Aude Sturny-Leclère

Cryptococcus neoformans Host Adaptation: Toward Biological Evidence of Dormancy

ABSTRACT Cryptococcosis is an opportunistic infection due to the ubiquitous yeast Cryptococcus neoformans. This yeast interacts closely with innate immune cells, leading to various fates, including fungal persistence within cells, making possible the dissemination of the yeast cells with monocytes via a Trojan horse strategy. In humans, the natural history of the infection begins with primoinfection during childhood, which is followed by dormancy and, in some individuals, reactivation upon immunosuppression. To address the question of dormancy, we studied C. neoformans infection at the macrophage level (in vitro H99-macrophage interaction) and at the organ level in a murine model of cryptococcosis. We analyzed the diversity of yeast adaptation to the host by characterizing several C. neoformans populations with new assays based on flow cytometry (quantitative flow cytometry, multispectral imaging flow cytometry, sorting), microscopy (dynamic imaging), and gene expression analysis. On the basis of parameters of multiplication and stress response, various populations of yeast cells were observed over time in vivo and in vitro. Cell sorting allowed the identification of a subpopulation that was less prone to grow under standard conditions than the other populations, with growth enhanced by the addition of serum. Gene expression analysis revealed that this population had specific metabolic characteristics that could reflect dormancy. Our data suggest that dormant yeast cells could exist in vitro and in vivo. C. neoformans exhibits a huge plasticity and adaptation to hosts that deserves further study. In vitro generation of dormant cells is now the main challenge to overcome the limited number of yeast cells recovered in our models. IMPORTANCE Cryptococcus neoformans is a sugar-coated unicellular fungus that interacts closely with various cells and organisms, including amoebas, nematodes, and immune cells of mammals. This yeast is able to proliferate and survive in the intracellular environment. C. neoformans causes cryptococcosis, and yeast dormancy in humans has been suggested on the basis of epidemiological evidence obtained years ago. By studying an in vitro model of yeast-macrophage interaction and murine models of cryptococcosis, we observed that yeast cells evolve in heterogeneous populations during infection on the basis of global metabolic activity. We compared the growth ability and gene expression of yeast cells belonging to various populations in those two models. We eventually found a population of yeast cells with low metabolism that fit some of the criteria for dormant cells. This paves the way for further characterization of dormancy in C. neoformans. Cryptococcus neoformans is a sugar-coated unicellular fungus that interacts closely with various cells and organisms, including amoebas, nematodes, and immune cells of mammals. This yeast is able to proliferate and survive in the intracellular environment. C. neoformans causes cryptococcosis, and yeast dormancy in humans has been suggested on the basis of epidemiological evidence obtained years ago. By studying an in vitro model of yeast-macrophage interaction and murine models of cryptococcosis, we observed that yeast cells evolve in heterogeneous populations during infection on the basis of global metabolic activity. We compared the growth ability and gene expression of yeast cells belonging to various populations in those two models. We eventually found a population of yeast cells with low metabolism that fit some of the criteria for dormant cells. This paves the way for further characterization of dormancy in C. neoformans.

Fungal-Induced Cell Cycle Impairment, Chromosome Instability and Apoptosis via Differential Activation of NF-κB

