Bacterial Outer Membrane Vesicles Induce Vitronectin Release Into the Bronchoalveolar Space Conferring Protection From Complement-Mediated Killing.

Magnus Paulsson,Anders Lindén,Karlhans F Che,Linda Sandblad,Johan Tham,Kristian Riesbeck,Yu-Ching Su,Ingemar Qvarfordt,Margaretha E Smith,Jonas Ahl

doi:10.3389/fmicb.2018.01559

Abstract

Pathogens causing pneumonia utilize the complement regulator vitronectin to evade complement-mediated killing. Although vitronectin is associated with several chronic lung diseases, the role of bronchoalveolar vitronectin in pneumonia has not been studied. This study sought to reveal the involvement of vitronectin in the bronchoalveolar space during pneumonia, to assess the effect of outer membrane vesicles and endotoxin on vitronectin release, and to determine whether bacterial pathogens utilize pulmonary vitronectin for evasion. Vitronectin was analyzed in cell-free bronchoalveolar lavage fluid harvested from patients with pneumonia (n = 8) and from healthy volunteers after subsegmental endotoxin instillation (n = 13). Vitronectin binding by Pseudomonas aeruginosa and Haemophilus influenzae was analyzed, and subsequent complement evasion was assessed by serum challenge. The effects of outer membrane vesicles on vitronectin production in mouse lungs and human type II alveolar epithelial cells (A549) were determined. We detected increased vitronectin concentrations in lavage fluid during pneumonia (p = 0.0063) and after bronchial endotoxin challenge (p = 0.016). The capture of vitronectin by bacteria significantly reduced complement-mediated lysis. Following challenge with vesicles, vitronectin was detected in mouse bronchoalveolar space, and mouse alveolar epithelial cells in vivo as well as A549 cells in vitro contained increased levels of vitronectin. Taken together, outer membrane vesicles and endotoxin from Gram-negative bacteria induce vitronectin, which is released into the bronchoalveolar space, and used for evasion of complement-mediated clearance.

Highlights

The complex interplay between bacteria and the pulmonary defenses of the host determines the success of the invading pathogens
Five of the pneumonia patients exhibited vitronectin concentrations exceeding this interval, with patient #2 demonstrating strongly elevated levels (88.7 μg/ml). These results indicated that the vitronectin concentrations in bronchoalveolar lavage fluid (BALF) samples were significantly higher in patients with clinical pneumonia relative to those in healthy controls
We present evidence that vitronectin is an “acute phase” protein in the lungs of humans and mice, the levels of which are elevated in the human bronchoalveolar space during clinical pneumonia

Summary

Introduction

The complex interplay between bacteria and the pulmonary defenses of the host determines the success of the invading pathogens. Pathogens have evolved to evade host defenses through strategies that include enhanced adhesion to the airway epithelium and recruitment of complement-regulatory proteins to decrease the bactericidal effect of serum (Lambris et al, 2008; Singh et al, 2010). Recent years it has been shown that a large part of the proinflammatory response induced by PAMPs in the lung is related to outer membrane vesicles. These nanoparticles are released into the bronchial lumen in large amounts, and contain major virulence factors mediating several functions at a distance from the parent bacterium (Sharpe et al, 2011; Park et al, 2013)

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