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

Abstract

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.

Introduction

Invasive fungal infections remain a substantial cause of morbidity and mortality in immunocompromised populations [1, 2]. A significant proportion of these patients will develop an invasive fungal infection despite controlling for classic risk factors. It has been suggested that underlying host genetics may explain some of this discrepancy [3–5]. Recent studies have demonstrated the role of genetic variant single nucleotide polymorphisms (SNPs) to be important in defining a host’s susceptibility to IFIs, especially during profound immunosuppression such as that induced by chemotherapy or following organ transplantation [6]. Further identification and evaluation of these genetic determinants will likely facilitate early appropriate prophylaxis and treatment of these high risk patients. In this review, we will highlight the prototypical but rare monogenic diseases that form much of the basis of our understanding of genetic susceptibility to specific fungal pathogens. Then we will review innovative approaches that have advanced our knowledge of the relationship between novel genetic variants of immunity and pathogenesis of selected pathogens within the clinical setting. A review of findings from in vitro systems or murine models is beyond the focus of this review and can be found elsewhere [2, 7]. However, data from murine systems of candidiasis can be effectively linked up to human genotypes to identify specific genetic susceptibility genes and SNPs. For example, the dysfunctional CXCR1-M280 and CXCR1-T276 alleles were found to be associated with an increased risk for systemic candidiasis and neutrophil killing of yeasts, respectively, in humans after being identified as important in murine candidiasis [8•, 9•].

Monogenic diseases

Much of our understanding of the molecular mechanisms underlying anti-fungal immunity was discovered by first investigating primary immunodeficiencies [10, 11]. This unique set of genetic deficiencies confers a predisposition to a narrow range of fungal pathogens, and has often been the presenting clinical manifestation of the immunodeficiency itself.

Candida

Candida species are commensal organisms on the skin and the mucous membranes of healthy individuals, but can become an opportunistic pathogen in the setting of a compromised immune system [12]. Candida species are now considered a common pathogen in bloodstream infections, and fungemias are associated with a mortality rate of approximately 40% [10]. Traditional risk factors for invasive infection include recent treatment with broad-spectrum antibiotics, administration of parenteral nutrition, presence of intravascular catheters, prolonged intensive care unit stay and neutropenia. Despite the aforementioned risk factors, only a minority of the population will develop an IFI in these circumstances. The causative etiology for developing an IFI is likely multifactorial; however, several monogenic diseases have been previously described to be associated with an increased susceptibility to infections with Candida species.

A notable example of a monogenic disorder, as described by Glocker, et al., involved the identification of a homozygous point mutation in CARD9 in a consanguineous family with known recurrent IFIs [13]. This mutated gene resulted in a premature stop codon (Q295X) and loss of function mutation, resulting in the lack of expression of the CARD9 protein in peripheral mononuclear cells. The mutated sequence was associated with a mean proportional reduction in Th17 cells as well as severe defects in Dectin-1 triggered TNF-α signaling. The activation of this conserved pathway highlights its importance in fungal recognition, stimulation of pro-inflammatory responses, and in Th17-cell differentiation.

Chronic mucocutanous candidiasis (CMC) represents another classic immunodeficiency associated with recurrent fungal infections related to a defect in IL-17 and IL-22 immunity that is required for mucocutaneous anti-fungal host defense [11]. Considered a heterogeneous group of disorders, CMC can be acquired primarily or secondarily; both autosomal dominant (AD) or recessive inheritance patterns can involve the skin, nails, and mucous membranes of affected individuals. Autosomal recessive autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) is a rare syndrome caused by a mutation in a thymic transcriptional regulator, AIRE, and manifests clinically in the form of impaired T-cell tolerance, self-reactive autoantibodies and CMC [14]. APECED patients form neutralizing antibodies against important anti-fungal cytokines including IL-17E, IL-17F, and IL-22. Furthermore, Liu et al. investigated the molecular mechanisms underlying STAT1 gain of function mutations known to characterize the autosomal dominant form of CMC [15]. These mutations increase STAT1 dependent responses through its impaired nuclear dephosphorylation, affecting cellular response to multiple cytokines and culminating in the curbed differentiation of IL-17 producing T-cells. In a follow-up study, Zheng and coworkers also reported that the negative effects of gain of function STAT1 mutations on STAT3 function were secondary to decreased histone acetylation, which affected STAT3 promoter binding and gene expression [16]. These investigators reported that the reduction in STAT3 gene expression explains the observed low Th17 responses that are the hallmark of the clinical syndrome of CMC. They noted a reversal in the cellular transcriptional profiles by inhibiting STAT1 activation or by enhancing histone, suggesting that targeting epigenetic modifiers may offer a unique treatment strategy for AD CMC. More extensive review of the role of genetic variants in the pathogenesis of CMC can be found elsewhere [12, 17–19].

