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. 2014 Aug;82(8):3141–3153. doi: 10.1128/IAI.01726-14

Surface Structure Characterization of Aspergillus fumigatus Conidia Mutated in the Melanin Synthesis Pathway and Their Human Cellular Immune Response

Jagadeesh Bayry a,b, Audrey Beaussart c, Yves F Dufrêne c, Meenu Sharma a,b, Kushagra Bansal a,b, Olaf Kniemeyer d,e, Vishukumar Aimanianda f, Axel A Brakhage d, Srini V Kaveri a,b, Kyung J Kwon-Chung g, Jean-Paul Latgé f,✉, Anne Beauvais f,✉
Editor: G S Deepe Jr
PMCID: PMC4136205  PMID: 24818666

Abstract

In Aspergillus fumigatus, the conidial surface contains dihydroxynaphthalene (DHN)-melanin. Six-clustered gene products have been identified that mediate sequential catalysis of DHN-melanin biosynthesis. Melanin thus produced is known to be a virulence factor, protecting the fungus from the host defense mechanisms. In the present study, individual deletion of the genes involved in the initial three steps of melanin biosynthesis resulted in an altered conidial surface with masked surface rodlet layer, leaky cell wall allowing the deposition of proteins on the cell surface and exposing the otherwise-masked cell wall polysaccharides at the surface. Melanin as such was immunologically inert; however, deletion mutant conidia with modified surfaces could activate human dendritic cells and the subsequent cytokine production in contrast to the wild-type conidia. Cell surface defects were rectified in the conidia mutated in downstream melanin biosynthetic pathway, and maximum immune inertness was observed upon synthesis of vermelone onward. These observations suggest that although melanin as such is an immunologically inert material, it confers virulence by facilitating proper formation of the A. fumigatus conidial surface.

INTRODUCTION

Melanin is a pigment that exists from humans to plants and has several functions, including resistance against environmental stress, such as UV light and oxidizing agents (1, 2). In airborne fungal spores, melanin helps in invasion of the host (3, 4) and contributes to the virulence of fungal pathogens (5, 6). Fungi produce different types of melanin: dihydroxynaphthalene (DHN)-melanin, pyomelanin, and DOPA-melanin. Aspergillus fumigatus produces the pigment DHN-melanin, responsible for the characteristic gray-green color of conidia. A. fumigatus is also able to produce a brownish pigment, pyomelanin, as an alternative melanin (7). Pyomelanin is produced via degradation of l-tyrosine with homogentisic acid as the main intermediate. On the other hand, Cryptococcus neoformans and Paracoccidioides brasiliensis synthesize DOPA-melanin (8). The production of melanin has been associated with the survival of the fungal species in the host (8, 9). DHN-melanin is hydrophobic and negatively charged, which modulates the binding capacity of conidia to host fibronectin and laminin present in the lungs (10). DHN-melanin is also essential for the proper assembly of cell wall layers in A. fumigatus. Pyomelanin was shown to protect the fungus from host defense mechanism, i.e., reactive oxygen intermediates and hence considered to be protecting the fungus against immune effector cells during infection (11). DOPA-melanin contributes to host death, fungal burden, and dissemination (8).

Genes responsible for the synthesis of DHN-melanin in A. fumigatus belong to a 19-kb cluster located on chromosome 2. Six genes have been identified in this cluster, and their functions were elucidated (Fig. 1) (4–6, 12–14). PKSP (ALB1; AFUA_2G17600) is the first gene of the pathway and codes for a polyketide synthase, which is responsible for catalyzing the synthesis of the heptaketide naphthopyrone from acetyl coenzyme A (acetyl-CoA) and malonyl-CoA. The heptaketide is then shortened by hydrolysis, reduction and dehydration by Ayg1p (AFUA_2G17550), Arp2p (AFUA_2G17560), and Arp1p (AFUA_2G17580), respectively. The generated product 1,3,6,8-tetrahydroxynaphthalene is reduced again by Arp2p and the resulting vermelone is oxidized by the copper oxidase Abr1p (AFUA_2G17540) to form the 1,8-DHN-melanin, which is polymerized by the laccase Abr2p (AFUA_2G17530) (12, 15).

FIG 1.

FIG 1

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Schematic representation of melanin biosynthetic pathway.

The role of the conidial melanin in the A. fumigatus virulence has been studied by using either melanin ghosts or the pigment-less mutant, wherein the PKSP gene, which encodes a protein involved in the first step of melanin biosynthesis, has been deleted (4, 9, 16–19), These reports demonstrated that melanin protects the conidia against reactive oxygen species, masks the recognition of various A. fumigatus pattern-associated molecular patterns, inhibits macrophage apoptosis and phagolysosome fusion, and attenuates the host immune response. All of these functions of melanin contribute to the increased survival of conidia in macrophages and promote the dissemination of A. fumigatus within the host.

However, the importance of melanin in the organization of the A. fumigatus conidial cell wall, the structural organization of the conidial surface due to the lack of melanin or in the presence of melanin intermediates, and the effect of melanin intermediate biosynthetic gene deletion on the activation of host immune cells are still unknown. In the present study, by using melanin mutants that are deleted in each of the genes of the melanin synthesis pathway, we analyzed the surface structure of conidia by biochemical and biophysical methods and explored the immunomodulatory role of these conidia on human dendritic cells (DCs), the professional antigen-presenting cells that act as sentinels of the immune system. We demonstrate for the first time that until the scytalone precursor was synthesized by Arp2p, the first three melanin biosynthetic gene deletion mutants (ΔpksP, Δayg1, and Δarp2) induce the maturation of DCs and cytokine production. Upon vermelone biosynthesis after dehydratation of the scytalone by Arp1p and reduction by Arp2, the subsequent mutants (Δabr1 and Δabr2) behaved like wild-type (WT) conidia, losing their capacity to prime the maturation of DCs and cytokine production. The Δarp1 conidia having scytalone but not vermelone on their surfaces were able to induce only a weak maturation of DCs. Further, we found that activation of DCs by ΔpksP, Δayg1, and Δarp2 conidia was in part due to amorphous proteinaceous conidial surface with patchy rodlets or surface exposed other cell wall components. Δarp1 conidia phenotype was intermediate between Δarp2 and Δabr1, whereas Δabr1, Δabr2, and WT conidia lack such material and have conidial surfaces covered with rodlets, which contribute for the masking of conidial recognition by the innate immune cells.

