Download Candida Infections: An Update on Host Immune Defenses and Anti

Infectious Diseases and Translational Medicine
• Review •
Candida Infections: An Update on Host
Immune Defenses and Anti-Fungal
Drugs
Ning Gao, Changbin Chen
ABSTRACT
From Key Laboratory of Molecular Virology and Immunology, Unit of Pathogenic Fungal Infection and Host Immunity, Institute Pasteur of Shanghai,
Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences,
Shanghai 200031, China.
Correspondence to: Changbin
Chen, Tel: (+86)02154923055, Fax:
(+86)02154923056, Email: cbchen@
ips.ac.cn.
Open access
DOI: 10.11979/idtm.201601005
Citation: Gao N, Chen CB. Candida Infections: An Update on Host Immune
Defenses and Anti-Fungal Drugs. Infect
Dis Transl Med, 2016; 2(1): 30-40.
Copyright © The Author(s) 2016. This
article is distributed under the terms
of the Creative Commons Attribution
4.0 International License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted use, distribution, and reproduction in any
medium, provided you give appropriate credit to the original author(s) and
the source, provide a link to the Creative Commons license, and indicate
if changes were made. The Creative
Commons Public Domain Dedication
waiver (http://creativecommons.org/
publicdomain/zero/1.0/) applies to the
data made available in this article, unless otherwise stated.
Received: Dec 29, 2015
Accepted: Jan 20, 2016
Published: Apr 10, 2016
30
Infections by fungal pathogens such as Candida albicans and non-albicans Candida species are becoming increasing prevalent in the human population. Such pathogens cause lifethreatening diseases with high mortality, particularly in immunocompromised patients. Host
defenses against fungal infections are provided by an exquisite interplay between innate and
adaptive immune responses. However, effective anti-fungal agents for Candida infections are
limited, and fungal drug resistance is a significant treatment challenge. In this review, we summarize the current understanding of host–fungal interactions, discuss the modes action of antifungal drugs, explore host defense mechanisms, and define the new challenges for treating
Candida infections.
Keywords: Candida spp.; Candida albicans; Drug resistance; Host immune defenses
I
nfections by pathogenic fungi have become an increasing problem in human health,
although their effects are not widely recognized and deaths caused by such infections
are often overlooked. Based on reports from the World Health Organization, most
people have experienced superficial fungal infections in their lifetimes[1]. In most cases,
these infections are easy to cure, but millions of individuals worldwide are suffering from
life-threatening invasive infections that are hard to diagnose and treat, a situation possibly
attributable to the immunosuppression caused by HIV-AIDS, cancer, metabolic disorders
like diabetes, and long-term antibiotic use. Invasive fungal infections are coming under the
spotlight because of their unacceptably high mortality rates and because more than 90% of
all the fungal-related deaths reported result from species that belong to one of the following
four genera: Candida, Cryptococcus, Aspergillus, and Pneumocystis (Table 1)[2]. These fungi
are saprophytes in soil and the general environment (e.g., Cryptococcus neoformans, Aspergillus fumigatus) or can be present as commensals in healthy organisms (e.g., Candida
species). Among them, candidemia, which is caused by infection with Candida species, is
one of the leading causes of blood-stream infections and has a mortality rate of more than
30%. There is an increased incidence of infections caused by non-albicans Candida species such as Candida tropicalis, Candida parapsilosis, and Candida glabrata. Although
these fungi are isolated much more frequently as the causative agents of invasive candidiasis worldwide, Candida albicans remains the most common isolate from hospitalized
Table 1. Statistics of the 4 most significant invasive fungal infections.
Disease (Most common species
Location
Estimated life-threatening
infections/year at that
location
Mortality rates
(% in infected
populations)
Candidiasis (Candida albicans)
Worldwide
＞ 400,000
46-75
Worldwide
＞ 200,000
30-95
Worldwide
＞ 1,000,000
20-70
Worldwide
＞ 400,000
20-80
Aspergillosis (Aspergillus
fumigatus)
Cryptococcosis (Cryptococcus
neoformans)
Pneumocystis (Pneumocystis
jirovecii)
Infect Dis Transl Med 2016;2(1):30-40.
Candida Infections: a Review
Figure 1. An overview of fungal pathogenicity mechanisms. (A)
Fungal cells adhere to the surfaces of host cells and invade the
host cell surfaces through damage of cell integrity or secretion of
proteinases. (B) Fungal proteins affecting the adherence to host
cells. Epa, ALS family and Hwp1 proteins can promote the adherence and enhance host-pathogen interactions. (C) Network of
transcriptional factors involved in regulation of fungal biofilm formation. Six key factors, such as Efg1, Bcr1, Tec1, Ndt80, Rob1, and
Brg1, form the regulation network to control C. albicans biofilm
development. In addition, Ume6 and Ywp1 are positive factors
and Nrg1 is a negative factor.
patients. In this review, we will focus on Candida species
as pathogens by dividing our discussion into three parts:
the first part concentrates on fungal infection mechanisms, the second part focusses on host–fungus interactions, and the final part looks at anti-fungal drugs.
1. MECHANISMS OF INFECTION BY
CANDIDA
To infect a diverse range of host niches, Candida requires
a wide range of virulence factors and fitness attributes.
In Fig. 1, we have summarized several major factors and
fitness traits for Candida.
1.1 Adherence. For most fungal infections, the ability
of the host to resist the physical clearing of the infectious
agent is important. During interactions with the host,
Candida spp. rely on their cell wall proteins for tissue
adhesion and invasion, biofilm formation, and evasion of
the host immune system[3, 4]. For successful infection, the
first step Candida has to achieve is adherence to host cell
surfaces[5]. Thus, adhesion is an extremely important step
in the infection process, and the extent of adhesion is dependent on the properties of the microbe, the host, and the
abiotic surface, with two such properties being cell-surface
hydrophobicity and cell wall composition[6, 7]. Dysfunction
in cell wall regulation may cause inappropriate exposure
or aberrant surface localization of adhesins, which can
reduce the adhesion properties of C. albicans[8]. Molecules are present in the most external layers of Candida
cells that are essential for its successful adherence to host
surfaces and these molecules must play a pivotal role in
the pathophysiology of candidiasis[9]. Below, we summarize some key factors involved in Candida adherence.
1.1.1 Lectin-like proteins: C. glabrata, a species of
yeast that is normally a commensal member of the human
microbiome, is localized predominantly in the mucosa of
different organs. However, like other Candida species,
this fungus can cause localized infections in patients
whose immune systems have been compromised[10, 11].
Adherence of C. glabrata to human epithelial cells relies
mainly on Epa (epithelial adhesins) proteins, which are
glycosylphosphatidylinositol-anchored cell proteins with
well-defined modular structures[12]. The C. glabrata cell
wall contains 67 adhesin-like proteins; these are classifiable into seven groups with the largest being the Epa
family, which contains 17–23 members depending on the
strain[13]. Several Epa protein members mediate adhesion
to epithelial and endothelial cells, and contribute to the
establishment of infection. Genome analysis indicates
that the EPA1 cluster harbors three gene loci (EPA1,
EPA2 and EPA3), and the transcriptional profiles of these
genes have revealed that EPA1 is induced in the first cell
division, EPA2 is induced by oxidative stress, and EPA3
is induced by biofilm formation or osmotic/nutritional
stressors[6, 13, 14]. Epa1p is a Ca2+-dependent lectin, and
its deletion significantly reduces adherence in vitro, supporting the importance of this protein in adherence[6, 15].
The structures and functions of Epa proteins have been
thoroughly studied at both the cellular and molecular
levels, and further information is available from recent
reviews[3, 4, 16, 17].
1.1.2 Agglutinin-like sequence (ALS) family: The
C. albicans ALS family includes eight genes (ALS1–7
and ALS9) that encode a family of cell-surface glycoproteins[5]. Functional studies of ALS family proteins have
focused on the notion that Als proteins act as the adhesins involved primarily in host–pathogen interactions in C.
albicans [18]. To date, two different approaches have been
used by researchers to test this hypothesis. One approach
evaluated the effect on adherence of knocking out individual Als proteins, while the other used heterogenous
overexpression of Candida ALS genes in S. cerevisiae.
The latter approach has a disadvantage in that it may
not accurately reflect the protein function in C. albicans
because of differences in codon usage and glycosylation
between Candida spp. and S. cerevisiae.
