Expression profile analysis reveals that Aspergillus fumigatus but not Aspergillus niger makes type II epithelial lung cells less immunological alert

Accelerating the understanding of Aspergillus terreus: Epidemiology, physiology, immunology and advances

Aspergillus species encompass a variety of infections, ranging from invasive aspergillosis to allergic conditions, contingent upon the immune status of the host. In this spectrum, Aspergillus terreus stands out due to its emergence as a notable pathogen and its intrinsic resistance to amphotericin-B. The significance of Aspergillus-associated infections has witnessed a marked increase in the past few decades, particularly with the increasing number of immunocompromised individuals. The exploration of epidemiology, morphological transitions, immunopathology, and novel treatment approaches such as new antifungal drugs (PC945, olorofim) and combinational therapy using antifungal drugs and phytochemicals (Phytochemicals: quercetin, shikonin, artemisinin), also using immunotherapies to modulate immune response has resulted in better outcomes. Furthermore, in the context COVID-19 era and its aftermath, fungal infections have emerged as a substantial challenge for both immunocompromised and immunocompetent individuals. This is attributed to the use of immune-suppressing therapies during COVID-19 infections and the increase in transplant cases. Consequently, this review aims to provide an updated overview encompassing the epidemiology, germination events, immunopathology, and novel drug treatment strategies against Aspergillus terreus-associated infections.

Novel Host Pathways Govern Epithelial Cell Invasion of Aspergillus fumigatus

Invasive aspergillosis is initiated when Aspergillus fumigatus adheres to and invades the pulmonary epithelial cells that line the airways and alveoli. To gain deeper insight into how pulmonary epithelial cells respond to A. fumigatus invasion, we used transcriptome sequencing (RNA-seq) to determine the transcriptional response of the A549 type II alveolar epithelial cell line to infection with strains CEA10 and Af293, two clinical isolates of A. fumigatus. Upstream regulator analysis of the data indicated that while both strains activated virtually identical host cell signaling pathways after 16 h of infection, only strain CEA10 activated these pathways after 6 h of infection. Many of the pathways that were predicted to be activated by A. fumigatus, including the tumor necrosis factor (TNF), interleukin-1α (IL-1α), IL-1β, IL-17A, Toll-like receptor 2 (TLR2), and TLR4 pathways, are known to be critical for the host defense against this fungus. We also found that the platelet-derived growth factor BB (PDGF BB) and progesterone receptor (PGR) pathways were activated by A. fumigatus. Using pharmacologic inhibitors, we determined that blocking the PDGF receptor or PGR inhibited the endocytosis of both strains of A. fumigatus in an additive manner. Both the PDGF BB and PGR pathways are also predicted to be activated by infection of A549 cells with other molds, such as Rhizopus delemar and Rhizopus oryzae. Thus, these pathways may represent a common response of pulmonary epithelial cells to mold infection. IMPORTANCE Invasive aspergillosis is a deadly invasive fungal infection that initiates when Aspergillus fumigatus spores are inhaled and come into contact with the epithelial cells that line the airways and alveoli. Understanding this fungus-host interaction is important for the development of novel therapeutics. To gain a deeper understanding of how these airway epithelial cells respond to A. fumigatus during infection, we used RNA-seq to determine the transcriptional response of alveolar epithelial cells to infection with two different clinical isolates of A. fumigatus. Our analysis identified new host response pathways that have not previously been tied to infection with A. fumigatus. Pharmacological inhibition of two of these pathways inhibited the ability of A. fumigatus to invade airway epithelial cells. These two pathways are also predicted to be activated by infection with other filamentous fungi. Thus, these pathways may represent a common response of alveolar epithelial cells to mold infection.

