Epithelial Expression of an Interstitial Lung Disease–Associated Mutation in Surfactant Protein-C Modulates Recruitment and Activation of Key Myeloid Cell Populations in Mice

Patients with idiopathic pulmonary fibrosis (IPF) often experience precipitous deteriorations, termed "acute exacerbations" (AE), marked by diffuse alveolitis and altered gas exchange, resulting in a significant loss of lung function or mortality. The missense isoleucine to threonine substitution at position 73 (I73T) in the alveolar type 2 cell-restricted surfactant protein-C (SP-C) gene ( SFTPC ) has been linked to clinical IPF. To better understand the sequence of events that impact AE-IPF, we leveraged a murine model of inducible SP-C I73T ( SP-C I73T/I73T Flp +/- ) expression. Following administration of tamoxifen to 8-12-wk-old mice, an upregulation of Sftpc I73T initiated a diffuse lung injury marked by increases in bronchoalveolar lavage fluid (BALF) protein and histochemical evidence of CD45 + and CD11b + cell infiltrates. Flow cytometry of collagenase-digested lung cells revealed a transient, early reduction in SiglecF hi CD11b low CD64 hi CD11c hi macrophages, countered by the sequential accumulation of SiglecF lo CD11b + CD64 - CD11c - CCR2 + Ly6C + immature macrophages (3 d), Ly6G + neutrophils (7 d), and SiglecF hi CD11b hi CD11c lo eosinophils (2 wk). By mRNA analysis, BALF cells demonstrated a time-dependent phenotypic shift from a proinflammatory (3 d) to an anti-inflammatory/profibrotic activation state, along with serial elaboration of monocyte and eosinophil recruitment factors. The i.v. administration of clodronate effectively reduced total BALF cell numbers, CCR2 + immature macrophages, and eosinophil influx while improving survival. In contrast, resident macrophage depletion from the intratracheal delivery of clodronate liposomes enhanced Sftpc I73T -induced mortality. These results using Sftpc I73T mice provide a detailed ontogeny for AE-IPF driven by alveolar epithelial dysfunction that induces a polycellular inflammation initiated by the early influx of proinflammatory CCR2 + Ly6C hi immature macrophages.