Genetic Susceptibility to Fungal Infections in Humans

Genetic Susceptibility to Fungal Infections

Reports of fungal infections have increased over the past decades, making them a major threat to human health. In this study, we review the effects of genetic defects on susceptibility to fungal diseases. To identify all relevant literature, we searched Google Scholar, PubMed, and Scopus and profiled studies published between 2008 and 2021. The results of several studies conducted on this subject have shown the significant effects of genetic variations such as hyper-IgE syndrome, Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy syndrome, dectin-1 deficiency, CARD9 mutations, STAT1 mutations, and IL17 mutationson the host immune system's response, which has an important impact on susceptibility to fungal infections. The underlying immune system-related genetic profile affects the susceptibility of individuals to different fungal infections; therefore, this subject should be further studied for better treatment of fungal diseases.

Penicillium and Talaromyces spp. emerging pathogens in dogs since 1990s

Penicillium and Talaromyces spp. are environmental saprophytic molds rarely encountered as infectious agents in humans and animals. This article summarizes the clinical features, treatment, and outcomes of proven infections caused by Penicillium or Talaromyces in four dogs in France. Two dogs had disseminated infections, while the other two had a localized form. All dogs had positive histopathological results showing the presence of hyaline septate hyphae and a positive fungal culture with typical Penicillium conidiophores. Talaromyces georgiensis (n = 1), Penicillium labradorum (n = 2), and Penicillium from section Ramosa series Raistrickiorum (n = 1), were identified based on Internal Transcribed Spacer (ITS) Sanger sequencing. The dogs were initially treated with ketoconazole or itraconazole. Second-line treatment was initiated in three dogs, but after several relapses, the prognosis remained poor. Since the 1990s, 18 cases of Penicillium or Talaromyces infections in dogs have been described worldwide. This series of four reports brings new cases to those already reported in the literature, which are probably underestimated in the world.

Bacterial Pigments and Their Multifaceted Roles in Contemporary Biotechnology and Pharmacological Applications

Synthetic dyes and colourants have been the mainstay of the pigment industry for decades. Researchers are eager to find a more environment friendly and non-toxic substitute because these synthetic dyes have a negative impact on the environment and people’s health. Microbial pigments might be an alternative to synthetic pigments. Microbial pigments are categorized as secondary metabolites and are mainly produced due to impaired metabolism under stressful conditions. These pigments have vibrant shades and possess nutritional and therapeutic properties compared to synthetic pigment. Microbial pigments are now widely used within the pharmaceuticals, food, paints, and textile industries. The pharmaceutical industries currently use bacterial pigments as a medicine alternative for cancer and many other bacterial infections. Their growing popularity is a result of their low cost, biodegradable, non-carcinogenic, and environmentally beneficial attributes. This audit article has made an effort to take an in-depth look into the existing uses of bacterial pigments in the food and pharmaceutical industries and project their potential future applications.

Pathogenesis and virulence of Candida albicans

Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.

Genetic Susceptibility to Fungal Infections and Links to Human Ancestry

Over the ages, fungi have associated with different parts of the human body and established symbiotic associations with their host. They are mostly commensal unless there are certain not so well-defined factors that trigger the conversion to a pathogenic state. Some of the factors that induce such transition can be dependent on the fungal species, environment, immunological status of the individual, and most importantly host genetics. In this review, we discuss the different aspects of how host genetics play a role in fungal infection since mutations in several genes make hosts susceptible to such infections. We evaluate how mutations modulate the key recognition between the pathogen associated molecular patterns (PAMP) and the host pattern recognition receptor (PRR) molecules. We discuss the polymorphisms in the genes of the immune system, the way it contributes toward some common fungal infections, and highlight how the immunological status of the host determines fungal recognition and cross-reactivity of some fungal antigens against human proteins that mimic them. We highlight the importance of single nucleotide polymorphisms (SNPs) that are associated with several of the receptor coding genes and discuss how it affects the signaling cascade post-infection, immune evasion, and autoimmune disorders. As part of personalized medicine, we need the application of next-generation techniques as a feasible option to incorporate an individual’s susceptibility toward invasive fungal infections based on predisposing factors. Finally, we discuss the importance of studying genomic ancestry and reveal how genetic differences between the human race are linked to variation in fungal disease susceptibility.

