Anti-Aspergillus Activities of the Respiratory Epithelium in Health and Disease

Abstract

Respiratory epithelia fulfil multiple roles beyond that of gaseous exchange, also acting as primary custodians of lung sterility and inflammatory homeostasis. Inhaled fungal spores pose a continual antigenic, and potentially pathogenic, challenge to lung integrity against which the human respiratory mucosa has developed various tolerance and defence strategies. However, respiratory disease and immune dysfunction frequently render the human lung susceptible to fungal diseases, the most common of which are the aspergilloses, a group of syndromes caused by inhaled spores of Aspergillus fumigatus. Inhaled Aspergillus spores enter into a multiplicity of interactions with respiratory epithelia, the mechanistic bases of which are only just becoming recognized as important drivers of disease, as well as possible therapeutic targets. In this mini-review we examine current understanding of Aspergillus-epithelial interactions and, based upon the very latest developments in the field, we explore two apparently opposing schools of thought which view epithelial uptake of Aspergillus spores as either a curative or disease-exacerbating event.

Keywords: Aspergillus fumigatus; airway epithelial cells (AECs); epithelial responses; fungal pathogenesis; internalization; morphotypes; respiratory epithelium; spore uptake.

Figure 1

Temporal and mechanistic basis of A. fumigatus attachment to, and uptake by, the respiratory epithelium. Respiratory aspergilloses commence with interaction of inhaled fungal particles with the respiratory epithelium. During vegetative growth of the pathogen, individual airway epithelial cells (AECs) may become iteratively exposed to one or multiple fungal morphotypes, as well as cell surface-associated and secreted fungal factors [8]. A. fumigatus conidia bind to AECs, extracellular matrix (ECM) components, and basement membrane components [21,22,23]. The immunoprotective conidial hydrophobin RodA is required for adherence to collagen [10,18,19,20,24] and the fucose-specific conidial lectin FleA mediates fucose-dependent binding of A. fumigatus conidia to airway mucin [25,26]. The cell surface protein CspA, which becomes unmasked during conidial germination, is also required for adhesion to A549-derived ECM [18]. The H-ficolin opsonin binds to A. fumigatus conidia via l-fucose, d-mannose and N-acetylglucosamine (GalNAc) on the conidial surface and moderates adhesion to A549 cells [27,28,29]. The secreted and hyphal cell wall-associated exopolysaccharide galactosaminogalactan (GAG) mediates adherence of A. fumigatus hyphae to fibronectin and epithelial cells [19]. CalA is the first identified invasin of A. fumigatus and is required for epithelial entry in a α5β1 integrin-dependent manner [17]. Spore internalisation is thought to be dependent on E-cadherin [30,31,32], and the actin regulators phospholipase D (PLD) [9] and cofilin-1 [33]. PLD co-localises with internalised conidia, as do the late endosomal/lysosomal markers LAMP-1, CD63, and cathepsin D [6,9]. Most internalised conidia are killed, but a few remain viable and eventually germinate to escape the phagolysosome without lysis of the host cell [6]. Induction of PLD following exposure to β-1,3-glucan on the surface of germinating conidia has been demonstrated to occur in a Dectin-1-dependent manner [9]. Conidial dihydroxynaphthalane (DHN) melanin increases internalisation of A. fumigatus spores by A549 cells and by preventing the acidification of AEC phagolysosomes, promotes spore viability [34].

Figure 2

Epithelial responses to A. fumigatus. Epithelial responses to A. fumigatus vary dramatically according to fungal morphotype and host cell origin, the latter denoted in red for alveolar epithelial cells and blue for bronchial epithelial cells. Contact with A. fumigatus spores prompts rapid cytoskeletal reorganisation and loss of focal adherence, possibly via spore-associated toxins [35]. In bronchial epithelial cells recognition of germinating conidia and hyphae leads to upregulation of Dectin-1 expression at the host cell membrane and phosphorylation-mediated activation of phosphatidylinositol3-kinase (PI3K), mitogen activated protein kinase (MAPK) p38 and ERK1/2 signalling, leading to TNFα, IL-8, and β-defensin expression (HBD-2 and -9) [36,37,38]. In AECs, challenge with A. fumigatus culture filtrate prompts MAPK ERK1/2, p38, and c-Jun N-terminal protein kinases (JNK) phosphorylation, in a partially fungal protease-dependent manner, leading to lytic death of the host cell [39,40]. JNK or p38 inhibitors protect against host damage [39]. The secreted products of cultured A. fumigatus also elicit remodelling of the respiratory epithelium involving heightened expression of classical Th2 cytokines (IL-4, IL-5, IL-13), serum IgE, collagen deposition, and neutrophil (N) and eosinophil (E) recruitment [41]. Gliotoxin exposure induces cytoskeletal collapse, cell peeling, and apoptosis. Gt activates the JNK pathway leading to apoptosis. JNK triple-phosphorylates the pro-apoptotic protein Bim, leading to pro-apoptotic Bak and Bax activation, mitochondrial membrane permeabilization, and apoptosis [42,43,44,45]. Italicised text indicates responses observed in whole animals or humans.