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Respiratory infections caused by the human fungal pathogen Aspergillus fumigatus are a major cause of mortality for immunocompromised patients. Exposure to these pathogens occurs through inhalation, although the role of the respiratory epithelium in disease pathogenesis has not been fully defined. Employing a primary human airway epithelial model, we demonstrate that fungal melanins potently block the post-translational secretion of the chemokines CXCL1 and CXCL8 independent of transcription or the requirement of melanin to be phagocytosed, leading to a significant reduction in neutrophil recruitment to the apical airway both in vitro and in vivo. Aspergillus -derived melanin, a major constituent of the fungal cell wall, dampened airway epithelial chemokine secretion in response to fungi, bacteria, and exogenous cytokines. Furthermore, melanin muted pathogen-mediated calcium fluxing and hindered actin filamentation. Taken together, our results reveal a critical role for melanin interaction with airway epithelium in shaping the host response to fungal and bacterial pathogens. Here, Reedy et al. use a human airway culture model to show that fungal melanin blocks the secretion of pro-inflammatory chemokines resulting in diminished immune responses to Aspergillus fumigatus and Pseudomonas aeruginosa .

Streptococcus pneumoniae ( Sp ), a leading cause of pneumonia, can spread from the lung into the bloodstream to cause systemic disease. Limitations in vaccine efficacy and a rise in antimicrobial resistance have spurred searches for host-directed therapies that limit pathologic host immune responses to Sp . Excessive polymorphonuclear leukocyte (PMN) infiltration into Sp -infected airways promotes systemic disease. Using stem cell-derived respiratory cultures that reflect bona fide lung epithelium, we identified eicosanoid hepoxilin A3 as a critical pulmonary PMN chemoattractant that is sufficient to drive PMN-mediated epithelial damage by inducing the release of neutrophil elastase. Inhibition of the release or activity of this protease in mice limited epithelial barrier disruption and bacterial dissemination, suggesting a new host-directed treatment for Sp lung infection.

Neutrophils are recognized to represent significant immune cell mediators for the clearance and elimination of the human-pathogenic fungal pathogen Candida . The sensing of fungi by innate cells is performed, in part, through lectin receptor recognition of cell wall components and downstream cellular activation by signaling components, including spleen tyrosine kinase (Syk). While the essential role of Syk in macrophages and dendritic cells is clear, there remains uncertainty with respect to its contribution in neutrophils. In this study, we demonstrated that Syk is critical for multiple cellular functions in neutrophils responding to major human-pathogenic Candida species. These data not only demonstrate the vital nature of Syk with respect to the control of fungi by neutrophils but also warn of the potential infectious complications arising from the recent clinical development of novel Syk inhibitors for hematologic and autoimmune disorders. Invasive fungal infections constitute a lethal threat, with patient mortality as high as 90%. The incidence of invasive fungal infections is increasing, especially in the setting of patients receiving immunomodulatory agents, chemotherapy, or immunosuppressive medications following solid-organ or bone marrow transplantation. In addition, inhibitors of spleen tyrosine kinase (Syk) have been recently developed for the treatment of patients with refractory autoimmune and hematologic indications. Neutrophils are the initial innate cellular responders to many types of pathogens, including invasive fungi. A central process governing neutrophil recognition of fungi is through lectin binding receptors, many of which rely on Syk for cellular activation. We previously demonstrated that Syk activation is essential for cellular activation, phagosomal maturation, and elimination of phagocytosed fungal pathogens in macrophages. Here, we used combined genetic and chemical inhibitor approaches to evaluate the importance of Syk in the response of neutrophils to Candida species. We took advantage of a Cas9-expressing neutrophil progenitor cell line to generate isogenic wild-type and Syk-deficient neutrophils. Syk-deficient neutrophils are unable to control the human pathogens Candida albicans , Candida glabrata , and Candida auris . Neutrophil responses to Candida species, including the production of reactive oxygen species and of cytokines such as tumor necrosis factor alpha (TNF-α), the formation of neutrophil extracellular traps (NETs), phagocytosis, and neutrophil swarming, appear to be critically dependent on Syk. These results demonstrate an essential role for Syk in neutrophil responses to Candida species and raise concern for increased fungal infections with the development of Syk-modulating therapeutics. IMPORTANCE Neutrophils are recognized to represent significant immune cell mediators for the clearance and elimination of the human-pathogenic fungal pathogen Candida . The sensing of fungi by innate cells is performed, in part, through lectin receptor recognition of cell wall components and downstream cellular activation by signaling components, including spleen tyrosine kinase (Syk). While the essential role of Syk in macrophages and dendritic cells is clear, there remains uncertainty with respect to its contribution in neutrophils. In this study, we demonstrated that Syk is critical for multiple cellular functions in neutrophils responding to major human-pathogenic Candida species. These data not only demonstrate the vital nature of Syk with respect to the control of fungi by neutrophils but also warn of the potential infectious complications arising from the recent clinical development of novel Syk inhibitors for hematologic and autoimmune disorders.