Although the three-domain structures of the ALS gene
family are similar, differences may exist in terms of their
expression profiles and functions. Some ALS genes appear to be regulated by obvious increases and decreases
at the mRNA level (ALS1, ALS2, and ALS3), whereas
others (ALS6 and ALS7) are consistently transcribed at
lower levels[19-21], suggesting that evaluated transcript levels alone may not be enough to explain the importance of
some ALS proteins in superficial adherence to the host. A
31
Infectious Diseases and Translational Medicine
substantial number of studies have supported the key role
played by Als proteins in host–fungus interactions. Als3
is a major component of the hyphal cell wall, but not for
the yeast phase[22]. Deletion of C. albicans ALS3 results
in striking adhesion and biofilm formation defects. ALS3
is also required for C. albicans cell binding to E-cadherin
on epithelial cells, and for N-cadherin on endothelial cells
and extracellular matrix proteins[23, 24]. ALS3 expression,
which is regulated by a number of transcription factors,
is strongly induced by Efg1, Tec1 and Bcr1 and inhibited
by Nrg1 and Tup1. Additionally, recent studies suggest
that the Cph1 transcription factor plays an essential role
in the ALS3 activation process[25, 26]. Knocking out ALS4
significantly slowed down germ tube formation and decreased fungal adhesion to endothelial cells. Interestingly,
similar to that of Als2p, ALS4 expression has no impact
on epithelial cell binding or biofilm formation in a catheter model[20]. Deletion of ALS5, ALS6 or ALS7, which
normally exhibit lower expression levels than other ALS
family genes, was found to significantly increase adhesion of C. albicans to human vascular endothelial cell
monolayers and buccal epithelial cells, indicating low
transcript levels are sufficient to translate enough protein
for the required function[21]. ALS9 contains two alleles
(ALS9-1 and ALS9-2), and based on sequence variations
within the 5 and 3′ domains of the ALS9 coding region,
this protein exhibits the greatest allelic variability in the
ALS gene family. ALS9-2, rather than ALS9-1, plays a
role in vascular endothelium adhesion, suggesting allelic
functional differences between strains[27]. Allelic diversity in ALS genes is an important area of study because
information about a protein encoded by a single allele
provides a context for studying allelic variation within
the larger population of C. albicans strains. Southern
blot hybridization and genomic DNA amplification using
degenerate PCR primers designed against the C. albicans
ALS sequences indicate that ALS genes probably exist in
C. dubliniensis and C. tropicalis, and potentially in C.
parapsilosis[28]. In addition to the comprehensive studies
that have outlined the structures and functions of the Als
proteins, and the role played by this gene family in C.
albicans biology and pathogenesis, ALS genes and proteins are attracting more attention largely because they
are potential targets for inclusion in an anti-Candida vaccine[29-32].
1.1.3 Hyphal wall protein 1 (HWP1): Hwp1, a
GPI anchor-dependent cell wall protein family member
responsible for adhesion processes and biofilm formation, is involved in the pathogenesis of C. albicans[33-35].
HWP1 expression is strongly induced during germ tube
formation but is absent during yeast growth, indicating
that its expression appears to be coordinately controlled
by the transcriptional activators and repressors that control morphology[36]. The N-terminal domain of Hwp1,
which mimics host cell transglutaminase substrates, can
form tight attachments to host epithelial cells through a
coiled disulfide bond, whereas the C-terminal domain,
which can be modified by glycosylphosphatidylinositol, is required for the N-terminal functioning in host
32
cell wall binding[37, 38]. As a hypha-specific gene, HWP1
expression during hyphae development and biofilm
formation is specifically regulated by a number of key
developmental regulators such as EFG1, TUP1, and
RBF1, but is not affected by CPH1. HWP1 expression
is dependent upon Efg1 and is repressed by Tup1, while
Rbf1 appears to act as an inducer of expression[39]. In addition, the Hwp1 adhesin protein may play a pivotal role
in C. albicans virulence and drug resistance, as shown
by the observation that its expression is regulated by the
Kex2 endoproteinase, which also regulates the activity
of C. albicans proteinases[40, 41]. Interestingly, Hwp1 and
Als3 (along with the closely related Als1) seem to have
distinct and complementary roles in cell-cell adhesion
and biofilm formation, because the Hwp1 function in
promoting cell adherence was attenuated in an als1−/−
als3−/− double mutant strain[42].
1.2 Biofilm formation. Biofilms display an organized
three-dimensional structure comprising a dense network
of yeast and filamentous cells embedded in an exopolymeric matrix of carbohydrates, proteins and nucleic
acids[43]. Biofilm formation is a step-wise process in
the host–fungal interaction, and occurs in surfaceassociated microbial communities that are surrounded
by an extracellular matrix, such as teeth and the foreign
surfaces of implanted medical devices as well as mucosal surfaces[7, 9]. Biofilm formation can be partitioned
into four temporal stages: adherence, initiation, maturation and dispersal[44, 45]. At the beginning, in the adherence step, yeast-form cells adhere to the substrate and
then propagate to form a basal layer and germ tubes form
to yield hyphae (the initiation stage). Next, the biofilm
forms the dense bilayers of yeast, hyphae and pseudohyphae (the maturation stage). Finally, the yeast-form cells
are released from the biofilm to attach and colonize
new sites in the surrounding environment (the dispersal
stage)[46]. Although these steps might occur concurrently
rather than sequentially during natural biofilm development in vivo, they provide a useful framework for analyzing the mechanisms involved in biofilm development.
A comprehensive study, based on high-throughput deepsequencing technologies, has unraveled a transcriptional
circuit governing biofilm formation[47, 48]. This complex,
but typical network, consists of six master transcriptional regulators (i.e., Efg1, Tec1, Bcr1, Ndt80, Brg1,
and Rob1) that control each other’s expression and bind
directly to the promoters of 44 additional biofilm regulators. The adherence step, for example, requires participation of Bcr1, Efg1 and some of their downstream
targets[34, 49-52]; these targets include the Ywp1 regulator
(which plays an inhibitory role in adherence)[53], the Tec1
and Bcr1 regulators, while Ece1 and Als9 (cell wall proteins) also affect the initiation of biofilm development[54].
Ume6 affects biofilm formation by inhibiting dispersal,
while the Nrg1 positive regulator promotes dispersal development[55-59].
1.3 Yeast and hyphal morphologies. The ability
to switch between yeast and hyphal forms represents
one of the most discussed and extensively investigated
Candida Infections: a Review
virulence attributes of the human pathogenic fungus C.
albicans[16, 60]. Numerous studies have shown that the
yeast-to-hyphal transition is important for virulence and
contributes to various distinct functions during the different stages of disease development, including adhesion to
epithelial and endothelial cells, invasion through induced
endocytosis and active penetration, iron acquisition and
utilization, escape from phagocytes and immune evasion, and mucosal immune activation and triggering of
specific sepsis-like immune responses during systemic
infection[17, 61, 62]. Therefore, fungal dimorphism is vital
for virulence at both superficial and systemic levels.
The ability to express hypha-specific genes accounts
for the pathogenic potential of hyphal formation and a
number of environmental signals trigger its development.
These signals include the presence of serum, elevated
temperature, neutral pH, the presence of certain nutrients, starvation signals, matrix embedding, CO2 and O2
levels, cell density, and contact with physical surfaces[16].
Interestingly, these signals resemble unfavorable growth
conditions or indicate the possibility of a hostile environment. Such environmental signals stimulate activation of
a range of signaling pathways that include the following:
inhibition of heat shock protein 90 (Hsp90) by elevated
temperatures and subsequent activation of Ras1[63], the
cAMP/PKA-signaling pathway via direct or indirect
activation of the Cyr1 adenylyl cyclase [64], mitogen
activated protein kinase (MAPK) signaling via Ras1/
Hst7 and Cek1[65], activation of the Rim101 pathway
by neutral to alkaline pH[66], Czf1 activation under nonembedded conditions via Rac1[67], hypoxia-induced Efg1/
Efh1 activation[68], and reactive oxygen species (ROS)
signaling induced by genotoxic stress[69]. Consequently,
activation of these signaling pathways triggers activation
or inhibition of the key transcriptional regulators (e.g.,
Efg1, Czf1, Cph1, Tec1, Flo8, and Nrg1) that control expression of the genes necessary for hyphae formation and
hypha-associated genes[70, 71].
The immune system is able to discriminate between
yeast and hyphal growth. Morphological recognition of
C. albicans, together with detection of fungal burden
and the damage caused by invading hyphae, may hold
the key to host discrimination between colonization and
infection, and may also explain the presence of different lifestyles of C. albicans, either as a commensal or a
pathogen.
