Pulmonary fibrosis is characterized by severe lung parenchymal remodeling, with the accumulation of activated myofibroblasts and extracellular matrix. Aberrant cell differentiation is often observed, with ectopic deposits of smooth muscle cells and diffuse ossification. Adipocytes are another type of mesenchymal cells found in fibrotic lungs, whose characteristics have not been fully investigated in depth. We aimed to:

– characterize distribution, extent and immunohistochemical phenotype of adipocytes in pulmonary fibrosis;

– in vitro, demonstrate an interaction between adipocytes and lung fibroblasts in IPF, and determine its mediators.

Fixed surgical samples from fibrotic and non-fibrotic (lobectomy for cancer) lungs were reviewed for adipocyte deposits (H&E staining). Immunostainings were performed to assess adipocyte phenotype: perilipin 1 (pan-adipocyte marker), fatty-acid binding protein 4 (FABP4, lipid chaperone), perilipin 4 (white adipocyte marker) and uncoupling protein 1 (brown adipocyte marker). Perihilar fat was used as control tissue. We developed an original procedure to generate primary lung adipocyte precursors from intrapulmonary fat deposits. In vitro, primary human lung fibroblasts were cocultured with adipocytes (Transwell® inserts), with and without TGF-β1 as a pro-fibrotic stimulus. Myofibroblastic differentiation was assessed using α smooth muscle actin and collagen 1 expression in Western-Blot. Adipokine production by lung adipocytes was measured using qPCR (cell lysates) and ELISA (supernatants).

Samples from 43 Idiopathic pulmonary fibrosis (IPF; 11 biopsies, 32 explants), 13 fibrotic hypersensitivity pneumonitis (fHP), 13 fibrotic idiopathic non specific interstitial pneumonia (fNSIP), and 47 nonfibrotic lungs were analyzed. Adipocytes were found in only 1 (2%) non-fibrotic lung. Subpleural clusters of adipocytes with a white phenotype (PLIN4+ UCP1-) were detected in 86% of IPF samples [8/11 biopsies, 29/32 explants], less frequently in fHP (69%) and fNSIP (23%) (P<0.001 for all). As compared with perihilar adipocytes, lung adipocytes were smaller, more circular, and expressed FABP4 less frequently (P<0.05). Presence of adipocytes was not related to age, BMI, or severity of fibrosis (assessed by FVC or DLCO). In coculture, IPF lung derived adipocytes inhibited myofibroblastic differentiation at baseline and inhibited the effect of TGF-β1, an effect that was not cyclooxygenase-2 dependent. Adipocytes expressed various adipokines with fibrosis-modulating effects, such as leptin (detected by qPCR and ELISA), chemerin and C1qTNF3, both assayed by qPCR, which could act as potential secreted mediators.

This study identifies sub-pleural adipocyte deposits as a common event in lung fibrosis, more frequently observed in UIP. In vitro results indicate an inhibitory paracrine effect of lung adipocytes on myofibroblastic differentiation suggesting that lung adipocyte differentiation is an antifibrotic response.