IL-7 Immunotherapy in a Nonimmunocompromised Patient With Intractable Fungal Wound Sepsis

A nonimmunocompromised patient developed life-threatening soft tissue infection with Trichosporon asahii, Fusarium, and Saksenaea that progressed despite maximum antifungal therapies and aggressive debridement. Interleukin-7 immunotherapy resulted in clinical improvement, fungal clearance, reversal of lymphopenia, and improved T-cell function. Immunoadjuvant therapies to boost host immunity may be efficacious in life-threatening fungal infections.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

CCR5-mediated Recruitment of NK Cells to the Kidney Is a Critical Step for Host Defense to Systemic Candida albicans Infection

C-C chemokine receptor type 5 (CCR5) regulates the trafficking of various immune cells to sites of infection. In this study, we showed that expression of CCR5 and its ligands was rapidly increased in the kidney after systemic Candida albicans infection, and infected CCR5^−/− mice exhibited increased mortality and morbidity, indicating that CCR5 contributes to an effective defense mechanism against systemic C. albicans infection. The susceptibility of CCR5^−/− mice to C. albicans infection was due to impaired fungal clearance, which in turn resulted in exacerbated renal inflammation and damage. CCR5-mediated recruitment of NK cells to the kidney in response to C. albicans infection was necessary for the anti-microbial activity of neutrophils, the main fungicidal effector cells. Mechanistically, C. albicans induced expression of IL-23 by CD11c⁺ dendritic cells (DCs). IL-23 in turn augmented the fungicidal activity of neutrophils through GM-CSF production by NK cells. As GM-CSF potentiated production of IL-23 in response to C. albicans, a positive feedback loop formed between NK cells and DCs seemed to function as an amplification point for host defense. Taken together, our results suggest that CCR5-mediated recruitment of NK cells to the site of fungal infection is an important step that underlies innate resistance to systemic C. albicans infection.

Primary immunodeficiencies and invasive fungal infection: when to suspect and how to diagnose and manage

PURPOSE OF REVIEW

Invasive fungal infections (IFIs) most often occur secondary to acquired immunodeficiency states such as transplantation, AIDS or immune-modulatory treatment for neoplastic and autoimmune disorders. Apart from these acquired conditions, several primary immunodeficiency disorders (PIDs) can present with IFIs in the absence of iatrogenic immunosuppression. This review highlights recent advances in our understanding of PIDs that cause IFIs, which may help clinicians in the diagnosis and management of such infections.

RECENT FINDINGS

A growing number of PIDs that cause varying combinations of invasive infections by commensal Candida, inhaled molds (primarily Aspergillus), Cryptococcus, Pneumocystis, endemic dimorphic fungi, dermatophytes, and/or agents of phaeohyphomycosis has uncovered the organ- and fungus-specific requirements for effective antifungal host defense in humans. Employing certain diagnostic algorithms tailored to the infecting fungus can facilitate the genetic diagnosis of the underlying PID, which has implications for the optimal management of affected patients.

SUMMARY

Heightened clinical suspicion is required for the diagnosis of underlying genetic defects in patients who develop IFIs in the absence of acquired immunodeficiency. Early initiation of antifungal therapy followed by long-term secondary prophylaxis is typically needed to achieve remission, but hematopoietic stem-cell transplantation may sometimes be necessary to promote immune restoration and infection control.

An exaggerated monocyte-derived cytokine response to Candida hyphae in patients with recurrent vulvovaginal candidiasis

Background

Recurrent vulvovaginal candidiasis (RVVC) affects up to 8% of women. The immunopathogenesis is poorly understood but it has been suggested that RVVC might be due to dysregulated innate immune response. The aim of this study was to compare cytokine profiles in stimulated primary mononuclear cells (PBMCs) from RVVC and healthy individuals.