Hyper-IgE syndrome (HIES) is the result of a heterogeneous group of missense mutations or in-frame deletions most commonly in the STAT3 transcription factor that clinically manifests as a distinct syndrome characterized by extreme elevations in IgE levels, eczema, recurrent pulmonary infections, invasive aspergillosis and cold staphylococcal skin abscesses [14]. Mutations in DOCK8 and Tyk2 have also been associated with HIES [20, 21]. Given the diverse roles of STAT3 in multiple biological processes, patients with HIES also have a constellation of non-immunologic features such as particular facial features, joint hyperextensibility, pathological fractures, delayed primary tooth exfoliation, pneumatoceles, coronary artery aneurysms, and Chiari’s malformation [22]. In 2007, two separate studies determined that mutations in STAT3 underlie the pathogenesis of HIES. These mutations may be hereditary or spontaneous (de novo), and result in increased levels of pro-inflammatory mediators and defective IL-6 signaling [22, 23]. Approximately 80% of patients with AD-HEIS develop CMC, due to the inability to induce Th17 CD4 differentiation that is crucial to downstream activation of cutaneous defense mechanisms. Patients with AD-HIES have been shown to have a reduced number of circulating Th17 cells and they generate a decreased amount of IL-17 and IL-22 following Candida stimulation. A reduction in these cytokines results in impaired priming of epithelial cells to produce host anti-microbial molecules including β-defensins, histatins, and neutrophil-recruiting chemoattractants important for anti-fungal defense [24, 25]. In addition to CMC, these patients are known to develop IFIs with filamentous molds including Aspergillus and Scedosporium. Pneumatoceles, parenchymal lung damage, and bronchiectasis resulting from a history of recurrent bacterial infections serve as a medium for these opportunistic mold infections [5, 26].

Aspergillus

Aspergillus spp are a group of filamentous environmental molds commonly found in soil and decaying vegetation. The spore form of these molds often enters the human host through inhalation and deposits into the bronchioles or alveolar spaces [14]. Without a robust host immune response, these spores can germinate and the hyphae invade local tissues, leading to established infection [6, 27]. Aspergillus fumigatus remains the most common and lethal species in most areas of the world and is thus responsible for approximately 90% of reported cases of invasive aspergillosis (IA) with a one-year survival rate as low as 20% in certain patient populations [28, 29]. Among the known primary immunodeficiencies, chronic granulomatous disease (CGD) has been associated with infections due to Aspergillus spp.; these mold infections can be lethal and are particularly persistent despite appropriate anti-fungal therapy [11, 30, 31]. In many cases, this inability to completely eliminate the fungus requires indefinite anti-fungal secondary suppression. These patients may also experience other fungal infections such as candidiasis as well as infection with the rare mold Rasamsonia (Geosmithia) argillacea species [32, 33]. CGD is a rare disorder characterized by specific defects in the NADPH subunits in phagocytes, resulting in defective superoxide production and compromised oxygen-dependent microbicidal activity [34]. It was initially thought that CGD had an X-linked (XL) pattern of inheritance, but later autosomal recessive (AR) forms of the disease were also recognized. Both forms involve primarily defects in the NADPH oxidase in either its membrane-associated (XL form) or cytosolic (AR form) components [35]. The sex-linked gene encodes for the gp 91 protein. The autosomal/recessive forms are transmitted through Neutrophil Cytosol Factor 1 (NCF1) encoding the p47 protein, and Cytochrome B-245 light chain (CYBA) encoding for the p22 protein. The types of mutations in these genes are varied and may include deletions, frameshifts, and nonsense and missense mutations. The inability of recruited macrophages and neutrophils to deploy oxidative killing mechanisms in response to inhaled conidia predisposes these patients to recurrent and/or persistent invasive fungal infections. While A. fumigatus has been the most commonly reported fungal pathogen in CGD, other Aspergillus spp., including A. nidulans may also cause infections in afflicted patients [11, 36]. The introduction of new triazole agents and the use of prophylactic anti-fungal regimens have substantially reduced the incidence and associated mortality of fungal infections in this population [37].