MATERIALS AND METHODS

Fungal strains and culture conditions.

The melanin precursor ΔpksP, Δayg1, Δarp2, Δarp1, Δabr1, and Δabr2 mutant strains and the WT strain B5233 have been maintained in silica gel at J. Kwon-Chung's laboratory at the National Institutes of Health until use (6, 14, 15). All strains were cultivated on malt-agar (2%) medium at ambient temperature for at least 15 days before collecting the resting conidia. Conidia were harvested from the culture medium using 0.05% Tween 20 in water. Conidial suspensions were filtered using BD Falcon filters (BD Biosciences) to remove any mycelium. For immunolabeling and DC experiments, resting conidia were fixed with paraformaldehyde (PFA)-fixed (2.5% [vol/vol] PFA in phosphate-buffered saline [PBS]) overnight at 4°C. The fixed conidia were subsequently washed three times with 0.1 M NH4Cl and once with PBS–0.1% Tween 20.

Melanin extraction.

The isolation of melanin from the WT conidia was performed as previously described (20, 21). After the fungi were gown on malt-agar medium for 15 days at ambient temperature, the conidia of each strain were collected in 0.05% Tween-water. Briefly, conidia were treated with a combination of proteolytic (proteinase K; Sigma) and glycohydrolytic (Glucanex; Novo) enzymes, denaturant (guanidine thiocyanate), and hot, concentrated HCl (6 M). This treatment resulted in an electron-dense layer similar in size and shape to the original conidial melanin layer without underlying cell components, for which reason these electron-dense materials were called “melanin ghosts” (20).

Extraction of the AS polysaccharide fraction from conidia.

Conidia were disrupted with 0.5-mm-diameter glass beads in a FastPrep (MP Biomedicals). The conidial cell wall fraction was recovered by centrifugation, washed with water, and then freeze-dried. The dried cell wall fraction was boiled in 50 mM Tris-HCl (pH 7.4) containing 50 mM EDTA, 2% sodium dodecyl sulfate (SDS), and 40 mM β-mercaptoethanol (10 min, twice) to remove proteins and extensively washed with water to obtain cell wall polysaccharides. From the latter, the alkali-soluble (AS) fraction was extracted as described earlier (22).

Extraction of conidial surface RodA protein (RodAp) involved in rodlet formation.

RodAp was extracted from the spore surface by incubating 109 dry conidia with 48% (vol/vol) hydrofluoric acid (HF) for 72 h at 4°C (23). The contents were centrifuged (10,000 × g, 10 min), and the supernatant obtained was dried under N2. The dried material was reconstituted in H2O, and an aliquot was subjected to SDS-PAGE (15% [wt/vol]) analysis and visualized by silver nitrate staining.

Analysis of proteins on the conidial surface.

Conidia were incubated in 0.5 M NaCl solution for 2 h at room temperature at a ratio of 1010 conidia per ml. The NaCl supernatant was recovered after centrifugation and directly subjected to SDS-PAGE (10% [wt/vol]). Two-dimensional (2D) gel electrophoretic analysis of the NaCl extract was carried out as described previously with slight modifications (24, 25). A 50- to 100-μg portion of protein was loaded onto IPG strips (11 cm, pH 3 to 7; GE Healthcare Life Sciences) by in-gel rehydration. After equilibration of the IPG strips, SDS-gel electrophoresis was carried out using Criterion AnykD TGX Stain-Free precast gels (Bio-Rad). Proteins were visualized by UV light and colloidal Coomassie blue staining (Candiano2004). After scanning, the gel images were analyzed with the software Delta 2D 4.3 (Decodon). Protein spots were excised and analyzed by mass spectrometry using an ultrafleXtreme MALDI-TOF/TOF device (Bruker Daltonics) as described previously (26).

Analysis of carbohydrate on the conidial surface by fluorescence microscopy.

The mannose moieties of glycoproteins on the resting conidial surface were labeled with concanavalin-fluorescein isothiocyanate (ConA-FITC; Sigma) at 0.1 mg/ml after incubating the resting conidia for 1 h at 37°C in 0.1 M carbonate buffer (pH 9.6) containing 0.1% Tween 20. The hexosamines were labeled with wheat germ agglutinin (WGA)-FITC at 0.1 mg/ml upon incubating the resting conidia for 1 h at the room temperature in PBS containing 0.1% Tween 20. For immunolabeling, PFA-fixed conidia were incubated with different antibodies as described previously (27). β-(1,3)-Glucan was labeled with dectin-1 (Fc-dectin1, 6 μg/ml), followed by FITC-conjugated GaHu-Fab2-human IgG (5 μg/ml; kindly provided by G. Brown, University of Aberdeen, Aberdeen, United Kingdom) (28).

Antibodies and reagents for human immunology.

Recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) were from Miltenyi Biotec (France). Fluorescein isothiocyanate (FITC)-conjugated monoclonal antibodies (MAbs) to CD80 and phycoerythrin (PE)-conjugated MAbs to CD83 and CD86 were from BD Biosciences (France), and PE-conjugated MAb to CD40 was from Becton Dickinson (France). Anti-Thr202/Tyr204 phospho-extracellular signal-related kinase 1/2 (ERK1/2) and anti-Thr180/Tyr182 phospho-p38 mitogen-activated protein (MAPK) antibodies were purchased from Cell Signaling Technology (USA). Anti-β-actin antibody (AC-15) was from Sigma-Aldrich (USA).

