Impaired RASGRF1/ERK-mediated GM-CSF response characterizes CARD9 deficiency in French-Canadians

Inhibition of EGFR Signaling Protects from Mucormycosis

Mucormycosis is a life-threatening, invasive fungal infection that is caused by various species belonging to the order Mucorales. Rhizopus species are the most common cause of the disease, responsible for approximately 70% of all cases of mucormycosis. During pulmonary mucormycosis, inhaled Rhizopus spores must adhere to and invade airway epithelial cells in order to establish infection. The molecular mechanisms that govern this interaction are poorly understood. We performed an unbiased survey of the host transcriptional response during early stages of Rhizopus arrhizus var. delemar (R. delemar) infection in a murine model of pulmonary mucormycosis using transcriptome sequencing (RNA-seq). Network analysis revealed activation of the host’s epidermal growth factor receptor (EGFR) signaling. Consistent with the RNA-seq results, EGFR became phosphorylated upon in vitro infection of human alveolar epithelial cells with several members of the Mucorales, and this phosphorylated, activated form of EGFR colocalized with R. delemar spores. Inhibition of EGFR signaling with cetuximab or gefitinib, specific FDA-approved inhibitors of EGFR, significantly reduced the ability of R. delemar to invade and damage airway epithelial cells. Furthermore, gefitinib treatment significantly prolonged survival of mice with pulmonary mucormycosis, reduced tissue fungal burden, and attenuated the activation of EGFR in response to pulmonary mucormycosis. These results indicate EGFR represents a novel host target to block invasion of alveolar epithelial cells by R. delemar, and inhibition of EGFR signaling provides a novel approach for treating mucormycosis by repurposing an FDA-approved drug.

Mucormycosis is an increasingly common, highly lethal fungal infection with very limited treatment options. Using a combination of in vivo animal models, transcriptomics, cell biology, and pharmacological approaches, we have demonstrated that Mucorales fungi activate EGFR signaling to induce fungal uptake into airway epithelial cells. Inhibition of EGFR signaling with existing FDA-approved drugs significantly increased survival following R. arrhizus var. delemar infection in mice. This study enhances our understanding of how Mucorales fungi invade host cells during the establishment of pulmonary mucormycosis and provides a proof-of-concept for the repurposing of FDA-approved drugs that target EGFR function.

Human CARD9: A Critical Molecule of Fungal Immune Surveillance

CARD9 is a signaling adaptor protein that is involved in the transduction of signals from a variety of innate pattern recognition receptors, including the C-type lectin receptors and intracellular NOD receptors and nucleic acid sensors. As a result, CARD9 has been shown in animal models to be an important regulator of immunity to bacteria, fungi, and viruses. Studies in humans with autosomal recessive CARD9 deficiency have indicated a highly specific role for this molecule in the activation of antifungal immune responses in the central nervous system, the oral mucosa, and the skin. Moreover, CARD9-dependent functions have recently been indicated to modulate the development of autoimmunity, inflammatory bowel diseases, and cancer. In this mini-review, we highlight the recent studies that have identified several novel functions of CARD9 in various disease contexts, and we summarize the contemporary understanding of the genetics and immunology of human CARD9 deficiency.

Inherited CARD9 Deficiency: Invasive Disease Caused by Ascomycete Fungi in Previously Healthy Children and Adults