Methods

PBMCs isolated from RVVC patients (n = 24) and healthy volunteers (n = 30) were stimulated with unspecific and pathogen-specific antigens. Cytokine production was assessed after 24 hours, 48 hours, and 7 days using ELISA.

Results

No significant differences in cytokine production were found in T helper 1 (Th1), Th2, and Th17 immunity in response to both unspecific and pathogen-specific stimulations. Tumor necrosis factor-α (TNF-α) production in response to C. albicans hyphae was significantly higher in patients than controls and within the patient group, a significant positive correlation was found between interleukin-1β (IL-1β) and both TNF-α and IL-6. Both IL-1β/IL-1Ra and TNF-α/IL-10 ratios in Candida hyphae-stimulated PBMCs were significantly higher in patients than controls.

Conclusions

Women affected by RVVC showed increased monocytes-derived cytokine production, which might contribute to an exaggerated vaginal immune response to Candida hyphae. RVVC patients show no defective Th-dependent adaptive immune response upon Candida stimulation.

Genetic variation and fungal infection risk: State of the art

Purpose of review

Fungal infections cause significant mortality in patients with acquired immunodeficiencies including AIDS, hematological malignancies, transplantation, and receipt of corticosteroids, biologics or small-molecule kinase inhibitors that impair key immune pathways. The contribution of several such pathways in antifungal immunity has been uncovered by inherited immunodeficiencies featuring profound fungal susceptibility. Furthermore, the risk of fungal infection in patients with acquired immunodeficiencies may be modulated by single nucleotide polymorphisms (SNPs) in immune-related genes. This review outlines key features underlying human genetic fungal predisposition.

Recent findings

The discovery of monogenic disorders that cause fungal disease and the characterization of immune-related gene SNPs that may regulate fungal susceptibility have provided important insights into how genetic variation affects development and outcome of fungal infections in humans.

Summary

Recognition of individualized genetic fungal susceptibility traits in humans should help devise precision-medicine strategies for risk assessment, prognostication and treatment of patients with opportunistic fungal infections.

Why are so many cases of invasive aspergillosis missed?

Invasive aspergillosis (IA) incidence is increasing in several countries like France, and numerous cases are indeed missed and still only diagnosed at autopsy as evidenced by recently published data. Such missed diagnoses are obviously encountered when appropriate diagnostic tools are not available especially in low resource areas or when biologists have not been trained enough in medical mycology (i.e., microscopic examination and culture in most of those areas). Besides logistical issues, which are indeed critical, IA may not be recognized because clinicians failed to consider that risk factors are evolving with the IA burden now observed among patients with chronic lymphoid malignancies or receiving new biotherapies, with diabetes mellitus or liver cirrhosis and/or acute alcoholic hepatitis, with patients from the intensive care unit (ICU) and among patients with some predisposing primary immune deficiencies now reaching the adult's age. This is also the case for human immunodeficiency virus (HIV)-infected patients who failed to meet the classical definitions of IA. From the radiology perspective, new entities of IA have also emerged which absolutely need to be recognized especially bronchial-based-IA among allogeneic stem cell transplant recipients. Finally, from the laboratory side, contribution and limits of indirect blood biomarkers should be integrated to the clinical life in order not to miss IA cases. To conclude, several diagnostic tools should be combined and a constant dialog between laboratory and clinics is crucial to appropriately diagnose IA.

Emerging Aspergillus Species Almost Exclusively Associated With Primary Immunodeficiencies

Invasive aspergillosis (IA) is the most serious mold infection encountered in patients with iatrogenic immunosuppression. IA is also a major cause of mortality and morbidity in individuals with primary immunodeficiency (PID). Although Aspergillus fumigatus is the most common etiologic agent of IA reported in PID patients, followed by A. nidulans, multiple poorly recognized Aspergillus species such as A. udagawae, A. quadrilineatus, A. pseudoviridinutans, A. tanneri, A. subramanianii, and A. fumisynnematus have been reported almost exclusively from patients with inborn defects in host antifungal defense pathways. Infection in PID patients exhibits patterns of disease progression distinct from those in iatrogenic immunosuppression. Specifically, the disease can be extrapulmonary and chronic with a tendency to disseminate in a contiguous manner across anatomical planes. It is also more refractory to standard antifungal therapy. This synopsis summarizes our understanding of emerging rare Aspergillus species that primarily affect patients with PIDs but not those with acquired immunodeficiencies.

