# Single-cell landscape of piglet lung response with Actinobacillus pleuropneumoniae

## TL;DR

This study uses single-cell RNA sequencing to explore how piglet lungs respond to a bacterial infection that causes severe lung fibrosis.

## Contribution

The study reveals new insights into immune cell dynamics and fibrosis mechanisms in pig lungs infected with Actinobacillus pleuropneumoniae.

## Key findings

- Infected pig lungs show increased monocytes, neutrophils, and pDCs with elevated inflammatory gene expression.
- A. pleuropneumoniae reduces macrophage numbers by blocking differentiation and inducing apoptosis.
- Pathological fibroblast-like cells and epithelial-mesenchymal transition contribute to fibrosis progression.

## Abstract

Pulmonary fibrosis is a prevalent, chronic, and fatal illness that poses considerable risks to life and health. Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is an archetypal bacteria responsible for inducing significant pulmonary fibrosis, resulting in substantial economic losses in the pig industry. Nevertheless, the immune response in pig lungs against this pathogen and the specific characteristics of fibrosis remain obscure. In this study, single-cell RNA sequencing (scRNA-seq) analysis of piglet lungs with or without A. pleuropneumoniae infection identified 18 subpopulations with different phenotypes. Monocytes, neutrophils, and plasmacytoid dendritic cells (pDCs) were enriched in the lungs post-infection and responded to infection by boosting IFN-γ-inducible and inflammatory-related gene expression. A. pleuropneumoniae reduces the number of macrophages by inhibiting monocyte differentiation into interstitial macrophages (IM) and alveolar macrophages (AM) and triggering AM endogenous apoptosis. Furthermore, we identified significantly augmented pathological fibroblast-like cells that contributed to the rapid development of pulmonary fibrosis. In contrast, epithelial cells were significantly decreased and included those with features of epithelial–mesenchymal transition differentiated into fibroblasts through the signaling of TGFB1 and HIF1A. Cell-to-cell communication analysis further indicated that the interaction between the epithelial, vascular endothelial, pDCs, and fibroblast subsets, except for COL3A1 fibroblasts, was enhanced mainly via CD74/(COPA or MIF) receptor ligands after infection. Our findings elucidate the key pathogenic mechanisms driving bacterial pneumonia, while establishing a comprehensive molecular resource for developing targeted strategies against A. pleuropneumoniae infection and related human fibrotic lung disorders.