Autosomal recessive CARD9 deficiency underlies life-threatening, invasive fungal infections in otherwise healthy individuals normally resistant to other infectious agents. In less than 10 years, 58 patients from 39 kindreds have been reported in 14 countries from four continents. The patients are homozygous (n = 49; 31 kindreds) or compound heterozygous (n = 9; 8 kindreds) for 22 different CARD9 mutations. Six mutations are recurrent, probably due to founder effects. Paradoxically, none of the mutant alleles has been experimentally demonstrated to be loss-of-function. CARD9 is expressed principally in myeloid cells, downstream from C-type lectin receptors that can recognize fungal components. Patients with CARD9 deficiency present impaired cytokine and chemokine production by macrophages, dendritic cells, and peripheral blood mononuclear cells and defective killing of some fungi by neutrophils in vitro. Neutrophil recruitment to sites of infection is impaired in vivo. The proportion of Th17 cells is low in most, but not all, patients tested. Up to 52 patients suffering from invasive fungal diseases (IFD) have been reported, with ages at onset of 3.5 to 52 years. Twenty of these patients also displayed superficial fungal infections. Six patients had only mucocutaneous candidiasis or superficial dermatophytosis at their last follow-up visit, at the age of 19 to 50 years. Remarkably, for 50 of the 52 patients with IFD, a single fungus was involved; only two patients had IFDs due to two different fungi. IFD recurred in 44 of 45 patients who responded to treatment, and a different fungal infection occurred in the remaining patient. Ten patients died from IFD, between the ages of 12 and 39 years, whereas another patient died at the age of 91 years, from an unrelated cause. At the most recent scheduled follow-up visit, 81% of the patients were still alive and aged from 6.5 to 75 years. Strikingly, all the causal fungi belonged to the phylum Ascomycota: commensal Candida and saprophytic Trychophyton, Aspergillus, Phialophora, Exophiala, Corynesprora, Aureobasidium, and Ochroconis. Human CARD9 is essential for protective systemic immunity to a subset of fungi from this phylum but seems to be otherwise redundant. Previously healthy patients with unexplained invasive fungal infection, at any age, should be tested for inherited CARD9 deficiency.

KEY POINTS

• Inherited CARD9 deficiency (OMIM #212050) is an AR PID due to mutations that may be present in a homozygous or compound heterozygous state. • CARD9 is expressed principally in myeloid cells and transduces signals downstream from CLR activation by fungal ligands. • Endogenous mutant CARD9 levels differ between alleles (from full-length normal protein to an absence of normal protein). • The functional impacts of CARD9 mutations involve impaired cytokine production in response to fungal ligands, impaired neutrophil killing and/or recruitment to infection sites, and defects of Th17 immunity. • The key clinical manifestations in patients are fungal infections, including CMC, invasive (in the CNS in particular) Candida infections, extensive/deep dermatophytosis, subcutaneous and invasive phaeohyphomycosis, and extrapulmonary aspergillosis. • The clinical penetrance of CARD9 deficiency is complete, but penetrance is incomplete for each of the fungi concerned. • Age at onset is highly heterogeneous, ranging from childhood to adulthood for the same fungal disease. • All patients with unexplained IFD should be tested for CARD9 mutations. Familial screening and genetic counseling should be proposed. • The treatment of patients with CARD9 mutations is empirical and based on antifungal therapies and the surgical removal of fungal masses. Patients with persistent/relapsing Candida infections of the CNS could be considered for adjuvant GM-CSF/G-CSF therapy. The potential value of HSCT for CARD9-deficient patients remains unclear.

The role of neutrophils in host defense against invasive fungal infections

Purpose of Review

Invasive fungal infections caused by the commensal yeast Candida and the ubiquitous, inhaled mold Aspergillus have emerged as major causes of morbidity and mortality in critically ill and immunosuppressed patient populations. Here, we review how neutrophils contribute to effective immunity against these infections.

Recent Findings

Studies in mouse models of invasive candidiasis and aspergillosis, and observations in hematological patients with chemotherapy-induced neutropenia and in patients with primary immunodeficiency disorders that manifest with these infections have highlighted the critical role of neutrophils and have identified key immune factors that promote neutrophil-mediated effective host defense against invasive fungal disease.

Summary

Neutrophils are crucial in host protection against invasive candidiasis and aspergillosis. Recent advances in our understanding of the molecular cues that mediate protective neutrophil recruitment and effector function against these infections hold promise for developing immune-based strategies to improve the outcomes of affected patients.

Candidiasis of the Central Nervous System in Neonates and Children with Primary Immunodeficiencies

PURPOSE OF REVIEW

Candida infections of the central nervous system (CNS) are a life-threatening complication of invasive infections that most often affect vulnerable groups of patients, including neonates and children with primary immunodeficiency disorders (PID). Here, we review the currently known risk factors for CNS candidiasis, focusing predominantly on the PID caused by biallelic mutations in CARD9.

RECENT FINDINGS

How the CNS is protected itself against fungal invasion is poorly understood. CARD9 promotes neutrophil recruitment and function, and is the only molecule shown to be critical for protection against CNS candidiasis in humans thus far.

