Abstract
Fungi are highly versatile organisms. Many are able to adapt to diverse environmental conditions with ease,
mounting transcriptional and physiological responses to an altering environment within very short time frames. This chapter
will assess current evidence to support the hypothesis that pathogenic fungi are able to recognise "a host". We will
consider the general importance of such mechanisms and how they might function in a range of infection scenarios. For
the purposes of this discussion, possible host recognition mechanisms will be categorised as either sensory or
molecular. We will draw upon reports from the fields of both human and phyto-pathogenic fungi to ask whether? and how?
fungal pathogens recognise the host.
Abstract
Fungal pathogens must evade intricate host defences in order to produce disease. At present, few "classical"
virulence factors have been defined that operate in this capacity. Comparatively more is known about the ways in which
host defence is altered by Blastomyces dermatitidis
BAD1 and Cryptococcus neoformans capsular polysaccharide
and melanin. The ability to undergo phenotypic switching
in vivo adds to the mechanisms by which C.
neoformans may thwart host responses. Pigment production also may be important for
Aspergillus fumigatus, although pksP may
contribute to virulence through other mechanisms. Additional understanding of ways by which fungi are protected from
elimination is obtained by the examination of phenotypes that indicate that subversion of the host's ability to mount
protective immune responses has occurred. The virulence determinant CPB1 is required by
Histoplasma capsulatum to evade intracellular killing, though other factors are involved in determining the outcome of the host-pathogen interaction
in the phagosome. The ability to produce inhibitory prostaglandins or induce prostaglandin production by the host
is another such phenotype, although one of which present knowledge is rudimentary. Examination of these
virulence factors and phenotypes illustrates the complexity of the mechanisms that protect fungi from elimination by the host.
Abstract
Candida albicans, a facultative pathogenic microorganism, has developed several virulence traits which
facilitate invasion of host tissues and avoidance of host defence mechanisms. One group of virulence factors that contributes
to this process are the hydrolytic enzymes which are extracellularly secreted by the fungus. The three most
significant hydrolytic enzymes produced by C. albicans
species are secreted aspartic
proteinases (Sap), phospholipases (PL) and lipases (Lip). They may play a central role in the pathogenicity of candidiasis. Their hydrolytic activity likely has
a number of possible functions in addition to the simple role of digesting molecules for nutrition. Saps as the
best-studied member of this group of hydrolytic enzymes contribute to host tissue invasion by
digesting or destroying cell membranes and by degrading host surface molecules. There is also some evidence that hydrolytic enzymes are able to attack
cells and molecules of the host immune system to avoid or resist antimicrobial activity. High hydrolytic activity with
broad substrate specificity has been found in several
Candida species, and most notably C.
albicans. This activity is attributed to multigene families with at least ten members for Saps and Lips and several members for phospholipase B. Distinct members of these gene families are differentially regulated in various
Candida infections. In the future, prevention
and control of Candida infections might be achieved by pharmacological or immunological tools specifically modulated
to inhibit virulence factors, e.g. the family of secreted aspartic proteinases.
Abstract
The human pathogenic fungi overcome numerous environmental stress conditions both outside and within the host
that challenge their ability to grow. That they survive is primarily a function of their ability to transmit stress signals
by signal transduction pathways that regulate their genomes to adapt to each stress condition. Adaptation of
commensal fungi occurs at mucosal surfaces such as the gastrointestinal tract, the oral cavity or vaginal canal. Here,
nutrient limitations, competitors, pH, and host factors exert their negative influences on the residing microbiota,
including fungi. Environmentally acquired fungi that cause human disease also encounter both similar and dissimilar
stress conditions that occur both within the host and in the environment. Likewise, these fungi must adapt or be eliminated.
In this chapter, the adaptation of human pathogenic fungi to oxidant, pH, and nutritional stress is discussed.