Generation and culture of human DCs.

Monocyte-derived DCs were generated as previously described (29, 30). Immature DCs (0.5 × 106 cells/well/ml) were cultured in the presence of GM-CSF and IL-4 (cytokines) alone, with cytokines and PFA-fixed conidia of the wild type or melanin mutants (1:1 ratio), with cytokines and 1 μg of melanin extracts, with cytokines and 1 μg of AS polysaccharide fraction of A. fumigatus cell wall (positive control), or with cytokines and NaCl extracts from 0.75 × 109 conidia for 48 h. Cells were harvested, and cell-free supernatants were stored at −80°C for cytokine analysis. Cells were labeled with fluorochrome-conjugated MAbs for surface marker analysis by using LSR II flow cytometry (BD Biosciences). Five thousand events were recorded for each sample, and data were analyzed by BD FACS DIVA software (BD Biosciences).

Mixed lymphocyte reaction.

CD4+ T cells were isolated from peripheral blood mononuclear cells of healthy donors using CD4 microbeads (Miltenyi Biotec). DCs were washed extensively and cocultured with 105 responder CD4+ T cells at DC/T cell ratios of 1:10, 1:20, 1:40, and 1:80. After 4 days, either cells were harvested and cell-free supernatants were stored at −80°C for cytokine analysis, or cells were pulsed for 16 to 18 h with 0.5 μCi of [3H]thymidine. Radioactive incorporation was measured by standard liquid scintillation counting. The proliferation of cells was measured as mean counts per minute (cpm) ± the standard error of the mean (SEM) of quadruplicate values after subtracting the values of responder T cell cultures alone.

Measurement of cytokines.

Cytokines were quantified in cell-free culture supernatants using CBA human inflammation and human Th1/Th2 kits (BD Biosciences).

Statistical analysis.

A two-sided, Student-t test was used for statistical analysis. A P value of <0.05 was considered significant (*, P < 0.05; **, P < 0.01).

Immunoblotting.

Immunoblotting was performed as described previously (31). DCs were washed with ice-cold PBS in radioimmunoprecipitation assay lysis buffer (50 mM Tris-HCl [pH 7.4]; 1% NP-40; 0.25% sodium deoxycholate; 150 mM NaCl; 1 mM EDTA; 1 mM phenylmethylsulfonyl fluoride; 1 μg of aprotinin, leupeptin, and pepstatin/ml; 1 mM Na3VO4; 1 mM NaF). Equal amounts of proteins from the total cell lysates were subjected to SDS-PAGE, followed by transfer of proteins to polyvinylidene difluoride membranes. Membranes were blocked in TBST buffer (0.02 M Tris-HCl [pH 7.5], 0.15 M NaCl, 0.1% Tween 20) containing 5% nonfat dried milk and investigated using a primary antibody overnight at 4°C. After washing with TBST, membranes were incubated with horseradish peroxidase-conjugated secondary antibody (Jackson Immunologicals, USA). The blots were then developed with an enhanced chemiluminescence detection system (Perkin-Elmer, USA) according to the manufacturer's instructions.

Analysis of the conidial surface by AFM.

Conidial surfaces were analyzed by atomic force microscopy (AFM) using a Multimode VIII instrument (Bruker, Santa Barbara, CA). Sample immobilization was achieved by mechanically trapping living conidia into porous polycarbonate membranes (it4ip SA; Belgium). After a concentrated suspension of cells was filtered, the membrane was rinsed with deionized water, carefully cut, and attached to a metallic puck using double-sided tape. The mounted sample was then transferred to the AFM liquid cell, while avoiding dewetting. Imaging was performed in contact mode under minimal applied force using oxide-sharpened microfabricated Si3N4 cantilevers (MSCT; Bruker) with a nominal spring constant of 0.01 N/m. Force measurements were carried out by chemical-force microscopy (32, 33) using gold tips (OMCL-TR4; Olympus, Tokyo, Japan) coated with hydrophobic thiols. To do so, cleaned tips were immersed for 12 h in 1 mM solutions of HS(CH2)11CH3 (Sigma) in ethanol, rinsed, and dried with N2 prior to use. The cantilevers spring constants were measured by the thermal noise method (Picoforce; Bruker). Force curves were analyzed in order to determine the adhesion force between the conidia and the AFM tip. These adhesion forces were plotted as bright pixels, with brighter colors indicating larger adhesion values. The data were processed using Nanoscope analysis (Bruker) and MATLAB software (MathWorks, Natick, MA). For each strain, several images were obtained on different conidia, and force measurements were obtained in duplicate using different tips.

RESULTS

A. fumigatus conidia from ΔpksP, Δayg1, and Δarp2 mutants induce maturation and activation of human DCs.

Conidia from individual melanin biosynthetic pathways gene deletion mutants were used to study the maturation and activation of DCs. Melanin-mutant conidia deficient in the early steps of the biosynthetic pathway (ΔpksP, Δayg1, and Δarp2) induced the maturation of DCs, as demonstrated by the significantly enhanced expression of CD83 and costimulatory molecules CD86, CD80, and CD40 (Fig. 2) (34, 35). The ΔpksP, Δayg1, and Δarp2 conidia also stimulated a panel of DC-cytokines such as tumor necrosis factor alpha (TNF-α), IL-1β, IL-6, and IL-10 (Fig. 3). Upon vermelone biosynthesis, the subsequent downstream melanin biosynthetic pathway mutant conidia (Δabr1 and Δabr2) behaved like the WT strain, becoming immunologically inert (Fig. 2 and 3). The Δarp1 conidia presented an intermediate phenotype, since they induced only modest changes in the expression of costimulatory molecules of DCs and the secretion of DC-cytokines (Fig. 2 and 3).

