Deficiency of Psgl-1 accelerates bleomycin (BLM)-induced lung fibrosis and inflammation in mice through activating PI3K/AKT

Highlights

• Psgl-1 deficiency accelerates BLM-induced lung injury in mice.

• Psgl-1 knockout elevates fibrosis, and shows a M2-like phenotype in macrophages.

• Psgl-1 knockout exhibits reduced bronchoalveolar infiltration and re-stimulatory capacity of pf mice T-cells.

• Psgl-1 deficiency elevates inflammatory cytokines expression and promotes macrophages transformation towards to M1 phenotype.

Abstract

Pulmonary fibrosis is a disease characterized by unremitting fibrosis accumulation in the lung, causing respiratory failure eventually. Presently, the immunological mechanism underlying pulmonary fibrosis still remains unclear. P-selectin glycoprotein ligand-1 (Psgl-1) is a leukocyte ligand that regulates recruitment and activation of multiple cell types, which is associated with phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway activation. In the present study, we attempted to clarify the AKT activation induced by Psgl-1 knockout in mice with bleomycin (BLM)-induced pulmonary fibrosis. The findings indicated that after BLM induction, myeloid Psgl-1-deficient mice exhibited enhanced transforming growth factor-β 1 (TGF-β1), α-SMA and various Collagen accumulation in comparison to the wild type mice, resulting in accelerated morbidity and declined survival rate. Further, Psgl-1-knockout mice showed decreased number of macrophages and T-cells responding to BLM treatment after assays of the alveolar lavage and cell composition in the lung. Notably, abnormal macrophage polarization was observed in Psgl-1-deficient mice, accompanied with enhanced cytokines secretion after BLM induction. The findings here possibly referred to an uncontrollable wound healing related to Psgl-1 knockout, which modulates inflammatory response and macrophage phenotypes, leading to fibrosis enhancement ultimately. However, further experiments are still necessary to characterize the precise molecular mechanism of Psgl-1 in mice fibrosis.

Introduction

The pulmonary fibrosis is characterized by increased fibroblasts proliferation, interstitial inflammation and promoted extracellular matrix synthesis and deposition [1]. The pulmonary fibrosis is an important cause of morbidity and mortality with a prognostic of 5-year life [2]. Thus, the new therapeutic strategies and the underlying molecular mechanism are remained to be investigated.

TGF-β1, mainly secreted by macrophages, is considered as the most important factor for fibrotic diseases progression. It regulates fibroblasts activation, enhances collagen accumulation, and suppresses extracellular matrix degradation [3]. Herein, impeding TGF-β1 release is a key target for various drugs' investigation to prevent fibrotic disease promotion [4]. According to previous studies, Interleukin (IL)-4 and IL-13 are involved in TGF-β1 secretion, resulting in collagen deposition, which could be reduced for IL-13 knockout [5]. Additionally, pro-inflammatory cytokines, such as IL-6, IL-1β and tumor necrosis factor-α (TNF-α), are shown to possess pro-fibrotic activity [6]. Recently, studies also indicated that targeting these cytokines could decrease collagen accumulation and leukocyte infiltration [5], [7].

P-selectin glycoprotein ligand-1 (Psgl-1) has been identified to regulate leukocyte rolling and adhesion during inflammation [8]. As suggested, binding of Psgl-1 to P-selectin plays an essential role in homing of lymphocytes to the lung, regulating attachment of lymphocytes [9]. Additionally, in models of melanoma cancer in which T cell dysfunction occurs, Psgl-1 deficiency caused programmed death 1 (PD-1) decreasing, improved T cell responses, and tumor control [10]. Therefore, Psgl-1 plays an important role in balancing viral control and immunopathology and also functions to regulate T cell responses in the tumor microenvironment. Previously, Psgl-1 is reported to be associated with PI3K/AKT activity [11]. However, it is still unclear if and how Psgl-1 could modulate PI3K/AKT in myeloid cells, regulating inflammatory response and fibrosis. Through BLM-induced mice with or without Psgl-1 expression, we explore the Psgl-1 and AKT in fibrosis development, the leukocyte recruitment and the macrophages phenotypes. Our study indicated that myeloid Psgl-1 knockout with enhanced AKT activation, significantly up-regulated morbidity and down-regulated the survival rate of mice challenged with BLM, accompanied with reduced weight loss and accelerated collagen accumulation in lung tissues in comparison to the wild type (WT) group. The results might be due to the decreasing of leukocyte recruitment. Finally, elevated fibrotic and inflammatory cytokines were observed in BLM-treated mice with Psgl-1 deficiency.