T-cell subs et is cytolysis of virus-infected cells, and it is widely believed that the re 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 b ased primarily on data obtained with hematopoetic cell lines as target cell s. There is also a growing body of evidence that Fas is involved in the tra nsduction 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 epithe lial cells. We present evidence that Fast-induced, Fas-mediated apoptosis d oes not directly contribute to T-cell-mediated cytolysis of alveolar epithe lial-derived cells, even though Fas is expressed and functional on these ce lls. We also demonstrated that the perforin-independent cytolytic activity of CD8(+) T cells against alveolar epithelial-derived cells is explained en tirely by tumor necrosis factor-alpha (TNF-alpha), which is expressed on CD 8(+) T cells. Furthermore, we show that bystander cytolysis of alveolar epi thelial-derived cells by antiviral CD8(+) T cells is entirely perforin-inde pendent. This activity is mediated exclusively by TNF-alpha. Both alveolar epithelial-derived cells and primary murine type II cells show susceptibili ty to apoptosis triggered by soluble TNF-alpha, without the need for transc riptional or translational inhibition. We also confirmed the resistance of alveolar type II cells to Fast in vivo by performing adoptive transfer of p erforin-deficient antiviral CD8(+) T cells into transgenic mice expressing a target antigen in type Il epithelial cells. Significant lung injury devel oped in the transgenic CD8(+) T-cell recipients, whether or not Fas was exp ressed in these animals. Furthermore, preincubation of the T cells with ant ibody to TNF-alpha completely abolished the injury. These results suggest t hat alveolar epithelial cells are relatively sensitive to T cell-triggered, TNF-alpha-mediated apoptosis, and resistant to apoptosis triggered by Fast . These observations may have important ramifications for understanding of the pathophysiology of interstitial and inflammatory lung diseases.