FIG 2.

FIG 2

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Effect of melanin biosynthetic pathways mutant conidia on the maturation of human DCs. Immature DCs (0.5 × 106 cells/ml) were cultured in the presence of cytokines GM-CSF and IL-4 (Ctr DC) or with cytokines and WT conidia or melanin biosynthetic pathway mutant conidia (ΔpksP, Δayg1, Δarp2, Δarp1, Δarb1, and Δarb2) at a 1:1 ratio for 48 h. The percent expression of CD83 (A) and CD86 (B) and the mean fluorescence intensities (MFI) of CD80 (C) and CD40 (D) were analyzed by flow cytometry. The data (means ± the SEM) are from four to five donors. The level of statistical significance is indicated (*, P < 0.05; **, P < 0.01).

FIG 3.

FIG 3

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Induction of DC cytokines by the melanin biosynthetic pathways mutant conidia. Immature DCs (0.5 × 106 cells/ml) were cultured in the presence of cytokines GM-CSF and IL-4 (Ctr DC) or with cytokines and WT conidia or melanin biosynthetic pathway mutant conidia (ΔpksP, Δayg1, Δarp2, Δarp1, Δarb1, and Δarb2) at a 1:1 ratio for 48 h. Cell-free culture supernatants were analyzed for the secretion of TNF-α (A), IL-1β (B), IL-6 (C), and IL-10 (D). The data (means ± the SEM) are from four donors and are presented as pg/ml. The level of statistical significance is indicated (*, P < 0.05; **, P < 0.01).

Melanin does not stimulate DC maturation.

As shown in Fig. 4, melanin ghosts from WT conidia did not induce DC maturation (Fig. 4A). We further verified the lack of activation of DCs by analyzing the intracellular signaling pathways and the ability of DCs to induce T cell proliferation and cytokines (Fig. 4B to D). We show that melanin ghost failed to phosphorylate p38 MAPK and ERK, thus confirming the immunological inert nature of melanin (Fig. 4B). The lack of activation of DCs by WT ghost extract was also reflected in the inability of WT ghost extract-treated DCs to promote T cell proliferation (Fig. 4C) and the T cell cytokines IL-2, interferon gamma (IFN-γ), and IL-5 (Fig. 4D). In contrast, the AS polysaccharide fraction [rich in α-(1,3)-glucan] of the A. fumigatus cell wall, used as a positive control, induced maturation of DCs, activated intracellular signaling pathways, and promoted DC-mediated T cell proliferation and cytokine production (Fig. 4). Taken together, these data indicated that conidial surface melanin, either in situ or in the extracted form (melanin ghosts), failed to stimulate DC activation and cytokine production.

FIG 4.

FIG 4

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Effect of dihydroxynaphthalene (DHN)-melanin on the phenotype, intracellular signaling pathways, and T cell stimulatory abilities of DCs. (A) Immature DCs (0.5 × 106 cells/ml) were cultured in the presence of cytokines GM-CSF and IL-4 (negative control, Ctr-DC) or with cytokines and 1 μg of DHN-melanin from WT conidia (Melanin-DC) or 1 μg of the AS polysaccharide fraction of A. fumigatus cell wall (AS-DC, positive control) for 48 h. The percent expression of CD83 and CD86 and the mean fluorescence intensity (MFI) of CD80 were analyzed by flow cytometry. The data (means ± the SEM) are from four donors. (B) DCs were treated with 1 μg of DHN-melanin from WT conidia (left panels) or 1 μg of the AS polysaccharide fraction of A. fumigatus cell wall (AS, positive control; right panels) for the indicated time periods. The phosphorylation of ERK1/2 (pERK1/2) and p38 MAPK (pp38) was analyzed by immunoblotting. Med, unstimulated conditio. (C) DCs were cultured in the presence of the cytokines GM-CSF and IL-4 (negative control, Ctr-DC) or cytokines and 1 μg of melanin extracts from WT conidia or cytokines and 1 μg of the alkali-soluble (AS) fraction (positive control, AS-DC) for 48 h. After extensive washing, DCs were cocultured with CD4+ T cells at various DC/T cell ratios. The T cell proliferation was quantified by measuring [3H]thymidine incorporation, and values are presented as cpm. (D) CD4+ T cell cytokines IL-2, IFN-γ, and IL-5 in the DC-T cell cocultures described above were quantified and are presented as pg/ml. The data (means ± the SEM) are from four donors. The level of statistical significance is indicated (*, P < 0.05; **, P < 0.01).

The surface rodlet layer is masked by an amorphous hydrophilic layer.

To investigate whether the structural modifications in the ΔpksP, Δayg1, Δarp2, and Δarp1 conidia could be responsible for stimulating the DCs, conidial surfaces were imaged by AFM (36, 37) (Fig. 5A to O). Rather than using an incident beam as in classical microscopy, AFM probes the small forces acting on the sample surface (37). Three-dimensional images are generated in buffer by scanning a sharp tip over the cell surface while sensing the interaction force between the tip and the surface. Originally invented for topographic imaging, AFM has evolved into a multifunctional molecular toolkit, enabling researchers not only to observe structural details of cells to near molecular resolution (36, 38) but also to measure their biophysical properties and interactions (39–41). In contrast to the WT conidia that are covered with rodlet structure (30, 42) (Fig. 5M to O), the ΔpksP mutant conidial surface was amorphous, without organized rodlet structure (Fig. 5A to C). Some patches of organized rodlet layers were observed on the Δayg1 conidial surface (Fig. 5D to F), and their percentage increased on Δarp2 conidia (Fig. 5G to I) to finally cover almost the entire surface of the Δarp1 conidia (Fig. 5J to L). However, the rodlet layer on Δarp1 conidia appeared to be less compact and less organized than that of the WT conidia (Fig. 5M and O). The mutants of further downstream melanin biosynthetic pathway genes (Δabr1 and Δabr2) presented organized and compact rodlets on the entire surface of their conidia, similar to WT conidia (data not shown).