Microbial pathogens have developed efficient strategies to compromise host immune responses. Cryptococcus neoformans is a facultative intracellular pathogen, recognised as the most common cause of systemic fungal infections leading to severe meningoencephalitis, mainly in immunocompromised patients. This yeast is characterized by a polysaccharide capsule, which inhibits its phagocytosis. Whereas phagocytosis escape and macrophage intracellular survival have been intensively studied, extracellular survival of this yeast and restraint of host innate immune response are still poorly understood. In this study, we have investigated whether C. neoformans affected macrophage cell viability and whether NF-κB (nuclear factor-κB), a key regulator of cell growth, apoptosis and inflammation, was involved. Using wild-type (WT) as well as mutant strains of C. neoformans for the pathogen side, and WT and mutant cell lines with altered NF-κB activity or signalling as well as primary macrophages for the host side, we show that C. neoformans manipulated NF-κB-mediated signalling in a unique way to regulate macrophage cell fate and viability. On the one hand, serotype A strains reduced macrophage proliferation in a capsule-independent fashion. This growth decrease, which required a critical dosage of NF-κB activity, was caused by cell cycle disruption and aneuploidy, relying on fungal-induced modification of expression of several cell cycle checkpoint regulators in S and G2/M phases. On the other hand, C. neoformans infection induced macrophage apoptosis in a capsule-dependent manner with a differential requirement of the classical and alternative NF-κB signalling pathways, the latter one being essential. Together, these findings shed new light on fungal strategies to subvert host response through uncoupling of NF-κB activity in pathogen-controlled apoptosis and impairment of cell cycle progression. They also provide the first demonstration of induction of aneuploidy by a fungal pathogen, which may have wider implications for human health as aneuploidy is proposed to promote tumourigenesis.

Anthrax lethal toxin down-regulates type-IIA secreted phospholipase A2 expression through MAPK/NF-κB inactivation

Bacillus anthracis, the etiological agent of anthrax, produces lethal toxin (LT) that displays a metallo-proteolytic activity toward the N-terminus of the MAPK-kinases. We have previously shown that secreted type-IIA phospholipase A(2) (sPLA(2)-IIA) exhibits potent anthracidal activity. In vitro expression of sPLA(2)-IIA in guinea pig alveolar macrophages (AMs), the major source of this enzyme in lung tissues, is inhibited by LT. Here, we examined the mechanisms involved in sPLA(2)-IIA inhibition by LT. We first showed that chemical inhibitors of p38 and ERK MAPKs reduced sPLA(2)-IIA expression in AMs indicating that these kinases play a role in sPLA(2)-IIA expression. LT inhibited IL-1beta-induced p38 phosphorylation as well as sPLA(2)-IIA promoter activity in CHO cells. Inhibition of sPLA(2)-IIA promoter activity was mimicked by co-transfection with dominant negative construct of p38 (DN-p38) and reversed by the active form of p38-MAPK (AC-p38). Both LT and DN-p38 decreased IL-1beta-induced NF-kappaB luciferase activity. This contrasted with the effect of AC-p38, which enhanced this activity. However, neither LT nor specific p-38 inhibitor interfered with LPS-induced IkappaBalpha degradation or NF-kappaB nuclear translocation in AMs. Subcutaneous administration of LT to guinea pig before LPS challenge reduced sPLA(2)-IIA levels in broncho-alveolar lavages and ears. We conclude that sPLA(2)-IIA expression is induced via a sequential MAPK-NF-kappaB activation and that LT inhibits this expression likely by interfering with the transactivation of NF-kappaB in the nucleus. This inhibition, which is operating both in vitro and in vivo, may represent a mechanism by which B. anthracis subvert host defense.

Copy Number Variation of Mitochondrial DNA Genes in Pneumocystis jirovecii According to the Fungal Load in BAL Specimens

