SASP from lung senescent fibroblasts induces immunesenescence and fibrosis
Authors: Cruz Tamara , Jia Minxue , Tabib Tracy , Sembrat John , Liu Jie , Bondonese Anna , Kavanagh John , Nayra Cardenes , Bruno Tullia , Mora Ana , Lafyatis Robert , Finkel Toren , Benos Takis , Rojas Mauricio 
Published: April 2020

Rationale

Lung aging is characterized by a progressive functional impairment. One of the main features of aging is the accumulation of senescent cells, a phenotype that has been described in chronic lung diseases such as idiopathic pulmonary fibrosis (IPF). Senescent cells produce the senescence‐associated secretory phenotype (SASP), which is highly variable. Understanding SASP components would provide a new insight into the different pathologies of age‐related diseases. It is known that SASP has an effect in the immune system, and features of immunosenescence have been described in IPF patients. In this study, we point toward immunosenescence as a possible mechanism perpetrating the accumulation of senescent cells in the IPF lung and focus on the interaction between senescent fibroblast and natural killer lymphocytes (NK).

Methods

Human lung fibroblasts were isolated from explanted lungs and healthy donors. Conditioned media (CM) from lung fibroblasts were used to study the effects of the SASP on healthy fibroblasts and NKs. We used the BioID technique for the identification of the secreted proteins to study the fibroblast SASP. Fresh lung tissue was analyzed by flow cytometry to determine the lung immune response and senescence status. Single cell RNAseq was performed on 3 controls and 3 IPF. Blood from IPF and controls was collected for the study of plasma cytokines and PBMCs were analyzed by flow cytometry.

Results

CM from IPF fibroblasts induce a pro‐fibrotic and senescent phenotype on healthy control fibroblasts. Mass spectrometry of the secreted proteins in the SASP show increased secretion of extracellular matrix proteins and growth factors. Analysis of lung homogenates shows a decrease in the number of NK cells in the lower lobe of IPF patients compared to healthy controls. Looking into the NK subpopulations, there was a decrease in circulatory/cytotoxic NK cells. Single cell RNAseq analysis of the NK cluster in IPF patients revealed the upregulation of ER stress and oxidative stress response genes and downregulation of genes involved in the immune response or immunosenescence. Analysis of the fibroblast cluster reveals a profound decrease in the lung capacity to produce chemokines to recruit the circulating/cytotoxic NK population. There were no differences in the NK proportion in peripheral blood, but there was an increase in the circulating/cytotoxic NK population. The determination of the plasma cytokines confirms a reduced capacity to produce chemokines in IPF. The effect of the lung microenvironment over the NK cells is being assesed in vitro using CM from IPF fibroblasts to determine NK migration capacity, killing activity, and senescence status.

Conclusions

IPF fibroblasts induce senescence and a pro‐fibrotic phenotype in vitro through their SASP. IPF patients have reduced numbers of NK cells in the lung with an impaired phenotype that appears to be controlled by the lung microenvironment through the SASP of senescence cells. This deficiency in the cytotoxic NK activity and senescence status in the lung could be one of the altered mechanisms perpetrating the accumulation of senescent cells in IPF lungs.

Support or Funding Information

National Institute for Heart, Blood and Lung R01 HL123766