Expression of SHP-1 and SOCS6 in patients with acute leukemia and their clinical implication

BACKGROUND

To investigate the expression and clinical relevance of Src homology region 2 domain-containing phosphatase-1 (SHP-1) and suppressor of cytokine signaling 6 (SOCS6) in acute leukemia (AL).

PATIENTS AND METHODS

The enrolled AL patients were divided into three groups (newly diagnosed, relapsed, and complete remission [CR]). Healthy donors were also included as a control group in this study. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was performed to measure messenger RNA (mRNA) expression of SHP-1 and SOCS6. Statistical analysis was conducted to analyze the correlation between mRNA levels of SHP-1 and SOCS6 with patient outcomes.

RESULTS

mRNA expression of SHP-1 was significantly lower in AL patients than that in healthy donors. The newly diagnosed or relapsed AL patients had lower mRNA levels of SHP-1 than the patients in CR. In contrast, SOCS6 mRNA expression was significantly higher in newly diagnosed or relapsed patients than that in patients in CR as well as healthy donors. However, mRNA levels of both SHP-1 and SOCS6 were positively correlated with the patient remission. The chemotherapy-induced remission rate was higher in patients with detectable SHP-1 or SOCS6 expression than in patients with undetectable SHP-1 or SOCS6 expression. Furthermore, the AL patients with detectable SHP-1 mRNA expression had lower incidence rate of invasive fungal infection.

CONCLUSION

The results suggest that expression patterns of SHP-1 and SOCS6 differ in AL patients. Despite the difference, expression of SHP-1 and SOCS6 is associated with favorable outcomes, suggesting an anticancer property of these two genes in AL.

Immunity to Commensal Fungi: Detente and Disease

Fungi are ubiquitous in our environment, and a healthy immune system is essential to maintain adequate protection from fungal infections. When this protection breaks down, superficial and invasive fungal infections cause diseases that range from irritating to life-threatening. Millions of people worldwide develop invasive infections during their lives, and mortality for these infections often exceeds 50%. Nevertheless, we are normally colonized with many of the same disease-causing fungi (e.g., on the skin or in the gut). Recent research is dramatically expanding our understanding of the mechanisms by which our immune systems interact with these organisms in health and disease. In this review, we discuss what is currently known about where and how the immune system interacts with common fungi.

PKC-δ activation in neutrophils promotes fungal clearance

The C-type lectin receptor dectin-1 and the integrin Mac-1 have key roles in controlling fungal infection. Here, we demonstrate that dectin-1- and Mac-1-induced activation of protein kinase Cδ in neutrophils, independent of the Card9 adaptor, is required for reactive oxygen species production and for intracellular killing upon Candida albicans uptake. Protein kinase Cδ was also required for zymosan-induced cytokine generation in neutrophils. In macrophages, protein kinase Cδ deficiency prevented fungi-induced reactive oxygen species generation but had no effect on activation of TGF-β-activated kinase-1, an effector of Card9, or nuclear factor κB activation, nor did it affect phagolysosomal maturation, autophagy, or intracellular C. albicans killing. In vivo, protein kinase Cδ-deficient mice were highly susceptible to C. albicans and Aspergillus fumigatus infection, which was partially rescued with adoptively transferred wild-type neutrophils. Thus, protein kinase Cδ activation downstream of dectin-1 and Mac-1 has an important role in neutrophil, but not macrophage, functions required for host defense against fungal pathogens.