Fungal infections in the lungs are lethal complications for those with compromised immune systems and have limited treatment strategies available. These options are restricted further by the increased prevalence of treatment-resistant fungi. Many studies focus on how our immune systems respond to these pathogens, yet airway epithelial cells remain an understudied component of fungal infections in the lungs. Here, the authors provide a transcriptional analysis of primary human airway epithelial cells stimulated by two distinct fungal pathogens, Aspergillus fumigatus and Coccidioides posadasii . These data will enable further mechanistic studies of the contribution of the airway epithelium to initial host responses and represent a powerful new resource for future investigations.

The lung epithelial lining serves as the primary barrier to inhaled environmental toxins, allergens, and invading pathogens. Pulmonary fungal infections are devastating and carry high mortality rates, particularly in those with compromised immune systems. While opportunistic fungi infect primarily immunocompromised individuals, endemic fungi cause disease in immune competent and compromised individuals. Unfortunately, in the case of inhaled fungal pathogens, the airway epithelial host response is vastly understudied. Furthering our lack of understanding, very few studies utilize primary human models displaying pseudostratified layers of various epithelial cell types at air-liquid interface. In this review, we focus on the diversity of the human airway epithelium and discuss the advantages and disadvantages of oncological cell lines, immortalized epithelial cells, and primary epithelial cell models. Additionally, the responses by human respiratory epithelial cells to invading fungal pathogens will be explored. Future investigations leveraging current human in vitro model systems will enable identification of the critical pathways that will inform the development of novel vaccines and therapeutics for pulmonary fungal infections.

Inhibition of Bruton's tyrosine kinase (BTK) through covalent modifications of its active site (e.g., ibrutinib [IBT]) is a preferred treatment for multiple B cell malignancies. However, IBT-treated patients are more susceptible to invasive fungal infections, although the mechanism is poorly understood. Neutrophils are the primary line of defense against these infections; therefore, we examined the effect of IBT on primary human neutrophil effector activity against Aspergillus fumigatus. IBT significantly impaired the ability of neutrophils to kill A. fumigatus and potently inhibited reactive oxygen species (ROS) production, chemotaxis, and phagocytosis. Importantly, exogenous TNF-α fully compensated for defects imposed by IBT and newer-generation BTK inhibitors and restored the ability of neutrophils to contain A. fumigatus hyphal growth. Blocking TNF-α did not affect ROS production in healthy neutrophils but prevented exogenous TNF-α from rescuing the phenotype of IBT-treated neutrophils. The restorative capacity of TNF-α was independent of transcription. Moreover, the addition of TNF-α immediately rescued ROS production in IBT-treated neutrophils, indicating that TNF-α worked through a BTK-independent signaling pathway. Finally, TNF-α restored effector activity of primary neutrophils from patients on IBT therapy. Altogether, our data indicate that TNF-α rescued the antifungal immunity block imposed by inhibition of BTK in primary human neutrophils.

Macrophages play a critical role in the elimination of fungal pathogens. They are sensed cell surface pattern-recognition receptors and are phagocytosed into newly formed organelles called phagosomes. Phagosomes mature through the recruitment of proteins and lysosomes, resulting in addition of proteolytic enzymes and acidification of the microenvironment. Our earlier studies demonstrated an essential role of Dectin-1-dependent activation of spleen tyrosine kinase (Syk) in the maturation of fungal containing phagosomes. The absence of Syk activity interrupted phago-lysosomal fusion resulting in arrest at an early phagosome stage. In this study, we sought to define the contribution of Syk to the control of phagocytosed live in primary macrophages. To accurately measure intracellular yeast division, we designed a carboxyfluorescein succinimidyl ester (CFSE) yeast division assay in which bright fluorescent parent cells give rise to dim daughter cells. The CFSE-labeling of did not affect the growth rate of the yeast. Following incubation with macrophages, internalized CFSE-labeled were retrieved by cellular lysis, tagged using ConA-647, and the amount of residual CFSE fluorescence was assessed by flow cytometry. remained undivided (CFSE bright) for up to 18 h in co-culture with primary macrophages. Treatment of macrophages with R406, a specific Syk inhibitor, resulted in loss of intracellular control of with initiation of division within 4 h. Delayed Syk inhibition after 8 h was less effective indicating that Syk is critically required at early stages of macrophage-fungal interaction. In conclusion, we demonstrate a new method of tracking division of using CFSE labeling. Our results suggest that early Syk activation is essential for macrophage control of phagocytosed .

