Osteopontin-expressing macrophages in dogs affected with canine idiopathic pulmonary fibrosis, a model for human idiopathic pulmonary fibrosis

Canine idiopathic pulmonary fibrosis (CIPF) affects West Highland white terriers (WHWTs) and mimics idiopathic pulmonary fibrosis. In this study, single cell RNA-sequencing (scRNA-seq) was used to compare transcriptomic expression of broncho-alveolar lavage fluid (BALF) macrophages sampled from CIPF and healthy WHWTs. Osteopontin (SPP1) transcript, encoding a glycoprotein associated with fibrosis in human, was further analyzed at the protein level by immunohistochemistry in lung tissue and ELISA in BALF and serum from CIPF WHWTs, healthy WHWTs and healthy terriers other than WHWTs.
Two pro-fibrotic macrophages clusters overexpressing SPP1 were identified by scRNA-seq: one in a larger proportion of CIPF (13.5±4.7%) compared with healthy WHWTs (2.9±0.2%, P-value:0.009); one enriched in pro-fibrotic transcripts in CIPF compared with healthy WHWTs (enrichment score: 2.01; q-value:0.008). SPP1 immunolabelling was found in all dogs in ciliated cells, smooth muscular cells and macrophages. In CIPF WHWTs, pneumocytes II and lung interstitium were also labelled, while it was the case in half of healthy WHWTs and absent in terriers. SPP1 serum concentration was higher in CIPF (2.15ng/mL [0.87-5.13]) compared with healthy WHWTs (0.63ng/mL [0.41-1.63]; P-value:0.013) and terriers (0.31ng/mL [0.19-0.51]; P-value:<0.0001), and in healthy WHWTs compared with terriers (P-value:0.002). Higher SPP1 BALF level was found in CIPF (0.34ng/mL [0.15-0.52]; P-value:<0.0001) and healthy WHWTs (0.25ng/mL [0.14-0.40]; P-value:0.002) compared with terriers (0.02ng/mL [0.01-0.08]).
Our results support that profibrotic macrophage clusters contribute to CIPF pathogenesis. The overexpression of SPP1 by macrophages but also by other lung cells might trigger increased SPP1 concentrations in BALF and serum, making SPP1 a possible biomarker for CIPF.