1.4 pH adaptation. The pH of the human host differs
dramatically according to the anatomical site. Normally,
the pH in the blood and tissues is somewhat neutral
(pH 7.4 ±e0.1), whereas the human vaginal cavity is
acidic (pH ~4) and the pH along the digestive tract differs significantly, ranging from pH 2 to 8[72]. C. albicans
is able to survive in both the acidic and basic pH of the
host environment, and this ability enables this fungus to
colonize different host niches, such as the acidic vagina
or the neutral oropharyngeal tract. Accumulating evidence has demonstrated that tolerance of different pH
levels is important for C. albicans pathogenicity. PHR1
and PHR2 are two pH-regulated C. albicans genes that
are differentially expressed according to the pH conditions in the host. Transcriptional profiling has shown that
PHR2 is expressed under acidic pH, while PHR1 expression is enriched at neutral and basic pH[73, 74]. Deletion of
PHR2 leads to growth inhibition at acidic pH conditions,
whereas deletion of PHR2 results in an inability to grow
at neutral and basic pH conditions. Importantly, the contribution of each gene to virulence depends on the host
niche, as noted by the observation that a PHR2 deletion
mutant was avirulent in the vagina, whereas the PHR1
deletion mutant showed reduced virulence in a systemic
infection model. Furthermore, the RIM101 gene, which
encodes a pH-regulated transcription factor and is well
characterized in other fungi such as Cryptococcus neoformans, when deleted also reduced fungal virulence
in a systemic mouse model and in an endothelial-celldamage-model[66]. More recently, researchers found that
C. albicans is not only able to sense and adapt to a wide
range of pH conditions, but can also actively modulate
environmental pH. A good example of such modulation
is the research that showed that C. albicans is able to utilize amino acids to promote neutralization of phagosomal
pH and that increasing pH levels resulted in hyphae development and escape from macrophages[75].
2. HOST DEFENSE MECHANISMS
Of the approximately 200 Candida species that have
been described to date, only five species (C. albicans, C.
glabrata, C. parapsilosis, C. tropicalis, and C. krusei)
are attributed to the vast majority of Candida infections,
suggesting that these species in particular are well adjusted for survival within the human host. For example,
C. glabrata and C. tropicalis are more commonly seen
in patients with hematological or solid organ malignancies and neutropenia, while C. krusei infections occur
predominately in patients with hematopoietic stem cell
transplantations[76]. C. parapsilosis, which causes infections mainly in neonates rather than in adults, is a common pathogen of catheter-related infections[76]. To fight
fungal infections, the human host has evolved several
different effective strategies, including mechanical (e.g.,
epithelial) barriers, as well as biochemical, chemical and
physical antagonists (e.g., bile, mucus, pH, and antimicrobial peptides), microbial competition (normal human
microbiota), and the innate and adaptive immune systems. The increased prevalence of fungal infections has
prompted the development of one branch of fungi-related
human disease: research that involves investigations into
the two major host anti-fungal protective mechanisms.
One of these mechanisms was active early in the evolution of multicellular organisms (innate immunity), while
the other involves the sophisticated adaptive mechanisms
that are induced specifically during infection and disease
(adaptive immunity). Fig.2 shows most of the major
events involved in host immune recognition upon fungal
infection. Defense against fungal infection is supported
by an exquisite interplay between the innate and adaptive
arms of the host immune system, which act together to
33
Infectious Diseases and Translational Medicine
Figure 2. Innate and adaptive immunity involved in host antifungal defense activities. (A) Innate immunity. Recognition of Candida species is mediated by TLRs, CLRs and NOD receptors. Both
TLR4/TLR2 can induce production of proinflammatory signals
and cytokines through the MyD88-dependent NF-κB pathway.
However, TLR3-mediated recognition is through activation of the
IRF3 transcription factor. The CLR receptors, such as Dectin-1,
Dectin-2, Dectin-3 and Mincle, stimulate release of inflammatory
cytokines by activating T cell lineage-specific tyrosine kinase (Syk)
and downstream complex (CARD9-Bcl10-MALT1) to initiate the
canonical and non-canonical NF-κB pathways. Dectin-1 promotes
maturation of inflammasome to induce production of cytokines.
DC-specific intracellular adhesion molecule-grabbing non-integrin
(DC-SIGN) receptor can induce the TH cell responses through Raf1
activation. In addition, NLRP3 stimulates ROS production through
an Erk dependent pathway, and NOD2 receptor activates inflammatsome by affecting maturation of the pro-IL-1β via caspase-1.
(B) Adaptive immunity. T cells form an integral part of adaptive
immunity, which classified into two distinct lineages (CD4+ and
CD8+ cells). Based on the effector functions, CD4+ cells are further
subdivided into Th1, Th2, Th17, and Treg cells.
eliminate pathogens from the human body.
2.1 Innate immunity. Traditionally recognized as the
first line of defense against pathogens, innate immunity
utilizes the body surfaces and the mucosal epithelial surfaces of the respiratory, gastrointestinal and genito-urinary tracts as barriers against the continuous interactions
between fungi and host[77, 78]. The cell wall of Candida
spp. has two layers: the inner layer, which contains β-1,
3-glucan and β-1, 6-glucan, and the outer layer, which
is composed mainly of O- and N-linked glycoproteins[3].
The polysaccharide structures in the C. albicans cell wall
are recognized by two classes of membrane-bound pat34
tern-recognition receptors (PRRs): the Toll-like receptors
(TLRs) and the C-type lectin receptors (CLRs) [79]. The
several TLRs that have been extensively studied for their
roles in fungal recognition include TLR1, TLR2, TLR4,
TLR6, TLR7 and TLR9[80]. Remarkably, different TLRs
can recognize several structurally divergent pathogenassociated molecular pattern (PAMP) ligands in fungi.
For example, TLR4 can recognize mannans and mannoproteins from C. albicans [81-83], while TLR2 responds to
phospholipomannan from C. albicans[84]. However, other
TLRs, such as TLR1 and TLR6, may not be important for
host innate recognition and they do not seem to be essential for antifungal defense in candidiasis[85].
CLRs are another type of PRR family that can recognize both endogenous and exogenous ligands via the
characteristics of the conserved domain from CRD,
which allows them to recognize a range of molecules
such as carbohydrates, lipids and proteins. Dectin-1
specifically recognizes β-glucans [86], and the macrophage mannose receptor recognizes N-linked mannan
and mediates recognition and phagocytosis of different
yeast species by macrophages[82, 87], while α-mannan was
reported to be recognized by both yeast and hyphae[88-90].
DC-SIGN is another important dendritic cell (DC)specific receptor that is able to internalize C. albicans by
recognizing the mannan structure exposed on Candida
cells[91]. Also, galectin-3, which was found to be essential
for recognizing the β-1,2 mannosides of C. albicans [92],
associates with TLR-2 when macrophages interact with
yeasts [93]. A soluble CLR MBL (mannose-binding lectin)
was found to facilitate opsonophagocytosis by binding to
Candida mannan and to the surface of the C1q receptor
on the phagocyte[94]. Although the nature of the macrophage-inducible C-type lectin (Mincle) ligand has not
yet been characterized, this C-lectin receptor also binds
to yeast cell wall components and participates in the cell
response with TLR-2[95].
In addition to the membrane-bound receptors described above, several PRRs are able to recognize Candida intracellularly. TLR9 has been shown to recognize
C. albicans DNA and induce cytokine production in dendritic cells [96]. The nucleotide-binding domain, leucinerich, repeat-containing receptors (NOD-like receptors,
NLRs) are PRRs that recognize intracellular PAMPs, and
one of their main functions is to activate the inflammasome via caspase 1, thereby leading to processing and
activation of interleukin 1 (IL-1) family cytokines[97].
These different PRRs have been implicated in the
activation of highly characterized innate host responses
against fungal pathogens. Most TLRs (with the exception of TLR3) are able to signal through adaptor protein
MyD88, which is essential for responses against a broad
range of microbial components and crucial for the clearance of infections [79]. In contrast, TLR3 mediates the
activation of interferon (IFN)-regulatory factor 3 and this
activation is operated in a MyD88-independent manner.
Spleen tyrosine kinase (Syk), a primary signal transduction factor in the immune regulation of CLRs, operates to
either activate the classic CARD9-Bcl10-MALT1 com-
Candida Infections: a Review
plex to induce the canonical NF-κB pathway or activate
NIK to induce non-canonical NF-κB activation [98-100].
Although most CLRs like Dectin-1, Dectin-2 and Mincle
signal mainly through Syk activation, their major roles in
fungal infections are quite distinct. Dectin-1 was found
to play a major role in activating the inflammasome via
the induction of phagocytosis and other antimicrobial
mechanisms, such as NLRP3 activation via ERK-induced
ROS production[101]. With human DCs, host recognition
by Dectin-2 activates the NF-κB subunit c-Rel via Malt1
activation, thereby triggering a Th17 response. Th17
cells produce IL-17A/F cytokines that bind to the IL-17
receptor and induce secretion of a number of proinflammatory factors including IL-6, IL-8 and GM-CSF as well
as CXCL1 and CCL20 chemokines, which are involved
in macrophage and neutrophil recruitment[102]. Additionally, interplay between TLRs and CLR-mediated signaling is required for inducing optimal immunity and antifungal responses. Co-stimulation with both Dectin-1 and
TLR2 or TLR4 ligands, for example, leads to an increase
in TNF, IL-10 and IL-23 and a decrease in IL-12 production[103]. NLRs act as scaffold proteins that assist with
assembling the signal platforms required for triggering
NF-κB and MAPK pathways that control the activation
of inflammatory caspases and proinflammatory cytokine
release[104].
