Inflammatory Monocytes Orchestrate Innate Antifungal Immunity in the Lung

Aspergillus fumigatus is an environmental fungus that causes invasive aspergillosis (IA) in immunocompromised patients. Although -CC-chemokine receptor-2 (CCR2) and Ly6C-expressing inflammatory monocytes (CCR2(+)Mo) and their derivatives initiate adaptive pulmonary immune responses, their role in coordinating innate immune responses in the lung remain poorly defined. Using conditional and antibody-mediated cell ablation strategies, we found that CCR2(+)Mo and monocyte-derived dendritic cells (Mo-DCs) are essential for innate defense against inhaled conidia. By harnessing fluorescent Aspergillus reporter (FLARE) conidia that report fungal cell association and viability in vivo, we identify two mechanisms by which CCR2(+)Mo and Mo-DCs exert innate antifungal activity. First, CCR2(+)Mo and Mo-DCs condition the lung inflammatory milieu to augment neutrophil conidiacidal activity. Second, conidial uptake by CCR2(+)Mo temporally coincided with their differentiation into Mo-DCs, a process that resulted in direct conidial killing. Our findings illustrate both indirect and direct functions for CCR2(+)Mo and their derivatives in innate antifungal immunity in the lung. Author Summary Despite the significant impact of fungal infections to human health our understanding of immunity to these pathogens remains incomplete. Human mycoses are associated with high morbidity and mortality, even with modern antifungal therapies. Aspergillus fumigatus is the most common etiologic agent of invasive aspergillosis (IA), a serious infection that develops in immunodeficient patients. In this study we employ a combination of cell ablation strategies to examine the role of CCR2(+)Ly6C(+) inflammatory monocytes (CCR2(+)Mo) in innate responses against a pulmonary infection with A.fumigatus conidia. We find that CCR2(+)Mo and their derivative dendritic cells (Mo-DCs) are required for defense against IA and that mice lacking these cells succumb to infection with A.fumigatus. Our studies indicate that CCR2(+)Mo and Mo-DCs exert crucial innate antifungal defense by two main mechanisms: 1) CCR2(+)Mo and Mo-DCs are a significant source of inflammatory mediators that augment the killing capacity of neutrophils and 2) conidial uptake by CCR2(+)Mo is coincident with their differentiation into Mo-DCs that directly kill fungal conidia via partially NADPH oxidase-dependent mechanisms. In aggregate, our studies find a novel essential function for CCR2(+)Mo in innate defense against a pulmonary fungal pathogen by mediating indirect and direct containment of fungal cells at the portal of infection.