Neutrophil Attack Triggers Extracellular Trap-Dependent Candida Cell Wall Remodeling and Altered Immune Recognition

Pathogens hide immunogenic epitopes from the host to evade immunity, persist and cause infection. The opportunistic human fungal pathogen Candida albicans, which can cause fatal disease in immunocompromised patient populations, offers a good example as it masks the inflammatory epitope β-glucan in its cell wall from host recognition. It has been demonstrated previously that β-glucan becomes exposed during infection in vivo but the mechanism behind this exposure was unknown. Here, we show that this unmasking involves neutrophil extracellular trap (NET) mediated attack, which triggers changes in fungal cell wall architecture that enhance immune recognition by the Dectin-1 β-glucan receptor in vitro. Furthermore, using a mouse model of disseminated candidiasis, we demonstrate the requirement for neutrophils in triggering these fungal cell wall changes in vivo. Importantly, we found that fungal epitope unmasking requires an active fungal response in addition to the stimulus provided by neutrophil attack. NET-mediated damage initiates fungal MAP kinase-driven responses, particularly by Hog1, that dynamically relocalize cell wall remodeling machinery including Chs3, Phr1 and Sur7. Neutrophil-initiated cell wall disruptions augment some macrophage cytokine responses to attacked fungi. This work provides insight into host-pathogen interactions during disseminated candidiasis, including valuable information about how the C. albicans cell wall responds to the biotic stress of immune attack. Our results highlight the important but underappreciated concept that pattern recognition during infection is dynamic and depends on the host-pathogen dialog.

Opportunistic fungal infections, including those caused by C. albicans, have emerged as a significant global health burden and the disseminated form of these infections still have unacceptably high mortality rates despite modern antifungal treatments. The fungal cell wall controls its interaction with the host environment and immune recognition, although cell wall dynamics during infection are poorly understood. C. albicans organizes its cell wall to mask the inflammatory β-glucan as a form of immune evasion and it is known that during infection this β-glucan becomes exposed. Here, we investigated how β-glucan becomes exposed and discovered a dynamic interaction where host NETs provoke an active fungal response that disrupts cell wall architecture and unmasks β-glucan. We revealed an unexpected level of local fungal cell wall dynamics in response to immune mediated stress, suggesting this may represent a model that can be leveraged to identify novel drug targets. Our results highlight the understudied concept that the cell wall is a dynamic landscape during infection and can be influenced by the host.

Genetic Susceptibility to Fungal Infections: What is in the Genes?

The development of severe fungal infections has long been associated with traditional risk factors such as profound immunosuppression, yet it remains challenging to understand why under similar conditions only some patients will develop these infections while others will not. Recent studies have demonstrated the importance of host genetic variation in influencing the severity and susceptibility to invasive fungal infections (IFIs). In this review, we examine selected primary immunodeficiencies characterized by their vulnerability to a narrow range of fungal pathogens, and then focus on recently identified genetic polymorphisms associated with an increased susceptibility to IFIs.

Immune Interactions with Pathogenic and Commensal Fungi: A Two-Way Street

We are exposed to a wide spectrum of fungi including innocuous environmental organisms, opportunistic pathogens, commensal organisms, and fungi that can actively and explicitly cause disease. Much less is understood about effective host immunity to fungi than is generally known about immunity to bacterial and viral pathogens. Innate and adaptive arms of the immune system are required for effective host defense against Candida, Aspergillus, Cryptococcus, and others, with specific elements of the host response regulating specific types of fungal infections (e.g., mucocutaneous versus systemic). Here we will review themes and controversies that are currently shaping investigation of antifungal immunity (primarily to Candida and Aspergillus) and will also examine the emerging field of the role of fungi in the gut microbiome.

Opportunistic yeast pathogens: reservoirs, virulence mechanisms, and therapeutic strategies

Life-threatening invasive fungal infections are becoming increasingly common, at least in part due to the prevalence of medical interventions resulting in immunosuppression. Opportunistic fungal pathogens of humans exploit hosts that are immunocompromised, whether by immunosuppression or genetic predisposition, with infections originating from either commensal or environmental sources. Fungal pathogens are armed with an arsenal of traits that promote pathogenesis, including the ability to survive host physiological conditions and to switch between different morphological states. Despite the profound impact of fungal pathogens on human health worldwide, diagnostic strategies remain crude and treatment options are limited, with resistance to antifungal drugs on the rise. This review will focus on the global burden of fungal infections, the reservoirs of these pathogens, the traits of opportunistic yeast that lead to pathogenesis, host genetic susceptibilities, and the challenges that must be overcome to combat antifungal drug resistance and improve clinical outcome.

