PRF1 and chronic obstructive pulmonary disease: Several studies have shown that the number of CD8+ T cells found in the lungs of patients with COPD correlates with disease severity [43], and lung CD8+ T cells may directly cause cytotoxicity contributing to emphysema by inducing apoptosis through secretion of perforin, granzyme, and by Fas/Fas ligand (FasL) interactions [44].