Differential Interactions of Serum and Bronchoalveolar Lavage Fluid Complement Proteins with Conidia of Airborne Fungal Pathogen Aspergillus fumigatus

Infect Immun. 2020 Aug 19;88(9):e00212-20. doi: 10.1128/IAI.00212-20. Print 2020 Aug 19.

Abstract

Even though both cellular and humoral immunities contribute to host defense, the role played by humoral immunity against the airborne opportunistic fungal pathogen Aspergillus fumigatus has been underexplored. In this study, we aimed at deciphering the role of the complement system, the major humoral immune component, against A. fumigatus Mass spectrometry analysis of the proteins extracted from A. fumigatus conidial (asexual spores and infective propagules) surfaces opsonized with human serum indicated that C3 is the major complement protein involved. Flow cytometry and immunolabeling assays further confirmed C3b (activated C3) deposition on the conidial surfaces. Assays using cell wall components of conidia indicated that the hydrophobin RodAp, β-(1,3)-glucan (BG) and galactomannan (GM) could efficiently activate C3. Using complement component-depleted sera, we showed that while RodAp activates C3 by the alternative pathway, BG and GM partially follow the classical and lectin pathways, respectively. Opsonization facilitated conidial aggregation and phagocytosis, and complement receptor (CR3 and CR4) blockage on phagocytes significantly inhibited phagocytosis, indicating that the complement system exerts a protective role against conidia by opsonizing them and facilitating their phagocytosis mainly through complement receptors. Conidial opsonization with human bronchoalveolar lavage fluid (BALF) confirmed C3 to be the major complement protein interacting with conidia. Nevertheless, complement C2 and mannose-binding lectin (MBL), the classical and lectin pathway components, respectively, were not identified, indicating that BALF activates the alternative pathway on the conidial surface. Moreover, the cytokine profiles were different upon stimulation of phagocytes with serum- and BALF-opsonized conidia, highlighting the importance of studying interaction of conidia with complement proteins in their biological niche.

Keywords: Aspergillus fumigatus; Aspergillus fumigatus conidia; cell wall; complement receptors; complement system; cytokines; host-pathogen interactions; humoral immunity; macrophages; polysaccharides; proteomics.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aspergillosis / genetics
  • Aspergillosis / immunology
  • Aspergillosis / microbiology
  • Aspergillus fumigatus / chemistry
  • Aspergillus fumigatus / immunology*
  • Bronchoalveolar Lavage Fluid / chemistry
  • Bronchoalveolar Lavage Fluid / immunology*
  • Bronchoalveolar Lavage Fluid / microbiology
  • Cell Wall / chemistry
  • Cell Wall / immunology
  • Complement Activation / drug effects
  • Complement C3 / genetics
  • Complement C3 / immunology*
  • Cytokines / biosynthesis
  • Cytokines / immunology
  • Fungal Polysaccharides / immunology
  • Fungal Polysaccharides / isolation & purification
  • Fungal Polysaccharides / pharmacology*
  • Galactose / analogs & derivatives
  • Host Microbial Interactions / immunology
  • Humans
  • Immunity, Cellular
  • Immunity, Humoral
  • Integrin alphaXbeta2 / genetics
  • Integrin alphaXbeta2 / immunology
  • Macrophage-1 Antigen / genetics
  • Macrophage-1 Antigen / immunology
  • Macrophages / drug effects*
  • Macrophages / immunology
  • Macrophages / microbiology
  • Mannans / immunology
  • Mannans / isolation & purification
  • Mannans / pharmacology
  • Opsonin Proteins / pharmacology
  • Phagocytosis / drug effects
  • Primary Cell Culture
  • Protein Binding
  • Reactive Oxygen Species
  • Serum / chemistry
  • Serum / immunology*
  • Serum / microbiology
  • Spores, Fungal / chemistry
  • Spores, Fungal / immunology*
  • beta-Glucans / immunology
  • beta-Glucans / isolation & purification
  • beta-Glucans / pharmacology

Substances

  • Complement C3
  • Cytokines
  • Fungal Polysaccharides
  • Integrin alphaXbeta2
  • Macrophage-1 Antigen
  • Mannans
  • Opsonin Proteins
  • Reactive Oxygen Species
  • beta-Glucans
  • galactomannan
  • Galactose