Inherited IL-17RC deficiency in patients with chronic mucocutaneous candidiasis

Autosomal-recessive IL-17RA, IL-17RC, and ACT1 deficiencies and autosomal-dominant IL-17F deficiency in humans underlie susceptibility to chronic mucocutaneous candidiasis.

Chronic mucocutaneous candidiasis (CMC) is characterized by recurrent or persistent infections of the skin, nail, oral, and genital mucosae with Candida species, mainly C. albicans. Autosomal-recessive (AR) IL-17RA and ACT1 deficiencies and autosomal-dominant IL-17F deficiency, each reported in a single kindred, underlie CMC in otherwise healthy patients. We report three patients from unrelated kindreds, aged 8, 12, and 37 yr with isolated CMC, who display AR IL-17RC deficiency. The patients are homozygous for different nonsense alleles that prevent the expression of IL-17RC on the cell surface. The defect is complete, abolishing cellular responses to IL-17A and IL-17F homo- and heterodimers. However, in contrast to what is observed for the IL-17RA– and ACT1-deficient patients tested, the response to IL-17E (IL-25) is maintained in these IL-17RC–deficient patients. These experiments of nature indicate that human IL-17RC is essential for mucocutaneous immunity to C. albicans but is otherwise largely redundant.

CX3CR1 is dispensable for control of mucosal Candida albicans infections in mice and humans

Candida albicans is part of the normal commensal microbiota of mucosal surfaces in a large percentage of the human population. However, perturbations of the host's immune response or bacterial microbiota have been shown to predispose individuals to the development of opportunistic Candida infections. It was recently discovered that a defect in the chemokine receptor CX3CR1 increases susceptibility of mice and humans to systemic candidiasis. However, whether CX3CR1 confers protection against mucosal C. albicans infection has not been investigated. Using two different mouse models, we found that Cx3cr1 is dispensable for the induction of interleukin 17A (IL-17A), IL-22, and IL-23 in the tongue after infection, as well as for the clearance of mucosal candidiasis from the tongue or lower gastrointestinal (GI) tract colonization. Furthermore, the dysfunctional human CX3CR1 allele CX3CR1-M280 was not associated with development of recurrent vulvovaginal candidiasis (RVVC) in women. Taken together, these data indicate that CX3CR1 is not essential for protection of the host against mucosal candidiasis, underscoring the dependence on different mammalian immune factors for control of mucosal versus systemic Candida infections.

Mononuclear phagocyte-mediated antifungal immunity: the role of chemotactic receptors and ligands

Over the past two decades, fungal infections have emerged as significant causes of morbidity and mortality in patients with hematological malignancies, hematopoietic stem cell or solid organ transplantation and acquired immunodeficiency syndrome. Besides neutrophils and CD4(+) T lymphocytes, which have long been known to play an indispensable role in promoting protective antifungal immunity, mononuclear phagocytes are now being increasingly recognized as critical mediators of host defense against fungi. Thus, a recent surge of research studies has focused on understanding the mechanisms by which resident and recruited monocytes, macrophages and dendritic cells accumulate and become activated at the sites of fungal infection. Herein, we critically review how a variety of G-protein coupled chemoattractant receptors and their ligands mediate mononuclear phagocyte recruitment and effector function during infection by the most common human fungal pathogens.

Host genetics and opportunistic fungal infections

Current knowledge on the human pathophysiology of fungal infections highlights the crucial role of genetic pitfalls in specific immunity pathways that determine, together with other risk factors, the predisposition to and clinical outcome of fungal disease. In several studies, associations between gene polymorphisms and genetic errors have been implicated in an immunodeficiency phenotype and an increased incidence of opportunistic fungal diseases. The major challenge is to fully understand the complex interactions between genetic variations and multiple factors, and their relative contributions to the final clinical fungal disease phenotype. The aim of this review is to present updated knowledge on immunity genetics and susceptibility to medically relevant fungal diseases, such as those caused by Candida, Aspergillus, and certain other more rare fungi.

