TLR9 and idiopathic pulmonary fibrosis: Collectively, these studies suggest that TLR9 expression is enhanced in lung fibroblasts derived from rapid progressor IPF lung biopsies and the activation of this receptor leads to increased fibrotic response of human lung fibroblasts, the differentiation of monocytes into fibrocytes, and the EMT of alveolar type II cells leading to dysregulated ECM production and the deterioration of the normal lung architecture.