# Development of peribronchiolar fibrosis is associated with local immune and MAPK pathway activation in a murine model of deployment-related constrictive bronchiolitis

## TL;DR

This study explores the development of a lung disease in mice, finding that immune activation and a specific signaling pathway are involved in airway fibrosis.

## Contribution

The study identifies immune and MAPK pathway activation as key factors in the development of fibrosis in a mouse model of deployment-related constrictive bronchiolitis.

## Key findings

- Sustained club cell injury leads to small airway wall thickening due to collagen deposition.
- Proteins involved in immune activation and MAPK signaling are upregulated in injured airways.
- PD-L1+ cells and p-p90RSK+ epithelial cells increase in response to club cell injury.

## Abstract

Deployment-related constrictive bronchiolitis (DRCB), a chronic fibrotic small airway disease, has been reported in military personnel following deployment to Southwest Asia and Afghanistan. Veterans diagnosed with DRCB indicate exposure to inhalational hazards, yet the molecular pathophysiology of this disorder remains enigmatic. Club cells are local progenitors critical for repair of small airway epithelium after inhalational injury. We have previously modeled DRCB using transgenic CC-DTA mice in which sustained club cell injury induces murine constrictive bronchiolitis (mCB) that recapitulates many of the histopathologic abnormalities observed in DRCB including peribronchiolar inflammation and fibrosis. The aim of the current study was to identify molecular pathways activated at the site of injury during the development of mCB.

CC-DTA and control mice were exposed to doxycycline on protocol day 0–10 to induce club cell injury. Protein digital spatial profiling restricted to small airways was performed on lung sections harvested on protocol day 0, 10, and 20 and targeted a panel of 57 proteins focused on immune modulation and MAPK signaling.

Sustained club cell injury mediated small airway wall thickening due to enhanced deposition of subepithelial collagen fibers. Principal component analysis separated small airways of CC-DTA from control mice on day 10 and 20. Upregulated protein expression in small airways of CC-DTA versus control mice was found to be most numerous and prominent on day 20 and included an array of proteins involved in myeloid or lymphoid cell activation, signal transduction in response to extracellular stress stimuli, and regulation of cell proliferation, differentiation and survival. Immunofluorescence validated an increase in programmed death-ligand 1+ (PD-L1+) cells and in phosphorylated-p90 ribosomal S6 kinase+ (p-p90RSK+, a downstream effector of the MAPK/ERK pathway) epithelial cells in response to sustained club cell injury.

We identified features of chronic innate and adaptive immune activation and aberrant collagen deposition within injured small airways during the development of mCB. Further, our findings implicate the PD-1/PD-L1 axis and the MAPK/ERK pathway in the pathogenesis of DRCB and may reveal potential therapeutic targets for combating this disease.