Pneumocystis jirovecii is an unculturable fungus and the causative agent of Pneumocystis pneumonia, a life-threatening opportunistic infection. Although molecular diagnosis is often based on the detection of mtLSU rRNA mitochondrial gene, the number of copies of mitochondrial genes had not been investigated. We developed and optimized six real-time PCR assays in order to determine the copy number of four mitochondrial genes (mtSSU rRNA, mtLSU rRNA, NAD1, and CYTB) in comparison to nuclear genome (DHPS and HSP70) and tested 84 bronchoalveolar fluids of patients at different stages of the infection. Unexpectedly, we found that copy number of mitochondrial genes varied from gene to gene with mtSSU rRNA gene being more represented (37 copies) than NAD1 (23 copies), mtLSU rRNA (15 copies) and CYTB (6 copies) genes compared to nuclear genome. Hierarchical clustering analysis (HCA) allowed us to define five major clusters, significantly associated with fungal load (p = 0.029), in which copy number of mitochondrial genes was significantly different among them. More importantly, copy number of mtLSU rRNA, NAD1, and CYTB but not mtSSU rRNA differed according to P. jirovecii physiological state with a decreased number of copies when the fungal load is low. This suggests the existence of a mixture of various subspecies of mtDNA that can harbor different amplification rates. Overall, we revealed here an unexpected variability of P. jirovecii mtDNA copy number that fluctuates according to P. jirovecii’s physiological state, except for mtSSU that is the most stable and the most present mitochondrial gene.

Variation in copy number of the 28S rDNA of Aspergillus fumigatus measured by droplet digital PCR and analog quantitative real-time PCR

Droplet digital PCR (ddPCR) after DNA digestion yielded a 28S rDNA copy number of 61 to 86 copies/genome when testing 10 unrelated Aspergillus fumigatus isolates, higher than with quantitative PCR. Unfortunately, ddPCR after DNA digestion did not improve the sensitivity of our PCR assay when testing serum patients with invasive aspergillosis.

Wild-derived mouse strains, a valuable model to study B cell responses.

In the present report, we revisited the B cell responsiveness of 7 wild-derived mouse strains to various toll-like receptor ligands (TLR-L). We found that 2 of them, namely PWK and STF presented profound defects in B cell proliferative responses to most of the TLR-L. Yet, their macrophage responses were largely unaffected, suggesting that regulation of TLR pathways are distinct in B cells and macrophages. We also showed that, anti-CD40 mAbs rescued the low proliferative responses to CpG in both PWK and STF B cells. In the other hand, CpG synergized with LPS to induce high levels of proliferation in STF B cells, which did not respond to LPS alone. Cytokine or immunoglobulin (Ig) productions, in vitro, were less impaired than the proliferative responses to LPS or CpG alone. In STF B cells, both ERK, P38 and JNK pathways were affected following in vitro TLR4 or TLR9 signaling. Moreover, while the basal levels of Ig secreting cells and of serum Igs were similar to that of control mice, antibody responses to both TI and TD antigens were severely affected, mainly in STF mice. Our findings therefore highlight the relevance of wild-derived mouse strains and TLR-L to study B cell physiology.

Titan cells formation in Cryptococcus neoformans is finely tuned by environmental conditions and modulated by positive and negative genetic regulators

The pathogenic fungus Cryptococcus neoformans exhibits morphological changes in cell size during lung infection, producing both typical size 5 to 7 μm cells and large titan cells (> 10 μm and up to 100 μm). We found and optimized in vitro conditions that produce titan cells in order to identify the ancestry of titan cells, the environmental determinants, and the key gene regulators of titan cell formation. Titan cells generated in vitro harbor the main characteristics of titan cells produced in vivo including their large cell size (>10 μm), polyploidy with a single nucleus, large vacuole, dense capsule, and thick cell wall. Here we show titan cells derived from the enlargement of progenitor cells in the population independent of yeast growth rate. Change in the incubation medium, hypoxia, nutrient starvation and low pH were the main factors that trigger titan cell formation, while quorum sensing factors like the initial inoculum concentration, pantothenic acid, and the quorum sensing peptide Qsp1p also impacted titan cell formation. Inhibition of ergosterol, protein and nucleic acid biosynthesis altered titan cell formation, as did serum, phospholipids and anti-capsular antibodies in our settings. We explored genetic factors important for titan cell formation using three approaches. Using H99-derivative strains with natural genetic differences, we showed that titan cell formation was dependent on LMP1 and SGF29 genes. By screening a gene deletion collection, we also confirmed that GPR4/5-RIM101, and CAC1 genes were required to generate titan cells and that the PKR1, TSP2, USV101 genes negatively regulated titan cell formation. Furthermore, analysis of spontaneous Pkr1 loss-of-function clinical isolates confirmed the important role of the Pkr1 protein as a negative regulator of titan cell formation. Through development of a standardized and robust in vitro assay, our results provide new insights into titan cell biogenesis with the identification of multiple important factors/pathways.