FIG 5.

FIG 5

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AFM imaging reveals that the loss of melanin correlates with the lack of exposed rodlet layer. AFM deflection images of the surfaces of ΔpksP (A to C), Δayg1 (D to F), Δarp2 (G to I), Δarp1 (J to L), and WT (M to O) conidia recorded in deionized water at low (A, D, G, J, and M), medium (B, E, H, K, and N), and high (C, F, I, L, and O) resolutions. Black labels “R” and “A” indicate regions made of rodlets and amorphous materials, respectively.

To investigate whether RodAp (responsible for the formation of rodlet layer on the conidial surface) is still present on the ΔpksP mutant conidial surface, all of the mutant conidia, as well as the WT conidia, were treated with HF (30). RodAp could be extracted from the ΔpksP mutant, other melanin mutants, and WT conidia. Their SDS-PAGE profiles (Fig. 6) showed that two bands of RodAp classically seen in the HF extracts of conidia were present in all of the melanin pathway mutants and WT conidia (30). A band at 18 kDa could also be observed in ΔpksP, Δayg1, and Δarp2 HF extracts (Fig. 6). Mass spectrometry (MS) and tandem MS (MS/MS) analyses showed that this 18-kDa protein corresponds to the Aspf1 antigen, suggesting the loose architecture of rodlets in these mutants. These data confirmed AFM observations that the RodAp were present but hidden by an amorphous material on the surfaces of ΔpksP, Δayg1, and Δarp2 mutant conidia.

FIG 6.

FIG 6

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SDS-PAGE (15% gel) profile of the RodAp extracted from the WT and melanin biosynthetic pathway mutant conidial surfaces using hydrofluoric acid (HF). Protein bands were revealed by silver staining. RodAp*, degraded form of RodAp due to HF treatment.

Because the presence of this amorphous material covers the hydrophobic rodlets, we sought to determine whether the observed surface changes correlated with differences in the conidial hydrophobic adhesive properties. To understand this, we mapped and quantified the nanoscale adhesion properties of WT and ΔpksP mutant conidia by AFM with hydrophobic tips. The presence of this unorganized material on the ΔpksP mutant conidial surface was associated with a dramatic reduction in their conidial surface hydrophobicity (Fig. 7). For the WT, force-distance curves recorded across the cell surface revealed large adhesion forces, ranging from 0.2 to 6 nN (Fig. 7M to O). In contrast, structural changes in ΔpksP conidia caused profound modifications in the cell surface physicochemical properties (Fig. 7A to C). Force-distance curves and force maps showed the absence of adhesion forces over the entire surface of the ΔpksP mutant conidia, indicating that this mutant is hydrophilic. The adhesion force of the other mutants of the melanin pathway increased with the rank of the mutant in the pathway, from a low adhesion with the ΔpksP and Δayg1 conidia (Fig. 7A to F) to a maximal adhesion with WT conidia (Fig. 7M to O). The low adhesion capacities of the ΔpksP, Δayg1, and Δarp2 conidia indicated a modification of the cell surface hydrophobicity that could have influenced conidial recognition by DCs.

FIG 7.

FIG 7

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Structural modifications influence the conidial surface hydrophobicity. AFM deflection images (A, D, G, J, and M) recorded in deionized water, together with adhesion force maps (x-y, 1 by 1 μm; z-range, 3 nN) (B, E, H, K, and N) and corresponding adhesion histograms (n = 1,024) (C, F, I, L, and O) recorded with hydrophobic tips on the surfaces of ΔpksP (A to C), Δayg1 (D to F), Δarp2 (G to I), Δarp1 (J to L), and WT (M to O) conidia.

Proteins are present in the amorphous hydrophilic layer of ΔpksP and Δayg1 mutant conidia.

We then investigated the chemical nature of the amorphous layer present on the surfaces of ΔpksP, Δayg1, and Δarp2 mutant conidia. A strong labeling with ConA was observed only with the ΔpksP conidia, suggesting that its surface layer is rich in glycoconjugates. However, labeling with ConA was either low or negative in other melanin mutants, including Δayg1 and Δarp2 mutants, and WT conidia (Fig. 8; data not shown). The surface amorphous material could be extracted by incubating mutant conidia (mutants for the initial steps of melanin biosynthesis) with 0.5 M NaCl for 2 h, and they were positive for protein test, suggesting the presence of glycoproteins in this conidial surface amorphous material. As shown in Fig. 9, the amount of proteins present in the extract was very high in ΔpksP mutant, followed by Δayg1 mutant. Extract from ΔpksP mutant contained 53 μg of protein per 3 × 109 conidia, whereas Δayg1 conidial extract contained 12 μg of protein. The surface extracts of other mutants conidia, i.e., Δarp2 and Δarp1 mutants contained 3.8 μg of protein, while in Δabr1, Δabr2 and WT extracts the amount of proteins was too low to detect. These results thus indicate that smaller amounts of proteins on the surface layers of Δayg1, Δarp2, and Δarp1 conidia reflected in the low or negative ConA-FITC staining of these conidia.

FIG 8.

FIG 8

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ConA-FITC labeling of ΔpksP, Δayg1, and WT strain resting conidia. Note the increase in ConA labeling on the ΔpksP mutant conidial surface compared to WT and Δayg1 conidia. Scale bar, 10 μm.

FIG 9.

FIG 9

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NaCl-extracted proteins from the surfaces of mutant and WT resting conidia. SDS-PAGE (10% gel) of proteins extracted after 2 h of incubation of resting conidia in 0.5 M NaCl was performed.