IFIs have also been reported among patients with a syndrome of monocytopenia, B-cell and NK-cell lymphopenias (known as MonoMAC) and further susceptibility to mycobacteria, papillomaviruses and myelodysplasia. The incidence of IFIs and specifically IA in this population has been estimated at 18–43% and 17%, respectively [5]. Cryptococcal and Histoplasma spp. infections have also been reported [38]. MonoMAC results from specific mutations in the transcription factor, GATA2, which affects a number of genes important in the proliferation and growth factor responsiveness of early hematopoietic cells [39]. Although these patients have a complex hematopoietic clinical syndrome including lymphocytopenia, they still have evidence of immune cells in tissue compartments at sites of inflammation and normal immunoglobulin levels. This histopathology could represent local persistence of previously produced cells or possibly local proliferation from tissue resident macrophages. Functionally, affected neutrophils exhibit a spectrum of defects including uncharacteristic surface antigen expression, abnormal granule contents and even frank dysplasia; monocytopenia and monocyte dysfunction appear to be linked through an undefined mechanism to this susceptibility profile.

Polygenic diseases

Over the past decade, there has been substantial progress in understanding the role of the numerous genes that mediate host-pathogen interactions in fungal infections. We have begun to appreciate the sophisticated interplay between innate and adaptive immune cells along with numerous other key effector cells that constitute the host immune response to fungal invasion, and how certain genetic defects in these pathways can lead to increased susceptibility to infection [19]. In vitro and murine models have been paramount in generating the framework of this exchange; however, we are only recently able to begin to explore the application of these genetic findings in humans (Table 1). The ability to identify additional genetic risk factors would likely prove to be very beneficial during times of profound immunosuppression in that such information might personalize prophylactic anti-fungal regimens or augment therapeutic management plans in high risk patients.

Table 1.

SNPs associated with Candida, Aspergillus, and Cryptococcus infections

Pathogen/Disease	Gene	Molecular Phenotype	Refs
Candida
Candidemia, invasive candidiasis	CCL8	Defective type I IFN pathway	[49]
	CXCR1	Impaired neutrophil effector function	[9]
	IL-10	↑ Candida-induced IL-10 production	[45]
	IL-12B	↓ Candida-induced IFN-γ production	[45]
	PSMB8	Defective type I IFN pathway	[49]
	SP110	Defective type I IFN pathway	[49]
	STAT1	Defective type I IFN pathway	[49]
	TLR1	↓ IL-1β, IL-6, IL-8 after stimulation	[42]
	TLR2	↓ IFN-γ and IL-8	[44]
	TLR4	↑ IL-10 production	[40]
Candida colonization, Candida carriage	Dectin-1	↓ IL-1β and TH17 responses	[85, 86]
	DEFB1	Unknown	[87, 88]
CDC	IL-4	Unknown	[47]
CMC	PTPN22	Unknown	[89]
	TLR3	↓ IFN-γ levels	[90, 91]
IAC	MBL	↓ MBL levels	[92]
RVVC	IL-4	↑ vaginal IL-4, ↓ NO and MBL levels	[93]
	MBL	↓ vaginal MBL levels	[94, 95]
	NLPR3	Impaired IL-1β production	[96]
Aspergillus
IA	AGER	Enhanced expression of RAGE	[97]
	CLEC7A	Defective cytokine production Defective expression of dectin-1	[1, 66, 67]
	CXCL-10	Impaired expression of CXCL-10	[60]
	Dectin-1	Unknown	[1, 66]
	IL-10	Unknown	[63, 64]
	MBL	Variable MBL levels	[98, 99]
	PLG	Unknown	[70]
	PTX3	↓ PTX3 levels	[65]
	S100B	Enhanced secretion of S100B	[97]
	TLR1	Unknown	[61]
	TLR3	Defective antigen presentation and activation of CD8 T-cell responses	[100]
	TLR4	Unknown	[51, 56]
	TLR6	Unknown	[61]
	TNFR1	Impaired expression of TNFR1 mRNA	[101]
	TNFR2	Unknown	[102]
Cryptococcus
Cryptococcosis	FCGR(2A/3A)	Unknown	[79, 80]
	IL12RB1	Defective IL-12 signaling ↓ IL-12Rβ1 expression on cell surface
	MBL	↓ MBL levels	[81]

