Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a TNF family member, which has been closely associated with regulation of the immune response and pathogenesis of autoimmune diseases. The expression of TRAIL was found in CD8 T cells that had undergone oligoclonal expansion in the lungs of patients with systemic sclerosis (scleroderma) and were able to stimulate collagen production in lung fibroblasts in vitro. Among the family members, TRAIL displays highest homology to Fas ligand, the receptor of which may mediate not only cell apoptosis but also the proliferation of normal human fibroblasts.

Considering structural and functional similarities between TRAIL and Fas ligand, we examined the effects of soluble multimerized TRAIL on normal human lung fibroblasts.

TRAIL at doses in excess of 100 ng/ml induced apoptotic death of fibroblasts. However, at lower concentrations it was found to stimulate collagen production. Collagen alpha2(I) mRNA expression was assessed by real-time RT-PCR; and total soluble collagen was measured in culture supernatants using the Sircol assay. Both alpha2(I) collagen mRNA and total soluble collagen were increased upon TRAIL stimulation, with peak response (more than fourfold increase in mRNA) at 1 ng/ml TRAIL. DNA microarray analysis revealed TRAIL-induced increase in the expression of a number of genes involved in tissue remodeling, including those related to the transforming growth factor (TGF)-β pathway. The microarray results were confirmed by Northern blot analysis of TGF-β₁ mRNA expression and by measurements of total active TGF-β₁ in culture supernatants. In addition, antibody-mediated blocking of TGF-β has shown to abrogate TRAIL-induced collagen synthesis.

These data suggest that TRAIL can enhance extracellular matrix production in fibroblasts by triggering TGF-β expression, which acts in an autocrine manner. If this process continues uncontrolled, it may contribute to the development of fibrosis, particularly in the lungs of patients with scleroderma.