Oxidant stress is exerted by both the host and by the environment. Fungi utilize a number of mechanisms to overcome this barrier and resume their growth, including oxidant-degrading enzymes such as, catalase, peroxidase, and
superoxide dismutase as well as melanin that acts as an oxidant-neutralizing biomolecule. Further, oxidants that are
generated during metabolism of fungi (or by the host, conceivably) are neutralized by other enzymes that restore a
reduced condition to the cellular physiology. Signal pathways that adapt cells to oxidant stress include the 2-component
signal transduction. A second stress condition that in the host may play a significant role in preventing fungal growth is pH.
For example, the pH of the human vaginal canal and ecological niches within a site such as the oral cavity are acidic.
Studies on pH response pathways in human pathogens have been limited to
Candida species, but there are striking similarities in the method in which
Candida and the non-pathogens, Saccharomyces
cerevisiae and Aspergillus nidulans, respond to pH extremes. Finally, we discuss the mechanisms whereby fungi adapt to various types of nutrient stress.
Most response mechanisms to nutrient stress are processed by several signal pathways, including the
cAMP-protein kinase A, Snf1 and the TOR pathways. Each is discussed below. The challenge of future studies is to demonstrate
the significance of these response elements to the challenge of the host. In some cases, as with the 2-component
signal transduction as well as the TOR pathway and pH response pathways, relationships to the establishment of disease
are currently being studied.
Abstract
In the past decade, a dramatic shift has occurred in our mechanistic understanding of innate immunity. Precisely,
the appreciation that activation of the innate immune system initiates, amplifies and drives antigen-specific immune
responses together with the identification of discrete cell types, specific receptors and the signalling pathways involved in
the activation of innate immunity has provided a multitude of new targets for exploitation by the development of
adjuvants for vaccines. It has became apparent that understanding how immune responses are activated will result in the
construction of better vaccines and immunomodulatory strategies that are effective at eliciting acquired protective immunity
to fungi. The model has brought dendritic cells to center stage as promising targets for immunotherapy intervention,
and vaccine development and has shifted the emphasis from the "antigen" towards the "adjuvant". Thus, the promise of
a fungal vaccine will demand an adjuvant capable of stimulating the appropriate type of immune response best
tailored to combating an infection. This objective can be achieved by targeting and manipulating cells and pathways of
the innate immune system.
Abstract
The majority of human Candida
infections occur at mucosal surfaces. Mucosal candidiasis (oropharyngeal and
vaginal) is almost invariably observed in acquired immune deficiency syndrome (AIDS) patients. Vulvovaginal candidiasis is
a widespread, common disease affecting a large proportion of healthy women with some of them affected by
recurrent often intractable forms of the disease. However, in contrast to systemic candidiasis, relatively little is known about
the role of mucosal immunity in protection against
Candida. Therefore, understanding the components of the
host-fungus interaction at the mucosal level can lead to a better understanding of the pathogenesis of mucosal candidiasis and
result in the optimization of preventive and therapeutic antifungal strategies.
C. albicans is capable of colonizing and
persisting on mucosal surfaces and also of stimulating mucosal responses. The transition from asymptomatic colonization
to symptomatic candidiasis occurs in presence of factors that enhance
Candida virulence and/or are a result of a loss
of defense mechanisms. Both humoral and cellular factors have been suggested to confer protection.This review
summarizes the salient features of immune responses to
Candida at the mucosal level with emphasis on the T and B cells
and molecular interaction involved in their regulation. It also focuses on the defense mechanisms characterized in
several animal models of candidiasis. In fact, animal models have provided an important contribution to the study of
various aspects of the host's immune response against
C. albicans at the mucosal level.
Abstract
The immunomodulation and immunotherapy of fungal infections depend on the recognition of antigens that can
elicit protective antibody responses and/or activate and induce proliferation of effector cells of the immune system
mediated by cytokines and other soluble factors. Antibodies and cellular immune responses can be induced by a wide variety
of antigens but only a few will be effective in the immunoprotection against systemic mycoses. Researchers in this
area are, therefore, continuously engaged in seeking for new compounds that show promising results in the treatment
of experimental infections. Antibodies are believed to have protective role in
Candida albicans, Cryptococcus
neoformans and Pneumocystis carinii infections but less so in other systemic mycoses. Mixtures of antibodies, however,
may contain protective, nonprotective and disease enhancing types that should be sorted out for the host benefit.