CDC – chronic disseminated candidiasis; CMC – chronic mucocutaneous candidiasis; CXCL – chemokine (C-X-C motif) ligand; DC – disseminated candidiasis; IA – invasive aspergillosis; IAC – intra-abdominal candidiasis – IAC; IFN – interferon; IL – interleukin; MBL – mannose-binding lectin; PLG – plasminogen; RAGE – receptor for advanced glycation end products; RVVC – recurrent vulvovaginal candidiasis; SNP – single nucleotide polymorphism; TLR – Toll-like receptor; TNFR – tumor necrosis factor receptor.

Candida

Toll-like receptors (TLRs) are known to be key players in fungal immune recognition and have been well studied in models of Candida infection [2]. An increased susceptibility to Candida bloodstream infections was found to be associated with TLR4 D299G and T399I polymorphisms in non-neutropenic patients, but this finding was not supported in a study involving a larger patient cohort [40, 41]. Instead, Plantinga and coworkers found three SNPs in TLR1 were significantly associated with an increased susceptibility to candidemia in a population of hospitalized Caucasian subjects, while no appreciated associations were noted in other TLRs including 2, 4, 6, 9 and the adaptor molecules MyD88 and TIRAP [42]. Moreover, there was no association between any of the SNPs analyzed and outcomes of infection including dissemination, persistence of infection, or mortality, implying that TLR1 may be more important in early host immune response rather than determining the outcome of an established infection. However, this finding was in contrast to a prior report, which demonstrated that TLR1 knock-out (TLR1 −/−) mice did not show increased susceptibility to infection in a model of disseminated candidiasis [43]. Explanations that may account for the anti-fungal role of TLR1 in humans have been related to possible alterations in the signaling profiles of other TLRs such as TLR2 or TLR6 or through downstream effector molecules such as beta-defensin-3, which activates immune cells through TLR1/TLR2 [17]. Lastly, patients with polymorphisms in TLR2 were shown to have decreased IFN-γ and IL-8 plasma levels during Candida sepsis [44].

Johnson et al. examined the role of cytokine gene polymorphisms in a cohort of candidemia patients, and discovered that none of the cytokine SNPs examined (IFNG, IL10, IL12B, IL18, IL1B, IL8) was associated with an increased susceptibility to candidemia [45]. Of the cytokines tested, the persistence of candidemia was found to be associated with SNPs in cytokine genes, IL-12B and IL-10, but no association was appreciated between polymorphisms in cytokine genes and disseminated disease or 30-day mortality. Follow-up in vitro studies using peripheral blood mononuclear cells (PBMCs) demonstrated lower cytokine transcription compared to healthy individuals in response to Candida stimulation. Sun and coworkers found a similarly increased risk for invasive Candida infections with a polymorphism in IL-10, and demonstrated that increased IL-10 production may be a predisposing factor for prolonged candidemia [46].

Using a candidate gene approach, Choi and coworkers investigated genetic variations that contribute to the risk of developing chronic disseminated candidiasis (CDC) in patients with acute leukemia [47]. A common haplotype of IL4 (-1089T/-589C/-33C) was found to be associated with a greater risk of developing CDC, whereas no association with the other investigated variant genotypes of IL10, IL12A, TGFB1, and TNF was appreciated. Another interesting finding of the study was that a different haplotype (-1089T/-589T/-33T), which corresponds to individuals that are heterozygous for the IL4-589 polymorphism, was protective against CDC.

Polymorphisms in TNF-α and β-defensin 1 were shown to be associated with an increased susceptibility to intra-abdominal candidiasis (IAC) among a cohort of high-risk surgical intensive care unit (ICU) patients [48•]. Prevalence of “heavy” Candida colonization was associated with the TLR4 D229G SNP, but no such correlation was found for this SNP with IAC. Interestingly, the SNPs found to be linked with fungal colonization varied from those associated with IAC in this population. This finding suggests that although colonization is considered a predisposing risk factor to invasive infection, there are likely distinct mechanisms that contribute to this pathogenic transition [48•].

