Revisiting Cell Death Responses in Fibrotic Lung Disease: Crosstalk between Structured and Non-Structured Cells

Fibrosis is a life-threatening disorder caused by excessive formation of connective tissue that can affect several critical organs. Innate immune cells are involved in the development of various disorders, including lung fibrosis. To date, several hematopoietic cell types have been implicated in fibrosis, including pro-fibrotic monocytes like fibrocytes and segregated-nucleus-containing atypical monocytes (SatMs), but the precise cellular and molecular mechanisms underlying its development remain unclear. Repetitive injury and subsequent cell death response are triggering events for lung fibrosis development. Crosstalk between lung structured and non-structured cells is known to regulate the key molecular event. We recently reported that RNA-binding motif protein 7 (RBM7) expression is highly upregulated in the fibrotic lung and plays fundamental roles in fibrosis development. RBM7 regulates nuclear degradation of NEAT1 non-coding RNA, resulting in sustained apoptosis in the lung epithelium and fibrosis. Apoptotic epithelial cells produce CXCL12, which leads to the recruitment of pro-fibrotic monocytes. Apoptosis is also the main source of autoantigens. Recent studies have revealed important functions for natural autoantibodies that react with specific sets of self-antigens and are unique to individual diseases. Here, we review recent insights into lung fibrosis development in association with crosstalk between structured cells like lung epithelial cells and non-structured cells like migrating immune cells, and discuss their relevance to acquired immunity through natural autoantibody production.