Hyper-responsiveness of IPF/UIP fibroblasts: Interplay between TGFβ1, IL-13 and CCL2

One of the hallmarks of idiopathic pulmonary fibrosis with a usual interstitial pneumonia histological pathology (IPF/UIP) is excess collagen deposition, due to enhanced fibroblast extracellular matrix synthetic activity. Studies using murine models of lung fibrosis have elucidated a pro-fibrotic pathway involving IL-13 driving CCL2, which in turn drives TGFβ1 in lung fibroblasts. Therefore, we sought to determine whether this pathway exists in the human fibrotic setting by evaluating human IPF/UIP fibroblasts. IPF/UIP fibroblasts have an increased baseline fibrotic phenotype compared to non-fibrotic fibroblasts. Interestingly, non-fibrotic fibroblasts responded in a pro-fibrotic manner to TGFβ1 but were relatively non-responsive to IL-13 or CCL2, whereas, IPF/UIP cells were hyper-responsive to TGFβ1, IL-13 and CCL2. Interestingly, TGFβ1, CCL2 and IL-13 all upregulated TGFβ receptor and IL-13 receptor expression, suggesting an ability of the mediators to modulate the function of each other. Furthermore, in vivo, neutralization of both JE and MCP5, the two functional orthologs of CCL2, during bleomycin-induced pulmonary fibrosis significantly reduced collagen deposition as well as JE and CCR2 expression. Also in the bleomycin model, CTGF, which is highly induced following TGFβ stimulation, was attenuated with anti-JE/anti-MCP5 treatment. Overall this study demonstrates an interplay between TGFβ1, IL-13 and CCL2 in IPF/UIP, where these three mediators feedback on each other, promoting the fibrotic response.