Identification of environmental and genetic factors important for Cryptococcus neoformans titan cell formation using new in vitro inducing conditions

A remarkable aspect of the human fungal pathogen Cryptococcus neoformans is the morphological changes triggered by the interaction with the host. During infection, a specific morphological change in cell size is observed, particularly in lung tissue, from regular 5 to 7 µm cells (RCs) to titan cells (TCs, > 10 µm and up to 100 µm). However, the stable and reproducible generation of large quantity of TCs was only possible during experimental infection. We implemented here in vitro conditions allowing TCs generation with the aim to understand the ancestry of TC, the environmental determinants of TCs formation and perform easily phenotype/genotype analysis to uncover genetic factors underlying TCs formation. This paper reports robust in vitro conditions that generates yeasts cells harboring the main characteristics of TCs: large (>10 µm), uninucleate and polyploid cells harboring a single large vacuole with a dense capsule surrounding a thickened melanized cell wall. We observed that TCs formation begins as soon as 8 hours after induction, with a maximal proportion of TCs obtained after 24 hours. TCs derived for the initial oldest cells (mother cells) rather than from daughter cells. Hypoxia, nutrient starvation and low pH stresses were the main factors that trigger TCs formation, while quorum sensing factors like the initial inoculum concentration and the addition of pantothenic acid also impacted TCs formation. Antifungal drugs, resulting in inhibition of ergosterol (fluconazole), or proteins and nucleic acids (flucytosine), altered TCs formation, as did hosts-related products such as serum, phospholipids and anti-capsular antibodies in our settings. We then explored genetic factors important for TCs formation using two approaches. Using strains from H99 lineage strains among which few genetic differences have been described, we showed that TCs formation was dependent on Lmp1, Sgf29 and Gpr4/5-Rim101 proteins. Then, based on a the analysis of natural Pkr1 loss-of-function clinical isolates and serial clinical isolates, we observed the important role of Pkr1 protein, a negative regulator of the cyclic adenosine monophosphate / protein kinase A (cAMP/PKA), for TCs formation. These strains also emphasize the structural and functional relationship between Pkr1 and Pka. Our results provide new insights into TCs biogenesis with identification of multiple important factors/pathways involved. The implementation of such standardized and robust in vitro conditions pave the way for future research focusing on the genetic basis of TCs biogenesis, biology of TCs and the ontology of morphological changes in Cryptococcus neoformans. Author Summary Cryptococcus neoformans is a yeast that is capable of morphological change upon interaction with host. Particularly, in the lung of infected mice, a subpopulation of yeast evolves toward gigantism (titan cells size from 10 to 100 µm) that include other characteristics such as thickened cell wall, dense capsule, polyploidization, large vacuole with peripheral nucleus and cellular organelles. The generation of large number of such cells outside the lung of mice have been described but was not reproducible nor standardized. We here report standardized, reproducible, robust conditions of generation of titan cells and explored the environmental and genetic factors underlying the genesis of these cells. We showed that Titan cells were generated upon stresses such as change in the medium, nutrient deprivation, hypoxia and low pH. Using collection of well characterized reference strains and clinical isolates, we validated with our model the AMPc/PKA/Rim pathway as the major genetic determinant of titan cell formation. This study opens the way for a more comprehensive picture of the ontology of morphological changes in Cryptococcus neoformans.The pathogenic fungus Cryptococcus neoformans exhibits morphological changes in cell size during lung infection, producing both typical size 5 to 7 microns cells and large titan cells (> 10 microns and up to 100 microns). We found and optimized in vitro conditions that produce titan cells in order to identify the ancestry of titan cells, the environmental determinants, and the key gene regulators of titan cell formation. Titan cells generated in vitro harbor the main characteristics of titan cells produced in vivo including their large cell size (>10 microns), polyploidy with a single nucleus, large vacuole, dense capsule, and thick cell wall. Here we show titan cells derived from the enlargement of progenitor cells in the population independent of yeast growth rate. Change in the incubation medium, hypoxia, nutrient starvation and low pH were the main factors that trigger titan cell formation, while quorum sensing factors like the initial inoculum concentration, pantothenic acid, and the quorum sensing peptide Qsp1p also impacted titan cell formation. Inhibition of ergosterol, protein and nucleic acid biosynthesis altered titan cell formation, as did serum, phospholipids and anti-capsular antibodies in our settings. We explored genetic factors important for titan cell formation using three approaches. Using H99-derivative strains with natural genetic differences, we showed that titan cell formation was dependent on LMP1 and SGF29 genes. By screening a gene deletion collection, we also confirmed that GPR4/5-RIM101, and CAC1 genes were required to generate titan cells and that the PKR1, TSP2, USV101 genes negatively regulated titan cell formation. Furthermore, analysis of spontaneous Pkr1 loss-of-function clinical isolates confirmed the important role of the Pkr1 protein as a negative regulator of titan cell formation. Through development of a standardized and robust in vitro assay, our results provide new insights into titan cell biogenesis with the identification of multiple important factors/pathways.