Recent studies have utilized functional genomics coupled to case-control association studies and genome-wide associated studies (GWAS) as a comprehensive method for the identification of the genetic variants affiliated with increased susceptibility to invasive Candida infections exclusively in human studies. Smeekens and colleagues were able to compare the transcriptional profile produced after stimulating primary leukocytes with Candida albicans as compared to other unrelated inflammatory stimuli thereby identifying an expression signature of 101 transcripts (95 genes) that described a distinct molecular response to C. albicans [49•]. Analysis revealed an overrepresentation of the IFN signaling pathway. Using genotyping results from a cohort of patients with candidemia, several genes were identified from the transcripts analyzed and were found to have a significant association with susceptibility to systemic candidiasis in four genes (CCL8, STAT1, PSMB8, and SP110). To further validate the role of IFN-related genes in the anti-Candida immune response, correlation between cytokine production with SNP variants showed a significant association at both IRF1 and STAT1 regions. Immunological studies showed that type I IFNs, induced in the setting of Candida-induced inflammation, modulate the cytokine profile from a Th17 to a Th1 response, underscoring the role of this pathway in anti-Candida host response in humans.

The first GWAS evaluating genetic susceptibility to candidiasis was performed using the largest patient cohort to date [50]. This analysis revealed significant association between novel SNPs in the CD58, LCE4A-C1orf68, and TAGAP loci. This risk of candidemia was increased 19.4-fold in those individuals carrying two or more risk alleles. Mechanistically, CD58 was found to inhibit fungal germination at the level of the phagosome as well as modulate Candida-specific cytokine production, findings that highlight a new role for this molecule in mediating host responses. Investigations into the role of TAGAP in fungal infection demonstrated the inability of TAGAP (−/−) mice to eradicate the fungus from the liver and kidney at later stages of infection, underscoring its role in anti-fungal host defense. An interesting conclusion of this study is the observation that the genes identified as novel risk factors for candidemia are also noted to be associated with autoimmune diseases such as rheumatoid arthritis, psoriasis, and Crohn’s disease, suggesting that some consideration be given to the notion that a predisposition to such conditions may have been evolutionarily driven by exposure to pathogens [50].

Aspergillus

The estimated incidence of IA in the hematopoietic stem cell transplant (HSCT) population is approximately 10–12%, and despite significant advances in anti-fungal therapy, the 1-year mortality related to these infections remains high at 50–80%. IA is the leading cause of infection-related deaths among patients with allogeneic hematopoietic-stem cell transplants [51]. Several studies have demonstrated that specific SNPs associated with genes governing the immune system have affected the course and outcome of patients with Aspergillus infections [52].

Most notable of these was the study published by Bochud and coworkers, suggesting an association between allelic variations in TLR4 haplotype S4 among donors and the risk of IA among recipients of HSCT [51]. The TLR4 S4 haplotype (6% haplotype frequency) was discovered to be present in carriers of two SNPs (1063 A/G and 1363 C/T) in strong linkage disequilibrium, and was subsequently confirmed by the investigators in a companion validation study. Additional subgroup analysis also revealed that the association between IA and the S4 haplotype was most significant in unrelated donors as compared to related donors, for which the underlying explanation for this difference is unknown. Moreover, the cumulative incidence of IA as well as the incidence of death not related to relapse were increased in patients with seropositivity for CMV, donor positivity for the S4 haplotype, or both, as compared to patients who were negative for CMV or the S4 haplotype. The authors postulated that the observed SNPs affect TLR4 function thereby explaining the observed clinical phenotype, as studies have demonstrated TLR4 involvement in fungal ligand recognition, detection of lipopolysaccharide, and innate immune activation [53–55]. The genetic susceptibility to IA found to be associated with polymorphisms at 1063A/G and 1363C/T was further confirmed by Koldehoff and coworkers, who demonstrated that either probable or proven IA in transplant recipients occurred if either recipients or donors harbored one of the aforementioned TLR4 variants [56•]. However, investigations into the functional consequences of the Asp299Gly (1063 A/G) polymorphism have yet to find a consistent signaling response following TLR4 stimulation [57–59]. Increased risk for IA after allogeneic HSCT has been shown to be associated with polymorphic variants in CXCL10, TLR1, and TLR6 [52, 60, 61].

