MyD88 Signaling Contributes to Early Pulmonary Responses to Aspergillus fumigatus

Abstract

ABSTRACT Toll-like receptors and the β-glucan receptor, dectin-1, mediate macrophage inflammatory responses to Aspergillus fumigatus through MyD88-dependent and -independent signaling mechanisms; however, pulmonary inflammatory responses in MyD88-deficient mice challenged with A. fumigatus are poorly defined. The role of MyD88 signaling in early pulmonary inflammation and fungal clearance was evaluated in C57BL/6J wild-type (WT) and MyD88-deficient (MyD88 −/− ) mice. Early (<48 h) after infection, MyD88 −/− mice had higher fungal burdens than those of WT mice, although fungal burdens rapidly declined (>72 h) in both. MyD88 −/− mice had less consolidated inflammation, with fewer NK cells, in lung tissue early (24 h) after infection than did WT mice. At the latter time point, MyD88 −/− mouse lungs were characterized by a large amount of necrotic cellular debris and fibrin, while WT lungs had organized inflammation. Although there were equivalent numbers of macrophages in WT and MyD88 −/− mouse lung tissues, MyD88 −/− cells demonstrated delayed uptake of green fluorescent protein-expressing A. fumigatus (GFP-Af293); histologically, MyD88 −/− mouse lungs had more hyphal invasion of terminal airways and vessels, the appearance of bronchiolar epithelial cell necrosis, and necrotizing vasculitis. MyD88 −/− lung homogenates contained comparatively decreased amounts of interleukin-1β (IL-1β), IL-6, KC, and gamma interferon and paradoxically increased amounts of tumor necrosis factor alpha and macrophage inflammatory protein 1α. These data indicate that the MyD88-dependent pathway mediates acute pulmonary fungal clearance, inflammation, and tissue injury very early after infection. Resolution of abnormalities within a 3-day window demonstrates the importance of redundant signaling pathways in mediating pulmonary inflammatory responses to fungi.