Novel Treatment Approach for Aspergilloses by Targeting Germination

Germination of conidia is an essential process within the Aspergillus life cycle and plays a major role during the infection of hosts. Conidia are able to avoid detection by the majority of leukocytes when dormant. Germination can cause severe health problems, specifically in immunocompromised people. Aspergillosis is most often caused by Aspergillus fumigatus (A. fumigatus) and affects neutropenic patients, as well as people with cystic fibrosis (CF). These patients are often unable to effectively detect and clear the conidia or hyphae and can develop chronic non-invasive and/or invasive infections or allergic inflammatory responses. Current treatments with (tri)azoles can be very effective to combat a variety of fungal infections. However, resistance against current azoles has emerged and has been increasing since 1998. As a consequence, patients infected with resistant A. fumigatus have a reported mortality rate of 88% to 100%. Especially with the growing number of patients that harbor azole-resistant Aspergilli, novel antifungals could provide an alternative. Aspergilloses differ in defining characteristics, but germination of conidia is one of the few common denominators. By specifically targeting conidial germination with novel antifungals, early intervention might be possible. In this review, we propose several morphotypes to disrupt conidial germination, as well as potential targets. Hopefully, new antifungals against such targets could contribute to disturbing the ability of Aspergilli to germinate and grow, resulting in a decreased fungal burden on patients.

Expression of immune response genes in human corneal epithelial cells interacting with Aspergillus flavus conidia

Background

Aspergillus flavus, one of the causative agents of human fungal keratitis, can be phagocytosed by human corneal epithelial (HCE) cells and the conidia containing phagosomes mature into phagolysosomes. But the immunological responses of human corneal epithelial cells interacting with A. flavus are not clear. In this study, we report the expression of immune response related genes of HCE cells exposed to A. flavus spores using targeted transcriptomics.

Methods

Human corneal epithelial cell line and primary cultures were grown in a six-well plate and used for coculture experiments. Internalization of the conidia was confirmed by immunofluorescence microscopy of the colocalized endosomal markers CD71 and LAMP1. Total RNA was isolated, and the quantity and quality of the isolated RNA were assessed using Qubit and Bioanalyzer. NanoString nCounter platform was used for the analysis of mRNA abundance using the Human Immunology panel. R-package and nSolver software were used for data analysis. KEGG and FunRich 3.1.3 tools were used to analyze the differentially expressed genes.

Results

Different morphotypes of conidia were observed after 6 h of coculture with human corneal epithelial cells and found to be internalized by epithelial cells. NanoString profiling showed more than 20 differentially expressed genes in immortalized human corneal epithelial cell line and more than ten differentially expressed genes in primary corneal epithelial cells. Distinct set of genes were altered in their expression in cell line and primary corneal epithelial cells. KEGG pathway analysis revealed that genes associated with TNF signaling, NF-KB signaling, and Th17 signaling were up-regulated, and genes associated with chemokine signaling and B cell receptor signaling were down regulated. FunRich pathway analysis showed that pathways such as CDC42 signaling, PI3K signaling, and Arf6 trafficking events were activated by the clinical isolates CI1123 and CI1698 in both type of cells.

Conclusions

Combining the transcript analysis data from cell lines and primary cultures, we showed the up regulation of immune defense genes in A. flavus infected cells. At the same time, chemokine signaling and B cell signaling pathways are downregulated. The variability in the expression levels in the immortalized cell line and the primary cultures is likely due to the variable epigenetic reprogramming in the immortalized cells and primary cultures in the absence of any changes in the genome. It highlights the importance of using both cell types in host-pathogen interaction studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08218-5.

Distinct Cohorts of Aspergillus fumigatus Transcription Factors Are Required for Epithelial Damage Occurring via Contact- or Soluble Effector-Mediated Mechanisms