New insights into innate immune control of systemic candidiasis

Systemic infection caused by Candida species is the fourth leading cause of nosocomial bloodstream infection in modern hospitals and carries high morbidity and mortality despite antifungal therapy. A recent surge of immunological studies in the mouse models of systemic candidiasis and the parallel discovery and phenotypic characterization of inherited genetic disorders in antifungal immune factors that are associated with enhanced susceptibility or resistance to the infection have provided new insights into the cellular and molecular basis of protective innate immune responses against Candida. In this review, the new developments in our understanding of how the mammalian immune system responds to systemic Candida challenge are synthesized and important future research directions are highlighted.

Antifungal drug development: challenges, unmet clinical needs, and new approaches

Invasive, life-threatening fungal infections are an important cause of morbidity and mortality, particularly for patients with compromised immune function. The number of therapeutic options for the treatment of invasive fungal infections is quite limited when compared with those available to treat bacterial infections. Indeed, only three classes of molecules are currently used in clinical practice and only one new class of antifungal drugs has been developed in the last 30 years. Here we summarize the unmet clinical needs of current antifungal therapy, discuss challenges inherent to antifungal drug discovery and development, and review recent developments aimed at addressing some of these challenges.

Primary immunodeficiencies underlying fungal infections

PURPOSE OF REVIEW

We review the primary immunodeficiencies (PIDs) underlying an increasing variety of superficial and invasive fungal infections. We also stress that the occurrence of such fungal infections should lead physicians to search for the corresponding single-gene inborn errors of immunity. Finally, we suggest that other fungal infections may also result from hitherto unknown inborn errors of immunity, at least in some patients with no known risk factors.

RECENT FINDINGS

An increasing number of PIDs are being shown to underlie fungal infectious diseases in children and young adults. Inborn errors of the phagocyte NADPH oxidase complex (chronic granulomatous disease), severe congenital neutropenia (SCN) and leukocyte adhesion deficiency type I confer a predisposition to invasive aspergillosis and candidiasis. More rarely, inborn errors of interferon-γ immunity underlie endemic mycoses. Inborn errors of interleukin-17 immunity have recently been shown to underlie chronic mucocutaneous candidiasis (CMC), while inborn errors of caspase recruitment domain-containing protein 9 (CARD9) immunity underlie deep dermatophytosis and invasive candidiasis.

SUMMARY

CMC, invasive candidiasis, invasive aspergillosis, deep dermatophytosis, pneumocystosis, and endemic mycoses can all be caused by PIDs. Each type of infection is highly suggestive of a specific type of PID. In the absence of overt risk factors, single-gene inborn errors of immunity should be sought in children and young adults with these and other fungal diseases.

CX3CR1-dependent renal macrophage survival promotes Candida control and host survival

Systemic Candida albicans infection causes high morbidity and mortality and is associated with neutropenia; however, the roles of other innate immune cells in pathogenesis are poorly defined. Here, using a mouse model of systemic candidiasis, we found that resident macrophages accumulated in the kidney, the main target organ of infection, and formed direct contacts with the fungus in vivo mainly within the first few hours after infection. Macrophage accumulation and contact with Candida were both markedly reduced in mice lacking chemokine receptor CX3CR1, which was found almost exclusively on resident macrophages in uninfected kidneys. Infected Cx3cr1-/- mice uniformly succumbed to Candida-induced renal failure, but exhibited clearance of the fungus in all other organs tested. Renal macrophage deficiency in infected Cx3cr1-/- mice was due to reduced macrophage survival, not impaired proliferation, trafficking, or differentiation. In humans, the dysfunctional CX3CR1 allele CX3CR1-M280 was associated with increased risk of systemic candidiasis. Together, these data indicate that CX3CR1-mediated renal resident macrophage survival is a critical innate mechanism of early fungal control that influences host survival in systemic candidiasis.