SUMMARY

Fundamental insights into the pathogenesis of CNS candidiasis gained from studying rare CARD9-deficient patients has significant implications for other patients at risk for this disease, such as CARD9-sufficient neonates. These findings will be important for the development of adjunctive immune-based therapies, which are urgently needed to tackle the global burden of invasive fungal diseases.

The Role of AIRE in the Immunity Against Candida Albicans in a Model of Human Macrophages

Autoimmune-polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a primary immunodeficiency caused by mutations in the autoimmune regulator gene (AIRE). Patients with AIRE mutations are susceptible to Candida albicans infection and present with autoimmune disorders. We previously demonstrated that cytoplasmic AIRE regulates the Syk-dependent Dectin-1 pathway. In this study, we further evaluated direct contact with fungal elements, synapse formation, and the response of macrophage-like THP-1 cells to C. albicans hyphae to determine the role of AIRE upon Dectin receptors function and signaling. We examined the fungal synapse (FS) formation in wild-type and AIRE-knockdown THP-1 cells differentiated to macrophages, as well as monocyte-derived macrophages from APECED patients. We evaluated Dectin-2 receptor signaling, phagocytosis, and cytokine secretion upon hyphal stimulation. AIRE co-localized with Dectin-2 and Syk at the FS upon hyphal stimulation of macrophage-like THP-1 cells. AIRE-knockdown macrophage-like THP-1 cells exhibited less Dectin-1 and Dectin-2 receptors accumulation, decreased signaling pathway activity at the FS, lower C. albicans phagocytosis, and less lysosome formation. Furthermore, IL-1β, IL-6, or TNF-α secretion by AIRE-knockdown macrophage-like THP-1 cells and AIRE-deficient patient macrophages was decreased compared to control cells. Our results suggest that AIRE modulates the FS formation and hyphal recognition and help to orchestrate an effective immune response against C. albicans.

Host Control of Fungal Infections: Lessons from Basic Studies and Human Cohorts

In the last few decades, the AIDS pandemic and the significant advances in the medical management of individuals with neoplastic and inflammatory conditions have resulted in a dramatic increase in the population of immunosuppressed patients with opportunistic, life-threatening fungal infections. The parallel development of clinically relevant mouse models of fungal disease and the discovery and characterization of several inborn errors of immune-related genes that underlie inherited human susceptibility to opportunistic mycoses have significantly expanded our understanding of the innate and adaptive immune mechanisms that protect against ubiquitous fungal exposures. This review synthesizes immunological knowledge derived from basic mouse studies and from human cohorts and provides an overview of mammalian antifungal host defenses that show promise for informing therapeutic and vaccination strategies for vulnerable patients.

Impaired Specific Antifungal Immunity in CARD9-Deficient Patients with Phaeohyphomycosis

Phaeohyphomycosis is a group of severe infections caused by dematiaceous fungi. We previously identified CARD9 deficiencies in four Chinese patients with phaeohyphomycosis caused by Phialophora verrucosa. In this study, we sought to identify the genetic and immunological mechanisms underlying rare dematiaceous fungal infections in three otherwise healthy patients with phaeohyphomycosis caused by Exophiala spinifera, Ochroconis musae, and Corynespora cassiicola. CARD9 sequencing in these patients showed one mutation (p.S23X) that, to our knowledge, has not been characterized and two previously characterized mutations (p.D274fsX60 and p.L64fsX59) that led to lack of CARD9 protein expression. Patient-derived CARD9-deficient cells showed a selective impairment of proinflammatory cytokine and chemokine production, NF-κB activation, and T helper type 22- and T helper type 17-associated responses upon fungus-specific stimulation, whereas phagocytosis and reactive oxygen species production were intact. Consistently, Card9-knockout mice were highly susceptible to phaeohyphomycosis and exhibited immune deficiencies similar to those of patients, including diminished NF-κB and p38 MAPK activation in local and in vitro functional studies. This work clarifies the association between inherited CARD9 deficiencies and phaeohyphomycosis, and furthers current knowledge on the spectrum and pathophysiology of diseases resulting from CARD9 deficiencies.

