Modulation of Myeloid Cell Recruitment and Activation By Alveolar Epithelial Type 2 Cells Drives Early Inflammation in a Murine Model of Mutant Surfactant Protein‐C Pulmonary Fibrosis

Abstract

The missense isoleucine to threonine substitution at position 73 [I73T] in the Surfactant Protein‐C [SP‐C] gene [SFTPC], an alveolar type‐2 cell [AT2] restricted protein, has been linked to sporadic and familial cases of idiopathic pulmonary fibrosis (IPF). Many patients with idiopathic pulmonary fibrosis (IPF) experience one or more episodes of precipitous deterioration. These “acute exacerbations” are marked by altered gas exchange, neutrophilic/eosinophilic alveolitis, diffuse alveolar damage, and changes on HRCT. To temporally model the impact of SP‐CI73T expression on lung homeostasis, we generated an inducible SP‐CI73T expressing line (IER‐SP‐CI73T) by crossing a hypomorphic SP‐CI73T‐Neo founder line to a R26FlpOER line. Intraperitoneal tamoxifen administration to hypomorphic IER‐SP‐CI73T mice resulted in early increases in mutant sftpcI73T mRNA and proSP‐CI73T protein expression accompanied by allele and tamoxifen dose dependent weight loss and mortality. In this early phase (3–14 d), IER‐SP‐CI73T mice exhibited histological evidence of diffuse lung injury, increased vascular leak, elevations in total bronchoalveolar lavage (BAL) cell counts, and distinct temporal increases in BAL MCP‐1, IL‐5, Eotaxin, IL‐6, CCL‐17, KC/CXCL‐1 and TGF‐β. By 6 weeks, lungs of surviving IER‐SP‐CI73T mice showed marked alveolar septal thickening, diffuse collagen deposition, and foci of αSMA+ cells consistent with an IPF phenotype. RNA‐sequencing profiling and PCR analysis of AT2 cells from IER‐SP‐CI73T mice confirmed an epithelial contribution to these recruitment and activation factors starting at 3d. To test the downstream effects of AT2‐effector cell crosstalk, we analyzed BAL and whole lung digests via flow cytometry. Tamoxifen induction resulted in sequential accumulation of SiglecFloCD11b+CD64−CD11c−Ly6C+ monocytes (3d), LY6G+ neutrophils (7d) and SiglecFhiCD11bhiCD11clo eosinophils (2wk) in the lung. Conversely, there was significant reduction in the number of SiglecFhiCD11bintCD64hiCD11chiCD206hi resident macrophages in lung tissue digests of IER‐SP‐CI73T mice. Intratracheal and intravenous clodronate depletion was used to test the role of resident macrophages and infiltrating monocytes, respectively, in the development of the injury. Notably, early intratracheal clodronate administration was associated with enhanced mortality, corroborating the notion that resident alveolar macrophages are fundamental in the early inflammatory response and crosstalk with lung epithelia. Conversely, monocyte depletion via IV clodronate appeared to be beneficial with respect to survival and BAL cell counts at 3d and 7d suggesting a deleterious role for these proinflammatory infiltrating cells in SP‐CI73T mediated lung injury. Taken together, our findings support the concept that epithelial dysfunction is a key component of IPF and highlights the importance of AT2 crosstalk with immune compartments in the lung to mediate blood borne cell recruitment, as well as modulate the resultant acute exacerbations and fibrotic remodeling.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.