Date Published: April 8, 2016
Publisher: Public Library of Science
Author(s): Sheena C. Kerr, Gregory J. Fischer, Meenal Sinha, Orla McCabe, Jonathan M. Palmer, Tsokyi Choera, Fang Yun Lim, Michaela Wimmerova, Stephen D. Carrington, Shaopeng Yuan, Clifford A. Lowell, Stefan Oscarson, Nancy P. Keller, John V. Fahy, Donald C Sheppard.
The immune mechanisms that recognize inhaled Aspergillus fumigatus conidia to promote their elimination from the lungs are incompletely understood. FleA is a lectin expressed by Aspergillus fumigatus that has twelve binding sites for fucosylated structures that are abundant in the glycan coats of multiple plant and animal proteins. The role of FleA is unknown: it could bind fucose in decomposed plant matter to allow Aspergillus fumigatus to thrive in soil, or it may be a virulence factor that binds fucose in lung glycoproteins to cause Aspergillus fumigatus pneumonia. Our studies show that FleA protein and Aspergillus fumigatus conidia bind avidly to purified lung mucin glycoproteins in a fucose-dependent manner. In addition, FleA binds strongly to macrophage cell surface proteins, and macrophages bind and phagocytose fleA-deficient (∆fleA) conidia much less efficiently than wild type (WT) conidia. Furthermore, a potent fucopyranoside glycomimetic inhibitor of FleA inhibits binding and phagocytosis of WT conidia by macrophages, confirming the specific role of fucose binding in macrophage recognition of WT conidia. Finally, mice infected with ΔfleA conidia had more severe pneumonia and invasive aspergillosis than mice infected with WT conidia. These findings demonstrate that FleA is not a virulence factor for Aspergillus fumigatus. Instead, host recognition of FleA is a critical step in mechanisms of mucin binding, mucociliary clearance, and macrophage killing that prevent Aspergillus fumigatus pneumonia.
Aspergillus fumigatus (A. fumigatus) is an ubiquitous opportunistic pathogen that causes invasive and often fatal lung infection, particularly in immunocompromised patients . Aspergillus fumigatus produces small hydrophobic conidia that are easily inhaled into the lungs and require robust host defense mechanisms to prevent infection. The mechanisms of clearance of conidia from the lung are incompletely understood but phagocytosis by macrophages is known to be important [2–5]. Macrophages express Dectin-1, a C-type lectin that recognizes β-1-3 glucan on the surface of A. fumigatus conidia. Although the amount of surface accessible β-1-3 glucan is low on resting conidia, it is much higher in swollen conidia that appear early during germination and infection . Binding of β-glucan by Dectin-1 promotes macrophage killing of A. fumigatus conidia, and other macrophage receptors, such as the mannose receptor and toll-like receptors (TLR) -2 and -4, cooperate in this killing effect [7, 8]. Notably, however, the phagocytosis of A. fumigatus conidia by macrophages is incompletely blocked by inhibitors of Dectin-1, mannose receptor, and TLR-2/4 , which means that macrophages must employ additional mechanisms to phagocytose and kill A. fumigatus.
We have discovered that FleA produced by A. fumigatus conidia allows airway mucins to bind conidia and macrophages to effectively phagocytose them. Notably, when we engineer conidia that lack FleA, the resultant ΔfleA conidia show increased virulence in a mouse model of A. fumigatus pneumonia. Together these data uncover a novel mechanism of host defense against A. fumigatus infection in which fucosylated receptors in the airway mucus gel and on the surface of macrophages bind FleA to hasten the elimination of A. fumigatus conidia.