Quantitative aspects are also critical for the success of antibody therapy. Monoclonal antibodies with defined specificity are
essential in these studies. Other fungal agents such as
Blastomyces dermatitidis, Histoplasma
capsulatum, Coccidioides immitis and
Paracoccidioides brasiliensis express glycoprotein antigens that induce protective cell-mediated reactions.
Such adaptive immunity runs in parallel with the innate immunity to control fungal infections. Adaptive immunity
involves mainly T CD4+ lymphocytes of the Th-1 subtype, producing
IFN-g. In experimental histoplasmosis T
CD8+ cells have also a role in immunoprotection. In terms of human immunoprotection, promiscuous peptides (with degenerate
binding to MHC class II molecules) are the ones to be looked for while trying to transpose the results obtained in the mouse
to human patients.
Abstract
There is growing evidence that
Candida albicans can achieve phenotypic variability by genomic rearrangements.
For years, gross chromosomal rearrangements, as those represented by variation in the electrophoretical karyotypes
especially among clinical isolates, have been the most widely studied. The establishment of a direct correlation between
karyotype and phenotype is difficult because a chromosomal translocation can result in a modification of many traits as
many genes have changed their genomic environment and likely their expression patterns. However, specific
aneuploidies have been unambiguously correlated with specific mutants. The ability of
C. albicans to generate these alterations
in karyotype is not surprising in light of the genome structure, in particular the high frequency of repetitive
elements belonging to several categories, as well as the diploid condition of the organism. In addition to gross
rearrangements, more subtle changes such as gene conversion with or without crossover, other types of homologous or
illegitimate recombination, substitution of bases, minor deletions or additions, etc, may also create genomic variability by
affecting a single gene or a reduced number of genes. Adaptive mutagenesis appears especially advantageous for
C. albicans since it would facilitate a rapid adaptation to the adverse conditions present in the human host. In fact, the behavior
of the organism towards stress as well as the nature and distribution of some repeats, such as those present in MRS
or ALS7 support that concept. Genotypic changes through a sexual interchange of genetic information are
potentially possible since a sexual cycle has been recently discovered in
C. albicans. However, recombination between
homolog chromosomes received from each parent has not been reported so far and, therefore, the variability could be
restricted to new combinations of chromosomes. Prezygotic and postzygotic barriers seem to account for the apparent
inability of C. albicans diploids to return to the haploid state or for a tetraploid to return to the diploid state using meiosis.
Abstract
Invasive fungal infections have become one of the leading causes of death among patients with aggressive
hematological malignances, transplant recipients, and other immunocompromised patients such as those with
Acquired Immunodeficiency Syndrome (AIDS). The emergence of various opportunistic fungal infections and, for several
human fungal pathogens, the rapid development of drug resistance, has prompted the search for new broad-spectrum
antifungal agents that are minimally toxic and unlikely to result in the development of resistance. Current choices include
the polyenes, of which amphotericin B is the most commonly used for treating invasive fungal disease, as well as
a nucleoside analogue (5-fluorocytosine), and the azoles. With the exception of 5-fluorocytosine, the polyenes and
azoles act by interfering with the structural or functional integrity of the fungal plasma membrane. However, while the
target for the polyenes is the fungal sterol ergosterol, the selective nature of amphotericin B is compromised since the
compound also binds to human cholesterol. Thus, toxicity is an invariable consequence of patient management when this drug
is used, and new therapies are sought to avoid this problem. The azoles represent an impressive line of drugs that
have found use in treating both superficial and invasive fungal disease that also cause fewer side effects than amphotericin B. However, there are problems with toxicity (ketoconazole, itraconazole), resistance, especially among the
non-Candida albicans species of
Candida (fluconazole), and a lack of efficacy against other important pathogens
(Aspergillus fumigatus, fluconazole). The specificity of fungal cell wall targets has been exploited in the development of the
b-1,3-glucan inhibitor caspofungin. Other drugs, either experimental or in clinical trials, include those that target
unique fungal components, such as nikkomycins (chitin synthesis inhibitors), pradamicins, allylamines, sordarins and
cationic peptides. Newer targets that have been suggested include the sphingolipids, protein translational machinery that
targets myristoylation of proteins, and DNA topoisomerases. A more recent approach to drug discovery utilizes genomics
to search for targets (genomic mining) that offer specificity and minimal toxicity. In this chapter, first we review
the antifungal agents that are the currently used favorites in treatment. Secondly, experimental targets are discussed,
and finally, genomic approaches are mentioned.