Extracted protein mixture of ΔpksP mutant was then subjected to proteomic analysis. Forty-one proteins were identified in the extract, and in silico analysis of these proteins by SigPred (http://www.cbs.dtu.dk/services/SignalP/) and CADRE (http://www.cadre-genomes.org.uk/Aspergillus_fumigatus/) revealed that all of them had a signal peptide (Table 1). Extracellular proteins, normally secreted during the vegetative growth of A. fumigatus, such as Cat1p, Exg1p, ExoG2p, Aspf1p, and ChiB1, were identified in the NaCl extract of ΔpksP resting conidia (43–46). Of the 41 proteins, nine were glycosylhydrolases. RodAp was also identified in the NaCl extract. Other proteins, such as proteasome components, translation elongation factors, pyruvate dehydrogenases, adenosine deaminase, and protein disulfide isomerase, normally found in intracellular compartments were present in very small amounts since they were identified only once or twice in the proteomic survey.

TABLE 1.

Identification of proteins extracted from the ΔpksP mutant conidial surfacea

Locus tagb Protein sequence identification no. Name
AFUA_6G12070 gi|70992475 FAD binding domain protein
AFUA_5G09580 gi|83305637 Hydrophobin RodAp
AFUA_3G02270 gi|70986104 Mycelial catalase Cat1p
AFUA_8G00630 gi|70983229 Conserved hypothetical protein
AFUA_1G05770 gi|70990956 Exo-β-1,3-glucosidase ExoG2p
AFUA_4G09280 gi|70994100 Conserved hypothetical protein
AFUA_1G03600 gi|70990522 Exo-β-1,3-glucanase Exg1p
AFUA_3G15090 gi|146323525 Adenosine deaminase family protein
AFUA_2G11440 gi|71001590 Proteasome component Pup2p
AFUA_6G06350 gi|70991357 Proteasome subunit alpha type 3
AFUA_6G04790 gi|70984160 Proteasome component Pre5p
AFUA_7G04650 gi|70987129 Proteasome component Pre3p
AFUA_3G11300 gi|70999540 Proteasome component Prs2p
AFUA_6G06450 gi|70991377 Proteasome component Pre4p
AFUB_042910 gi|159128006 Cytoskeleton assembly control protein Sla2p, putative
AFUA_7G06140 gi|70986816 Putative secreted 1,4-β-d-glucan glucanhydrolase
AFUA_1G12610 gi|70995752 Hsp70 chaperone Hsp88p
AFUA_8G05020 gi|70983560 β-N-Acetylhexosaminidase NagAp
AFUA_1G14560 gi|70996140 Mannosidase MsdSp
AFUA_5G12780 gi|70997246 Kelch repeat protein
AFUA_2G10660 gi|71001436 Mannitol-1-phosphate dehydrogenase
AFUA_6G06440 gi|70991375 Proteasome component Prs3p
AFUA_5G02330 gi|70985206 Major allergen and cytotoxin AspF1
AFUA_1G13670 gi|70995964 Conserved hypothetical protein
AFUA_3G13465 gi|146323476 Conserved hypothetical protein
AFUA_2G11900 gi|71001682 Pyruvate dehydrogenase kinase
AFUA_4G03490 gi|70982015 Tripeptidyl-peptidase (TppAp)
AFUA_1G04130 gi|70985206 FG-GAP repeat protein
AFUA_3G00590 gi|70985747 Asp-hemolysin
AFUB_018320 gi|159128674 β-Fructofuranosidase, putative
AFUA_1G12170 gi|70995660 Translation elongation factor EF-Tu
AFUA_5G00700 gi|70985526 Hypothetical protein AFUA_5G00700
AFUA_8G01980 gi|70982961 Conserved hypothetical protein
AFUA_1G13670 gi|70995964 Conserved hypothetical protein
AFUA_2G06150 gi|70989789 Protein disulfide isomerase Pdi1p
AFUA_6G04570 gi|70984206 Translation elongation factor eEF-1 subunit gamma
AFLA_093140 gi|238499799 Histone H3 methyltransferase, putative
AFUA_3G07810 gi|71000275 Succinate dehydrogenase subunit Sdh1p
AFUA_8G01410 gi|70983075 Class V chitinase ChiB1p
AFUA_3G11690 gi|70999466 Fructose-bisphosphate aldolase, class II
AFUB_007340 gi|159130919 Pyruvate dehydrogenase E1 component α-subunit, putative
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a

Identification was accomplished using MS/MS and MS with a mascot score above a threshold of 54.

b

A. fumigatus locus tag (www.aspergillusgenome.org).

In order to determine whether the proteins present on the surface of ΔpksP and Δayg1 conidia are responsible for the activation of DC by these mutant conidia, we incubated DCs with the NaCl extracts of ΔpksP, Δayg1, Δarp2, and Δarp1 mutant and WT conidia. As expected, NaCl extract of ΔpksP and Δayg1 mutants induced the maturation of DCs, whereas NaCl extracts of Δarp2, Δarp1, and WT strains did not (Fig. 10). These results demonstrated that the surface protein layer was responsible at least in part for the induction of DC maturation following incubation of cells with resting conidia of ΔpksP and Δayg1 mutants. However, the amount of proteins present on the surfaces of Δarp2 and Δarp1 conidia was too low to stimulate DCs (Fig. 10). Although NaCl extracts from ΔpksP and Δayg1 mutant conidial surfaces induced maturation of the DCs based on the phenotype analysis of cells (Fig. 10), they did not induce the production of cytokines such as IL-1β, IL-10, or IL-6 (data not shown). The level of production of above cytokines was on par with that of control DCs. These data suggest that signals provided by NaCl extracts of ΔpksP and Δayg1 mutant conidia were not sufficient to induce the functional activation of the DCs. On the other hand, ΔpksP, Δayg1, and Δarp2 mutant conidia induced various DC cytokines, which could be due to the exposure of cell wall polysaccharides on their conidial surfaces.