2.2 Adaptive immunity. Generally, acquired immune
responses are slower to act than innate immune responses. Acquired immunity is mediated mainly by B cells
that produce specific antibodies, CD8+ T cells that kill
pathogen-infected cells, and CD4+ T cells that produce
effector cytokines, and these cells are able to respond
to a wide range of potential antigens[105, 106]. As specialized antigen presenting cells, DCs play a central role in
immune defense against pathogens. Immature DCs are
recruited to the site of infection in response to chemokines and antimicrobial peptides. It has been reported
that DCs are vital for the initiation of adaptive T-cellmediated immune protection against C. albicans and an
important conclusion is that mixed populations of DCs
have non-redundant functionalities and can drive CD4+
and CD8+ T-cell responses against C. albicans. DCs recognize C. albicans through interactions between PRRs
expressed on the surface of the DCs and PAMPs exposed
on the fungal cell wall[107]. Upon recognition, activated
DCs induce different antigen-specific immune responses.
For example, induction of Th17 responses requires the
presentation of C. albicans antigens by Langerhans cells.
However, this interaction has no impact on the development of CD8+ cytotoxic T-lymphocyte (CTL) responses.
Surprisingly, evidence has shown that specific Langerin+ dermal DCs are able to trigger both Th1 and CTL
responses, and activation of CTL responses suggests
that fungal antigens are cross-presented via the MHC I
pathway, thereby promoting antigen display to CD8+ Tlymphocytes[108].
Several immune and non-immune cells are also
known to contribute to antifungal responses and these
include epithelial cells, neutrophils, natural killer cells,
DCs, monocytes, macrophages and T cells. Epithelial
cells play an important role as barriers against tissue invasion by fungi. After sensing the cell wall component
signals of Candida yeast or hyphae, epithelial cells produce cytokines via MAPK1- and FOS-dependent pathways, leading to the recruitment of phagocytic immune
cells[109]. Neutrophil activation, which is essential for
fungal clearance, occurs through the mechanisms of both
oxidative and non-oxidative effectors that are capable
of killing fungi[110]. Macrophages produce the inflammatory cytokines and chemokines that recruit and activate
other immune cells, such as blood monocytes, at the site
of infection. Additionally, natural killer cells are in most
cases sufficient contributors to the rapid innate immune
response that prevents surface colonization and tissue invasion of the host.
3. ANTI-FUNGAL AGENTS
Fungal infections pose a continuous and serious threat to
human health and longevity. Although C. albicans is recognized as the most common Candida pathogen, other
non-albicans Candida species are being isolated with increasing frequency from patients with non-fungal clinical
diseases. The dramatic increase in fungal infections has
intensified the search for new, safer, and more efficacious
agents to combat serious fungal infections. In general,
the anti-fungal drugs currently available can be classified
into the following five categories: azoles, ployenes, allylamines, flucytosine and echinocandins. Unravelling the
mechanisms of action of the different anti-fungal agents
listed above will significantly improve our understanding
of the mechanisms of fugal resistance to them. Indeed,
determining resistance mechanisms has enabled or enhanced our understanding of the specific mechanisms of
action of many drugs.
3.1 Azoles. Azoles (e.g., fluconazole, voriconazole,
itraconazole and posaconazole) and allylamines (e.g.,
terbinafine) inhibit ergosterol biosynthesis by binding to
the 14-α-lanosterol demethylase (14α-DM) in yeasts or
14-α-sterol demethylase (CYP51p) in molds (cytochrome
P450 enzymes encoded by the EFG11 gene or CYP51A
and CYP51B genes, respectively)[111-113]. Fluconazole, an
inhibitor of cytochrome CYP3A4, is recommended as
the primary treatment for candidemia and remains the
most frequently used anti-fungal agent because it is safe,
cheap and tolerable[114]. Voriconazole, an inhibitor of the
cytochrome enzymes CYP2C19 and CYP2C9, is an azole
with a broader spectrum than fluconazole. Intraconazole
is out-of-date, but a parenteral form has become available recently. Fungal resistance to azoles has been attributed to a number of mechanisms, such as activation of
efflux pumps, genetic alterations and genomic instability,
transient gene overexpression, impaired drug penetration, modification and up-regulation of the target enzyme
concentration, and ergosterol replacement[111, 115, 116].
3.2 Polyenes. Polyenes like amphotericin B bind to
ergosterol, which is the major component of the fungal
plasma membrane, by creating large pores that allow
35
Infectious Diseases and Translational Medicine
leakage of cell contents and disrupt cell function. For
a long time, amphotericin B was considered to be the
‘gold standard’ in the treatment of invasive fungal infections[117]. The molecular structure of amphotericin
B shows it has poor water solubility but excellent lipid
solubility [118]. The two-domain structure of this polyene comprises a polyene hydrocarbon chain and a ployhydroxyl chain, both of which are important for its antifungal effect. These domains in amphotericin B mediate
the interaction between eight amphotericin B and eight
ergosterol molecules by forming a pore, and this results
in the rapid efflux of K+, inhibition of fungal glycolysis and subsequent Mg2+ efflux[117, 119]. Amphotericin B
stimulates immune cells via Toll-like receptor TLR2 and
CD14 protein, the latter of which is a transmembrane
signaling protein on the surface of mononuclear cells. By
triggering the intracellular signaling pathways involving
the adapter protein MyD88 and NF-κB, amphotericin B
induces the expression and release of inflammatory cytokines, such as pro-inflammatory cytokines (e.g., interleukin (IL)-1β, TNF-α, IL-6 and IL-1Rα) and chemokines
(e.g., IL-8, MCP-1 and MIP-1β)[120].
3.3 Allylamines. The allylamines are a class of synthetic anti-fungal agent used in clinical applications.
Allylamines, which are represented mainly by naftifine
and terbinafine, possess activity against a wide range of
filamentous, dimorphic and yeast-like fungi. The primary
mode of action of these anti-fungals is the inhibition of
fungal ergosterol biosynthesis at the point of squalene
epoxidation. Squalene epoxidase is another enzyme required for ergosterol synthesis[121].
3.4 Flucytosine. Flucytosine (e.g., 5-fluorocytosine)
is a base pyrimidine analogue that functions to inhibit
cellular pyrimidine metabolism, transcription, DNA
replication and protein synthesis[122]. This agent is active mainly against Candida spp., C. neoformans and
some molds. Normally, 5-flucytosine use is combined
with other antifungal agents such as amphotericin B or
fluconazole to attenuate secondary resistance to it[123, 124].
Two kinds of resistance have been found to be mediated
by 5-flucytosine in clinically relevant Candida species:
primary resistance is related to decreased drug uptake by
cytosine permease, which is encoded by the FCY2 gene,
while secondary resistance is limited by alterations in the
enzyme activities of cytosine deaminase or uracil phosphoribosyltransferase, which are encoded by FCY1 and
FUR1 genes, respectively[125]. In a study that sought to
decipher the resistance mechanism of flucytosine, a chemogenomics analysis of C. albicans identified 183 genes
that may contribute to flucytosine resistance[126]. Among
these genes, 5 are involved in DNA repair, 8 are involved
in chromatin remodeling, and 17 appear to participate in
RNA metabolism. Moreover, another study revealed that
a family of two genes, CgFPS1 and CgFPS2, which both
encode aquaglyceroporin-regulated glycerol transport,
were found to mediate 5-flucytosine resistance by decreasing the accumulation of 5-flucytosine in C. glabrata
cells[127].
3.5 Echinocandins. Echinocandins (i.e., caspofungin,
36
anidulafungin and micafungin), a group of unique antifungal agents, target the fungal cell wall through noncompetitive inhibition of the biosynthesis of β-1, 3-Dglucan, a major structural component of the fungal cell
wall and a fungal-specific target [128, 129]. Caspofungin
received FDA approval in 2002, micafungin in 2005,
and anidulafungin in 2006. These three agents are approved for treatment of serious fungal infections, such as
esophageal candidiasis, candidemia and other Candida
infections, but not infections by Aspergillus spp. In Candida spp., the β-1, 3-D-glucan synthase is a multi-subunit
enzyme complex and the UDP-glycosyltransferases are
major ones. The β-glucan synthase complex has a minimum of two subunits: Rho (regulatory subunit, which is
a GTP-binding protein in the Rho/Rac subfamily of Raslike GTPases) and Fksp (catalytic subunit), and these
two subunits are the major targets of echinocandins[128].