Abstract
Several human pathogenic fungi including
Candida albicans and its related species become resistant to
antifungals and, as a result, the resistance interferes with successful chemotherapy. The resistance of these pathogenic fungi is
not restricted to the commonly used triazole compounds but is even encountered, though not often, with polyene
derivatives as well. Research during past decade confirms that the antifungal resistance phenomenon resembles
multidrug resistance (MDR), a common occurrence in cancer cells. Similar to human P-glycoprotein (P-gp /MDR1) that
encodes a drug extrusion pump, many of the pathogenic fungi also have such efflux pump proteins. The result of
transcriptional activation of these encoding genes and their over expression is a rapid efflux of a drug. As a result of enhanced
efflux activity, the resistant fungal cell accumulates much less drug intracellularly and is able to tolerate and grow in
presence of antifungals. It is now established that antifungal resistance in fungi is not restricted to a single mechanism but
is rather a multifactorial phenomenon. This chapter focuses on some of the molecular mechanisms underlying
antifungal resistance.
Abstract
After attachment to a surface, many fungi can produce biofilms, a mixture of cells that coexist as an organized
community that is enveloped within an exopolymeric matrix. Biofilms represent a protective environment and thus biofilm
formation carries important clinical consequences. Indeed, the majority of human infections are associated with biofilm
formation. Candida albicans remains the fungal species most commonly associated with biofilm formation on both host
tissues and medical devices. Mature C.
albicans biofilms consist of a dense network of yeasts cells and filamentous
elements embedded within a three-dimensional structure consisting of exopolymeric material, microcolonies, and water
channels. Sessile fungal cells within the biofilm are resistant to a range of antifungal agents currently in clinical use, namely
the triazoles and polyenes; the molecular basis of antifungal drug resistance in biofilms is multifactorial.
Abstract
The search for new antifungals has recently focused upon drugs that may interfere with the synthesis of the fungal
cell wall b-1,3-glucan, a structure unique to most human fungal pathogens. The specificity of these drugs should mean
that they are less toxic than the currently available antifungals. Among these inhibitors, echinocandins are fungal
secondary metabolites composed of a cyclic hexapeptide core with a lipid side chain that is responsible for antifungal
activity. Three echinocandin derivatives are currently in clinical development, caspofungin, anidulafungin and micafungin. It
is expected that properties such as electrostatic potential, dipole moment, and others will be important factors in
determining the therapeutic action of these drugs. In this work, we have used a non-local density functional in the calculation of
the stable molecular conformation in water, the molecular electrostatic potential, and related properties for each of
the three echinocandins mentioned above. Molecular Mechanics calculations indicated that the side chain tail opposite
the polypeptide ring in flat structural representations is adjacent to this ring in a low-energy 3D conformation. In each
of the inhibitors, the electrostatic equipotential surface shows a complex and convoluted shape as a result of the
presence of the highly electronegative atoms such as O and N in the polypeptides and in some of the tails.
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