Key Points Invasive fungal infections are one of the most devastating consequence of the rapidly increasing number of immunocompromised patients. Despite the rising incidence and mortality of infections with Cryptococcus neoformans , Candida albicans and Aspergillus fumigatus , the antifungal armamentarium remains limited, and novel targets focused on fundamental molecular pathogenesis are needed. The calcineurin pathway is a conserved stress-response signalling pathway that has revolutionized today's immunosuppression. The calcineurin inhibitors FK506 and cyclosporine A (CsA) bind to human calcineurin and block signal transduction and T-cell activation, leading to efficient immunosuppression that can prevent organ rejection. Additionally, calcineurin inhibitors have been used clinically to treat a myriad of other conditions. In this article, the authors propose that inhibiting fungal calcineurin pathways could be an effective method of halting the growth of invasive fungal pathogens, thereby preventing or treating disease. The two currently available inhibitors FK506 and CsA delivered in their present intravenous formulations will not solve this problem, as they possess inherent and well-utilized cross-reactive human immunosuppressive activity. However, molecular dissection of the fungal calcineurin pathway is yielding promising results and concepts for new drug development. Current calcineurin inhibitors have been shown to possess antifungal activity against the major fungal pathogens, but under different conditions. Several calcineurin pathway genes in each of the major human fungal pathogens have also been disrupted and show effects on fungal growth and virulence. However, the effects are not uniform, highlighting that while the pathway is genetically conserved, the divergent disruption phenotypes require each individual pathogen to be studied in detail. In C. neoformans , calcineurin inhibition leads to temperature-sensitive growth and fungal clearing in animal models and at the relevant temperatures in patients. Additionally, calcineurin inhibition leads to defects in hyphal elongation and therefore the inability to mate. In C. albicans , calcineurin A is required for growth under stress, growth in serum and virulence in systemic animal models. In A. fumigatus calcineurin A is not essential, in contrast to its essential role in the less pathogenic A. nidulans . Disruption of calcineurin A in A. fumigatus yielded extremely blunted hyphae that were unable to invade animal tissue and resulted in near avirulence in multiple animal models. Molecular findings over the past decade have pointed to the fungal calcineurin pathway as a novel antifungal target with impressive effects at halting growth and inhibiting virulence in the three major fungal pathogens that affect immunocompromised patients. The future will hold further detailed molecular dissection of the pathway to optimize fungal killing and effectively harness the calcineurin pathway to both prevent and treat invasive fungal infections. As improvements continue to be made in medical care, so the number of immunocompromised patients increases, and invasive fungal infections caused by Cryptococcus neoformans , Candida albicans and Aspergillus fumigatus are a particular problem for this group of patients. Here, the prospects for harnessing the fungal calcineurin pathway in the development of novel antifungals to tackle invasive fungal infections are discussed. The number of immunocompromised patients with invasive fungal infections continues to increase and new antifungal therapies are not keeping pace with the growing incidence of these infections and their associated mortality. Calcineurin inhibition is currently used to exert effective immunosuppression following organ transplantation and in treating various other conditions. However, the calcineurin pathway is also intricately involved in the growth and pathogenesis of the three major fungal pathogens of humans, Cryptococcus neoformans , Candida albicans and Aspergillus fumigatus , and the exploitation of fungal calcineurin pathways holds great promise for the future development of novel antifungal agents. This Review summarizes our current understanding of calcineurin biology in these fungal species, and its exciting potential role in treating invasive fungal infections.

The host type I interferon (IFN) pathway is a major signature of inflammation induced by the human fungal pathogen, Candida albicans . However, the molecular mechanism for activating this pathway in the host defence against C. albicans remains unknown. Here we reveal that mice lacking cyclic GMP–AMP synthase (cGAS)–stimulator of IFN genes (STING) pathway components had improved survival following an intravenous challenge by C. albicans . Biofilm-associated C. albicans DNA packaged in extracellular vesicles triggers the cGAS–STING pathway as determined by induction of interferon-stimulated genes, IFNβ production, and phosphorylation of IFN regulatory factor 3 and TANK-binding kinase 1. Extracellular vesicle-induced activation of type I IFNs was independent of the Dectin-1/Card9 pathway and did not require toll-like receptor 9. Single nucleotide polymorphisms in cGAS and STING potently altered inflammatory cytokine production in human monocytes challenged by C. albicans . These studies provide insights into the early innate immune response induced by a clinically significant fungal pathogen. This work reveals a mechanism of cGAS- and STING-dependent type I IFN induction in response to biofilm-associated Candida albicans DNA packaged in extracellular vesicles.