Mycobacterium abscessus Glycopeptidolipid Prevents Respiratory Epithelial TLR2 Signaling as Measured by HβD2 Gene Expression and IL-8 Release

Lisa B Davidson,Douglas J Perkins,Prakasha Kempaiah,Thomas F Byrd,Rachid Nessar,Michael B Fessler

doi:10.1371/journal.pone.0029148

Abstract

Mycobacterium abscessus has emerged as an important cause of lung infection, particularly in patients with bronchiectasis. Innate immune responses must be highly effective at preventing infection with M. abscessus because it is a ubiquitous environmental saprophyte and normal hosts are not commonly infected. M. abscessus exists as either a glycopeptidolipid (GPL) expressing variant (smooth phenotype) in which GPL masks underlying bioactive cell wall lipids, or as a variant lacking GPL which is immunostimulatory and invasive in macrophage infection models. Respiratory epithelium has been increasingly recognized as playing an important role in the innate immune response to pulmonary pathogens. Respiratory epithelial cells express toll-like receptors (TLRs) which mediate the innate immune response to pulmonary pathogens. Both interleukin-8 (IL-8) and human β-defensin 2 (HβD2) are expressed by respiratory epithelial cells in response to toll-like receptor 2 (TLR2) receptor stimulation. In this study, we demonstrate that respiratory epithelial cells respond to M. abscessus variants lacking GPL with expression of IL-8 and HβD2. Furthermore, we demonstrate that this interaction is mediated through TLR2. Conversely, M. abscessus expressing GPL does not stimulate expression of IL-8 or HβD2 by respiratory epithelial cells which is consistent with “masking” of underlying bioactive cell wall lipids by GPL. Because GPL-expressing smooth variants are the predominant phenotype existing in the environment, this provides an explanation whereby initial M. abscessus colonization of abnormal lung airways escapes detection by the innate immune system.

Highlights

Mycobacterium abscessus, a nontuberculous mycobacterium, is an important emerging pathogen causing fibrocavitary lung disease which is often indistinguishable from disease caused by M. tuberculosis [1,2,3]
We demonstrate that respiratory epithelial cell toll-like receptors (TLRs) do not recognize the colonizing phenotype of M. abscessus which expresses GPL, and that loss of GPL through targeted deletion of the mmpLb4 gene converts M. abscessus to a phenotype which is recognized by toll-like receptor 2 (TLR2) on respiratory epithelial cells
In this study we demonstrate that a naturally occurring GPLexpressing smooth M. abscessus variant (390S) does not stimulate the innate immune response of respiratory epithelial cells, while rough variants lacking GPL (390R, 390V) stimulate respiratory epithelial cells through TLR2, resulting in gene expression of the downstream effector molecule human b-defensin 2 (HbD2) and release of IL-8

Summary

Introduction

Mycobacterium abscessus, a nontuberculous mycobacterium, is an important emerging pathogen causing fibrocavitary lung disease which is often indistinguishable from disease caused by M. tuberculosis [1,2,3]. It has been recognized that respiratory epithelial cells lining the lung airways play a critical role in surveillance and the innate immune response [15]. In this study we demonstrate the utility of measuring HbD2 gene expression, as well as IL-8 release, as readouts of respiratory epithelial cell responses to M. abscessus. Using these assays, we demonstrate that respiratory epithelial cell TLRs do not recognize the colonizing phenotype of M. abscessus which expresses GPL, and that loss of GPL through targeted deletion of the mmpLb4 gene converts M. abscessus to a phenotype which is recognized by toll-like receptor 2 (TLR2) on respiratory epithelial cells

Methods

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Discussion