A cell impedance-based real-time in vitro assay to assess the toxicity of amphotericin B formulations

&NA; Aerosolized liposomal amphotericin B (L‐AmB) has been investigated as prophylaxis against invasive aspergillosis. However, the clinical results are controversial and some trials suggest that toxicity could be a limitation for wider use. Our aim was to assess the dynamics of cell toxicity induced in a human alveolar epithelial cell line (A549) after exposure to L‐AmB (50 to 400 &mgr;g/ml) or amphotericin B deoxycholate (D‐AmB; 50 to 200 &mgr;g/ml) by monitoring real‐time A549 cell viability using an impedance‐based technology. Results were expressed as cell index values integrating cell adhesion, proliferation, and survival. In parallel, the gene expression of proinflammatory cytokines was quantified at 6 and 24 h after drug addition by real‐time RT‐PCR on cell lysates. No sustained reduction of cell indexes was observed with L‐AmB or empty liposomes, even at 400 &mgr;g/ml. Only the highest concentration tested of L‐AmB (400 &mgr;g/ml) yielded transient significant 6‐fold and 4‐fold induction of TNF‐&agr; and IL‐8 mRNAs, respectively. In contrast, D‐AmB induced a decrease in cell indexes and only the 50 &mgr;g/ml concentration of D‐AmB was followed by cell recovery, higher concentrations leading to cell death. Significant 4‐fold, 7‐fold and 3‐fold inductions of TNF‐&agr;, IL‐8 and IL‐33 mRNAs were also observed at 6 h with 50 &mgr;g/ml of D‐AmB. In conclusion, continuous cell impedance measurement showed no toxicity on overall cellular behavior although a slight proinflammatory cytokine expression is possible after L‐AmB challenge. Real‐time kinetics of cell impedance is an interesting tool for initial screening of cell toxicity. Graphical abstract Figure. No caption available. HighlightsReal‐time cell impedance was measured for in‐vitro toxicity screening on A549 cells.Liposomal amphotericin B (L‐AmB) induced no sustained reduction of cell indexes.Amphotericin B deoxycholate (D‐AmB) induced a sharp decrease in cell indexes.Only 400 &mgr;g/ml of L‐AmB transiently induced TNF‐&agr; and IL‐8 mRNAs.Significant induction of cytokine mRNAs was observed with 50 &mgr;g/ml of D‐AmB.