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Fungal Keratitis: Immune Evasion, Host-Pathogen Interactions, and Virulence Factors during Aspergillus Fumigatus Infection

de Jesus-Carrion, Steven
http://rave.ohiolink.edu/etdc/view?acc_num=case1417783811

Abstract Details

2015, Doctor of Philosophy, Case Western Reserve University, Pathology.
Corneal infections with Aspergillus fumigatus, A. flavus, Fusarium solani and F. oxysporum are major causes of corneal ulcers worldwide, resulting in visual impairment and blindness. During infection, these pathogenic molds synthesize cell wall components that mask immune recognition or confer resistance to leukocyte anti-microbial responses, resulting in increased virulence. To better understand how these fungal pathogens survive in the presence of a pronounced host immune response, we examined the role of the RodA protein and cell wall chitin polymers in fungal pathogenesis during corneal infection. Airborne spores (conidia) of A. fumigatus express the RodA protein, which forms an outer layer that coats the cell wall. We therefore determined if RodA acts as a virulence factor by masking the exposure of cell wall components that would otherwise induce a host immune response. Using a RodA deficient A. fumigatus mutant strain (ΔrodA), we observed increased surface exposure of ß-glucan and α-mannose on ΔrodA conidia than on the parent strain. We also found that ΔrodA conidia induced more nuclear translocation of the NFκB p65 subunit and cytokine production in bone marrow macrophages from C57BL/6, but not Dectin-1-/- or Dectin-2-/- mice. Using a murine model of fungal keratitis, we found that ΔrodA conidia induced significantly higher cytokine production and neutrophil infiltration into infected corneas, resulting in more rapid fungal killing compared with the parent strain. This response was dependent on Dectin-1 and Dectin-2 activation. Overall, these results identify the RodA protein as a virulence factor in A. fumigatus that prevents fungal recognition by masking Dectin-1 and Dectin-2 activation, resulting in impaired fungal clearance and worse clinical disease. Chitin is a cell wall polysaccharide present in all fungi, including A. fumigatus. Chitin is inert to the host immune system; however, fragmentation of chitin can lead to pro and anti-inflammatory responses. We therefore examined the role of chitin in A. fumigatus keratitis. In vivo, we found that expression of acidic mammalian chitinase (AMCase) co-localized with neutrophils in the corneal stroma. In vitro, mouse and human neutrophils expressed AMCase in response to Aspergillus hyphal extract (AspHE), and this up-regulation was dependent on Dectin-1 and spleen tyrosine kinase (Syk). AMCase expression was also up-regulated by IL-6 and IL-23 (IL-6/23). Moreover, chemical inhibition of AMCase using Bisdionin F or C resulted in impaired neutrophil killing of hyphae, suggesting that AMCase has anti-fungal activity against A. fumigatus. Conversely, chitin synthase mutant strains were more susceptible to neutrophil killing in vitro and following corneal infection. In contrast, we found that a chitin deacetylase mutant strain that has higher cell wall chitin was resistant to neutrophil killing and was more virulent than the parent strain. Pharmacological inhibition of chitin synthesis using nikkomycin Z reduced A. fumigatus virulence during infection and augmented neutrophil-mediated killing in vitro. Finally, chemical inhibition of both chitin and ß-glucan synthesis significantly increased susceptibility to neutrophil killing. These findings indicate than AMCase is an essential mediator of anti-fungal immunity, and also identifies chitin as a virulence factor that promotes resistance to neutrophil responses during A. fumigatus keratitis. In conclusion, we have shown that the RodA protein in A. fumigatus conidia masks the exposure of surface ß-glucan and α-mannose, which prevents Dectin-1 and Dectin-2 activation in resident corneal macrophages and enhances fungal survival during infection. Further, we established an anti-fungal role for neutrophil AMCase against hyphae and identified chitin as a virulence factor in A. fumigatus that confers resistance to neutrophil killing. These results indicate that structural cell wall components can circumvent immune recognition of A. fumigatus by host innate immune cells.
Eric Pearlman (Advisor)
George Dubyak (Committee Chair)
Mahmoud Ghannoum (Committee Member)
Amy Hise (Committee Member)
Derek Abbott (Committee Member)
Clive Hamlin (Committee Member)
154 p.
Immunology

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Citations

  • de Jesus-Carrion, S. (2015). Fungal Keratitis: Immune Evasion, Host-Pathogen Interactions, and Virulence Factors during Aspergillus Fumigatus Infection [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1417783811

    APA Style (7th edition)

  • de Jesus-Carrion, Steven. Fungal Keratitis: Immune Evasion, Host-Pathogen Interactions, and Virulence Factors during Aspergillus Fumigatus Infection. 2015. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1417783811.

    MLA Style (8th edition)

  • de Jesus-Carrion, Steven. "Fungal Keratitis: Immune Evasion, Host-Pathogen Interactions, and Virulence Factors during Aspergillus Fumigatus Infection." Doctoral dissertation, Case Western Reserve University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1417783811

    Chicago Manual of Style (17th edition)

case1417783811
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© 2014, some rights reserved.Creative Commons License Fungal Keratitis: Immune Evasion, Host-Pathogen Interactions, and Virulence Factors during Aspergillus Fumigatus Infection by Steven de Jesus-Carrion is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.