In C. albicans, echinocandin resistance was found to correlate with point mutations in FKS1[128]. However, this
type of resistance in C. glabrata is often caused by mutations in both FKS1 and FKS2 genes[130]. Interestingly,
studies have shown that two ‘hot spot’ regions in Fksp
can explain the mechanism of drug resistance against
echinocandin in C. albicans clinical isolates. These ‘hot
spots’ span amino acids Phe641 to Pro649 and Asp1357
to Leu1364, suggesting that FKS gene mutations in hot
spots account for most of the universally observed resistance to echinocandin [131].
Resistance to anti-fungal agents is the logical and
inevitable consequence of using these agents to treat
human infections. The availability of molecular genetic
tools will undoubtedly greatly improve our understanding of the mechanisms by which anti-fungal drug resistance emerges and spreads and promises to greatly
inform efforts to develop novel and effective compounds
for future use. In addition, the introduction of new antifungal agents (e.g., echinocandins, second-generation
triazoles) in the past decade has greatly increased the
number of treatments for invasive fungal infections, and
drug toxicity is no longer the major limiting factor in
treatment regimes. Although many of these newer antifungal agents have limitations in their activities, pharmacokinetics, unique predispositions for pharmacokinetic
drug-drug interactions and unusual toxicities associated
with their long-term use, we still believe that their benefits, in terms of delivering safer and more effective antifungal therapies, vastly outweigh these drawbacks.
4. CONCLUDING REMARKS
The rapidly increasing numbers of immune-compromised patients and the increase in fungal resistance to
fungicides mean that invasive fungal infections, which
have become an important problem in both the hospital
and the community and are costly to treat, are an everincreasing cause of morbidity and mortality. In the battle
against fungal infections, a concerted effort to raise
public awareness of fungal pathogens by clinicians, researchers, the pharmaceutical industry and public health
Candida Infections: a Review
officials is required.
As this review has sought to show, gaining better
knowledge of fungal pathogen biology and host immunity against these pathogens seems an equally important
goal, and the complexity of the interactions between
them is a fascinating area of research. Work on one gene
or one property, no matter how detailed and careful, will
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
WHO Model Prescribing Infor mation:
Drugs Used in Skin Diseases. 1997.
Brown GD, Denning DW, Gow NA, Levitz
SM, Netea MG, White TC: Hidden killers:
human fungal infections. Sci Transl Med 2012,
4(165):165rv113.
Chaffin WL: Candida albicans cell wall proteins. Microbiol Mol Biol Rev 2008, 72(3):495544.
Sundstrom P: Adhesins in Candida albicans.
Curr Opin Microbiol 1999, 2(4):353-357.
Kraneveld EA, de Soet JJ, Deng DM,
Dekker HL, de Koster CG, Klis FM, Crielaard W, de Groot PW: Identification and
differential gene expression of adhesin-like
wall proteins in Candida glabrata biofilms.
Mycopathologia 2011, 172(6):415-427.
Cormack BP, Ghori N, Falkow S: An adhesin of the yeast pathogen Candida glabrata
mediating adherence to human epithelial
cells. Science 1999, 285(5427):578-582.
Douglas LJ: Candida biofilms and their role
in infection. Trends Microbiol 2003, 11(1):3036.
Gow NA, Hube B: Importance of the Candida albicans cell wall during commensalism and infection. Curr Opin Microbiol 2012,
15(4):406-412.
Kojic EM, Darouiche RO: Candida infections of medical devices. Clin Microbiol Rev
2004, 17(2):255-267.
Turner SA, Butler G: The Candida pathogenic species complex. Cold Spring Harb Perspect Med 2014, 4(9):a019778.
Brunke S, Hube B: Two unlike cousins: Candida albicans and C. glabrata infection strategies. Cell Microbiol 2013, 15(5):701-708.
Frieman MB, McCaffery JM, Cormack BP:
Modular domain structure in the Candida
glabrata adhesin Epa1p, a beta1,6 glucancross-linked cell wall protein. Mol Microbiol
2002, 46(2):479-492.
de Groot PW, Kraneveld EA, Yin QY,
Dekker HL, Gross U, Crielaard W, de Koster
CG, Bader O, Klis FM, Weig M: The cell
wall of the human pathogen Candida glabrata: differential incorporation of novel
adhesin-like wall proteins. Eukaryot Cell 2008,
7(11):1951-1964.
Juarez-Cepeda J, Orta-Zavalza E, CanasVillamar I, Arreola-Gomez J, Perez-Cornejo
GP, Hernandez-Carballo CY, GutierrezEscobedo G, Castano I, De Las Penas A:
The EPA2 adhesin encoding gene is responsive to oxidative stress in the opportunistic
fungal pathogen Candida glabrata. Curr Genet
2015, 61(4):529-544.
Li F, Svarovsky MJ, Karlsson AJ, Wagner JP,
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
not be sufficient to obtain a comprehensive picture of
host–pathogen interactions. It will be exciting to witness the scientific advances in our understanding and
treatment of fungi and fungal diseases from use of highthroughput technologies such as genomics when combined with the use of more traditional research methods.
Marchillo K, Oshel P, Andes D, Palecek SP:
Eap1p, an adhesin that mediates Candida
albicans biofilm formation in vitro and in
vivo. Eukaryot Cell 2007, 6(6):931-939.
Mayer FL, Wilson D, Hube B: Candida albicans pathogenicity mechanisms. Virulence
2013, 4(2):119-128.
Polke M, Hube B, Jacobsen ID: Candida
survival strategies. Adv Appl Microbiol 2015,
91:139-235.
Hoyer LL: The ALS gene family of Candida
albicans. Trends Microbiol 2001, 9(4):176-180.
Zhao X, Oh SH, Cheng G, Green CB, Nuessen JA, Yeater K, Leng RP, Brown AJ, Hoyer
LL: ALS3 and ALS8 represent a single locus
that encodes a Candida albicans adhesin;
functional comparisons between Als3p and
Als1p. Microbiology 2004, 150(Pt 7):24152428.
Zhao X, Oh SH, Yeater KM, Hoyer LL:
Analysis of the Candida albicans Als2p and
Als4p adhesins suggests the potential for
compensatory function within the Als family. Microbiology 2005, 151(Pt 5):1619-1630.
Zhao X, Oh SH, Hoyer LL: Deletion of
ALS5, ALS6 or ALS7 increases adhesion of
Candida albicans to human vascular endothelial and buccal epithelial cells. Med Mycol
2007, 45(5):429-434.
Almeida RS, Brunke S, Albrecht A, Thewes S,
Laue M, Edwards JE, Filler SG, Hube B: the
hyphal-associated adhesin and invasin Als3
of Candida albicans mediates iron acquisition from host ferritin. PLoS Pathog 2008,
4(11):e1000217.
Phan QT, Myers CL, Fu Y, Sheppard DC,
Yeaman MR, Welch WH, Ibrahim AS, Edwards JE, Jr., Filler SG: Als3 is a Candida
albicans invasin that binds to cadherins and
induces endocytosis by host cells. PLoS Biol
2007, 5(3):e64.
Liu Y, Filler SG: Candida albicans Als3, a
multifunctional adhesin and invasin. Eukaryot Cell 2011, 10(2):168-173.
Cleary IA, Reinhard SM, Miller CL, Murdoch
C, Thornhill MH, Lazzell AL, Monteagudo
C, Thomas DP, Saville SP: Candida albicans
adhesin Als3p is dispensable for virulence in
the mouse model of disseminated candidiasis. Microbiology 2011, 157(Pt 6):1806-1815.
Nobile CJ, Mitchell AP: Regulation of cellsurface genes and biofilm formation by the C.
albicans transcription factor Bcr1p. Curr Biol
2005, 15(12):1150-1155.
Zhao X, Oh SH, Hoyer LL: Unequal contribution of ALS9 alleles to adhesion between
Candida albicans and human vascular endothelial cells. Microbiology 2007, 153(Pt 7):23422350.
Hoyer LL, Fundyga R, Hecht JE, Kapteyn
29.
30.
31.
32.
33.
34.
35.
36.
37.
JC, Klis FM, Arnold J: Characterization of
agglutinin-like sequence genes from non-albicans Candida and phylogenetic analysis of
the ALS family. Genetics 2001, 157(4):15551567.
Spellberg BJ, Ibrahim AS, Avenissian V,
Filler SG, Myers CL, Fu Y, Edwards JE, Jr.:
The anti-Candida albicans vaccine composed of the recombinant N terminus of
Als1p reduces fungal burden and improves
survival in both immunocompetent and immunocompromised mice. Infect Immun 2005,
73(9):6191-6193.