Inborn errors of immunity underlying fungal diseases in otherwise healthy individuals

It has been estimated that there are at least 1.5 million fungal species, mostly present in the environment, but only a few of these fungi cause human disease. Most fungal diseases are self-healing and benign, but some are chronic or life-threatening. Acquired and inherited defects of immunity, including breaches of mucocutaneous barriers and circulating leukocyte deficiencies, account for most severe modern-day mycoses. Other types of infection typically accompany these fungal infections. More rarely, severe fungal diseases can strike otherwise healthy individuals. Historical reports of fungi causing chronic peripheral infections (e.g. affecting the nails, skin, hair), and invasive diseases (e.g. brain, lungs, liver), in otherwise healthy patients, can be traced back to the mid-20^th century. These fungi typically cause endemic, but not epidemic diseases, are more likely to underlie sporadic than familial cases, and only threaten a small proportion of infected individuals. The basis of this 'idiosyncratic' susceptibility has long remained unexplained, but it has recently become apparent that 'idiopathic' fungal diseases, in children, teenagers, and even adults, may be caused by single-gene inborn errors of immunity. The study of these unusual primary immunodeficiencies (PIDs) has led to the identification of molecules and cells playing a crucial role in human host defenses against certain fungi at particular anatomic sites. A picture is emerging of inborn errors of IL-17 immunity selectively underlying chronic mucocutaneous candidiasis, with little inter-individual variability, and of inborn errors of CARD9 immunity underlying various life-threatening invasive fungal diseases, differing between patients.

Novel bi-allelic splice mutations in CARD9 causing adult-onset Candida endophthalmitis

CARD9 deficiency (CANDF2; OMIM# 212050) is an autosomal-recessive monogenic inborn error of immunity conferring susceptibility to invasive fungal diseases, including the very distinct syndrome of spontaneous central nervous system candidiasis, in which opportunistic yeast of the genus Candida infect the central nervous system (either brain parenchyma and/or meninges) in the absence of trauma, chemotherapy or underlying systemic disease. We present a patient with spontaneous endophthalmitis of the right eye due to Candida albicans; further investigations revealed concomitant cerebral abscess. She had a history of left endophthalmitis due to the dematiaceous mould, Aureobasidium pullulans, 15 years earlier. Targeted sequencing of the CARD9 gene revealed 2 novel variants (c.184G>A and c.288C>T). Analysis in silico predicted each variant altered splicing, which was confirmed by sequencing of cDNA from proband and carrier offsprings: c.184G>A results in a 4-base pair frameshift deletion with loss of allelic expression; c.288C>T results in an in-frame 36-bp pair deletion with detectable protein. CARD9 deficiency can present with a phenotype of spontaneous candidal endophthalmitis. We report 2 novel mutations in CARD9, both affecting splicing, expanding the range of morbid variants causing CARD9 deficiency, emphasising the importance of both genomic and cDNA sequencing for this condition.

Immunity against fungi

Pathogenic fungi cause a wide range of syndromes in immune-competent and immune-compromised individuals, with life-threatening disease primarily seen in humans with HIV/AIDS and in patients receiving immunosuppressive therapies for cancer, autoimmunity, and end-organ failure. The discovery that specific primary immune deficiencies manifest with fungal infections and the development of animal models of mucosal and invasive mycoses have facilitated insight into fungus-specific recognition, signaling, effector pathways, and adaptive immune responses. Progress in deciphering the molecular and cellular basis of immunity against fungi is guiding preclinical studies into vaccine and immune reconstitution strategies for vulnerable patient groups. Furthermore, recent work has begun to address the role of endogenous fungal communities in human health and disease. In this review, we summarize a contemporary understanding of protective immunity against fungi.

30 Years of NF-κB: A Blossoming of Relevance to Human Pathobiology

NF-κB was discovered 30 years ago as a rapidly inducible transcription factor. Since that time, it has been found to have a broad role in gene induction in diverse cellular responses, particularly throughout the immune system. Here, we summarize elaborate regulatory pathways involving this transcription factor and use recent discoveries in human genetic diseases to place specific proteins within their relevant medical and biological contexts.