FIG 10.

FIG 10

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Effect of NaCl extracts from the surfaces of mutant and WT resting conidia on the maturation of DCs. Immature DCs were cultured in the presence of the cytokines GM-CSF and IL-4 (Ctr DC) or cytokines and NaCl-extracts from the surfaces of WT resting conidia or mutant conidia (ΔpksP, Δayg1, Δarp2, and Δarp1) for 48 h. The percent expression of CD86 (A) and the mean fluorescence intensities (MFI) of CD80 (B) and CD40 (C) were analyzed by flow cytometry. The data (means ± the SEM) are from four donors. The level of statistical significance is indicated (*, P < 0.05, **, P < 0.01).

Glucosamine-containing components are exposed at the ΔpksP, Δayg1, and Δarp2 conidial surfaces.

To check whether any structural cell wall modification occurred and was responsible for DC activation by ΔpksP, Δayg1, and Δarp2 conidia, we labeled mutant and WT conidia with the β-(1,3)-glucan receptor dectin-1 and with the glucosamine (GlcN) recognizing lectin WGA. Mutants and WT conidia did not bind to dectin-1 (data not shown), suggesting that β-(1,3)-glucans were not exposed at the conidial surfaces. However, ΔpksP, Δayg1, and Δarp2 conidia were positive for WGA-FITC (Fig. 11), whereas Δabr1, Δabr2, and WT conidia were 3 to 9% WGA-FITC positive (Fig. 11). In line with the partial stimulation of DCs, Δarp1 mutant presented an intermediate phenotype: ca. 40% of the conidia were WGA positive. These results suggested that conidia with uniform exposure of glucosamine-containing components on the surface (in cases of ΔpksP, Δayg1, and Δarp2 mutant conidia) were able to induce DC activation, whereas conidia with low levels of exposure of GlcN content on the surface did not stimulate DCs (Δabr1, Δabr2, and WT). When GlcN exposure was intermediate as in Δarp1 conidia, such conidia were able to induce partial activation of DCs.

FIG 11.

FIG 11

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WGA-FITC labeling of mutant and WT resting conidia. Note the decreasing numbers of ConA-positive conidia from the first mutants of the melanin pathway to the last one. Scale bar, 10 μm.

Consequently, the absence of melanin or at least of the intermediate scytalone increased the permeability of the conidia to secreted proteins, which otherwise secreted normally during conidial germination, and exposed the GlcN polymers on the conidial surfaces. Proteins and GlcN polymers were responsible for the DC activation after incubation of cells with ΔpksP, Δayg1, and Δarp2 conidia. The Δabr1 and Δabr2 conidia were immunologically inert like their WT counterparts, whereas Δarp1 conidia presented an intermediate phenotype.

DISCUSSION

In fungal biology, melanin pigments are attributed with a variety of beneficial functions, including protection against exogenous stress, UV irradiation, and host defense mechanisms, including reactive oxygen species, lytic enzymes, and antimicrobial peptides (47), and hence considered to be one of the fungal virulence factors (48). Melanin, being extracellular, also contributes to the fungal spore structure (5). In A. fumigatus, melanin biosynthesis is reported to require a cluster of six genes (6, 13, 14). In the present study, we show for the first time the effect of the deletion of individual genes from the melanin biosynthetic gene cluster on the respective conidial surface morphology and the consequent immune responses to the mutant conidia.

The mutant conidia with a deletion in one of the first three genes of the melanin pathway (ΔpksP, Δayg1, and Δarp2) stimulated the maturation and elicited the production of IL-6, IL-1β, TNF-α, and IL-10 cytokines by DCs, whereas Δabr1, Δabr2, and WT conidia were not immunogenic. The Δarp1 mutant presented an intermediate phenotype since it induced partial activation of the DCs. Melanins as such were immunologically inert since the melanin ghost extracted from the WT conidia failed to activate DCs. These results suggested that the surfaces of ΔpksP, Δayg1, and Δarp2 resting conidia are covered with specific common compounds that are rare in Δarp1 conidia and are responsible for the maturation of DCs.

The major obvious phenotypes of ΔpksP, Δayg1, and Δarp2 resting conidia were the absence or very few patches of rodlets at the surface. Although the rodlets that immune-silence the resting conidia were present in the above mutant conidia, the masking of rodlets by an amorphous and hydrophilic layer enabled resting conidia to be immunogenic (24, 49). A few patches of amorphous and hydrophilic layers could also be observed on the Δarp1 conidial surface. When downstream genes of the DHN-melanin pathway were deleted, the appearance of the rodlets and the hydrophobicity of the conidia increased. Previous studies demonstrated that deletion of PKSP and ARP2 correlated with a decreased ability to bind laminin on the conidial surface (10). This result is explained by the low hydrophobicity of the conidia, which reduced the electronegative charge required for laminin binding.