Ibrahim AS, Spellberg BJ, Avanesian V, Fu
Y, Edwards JE, Jr.: The anti-Candida vaccine based on the recombinant N-terminal
domain of Als1p is broadly active against
disseminated candidiasis. Infect Immun 2006,
74(5):3039-3041.
Ibrahim AS, Spellberg BJ, Avenissian V, Fu Y,
Filler SG, Edwards JE, Jr.: Vaccination with
recombinant N-terminal domain of Als1p
improves survival during murine disseminated candidiasis by enhancing cell-mediated,
not humoral, immunity. Infect Immun 2005,
73(2):999-1005.
Spellberg BJ, Ibrahim AS, Avanesian V, Fu Y,
Myers C, Phan QT, Filler SG, Yeaman MR,
Edwards JE, Jr.: Efficacy of the anti-Candida rAls3p-N or rAls1p-N vaccines against
disseminated and mucosal candidiasis. J Infect
Dis 2006, 194(2):256-260.
Nobile CJ, Nett JE, Andes DR, Mitchell AP:
Function of Candida albicans adhesin Hwp1
in biofilm formation. Eukaryot Cell 2006,
5(10):1604-1610.
Nobile CJ, Andes DR, Nett JE, Smith FJ,
Yue F, Phan QT, Edwards JE, Filler SG,
Mitchell AP: Critical role of Bcr1-dependent
adhesins in C. albicans biofilm formation in
vitro and in vivo. PLoS Pathog 2006, 2(7):e63.
Daniels KJ, Lockhart SR, Staab JF, Sundstrom P, Soll DR: The adhesin Hwp1 and
the first daughter cell localize to the a/a
portion of the conjugation bridge during
Candida albicans mating. Mol Biol Cell 2003,
14(12):4920-4930.
Nantel A, Dignard D, Bachewich C, Harcus
D, Marcil A, Bouin AP, Sensen CW, Hogues
H, van het Hoog M, Gordon P, Rigby T,
Benoit F, Tessier DC, Thomas DY, Whiteway M: Transcription profiling of Candida
albicans cells undergoing the yeast-to-hyphal
transition. Mol Biol Cell 2002, 13(10):34523465.
Staab JF, Bahn YS, Tai CH, Cook PF, Sundstrom P: Expression of transglutaminase
substrate activity on Candida albicans germ
tubes through a coiled, disulfide-bonded
N-terminal domain of Hwp1 requires C-
37
Infectious Diseases and Translational Medicine
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
38
terminal glycosylphosphatidylinositol modification. J Biol Chem 2004, 279(39):4073740747.
Staab JF, Bradway SD, Fidel PL, Sundstrom
P: Adhesive and mammalian transglutaminase substrate properties of Candida albicans
Hwp1. Science 1999, 283(5407):1535-1538.
Sharkey LL, McNemar MD, SaporitoIrwin SM, Sypherd PS, Fonzi WA: HWP1
functions in the morphological development of Candida albicans downstream of
EFG1, TUP1, and RBF1. J Bacteriol 1999,
181(17):5273-5279.
Bader O, Schaller M, Klein S, Kukula J,
Haack K, Muhlschlegel F, Korting HC, Schafer W, Hube B: The KEX2 gene of Candida
glabrata is required for cell surface integrity.
Mol Microbiol 2001, 41(6):1431-1444.
Naglik J, Albrecht A, Bader O, Hube B: Candida albicans proteinases and host/pathogen
interactions. Cell Microbiol 2004, 6(10):915926.
Nobile CJ, Schneider HA, Nett JE, Sheppard DC, Filler SG, Andes DR, Mitchell
AP: Complementary adhesin function in C.
albicans biofilm formation. Curr Biol 2008,
18(14):1017-1024.
Ganguly S, Mitchell AP: Mucosal biofilms of
Candida albicans. Curr Opin Microbiol 2011,
14(4):380-385.
Hawser SP, Douglas LJ: Biofilm formation by Candida species on the surface of
catheter materials in vitro. Infect Immun 1994,
62(3):915-921.
Baillie GS, Douglas LJ: Role of dimorphism
in the development of Candida albicans biofilms. J Med Microbiol 1999, 48(7):671-679.
Uppuluri P, Chaturvedi AK, Srinivasan A,
Banerjee M, Ramasubramaniam AK, Kohler
JR, Kadosh D, Lopez-Ribot JL: Dispersion
as an important step in the Candida albicans
biofilm developmental cycle. PLoS Pathog
2010, 6(3):e1000828.
Nobile CJ, Fox EP, Nett JE, Sorrells TR,
Mitrovich QM, Hernday AD, Tuch BB,
Andes DR, Johnson AD: A recently evolved
transcriptional network controls biofilm
development in Candida albicans. Cell 2012,
148(1-2):126-138.
Finkel JS, Mitchell AP: Genetic control of
Candida albicans biofilm development. Nat
Rev Microbiol 2011, 9(2):109-118.
Fanning S, Xu W, Solis N, Woolford CA,
Filler SG, Mitchell AP: Divergent targets
of Candida albicans biofilm regulator Bcr1
in vitro and in vivo. Eukaryot Cell 2012,
11(7):896-904.
Fox EP, Nobile CJ: A sticky situation: untangling the transcriptional network controlling
biofilm development in Candida albicans.
Transcription 2012, 3(6):315-322.
Stichternoth C, Ernst JF: Hypoxic adaptation by Efg1 regulates biofilm formation
by Candida albicans. Appl Environ Microbiol
2009, 75(11):3663-3672.
Connolly LA, Riccombeni A, Grozer Z,
Holland LM, Lynch DB, Andes DR, Gacser
A, Butler G: The APSES transcription factor Efg1 is a global regulator that controls
morphogenesis and biofilm formation in
Candida parapsilosis. Mol Microbiol 2013,
90(1):36-53.
53. Ramon AM, Valentin E, Maicas S, Sentandreu R: Expression of YWP1, a gene that
encodes a specific Yarrowia lipolytica mycelial cell wall protein, in Saccharomyces cerevisiae. Fungal Genet Biol 1997, 22(2):77-83.
54. Dwivedi P, Thompson A, Xie Z, Kashleva H,
Ganguly S, Mitchell AP, Dongari-Bagtzoglou
A: Role of Bcr1-activated genes Hwp1
and Hyr1 in Candida albicans oral mucosal
biofilms and neutrophil evasion. PLoS One
2011, 6(1):e16218.
55. Banerjee M, Thompson DS, Lazzell A, Carlisle PL, Pierce C, Monteagudo C, Lopez-Ribot JL, Kadosh D: UME6, a novel filamentspecific regulator of Candida albicans hyphal
extension and virulence. Mol Biol Cell 2008,
19(4):1354-1365.
56. Zeidler U, Lettner T, Lassnig C, Muller M,
Lajko R, Hintner H, Breitenbach M, Bito
A: UME6 is a crucial downstream target
of other transcriptional regulators of true
hyphal development in Candida albicans.
FEMS Yeast Res 2009, 9(1):126-142.
57. Carlisle PL, Kadosh D: Candida albicans
Ume6, a filament-specific transcriptional
regulator, directs hyphal growth via a pathway involving Hgc1 cyclin-related protein.
Eukaryot Cell 2010, 9(9):1320-1328.
58. Cleary IA, Lazzell AL, Monteagudo C,
Thomas DP, Saville SP: BRG1 and NRG1
form a novel feedback circuit regulating
Candida albicans hypha formation and virulence. Mol Microbiol 2012, 85(3):557-573.
59. Uppuluri P, Pierce CG, Thomas DP, Bubeck
SS, Saville SP, Lopez-Ribot JL: The transcriptional regulator Nrg1p controls Candida
albicans biofilm formation and dispersion.
Eukaryot Cell 2010, 9(10):1531-1537.
60. Sudbery PE: Growth of Candida albicans
hyphae. Nat Rev Microbiol 2011, 9(10):737748.
61. Duhring S, Germerodt S, Skerka C, Zipfel
PF, Dandekar T, Schuster S: Host-pathogen
interactions between the human innate immune system and Candida albicans-understanding and modeling defense and evasion
strategies. Front Microbiol 2015, 6:625.
62. Conti HR, Gaffen SL: IL-17-Mediated
Immunity to the Opportunistic Fungal
Pathogen Candida albicans. J Immunol 2015,
195(3):780-788.
63. Robbins N, Uppuluri P, Nett J, Rajendran
R, Ramage G, Lopez-Ribot JL, Andes D,
Cowen LE: Hsp90 governs dispersion and
drug resistance of fungal biofilms. PLoS Pathog 2011, 7(9):e1002257.
64. Hogan DA, Muhlschlegel FA: Candida albicans developmental regulation: adenylyl
cyclase as a coincidence detector of parallel
signals. Curr Opin Microbiol 2011, 14(6):682686.