Damage to the lung epithelium is a unifying feature of disease caused by the saprophytic fungus Aspergillus fumigatus. However, the mechanistic basis and the regulatory control of such damage is poorly characterized. Previous studies have identified A. fumigatus mediated pathogenesis as occurring at early (≤ 16 hours) or late (>16 hours) phases of the fungal interaction with epithelial cells, and respectively involve direct contact with the host cell or the action of soluble factors produced by mature fungal hyphae. Both early and late phases of epithelial damage have been shown to be subject to genetic regulation by the pH-responsive transcription factor PacC. This study sought to determine whether other transcriptional regulators play a role in modulating epithelial damage. In particular, whether the early and late phases of epithelial damage are governed by same or distinct regulators. Furthermore, whether processes such as spore uptake and hyphal adhesion, that have previously been documented to promote epithelial damage, are governed by the same cohorts of epithelial regulators. Using 479 strains from the recently constructed library of A. fumigatus transcription factor null mutants, two high-throughput screens assessing epithelial cell detachment and epithelial cell lysis were conducted. A total of 17 transcription factor mutants were found to exhibit reproducible deficits in epithelial damage causation. Of these, 10 mutants were defective in causing early phase damage via epithelial detachment and 8 mutants were defective in causing late phase damage via epithelial lysis. Remarkably only one transcription factor, PacC, was required for causation of both phases of epithelial damage. The 17 mutants exhibited varied and often unique phenotypic profiles with respect to fitness, epithelial adhesion, cell wall defects, and rates of spore uptake by epithelial cells. Strikingly, 9 out of 10 mutants deficient in causing early phase damage also exhibited reduced rates of hyphal extension, and culture supernatants of 7 out of 8 mutants deficient in late phase damage were significantly less cytotoxic. Our study delivers the first high-level overview of A. fumigatus regulatory genes governing lung epithelial damage, suggesting highly coordinated genetic orchestration of host-damaging activities that govern epithelial damage in both space and time.

Variation of virulence of five Aspergillus fumigatus isolates in four different infection models

Conidia of Aspergillus fumigatus are inhaled by humans on daily basis. As a consequence, these conidia can cause infections that differ in severity ranging from allergic bronchopulmonary aspergillosis to invasive aspergillosis. In this study we compared virulence of five A. fumigatus isolates in four different infection models to address the predictive value of different model systems. Two of the A. fumigatus strains were isolated from dogs with a non-invasive sino-nasal aspergillosis (DTO271-B5 and DTO303-F3), while three strains were isolated from human patients with invasive aspergillosis (Af293, ATCC46645 and CEA10). Infection models used encompassed cultured type II A549 lung epithelial cells, Protostelium aurantium amoeba, Galleria melonella larvae and zebrafish embryos. No major differences in virulence between these five strains were observed in the lung epithelial cell model. In contrast, strain ATCC46645 was most virulent in the amoeba and zebrafish model, whereas it was much less virulent in the Galleria infection model. DTO303-F3 was most virulent in the latter model. In general, reference strain Af293 was less virulent as compared to the other strains. Genome sequence analysis showed that this latter strain differed from the other four strains in 136 SNPs in virulence-related genes. Together, our results show that virulence of individual A. fumigatus strains show significant differences between infection models. We conclude that the predictive value of different model systems varies since the relative virulence across fungal strains does not hold up across different infection model systems.

EphA2-Dependent Internalization of A. fumigatus Conidia in A549 Lung Cells Is Modulated by DHN-Melanin

Dectin-1 and ephrin type-A receptor 2 (EphA2) receptors recognize β-glucan present in the fungal cell wall. Inhibition of Dectin-1 with the monoclonal 2a11 antibody was shown to reduce internalization of conidia of the human pathogen Aspergillus fumigatus into epithelial cells. In this study, we investigated the role of the EphA2 receptor present on A549 epithelial type II lung cells in the interaction with A. fumigatus conidia. We assessed whether EphA2 is involved in association and internalization of conidia by receptor inhibition by an antibody or by using the kinase inhibitor dasatinib. A 50% reduction of internalization of conidia was observed when this receptor was blocked with either the EphA2-specific monoclonal antibody or dasatinib, which was similar when Dectin-1 was inhibited with the 2a11 monoclonal antibody. Inhibition of both receptors reduced the internalization to 40%. EphA2 inhibition was also assessed in a hydrophobin deletion strain (ΔrodA) that exposes more β-glucan and a dihydroxynaphthalene (DHN)-melanin deletion strain (ΔpksP) that exposes more glucosamine and glycoproteins. The ΔrodA strain behaved similar to the wild-type strain with or without EphA2 inhibition. In contrast, the ΔpksP mutant showed an increase in association to the A549 cells and a decrease in internalization. Internalization was not further decreased by EphA2 inhibition. Taken together, the presence of DHN-melanin in the spore cell wall results in an EphA2-dependent internalization of conidia of A. fumigatus into A549 cells.