Genetic variation in inflammatory cytokines has also been shown to be associated with susceptibility to IA. Sainz and coworkers examined the influence of IL-1 gene cluster polymorphisms on the susceptibility profile of hematological patients, and found that certain haplotypes were associated with an increased risk of developing IA (VNTR2/-889C/-511T) while other haplotypes were associated with resistance to infection (VNTR2/-889C/-511C) [62]. Genetic variation in the promoter region of IL-10 have also be shown to be associated with differential susceptibilities to IA in hematological patients; more specifically, the IL-10 -1082A allele was associated with increased susceptibility to IA while the -1082(AA) genotype was associated with resistance to develop IA [63]. Seo and coworkers investigated the association between three SNPs in the IL-10 promoter and the risk for IA following allogeneic HSCT. The risk for IA was variable among the different haplotypes, with the most striking finding that the ACC haplotype reduced the risk of IA approximately 9-fold [64].

A recent study published by Cunha et al. revealed HSCT recipients who received a donor with a homozygous haplotype (h2/h2) in PTX3 were found to have an associated increased risk of infection with Aspergillus as well as defective expression of PTX3, a soluble pattern recognition receptor shown to have a non-redundant role in modulating host response to fungal infection [65•]. The genetic deficiency of PTX3 was suggested to affect the anti-fungal capacity of neutrophils, presumably due to messenger RNA instability and altered regulation of PTX3 expression, ultimately resulting in impaired phagocytosis and clearance of the fungus. Exogenous addition of PTX3 to isolated PTX3-deficient neutrophils reversed the observed functional deficit, illustrating that the innate anti-fungal mechanisms of these cells are compromised by a lack of sufficient PTX3. This association was found regardless of HLA status of the door, T cell manipulation, acute GVHD and prophylaxis. Moreover, the h2/h2 haplotype was consistently associated with a defect in PTX3 expression in BAL fluid, lung biopsy specimens, and innate immune cells. These findings could not be replicated, however, in a small group of patients from a case-control study that previously confirmed the role of TLR4 mutations [3].

Other studies have investigated the association of a premature stop codon polymorphism in Dectin-1, a C-type lectin receptor present on human immune cells that recognizes the β-1,3 glucan motif on fungal species including Candida and Aspergillus, and the risk and clinical course of IA in both non-HSCT and post-HSCT patients [1]. Cunha and et al. found that the Y238X polymorphism in Dectin-1 increased susceptibility to IA among HSCT patients, with the high risk occurring when the polymorphism was present in both donors and recipients [66]. Similarly, in a hematological population, genotypic variation in Dectin-1 and DC-SIGN were associated with a significantly increased risk for IA [67]. Chia and coworkers discovered that the Y238X allele frequency was higher in non-HSCT patients with IA, and that heterozygosity for the polymorphism in HSCT recipients only showed a limited trend toward IA susceptibility but did not influence the clinical course of IA [1]. This was in contrast to the non-HSCT population, in which there was a stronger association towards enhanced susceptibility. Although this study demonstrated that Dectin-1 had limited influence on the susceptibility to IA in HSCT patients, thought to be partially attributable to redundancy in the innate immune system, this polymorphism may prove to be important in non-HSCT patients. Larger studies are needed to further clarify the relevance of this finding. Multiple in vitro studies and murine models have also demonstrated the requisite role of Dectin-1 in the host defense to Aspergillus [68, 69].

A novel method to identify candidate gene polymorphisms associated with an increased risk of IA following HSCT was conducted by Zaas and coworkers [70]. In this approach, the authors developed an exogenously suppressed murine model and investigated host genetic differences following pulmonary aspergillosis infection using computational haplotype genetic analysis. A SNP (Asp472Asn) in the gene encoding plasminogen was shown to be associated with an increased susceptibility to IA in the developed mouse model that was subsequently identified in the human homolog (PLG; Gene ID 5340). An association study of a cohort of allogeneic HSCT patients was then pursued, demonstrating that recipients that were homozygous or heterozygous for the Asn472 (AA or AG genotype) were at a significantly increased risk of developing IA following transplant. Interestingly, they also detected an apparent “gene-dosage effect,” in that heterozygous individuals were at a 3.0-fold increased risk while homozygous individuals were at a 5.6-fold risk of developing IA. Follow-up in vitro experiments showed that human plasminogen is able to directly bind to both swollen conidia and hyphal forms of Aspergillus, implicating the fibrinolytic pathway in the pathogenesis of IA.