Extrapulmonary Aspergillus infection in patients with CARD9 deficiency

Invasive pulmonary aspergillosis is a life-threatening mycosis that only affects patients with immunosuppression, chemotherapy-induced neutropenia, transplantation, or congenital immunodeficiency. We studied the clinical, genetic, histological, and immunological features of 2 unrelated patients without known immunodeficiency who developed extrapulmonary invasive aspergillosis at the ages of 8 and 18. One patient died at age 12 with progressive intra-abdominal aspergillosis. The other patient had presented with intra-abdominal candidiasis at age 9, and developed central nervous system aspergillosis at age 18 and intra-abdominal aspergillosis at age 25. Neither patient developed Aspergillus infection of the lungs. One patient had homozygous M1I CARD9 (caspase recruitment domain family member 9) mutation, while the other had homozygous Q295X CARD9 mutation; both patients lacked CARD9 protein expression. The patients had normal monocyte and Th17 cell numbers in peripheral blood, but their mononuclear cells exhibited impaired production of proinflammatory cytokines upon fungus-specific stimulation. Neutrophil phagocytosis, killing, and oxidative burst against Aspergillus fumigatus were intact, but neither patient accumulated neutrophils in infected tissue despite normal neutrophil numbers in peripheral blood. The neutrophil tissue accumulation defect was not caused by defective neutrophil-intrinsic chemotaxis, indicating that production of neutrophil chemoattractants in extrapulmonary tissue is impaired in CARD9 deficiency. Taken together, our results show that CARD9 deficiency is the first known inherited or acquired condition that predisposes to extrapulmonary Aspergillus infection with sparing of the lungs, associated with impaired neutrophil recruitment to the site of infection.

New clinical phenotypes of fungal infections in special hosts

Incidence of invasive fungal infections increases over time with the rise in at-risk populations; in particular, patients with acquired immunodeficiencies due to immunosuppressive therapies such as anti-tumour necrosis factor-α (TNF-α) treatment, cirrhosis or burns. Some primary immunodeficiencies (PID) can also predispose selectively to invasive fungal diseases. Conversely, some atypical fungal diseases can reveal new PID. Deep dermatophytosis, Candida central nervous system infections or gastrointestinal disease, or disseminated phaeohyphomycosis-revealed CARD9 deficiency. Most patients with inherited chronic mucocutaneous candidiasis were found to carry STAT1 gain-of-function mutations. The spectrum of fungal susceptibility and clinical presentation varies according to the PID. Among acquired immunodeficiencies, immunosuppressive treatments such as TNF-α blocker therapy, which has revolutionized autoimmune disorder treatment, may be complicated by endemic mycosis, aspergillosis, pneumocystosis or cryptococcosis. Burn patients with damaged skin barrier protection are susceptible to severe Candida infections and filamentous fungal infections (such as Aspergillus spp., Mucorales). Moreover, patients with cirrhosis are at increased risk of fungal infections. Therefore, physicians should think of any potential underlying acquired or inherited immunodeficiency in a patient developing an atypical fungal infection, or of a potential fungal disease in the context of an atypical presentation in specific hosts.

Mechanistic Insights into the Role of C-Type Lectin Receptor/CARD9 Signaling in Human Antifungal Immunity

Human CARD9 deficiency is an autosomal recessive primary immunodeficiency disorder caused by biallelic mutations in the gene CARD9, which encodes a signaling protein that is found downstream of many C-type lectin receptors (CLRs). CLRs encompass a large family of innate recognition receptors, expressed predominantly by myeloid and epithelial cells, which bind fungal carbohydrates and initiate antifungal immune responses. Accordingly, human CARD9 deficiency is associated with the spontaneous development of persistent and severe fungal infections that primarily localize to the skin and subcutaneous tissue, mucosal surfaces and/or central nervous system (CNS). In the last 3 years, more than 15 missense and nonsense CARD9 mutations have been reported which associate with the development of a wide spectrum of fungal infections caused by a variety of fungal organisms. The mechanisms by which CARD9 provides organ-specific protection against these fungal infections are now emerging. In this review, we summarize recent immunological and clinical advances that have provided significant mechanistic insights into the pathogenesis of human CARD9 deficiency. We also discuss how genetic mutations in CARD9-coupled receptors (Dectin-1, Dectin-2) and CARD9-binding partners (MALT1, BCL10) affect human antifungal immunity relative to CARD9 deficiency, and we highlight major understudied research questions which merit future investigation.