The amorphous and hydrophilic layer is composed of GlcN-containing components and deposited ConA-positive proteins mostly in ΔpksP conidia. These proteins were analyzed in the ΔpksP mutant, and their identification showed that they are usually secreted during vegetative growth. Most hydrolases (such as β-1,3-glucosidases, β-N-acetylhexosaminidase, and mannosidase), catalase, Aspf1, Asp-hemolysin, and chitinase found in the amorphous surface layer of the resting ΔpksP conidia were usually identified during mycelial growth in a protein-based medium (43–46). Their presence on the surfaces of resting conidia of ΔpksP and Δayg1 mutants is explained by modifications of the ionic strength of the hydrophobin layer resulting from the absence of melanin or at least the YWA1, a melanin biosynthetic intermediate that is synthesized by the combined activities of PksPp and Ayg1p. The easy removal of these hydrophilic glycoproteins by 0.5 M NaCl suggested that they adhered to the conidial cell wall through electrostatic binding. These surface proteins in the conidial amorphous layers were responsible for the DC maturation and cytokine production. Interestingly, cell wall structural modifications resulting from the absence of α-(1,3)-glucan, as in A. fumigatus Δags1 Δags2 Δags3 (Δags) mutant, also gave the similar conidial phenotype, a hydrophilic protein layer on the surface of the conidia that stimulated host defense reactions (24). However, the composition of proteins in this amorphous layer in triple Δags and ΔpksP deletion mutants was not the same, suggesting defined cell wall permeability defects due to the deletion of different cell wall component biosynthetic genes. When the melanin synthesis pathway was blocked farther downstream by gene deletion, fewer proteins were able to cross the conidial cell wall. Of note, DC cytokine production was less for ΔpksP mutant than for Δayg1 and Δarp2 mutant conidia (Fig. 3). This result suggests that glycoproteins were less stimulatory than GlcN residues present on the surfaces of the ΔpksP, Δayg1, and Δarp2 mutant conidia and that these GlcN residues are less exposed on ΔpksP mutant surfaces due to the larger amount of glycoproteins in this mutant.

The presence of GlcN residues on the surfaces of the ΔpksP, Δayg1, and Δarp2 mutant conidia could be explained by the unmasking of cell wall chitin due to the absence of melanin. The hexosamines present in the conidial cell wall were composed of long chains of fibrillar water insoluble chitin, amorphous and soluble chitin oligosaccharides (containing 10 to 15 N-acetylglucosamines), and deacetlylated chitin (chitosan) (A. Beauvais et al., unpublished results). Previous studies on different-sized chitin polymers have shown that >70-μm chitin polymers were immunologically inert with murine macrophages, whereas both intermediate-sized (40- to 70-μm) and small (<40-μm) chitin polymers stimulated TNF elaboration by macrophages (50), and only <40-μm chitin polymers induced IL-10 production. Small particles of chitosan were even better macrophage immune stimulators. Chitosan of <20 μm elicited the most IL-1β from bone marrow-derived macrophages (50). Although several chitin-binding proteins have been identified in mammalian cells, no chitin receptor had thus far been identified thus far. A recent study on the recognition of innate immune cells by Candida albicans chitin revealed that although there was no direct dectin-1 and chitin binding, chitin was capable of blocking dectin-1-mediated immune responses (51). Similarly, these small and/or deacetylated chitins were likely unmasked on the surfaces of the first three melanin mutants and were responsible for the DC maturation. Such an unmasking phenomenon was reported in chitin synthase ΔcsmA and ΔcsmB mutants, wherein the deletion of two of the chitin synthase genes (CSMA and CSMB) in A. fumigatus resulted in the increased exposure of WGA-positive components on the conidial surfaces (42). The ΔcsmA conidial surface was also amorphous and ConA positive; however, ConA-positive materials were not glycoproteins but rather due to the exposure of mannan-containing polymers. This further confirms the differential permeability defects due to the deletion of specific cell wall component biosynthetic genes.

Chai et al. (9) also demonstrated that melanin purified from WT conidia was also poorly immunogenic for the stimulation of cytokines by peripheral blood mononuclear cells. On the other hand, as seen in our study, ΔpksP conidia elicited significantly higher cytokine production, including IL-10, IL-6, and TNF-α. These researchers found that the blockage of dectin-1 with laminarin reduced cytokine production in response to ΔpksP conidia, which could be correlated with WGA-FITC positivity of ΔpksP, Δayg1, and Δarp2 mutant conidia and the observations that chitin can influence dectin-1-mediated immune responses (51). Jahn et al. (52) observed that the extent of macrophage phagocytosis and intracellular killing was significantly greater with ΔpksP conidia than with WT conidia. Thywissen et al. (18) showed that inside the phagolysosome, WT conidial DHN-melanin was responsible for the inhibition of the phagolysosomal acidification of mouse and human macrophages and neutrophils. The ΔpksP conidia, in contrast, were located in an acidic environment in the phagolysosome, which coincides with more effective killing of these conidia. The percentages of Δayg1, Δarp2, and Δabr2 conidia were lower in acidified phagolysosomes than those of ΔpksP conidia but higher than those of WT conidia. These results show that melanin intermediates formed by the downstream biosynthetic pathway increase conidial protection. However, the final product, the DHN-melanin, was important for the maximal protection since it facilitates the formation of a complete surface rodlet layer that hides conidia from their immediate recognition by the immune system.

Conclusion.

The absence of at least the scytalone intermediate of DHN-melanin is responsible for structural and chemical modifications of the cell surface, which will have an obvious impact on the immune response of the host toward the corresponding mutant. Our results also show that melanin is essential to acquire the right surface properties with precise charge and hydrophobicity that are necessary to have immunologically inert conidia due to an exposed rodlet layer.

ACKNOWLEDGMENTS

We thank Maria Pötsch for excellent technical support.

This study was partly supported by Aviesan grant BA-P1-09 Aspergillus. Research in the Unité des Aspergillus, Institut Pasteur, and the Institut National de la Santé et de la Recherche Médicale Unité 1138 was supported by European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement 260338 ALLFUN and ANR-10-BLAN-1309 HYDROPHOBIN. Work at the Université Catholique de Louvain was supported by the National Fund for Scientific Research, the Université Catholique de Louvain (Fonds Spéciaux de Recherche), the Federal Office for Scientific, Technical, and Cultural Affairs (Interuniversity Poles of Attraction Programme), and the Research Department of the Communauté Française de Belgique (Concerted Research Action).

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Footnotes

Published ahead of print 12 May 2014

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