Aspergillus Cell Wall Melanin Blocks LC3-Associated Phagocytosis to Promote Pathogenicity

Abstract

Concealing pathogen-associated molecular patterns (PAMPs) is a principal strategy used by fungi to avoid immune recognition. Surface exposure of PAMPs during germination can leave the pathogen vulnerable. Accordingly, β-glucan surface exposure during Aspergillus fumigatus germination activates an Atg5-dependent autophagy pathway termed LC3-associated phagocytosis (LAP), which promotes fungal killing. We found that LAP activation also requires the genetic, biochemical or biological (germination) removal of A. fumigatus cell wall melanin. The attenuated virulence of melanin-deficient A. fumigatus is restored in Atg5-deficient macrophages and in mice upon conditional inactivation of Atg5 in hematopoietic cells. Mechanistically, Aspergillus melanin inhibits NADPH oxidase-dependent activation of LAP by excluding the p22phox subunit from the phagosome. Thus, two events that occur concomitantly during germination of airborne fungi, surface exposure of PAMPs and melanin removal, are necessary for LAP activation and fungal killing. LAP blockade is a general property of melanin pigments, a finding with broad physiological implications.

Differential Kinetics of Aspergillus nidulans and Aspergillus fumigatus Phagocytosis

“…LAP utilizes autophagy machinery for more efficient killing of engulfed pathogens [19,20] and MHCII-mediated antigen presentation [28]. Specifically, LAP affects the downstream maturation of Aspergillus-containing phagosomes, including acidification [19][20][21]. Therefore, delayed LAP dynamics could indicate differential processing of the 2 Aspergillus species in the phagolysosomes.…”

Pharmacological modulation of autophagy: therapeutic potential and persisting obstacles

“…Similarly, mice bearing Atg7 -deficient or Becn1 -deficient macrophages, as well as Rubcn −/− mice, were less resistant to intra-nasal Aspergillus fumigatus infection than their control litter mates 10 . Moreover, some fungal pathogens, such as A. fumigatus , seem to have evolved strategies to block LAP, including the production of a cell wall component that specifically inhibits the activation of NADPH oxidases at the phagosome 202 . Atg5 , Atg7 and autophagy-related 16-like 1 ( Atg16l1 ) in macrophages were also required for mice to control infection caused by the eukaryotic parasite Toxoplasma gondii , although such a process was not accompanied by autophagic degradation 203–205 .…”

Immunity against fungi

“…Neutrophils rapidly internalize conidia and/or yeast cells and direct the production of ROS to the fungal phagosome (66). In macrophages, NADPH oxidase-dependent LC3-associated phagocytosis represents a potential clearance mechanism for A. fumigatus conidia (67,68). Nutritional immunity, exemplified by zinc and iron sequestration, is critical for macrophage defense against intracellular H. capsulatum yeast cells (69), against fungal keratitis (70), and in chronic granulomatous disease (CGD) patients with defects in NADPH oxidase (71)(72)(73).…”