65. Monge RA, Roman E, Nombela C, Pla J:
The MAP kinase signal transduction network in Candida albicans. Microbiology 2006,
152(Pt 4):905-912.
66. Ost KS, O’Meara TR, Huda N, Esher SK,
Alspaugh JA: The Cryptococcus neoformans
alkaline response pathway: identification of
a novel rim pathway activator. PLoS Genet
2015, 11(4):e1005159.
67. Hope H, Bogliolo S, Arkowitz RA, Bassilana
M: Activation of Rac1 by the guanine nucleotide exchange factor Dck1 is required for
invasive filamentous growth in the pathogen Candida albicans. Mol Biol Cell 2008,
19(9):3638-3651.
68. White SJ, Rosenbach A, Lephart P, Nguyen
D, Benjamin A, Tzipori S, Whiteway M,
Mecsas J, Kumamoto CA: Self-regulation of
Candida albicans population size during GI
colonization. PLoS Pathog 2007, 3(12):e184.
69. Schroter C, Hipler UC, Wilmer A, Kunkel
W, Wollina U: Generation of reactive oxygen species by Candida albicans in relation
to morphogenesis. Arch Dermatol Res 2000,
292(5):260-264.
70. Whiteway M, Bachewich C: Morphogenesis
in Candida albicans. Annu Rev Microbiol 2007,
61:529-553.
71. Liu H: Transcriptional control of dimorphism in Candida albicans. Curr Opin Microbiol 2001, 4(6):728-735.
72. Davis DA: How human pathogenic fungi
sense and adapt to pH: the link to virulence.
Curr Opin Microbiol 2009, 12(4):365-370.
73. Fonzi WA: PHR1 and PHR2 of Candida albicans encode putative glycosidases required
for proper cross-linking of beta-1,3- and beta-1,6-glucans. J Bacteriol 1999, 181(22):70707079.
74. Muhlschlegel FA, Fonzi WA: PHR2 of Candida albicans encodes a functional homolog
of the pH-regulated gene PHR1 with an inverted pattern of pH-dependent expression.
Mol Cell Biol 1997, 17(10):5960-5967.
75. Vylkova S, Lorenz MC: Modulation of
phagosomal pH by Candida albicans promotes hyphal morphogenesis and requires
Stp2p, a regulator of amino acid transport.
PLoS Pathog 2014, 10(3):e1003995.
76. Yapar N: Epidemiology and risk factors for
invasive candidiasis. Ther Clin Risk Manag
2014, 10:95-105.
77. Dr ummond RA, Gaffen SL, Hise AG,
Brown GD: Innate Defense against Fungal
Pathogens. Cold Spring Harb Perspect Med
2015, 5(6).
78. Netea MG, Brown GD, Kullberg BJ, Gow
NA: An integrated model of the recognition
of Candida albicans by the innate immune
system. Nat Rev Microbiol 2008, 6(1):67-78.
79. Plato A, Hardison SE, Brown GD: Pattern
recognition receptors in antifungal immunity.
Semin Immunopathol 2015, 37(2):97-106.
80. Netea MG, Ferwerda G, van der Graaf CA,
Van der Meer JW, Kullberg BJ: Recognition
of fungal pathogens by toll-like receptors.
Curr Pharm Des 2006, 12(32):4195-4201.
81. Netea MG, van der Meer JW, Kullberg BJ:
Both TLR2 and TLR4 are involved in the
recognition of Candida albicans. Reply to
“TLR2, but not TLR4, triggers cytokine
production by murine cells in response to
Candida albicans yeasts and hyphae” by
Gil and Gozalbo, Microbes and Infection 8
(2006) 2823-2824. Microbes Infect 2006, 8(1213):2821-2822; author reply 2823-2824.
82. Netea MG, Gow NA, Munro CA, Bates S,
Collins C, Ferwerda G, Hobson RP, Bertram
G, Hughes HB, Jansen T, Jacobs L, Buurman ET, Gijzen K, Williams DL, Torensma
Candida Infections: a Review
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
R, McKinnon A, MacCallum DM, Odds
FC, Van der Meer JW, Brown AJ, Kullberg
BJ: Immune sensing of Candida albicans
requires cooperative recognition of mannans
and glucans by lectin and Toll-like receptors.
J Clin Invest 2006, 116(6):1642-1650.
Tada H, Nemoto E, Shimauchi H, Watanabe T, Mikami T, Matsumoto T, Ohno N,
Tamura H, Shibata K, Akashi S, Miyake K,
Sugawara S, Takada H: Saccharomyces cerevisiae- and Candida albicans-derived mannan
induced production of tumor necrosis factor alpha by human monocytes in a CD14and Toll-like receptor 4-dependent manner.
Microbiol Immunol 2002, 46(7):503-512.
Jouault T, Ibata-Ombetta S, Takeuchi O,
Trinel PA, Sacchetti P, Lefebvre P, Akira S,
Poulain D: Candida albicans phospholipomannan is sensed through toll-like receptors.
J Infect Dis 2003, 188(1):165-172.
Netea MG, van de Veerdonk F, Verschueren
I, van der Meer JW, Kullberg BJ: Role of
TLR1 and TLR6 in the host defense against
disseminated candidiasis. FEMS Immunol Med
Microbiol 2008, 52(1):118-123.
Brown GD, Taylor PR, Reid DM, Willment JA, Williams DL, Martinez-Pomares
L, Wong SY, Gordon S: Dectin-1 is a major
beta-glucan receptor on macrophages. J Exp
Med 2002, 196(3):407-412.
Stahl PD, Ezekowitz RA: The mannose
receptor is a pattern recognition receptor
involved in host defense. Curr Opin Immunol
1998, 10(1):50-55.
McGreal EP, Rosas M, Brown GD, Zamze
S, Wong SY, Gordon S, Martinez-Pomares
L, Taylor PR: The carbohydrate-recognition
domain of Dectin-2 is a C-type lectin with
specificity for high mannose. Glycobiology
2006, 16(5):422-430.
Saijo S, Ikeda S, Yamabe K, Kakuta S, Ishigame H, Akitsu A, Fujikado N, Kusaka T,
Kubo S, Chung SH, Komatsu R, Miura N,
Adachi Y, Ohno N, Shibuya K, Yamamoto
N, Kawakami K, Yamasaki S, Saito T, Akira
S, Iwakura Y: Dectin-2 recognition of alphamannans and induction of Th17 cell differentiation is essential for host defense against
Candida albicans. Immunity 2010, 32(5):681691.
Sato K, Yang XL, Yudate T, Chung JS, Wu
J, Luby-Phelps K, Kimberly RP, Underhill
D, Cruz PD, Jr., Ariizumi K: Dectin-2 is a
pattern recognition receptor for fungi that
couples with the Fc receptor gamma chain
to induce innate immune responses. J Biol
Chem 2006, 281(50):38854-38866.
Cambi A, Netea MG, Mora-Montes HM,
Gow NA, Hato SV, Lowman DW, Kullberg
BJ, Torensma R, Williams DL, Figdor CG:
Dendritic cell interaction with Candida albicans critically depends on N-linked mannan.
J Biol Chem 2008, 283(29):20590-20599.
Jouault T, El Abed-El Behi M, Martinez-Esparza M, Breuilh L, Trinel PA, Chamaillard
M, Trottein F, Poulain D: Specific recognition of Candida albicans by macrophages
requires galectin-3 to discriminate Saccharomyces cerevisiae and needs association
with TLR2 for signaling. J Immunol 2006,
177(7):4679-4687.
93. Martinez-Esparza M, Sarazin A, Jouy N,
Poulain D, Jouault T: Comparative analysis
of cell wall surface glycan expression in Candida albicans and Saccharomyces cerevisiae
yeasts by flow cytometry. J Immunol Methods
2006, 314(1-2):90-102.
94. Brouwer N, Dolman KM, van Houdt M, Sta
M, Roos D, Kuijpers TW: Mannose-binding
lectin (MBL) facilitates opsonophagocytosis
of yeasts but not of bacteria despite MBL
binding. J Immunol 2008, 180(6):4124-4132.
95. Lee CG, Da Silva CA, Lee JY, Hartl D, Elias
JA: Chitin regulation of immune responses:
an old molecule with new roles. Curr Opin
Immunol 2008, 20(6):684-689.
96. Miyazato A, Nakamura K, Yamamoto
N, Mora-Montes HM, Tanaka M, Abe Y,
Tanno D, Inden K, Gang X, Ishii K, Takeda
K, Akira S, Saijo S, Iwakura Y, Adachi Y,
Ohno N, Mitsutake K, Gow NA, Kaku M,
Kawakami K: Toll-like receptor 9-dependent
activation of myeloid dendritic cells by Deoxynucleic acids from Candida albicans. Infect
Immun 2009, 77(7):3056-3064.
