Lower Oligomeric Form of Surfactant Protein D in Murine Acute Lung Injury Induces M1 Subtype Macrophages Through Calreticulin/p38 MAPK Signaling Pathway.

Dandan Li,Xiaoju Zhang,Zhilong Jiang,Linyue Pan

doi:10.3389/fimmu.2021.687506

Abstract

Surfactant protein D (SP-D) plays an important role in innate and adaptive immune responses. In this study, we found that the expression of total and de-oligomerized SP-D was significantly elevated in mice with lipopolysaccharide (LPS)-induced acute lung injury (ALI). To investigate the role of the lower oligomeric form of SP-D in the pathogenesis of ALI, we treated bone marrow-derived macrophages (BMDMs) with ALI-derived bronchoalveolar lavage (BAL) and found that SP-D in ALI BAL predominantly bound to calreticulin (CALR) on macrophages, subsequently increasing the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and expression of interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, IL-10, and CD80. However, anti-SP-D (aSP-D) and anti-calreticulin (aCALR) pretreatment reversed the SP-D binding and activation of macrophages induced by ALI BAL or de-oligomerized recombinant murine SP-D (rSP-D). Lack of signal transducer and activator of transcription (STAT)6 in STAT6-/- macrophages resulted in resistance to suppression by aCALR. Further studies in an ALI mouse model showed that blockade of pulmonary SP-D by intratracheal (i.t.), but not intraperitoneal (i.p.), administration of aSP-D attenuated the severity of ALI, accompanied by lower neutrophil infiltrates and expression of IL-1beta and IL-6. Furthermore, i.t. administration of de-oligomerized rSP-D exacerbated the severity of ALI in association with more pro-inflammatory CD45+Siglec-F(-) M1 subtype macrophages and production of IL-6, TNF-alpha, IL-1beta, and IL-18. The results indicated that SP-D in the lungs of murine ALI was de-oligomerized and participated in the pathogenesis of ALI by predominantly binding to CALR on macrophages and subsequently activating the pro-inflammatory downstream signaling pathway. Targeting de-oligomerized SP-D is a promising therapeutic strategy for the treatment of ALI and acute respiratory distress syndrome (ARDS).

Highlights

Acute lung injury (ALI) and its more severe form, acute respiratory distress syndrome (ARDS), are defined as severe complications with systemic inflammatory responses in the air spaces and lung parenchyma
Lung tissues and bronchoalveolar lavage (BAL) were collected from mice with LPS-induced ALI, and Surfactant protein D (SP-D) levels were evaluated by ELISA and Western blot analysis
The results showed that total SP-D was significantly elevated in the lung protein extracts of ALI mice compared to the phosphate buffered saline (PBS)-treated control mice (p < 0.05, n = 5) (Figure 1A)

Summary

Introduction

Acute lung injury (ALI) and its more severe form, acute respiratory distress syndrome (ARDS), are defined as severe complications with systemic inflammatory responses in the air spaces and lung parenchyma. Surfactant protein D (SP-D) is a collagen-containing C-type lectin and synthesized primarily by alveolar type II cells and unciliated bronchial epithelial cells. Serum SP-D is significantly increased in ALI/ARDS due to injury to the alveolar epithelial barrier and SP-D leakage into the blood circulation system. In addition to its role in promoting macrophage phagocytosis and clearing invading pathogens and dead cells [7, 8], SP-D is able to suppress macrophage activation through SIRP-alpha [9, 10]. Lack of SP-D in SP-D-/- mice induced lung fibrosis, alveolar epithelial injury, and early development of emphysema [11,12,13]. Administration of surfactant proteins or recombinant fragments of SP-D can attenuate the severity of lung injury and asthma in mice [14,15,16]

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Results

Conclusion