Cryptococcus

Crytococcus neoformans is considered an opportunistic fungal pathogen and is the leading cause of fungal meningitis in the world [10, 71]. This infection remains the fourth leading cause of death in HIV-infected patients for which the burden of Cryptococcus infection remains high. An estimated 1 million cases of cryptococcal meningitis occur annually in these patients worldwide [72]. Other risk factors for cryptococcosis include solid organ transplantation, hematological malignancy, and prolonged immunosuppression or chemotherapy. Previous studies have shown that inhalation of basidiospores or desiccated yeast cells can cause an initial pulmonary infection that has the predilection for dissemination to the central nervous system manifesting as meningoencephalitis [73]. Multiple studies have demonstrated an increased propensity for cryptococcal infection in patients with CD4 lymphopenia. Netea and coworkers analyzed ex vivo cytokine production from stimulated whole blood isolated from two patients with idiopathic CD4 lymphopenia and refractory cryptococcal meningitis [74]. They noted defective production of IFN-γ and TNF-α, two cytokines known to be involved in stimulation of anti-cryptoccocal mechanisms in phagocytes, suggesting that the deficiency of these pro-inflammatory mediators plays a crucial role in the development of IFIs in these patients. This concept was further supported when recombinant IFN-γ was administered as an adjunctive therapy to one of the progressively deteriorating patients that led to clinical recovery. Additionally, anti-IFN-γ antibodies have been isolated from a series of patients with crytococcosis and even the presence of auto-antibodies to granulocyte-macrophage colony-stimulating factor (GM-CSF) have been linked to cryptococcal infections [75, 76]. A prospective population study from Australia and New Zealand analyzed the host response and the epidemiological incidence of both C. neoformans and Cryptococcus gattii infection and found an increase in both types of infection in a predominately immunocompetent Aboriginal population in rural and semirural locations [77]. The authors noted that although the disease secondary to C. gattii was disproportionately high in this population suggesting an environmental exposure as the dominant risk factor, the increased incidence of C. neoformans was similarly increased, prompting the need for other risk factors such as genetic susceptibility to be further investigated.

Recent investigations into the genetic susceptibility of cryptococcosis have begun to identify potential human genes associated with cryptococcosis. Interestingly, a common theme has been polymorphisms in the Fc gamma receptors linked to cryptococcal disease [78–80•]. This work has emphasized the importance of B–cell immunology in the control of this encapsulated yeast. There has also been an association with genetic variability in the mannose-binding lectin complex and cryptococcosis, and we are starting to see reports of affected patients with mutations in cytokine genes for adaptive immunity such as IL12 [81, 82]. Thus both by phenotype and genotype, the search is underway for a detailed understanding of the genetic susceptibility to cryptococcosis, especially in the apparently immunocompetent host.

Conclusions

These studies continue to highlight the important role of the host’s underlying genetic profile in determining susceptibility to IFIs. Advances in knowledge about the human genome and vast improvements in genome technologies have facilitated this, and will continue to launch the field forward towards a better understanding of genetic risk to IFIs. However, the integration and subsequently translation of the aforementioned genetic findings are still very much in their infancy, as we have only begun to appreciate the complex and interwoven factors that govern host-pathogen interactions [34]. Studies involving large cohorts of patients are needed and outcomes analyses are mandatory to correlate the relative contribution of genetic findings to specific clinical outcomes. This is particularly important given the multiple challenges of genetic association studies including population heterogeneity and insufficient replication power [83]. The ability to stratify “at-risk” patients with selective genetic immunodeficiencies for susceptibility to specific pathogens has the great potential to dramatically alter clinical care in the future [84]. Dissecting the contribution that host genetic variation has on the susceptibility to fungal infection will be principal for personalizing medicine in infectious diseases. It will also serve as a catalyst for future research into the significance of genetics in other infectious phenotypes and population outcomes amid various management strategies.