97. Brodsky IE, Monack D: NLR-mediated control of inflammasome assembly in the host
response against bacterial pathogens. Semin
Immunol 2009, 21(4):199-207.
98. Geijtenbeek TB, Gringhuis SI: Signalling
through C-type lectin receptors: shaping
immune responses. Nat Rev Immunol 2009,
9(7):465-479.
99. Gross O, Gewies A, Finger K, Schafer M,
Sparwasser T, Peschel C, Forster I, Ruland J:
Card9 controls a non-TLR signalling pathway for innate anti-fungal immunity. Nature
2006, 442(7103):651-656.
100. LeibundGut-Landmann S, Gross O, Robinson MJ, Osorio F, Slack EC, Tsoni SV,
Schweighoffer E, Tybulewicz V, Brown GD,
Ruland J, Reis e Sousa C: Syk- and CARD9dependent coupling of innate immunity to
the induction of T helper cells that produce
interleukin 17. Nat Immunol 2007, 8(6):630638.
101. Poeck H, Ruland J: SYK kinase signaling and
the NLRP3 inflammasome in antifungal immunity. J Mol Med (Berl) 2010, 88(8):745-752.
102. Tesmer LA, Lundy SK, Sarkar S, Fox DA:
Th17 cells in human disease. Immunol Rev
2008, 223:87-113.
103. Dennehy KM, Willment JA, Williams DL,
Brown GD: Reciprocal regulation of IL23 and IL-12 following co-activation of
Dectin-1 and TLR signaling pathways. Eur J
Immunol 2009, 39(5):1379-1386.
104. Krishnaswamy JK, Chu T, Eisenbarth SC:
Beyond pattern recognition: NOD-like
receptors in dendritic cells. Trends Immunol
2013, 34(5):224-233.
105. Geginat J, Nizzoli G, Paroni M, Maglie S,
Larghi P, Pascolo S, Abrignani S: Immunity
to Pathogens Taught by Specialized Human
Dendritic Cell Subsets. Front Immunol 2015,
6:527.
106. Verma A, Wuthrich M, Deepe G, Klein B:
Adaptive immunity to fungi. Cold Spring Harb
Perspect Med 2015, 5(3):a019612.
107. Netea MG, Marodi L: Innate immune mechanisms for recognition and uptake of Candida species. Trends Immunol 2010, 31(9):346-
353.
108. Igyarto BZ, Haley K, Ortner D, Bobr A,
Gerami-Nejad M, Edelson BT, Zurawski
SM, Malissen B, Zurawski G, Berman J,
Kaplan DH: Skin-resident murine dendritic
cell subsets promote distinct and opposing
antigen-specific T helper cell responses. Immunity 2011, 35(2):260-272.
109. Moyes DL, Runglall M, Murciano C, Shen
C, Nayar D, Thavaraj S, Kohli A, Islam A,
Mora-Montes H, Challacombe SJ, Naglik JR:
A biphasic innate immune MAPK response
discriminates between the yeast and hyphal
forms of Candida albicans in epithelial cells.
Cell Host Microbe 2010, 8(3):225-235.
110. Aratani Y, Kura F, Watanabe H, Akagawa H,
Takano Y, Suzuki K, Dinauer MC, Maeda
N, Koyama H: Critical role of myeloperoxidase and nicotinamide adenine dinucleotide
phosphate-oxidase in high-burden systemic
infection of mice with Candida albicans. J
Infect Dis 2002, 185(12):1833-1837.
111. Peman J, Canton E, Espinel-Ingroff A: Antifungal drug resistance mechanisms. Expert
Rev Anti Infect Ther 2009, 7(4):453-460.
112. Loffler J, Kelly SL, Hebart H, Schumacher U,
Lass-Florl C, Einsele H: Molecular analysis
of cyp51 from fluconazole-resistant Candida
albicans strains. FEMS Microbiol Lett 1997,
151(2):263-268.
113. Nascimento AM, Goldman GH, Park S,
Marras SA, Delmas G, Oza U, Lolans K,
Dudley MN, Mann PA, Perlin DS: Multiple
resistance mechanisms among Aspergillus
fumigatus mutants with high-level resistance
to itraconazole. Antimicrob Agents Chemother
2003, 47(5):1719-1726.
114. Orozco AS, Higginbotham LM, Hitchcock
CA, Parkinson T, Falconer D, Ibrahim AS,
Ghannoum MA, Filler SG: Mechanism of
fluconazole resistance in Candida krusei. Antimicrob Agents Chemother 1998, 42(10):26452649.
115. Diaz-Guerra TM, Mellado E, CuencaEstrella M, Rodriguez-Tudela JL: A point
mutation in the 14alpha-sterol demethylase
gene cyp51A contributes to itraconazole resistance in Aspergillus fumigatus. Antimicrob
Agents Chemother 2003, 47(3):1120-1124.
116. Garcia-Effron G, Mellado E, Gomez-Lopez
A, Alcazar-Fuoli L, Cuenca-Estrella M,
Rodriguez-Tudela JL: Differences in interactions between azole drugs related to modifications in the 14-alpha sterol demethylase
gene (cyp51A) of Aspergillus fumigatus.
Antimicrob Agents Chemother 2005, 49(5):21192121.
117. Hamill RJ: Amphotericin B formulations: a
comparative review of efficacy and toxicity.
Drugs 2013, 73(9):919-934.
118. Kennedy AL, Wasan KM: Preferential distribution of amphotericin B lipid complex into
human HDL3 is a consequence of high density lipoprotein coat lipid content. J Pharm
Sci 1999, 88(11):1149-1155.
119. Moen MD, Lyseng-Williamson KA, Scott LJ:
Liposomal amphotericin B: a review of its
use as empirical therapy in febrile neutropenia and in the treatment of invasive fungal
infections. Drugs 2009, 69(3):361-392.
120. Sau K, Mambula SS, Latz E, Henneke P, Go-
39
Infectious Diseases and Translational Medicine
lenbock DT, Levitz SM: The antifungal drug
amphotericin B promotes inflammatory
cytokine release by a Toll-like receptor- and
CD14-dependent mechanism. J Biol Chem
2003, 278(39):37561-37568.
121. McClellan KJ, Wiseman LR, Markham A:
Terbinafine. An update of its use in superficial mycoses. Drugs 1999, 58(1):179-202.
122. Ghannoum MA, Rice LB: Antifungal agents:
mode of action, mechanisms of resistance,
and correlation of these mechanisms with
bacterial resistance. Clin Microbiol Rev 1999,
12(4):501-517.
123. Hospenthal DR, Bennett JE: Flucytosine
monotherapy for cryptococcosis. Clin Infect
Dis 1998, 27(2):260-264.
124. Espinel-Ingroff A: Mechanisms of resistance to antifungal agents: yeasts and
filamentous fungi. Rev Iberoam Micol 2008,
25(2):101-106.
40
125. Kontoyiannis DP, Lewis RE: Antifungal
drug resistance of pathogenic fungi. Lancet
2002, 359(9312):1135-1144.
126. Costa C, Ponte A, Pais P, Santos R, Cavalheiro M, Yaguchi T, Chibana H, Teixeira MC:
New Mechanisms of Flucytosine Resistance
in C. glabrata Unveiled by a Chemogenomics Analysis in S. cerevisiae. PLoS One 2015,
10(8):e0135110.
127. Beese-Sims SE, Lee J, Levin DE: Yeast Fps1
glycerol facilitator functions as a homotetramer. Yeast 2011, 28(12):815-819.
128. Douglas CM, D’Ippolito JA, Shei GJ, Meinz
M, Onishi J, Marrinan JA, Li W, Abruzzo
GK, Flattery A, Bartizal K, Mitchell A,
Kurtz MB: Identification of the FKS1 gene
of Candida albicans as the essential target of
1,3-beta-D-glucan synthase inhibitors. Antimicrob Agents Chemother 1997, 41(11):24712479.
129. Perlin DS: Resistance to echinocandin-class
antifungal drugs. Drug Resist Updat 2007,
10(3):121-130.
130. Cleary JD, Garcia-Effron G, Chapman SW,
Perlin DS: Reduced Candida glabrata susceptibility secondary to an FKS1 mutation
developed during candidemia treatment.
Antimicrob Agents Chemother 2008, 52(6):22632265.
131. Park S, Kelly R, Kahn JN, Robles J, Hsu MJ,
Register E, Li W, Vyas V, Fan H, Abruzzo
G, Flattery A, Gill C, Chrebet G, Parent SA,
Kurtz M, Teppler H, Douglas CM, Perlin
DS: Specific substitutions in the echinocandin target Fks1p account for reduced
susceptibility of rare laboratory and clinical
Candida sp. isolates. Antimicrob Agents Chemother 2005, 49(8):3264-3273.