Download Pathobiology of Aspergillus fumigatus, a saprotrophic pathogenic

真菌症フォーラム 第 9 回学術集会
招待講演①
Pathobiology of Aspergillus fumigatus, a saprotrophic pathogenic fungus
Claude Lamarre, Ph. D.
（Aspergillus Unit, Institut Pasteur, Paris, France）
Aspergillus fumigatus is a world wide saprotrophic species playing an essential role in recycling carbon
and nitrogen of decaying vegetal material. This fungal species has a very simple biological cycle ; one of its
characteristics is its high sporulating capacity which results in the ubiquitous presence of high concentrations of conidia (1-100 co/m3) in the atmosphere indoors and outdoors. Conidia of A. fumigatus are continuously inhaled by humans and are eliminated efficiently by innate immune mechanisms. Over the last years
due to the increase in the number of immunosuppressed patients and the degree of severity of immunosuppressive therapies. A. fumigatus has become the most prevalent airborne fungal pathogen causing severe
and usually fatal invasive infections in immunocompromised hosts in developed countries.
More recently, another polticical and environmental issue arised for A. fumigatus. Because of its natural
habitat, this species is found in high concentration in the environment of compost plants and in the commercial products from composting plants. This may represent a biological hazard for the population, specially since besides polypeptide allergens responsible for asthma and rhinitis, mycotoxins and ß1,3 glucans
that are known modulators of the immune system, are also produced by A. fumigatus in its natural environment.
In spite of its dramatic increase in incidence, the pathogenicity of A. fumigatus is still poorly understood.
Investigations to discover the initial sites of the infection, an essential event in the understanding of IA,
have been limited. The 2 putative pulmonary sites of infection , the epithelium and the alveola will be discussed.
To date, the study of the virulence of A. fumigatus is based on the analysis of mutants constructed by
gene disruption, and the detection of attenuated virulence of the mutant through an animal screen No true
virulence factors viz a gene or a protein essential for growth in vivo whose deletion does not affect mycelial
growth in vitro, have been identified to date in A. fumigatus. Avirulence has been associated to genes such
as PABA or PyrG. PABA or PyrG auxotrophs are not able to germinate in the lung environment because
these molecules are absent from the lung. Strains with reduced growth rate are less pathogenic than wild
type parental strains. Attenuated virulence of this fungus has been also associated to the disruption of
melanin genes involved in the resistance of the fungus to its killing by the phagocytic immune response
All virulence studies suggested that virulence of A. fumigatus looks polygenic. Further analysis of A.
fumigatus virulence would require multiple gene disruption the identification of global regulators or multiple genes expressed concomittantly in the same metabolic pathway. To date, the most efficient way to reach
this objective is a comparative transcriptome analysis of the fungus grown in vivo and in vivo. Such
approach is now feasible since the sequence of the entire genome of A. fumigatus, although not annotated
yet, is available at http://www.tigr.org/tdb/e2k1/afu1. A transcriptome projet based on a Pasteur initiative
will be described
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招待講演
招
待
講
演
The studies in our laboratory on the analysis of the pathobiology of A. fumigatus are focused toward the
study of the response of the major phagocytic cell of the lung, the alveolar macrophage against the fungus
and the identification of A. fumigatus virulence determinants.
Conidial engulfment by the alveolar macrophage (AM) is very quick (1-2h) and not affected by the
immune status of the host. Killing of conidia, mainly due to Reactive Oxidant intermediates (ROIs) starts 68 hours after phagocytosis. The killing rate is surprisingly slow with only a 10% or less reduction of conidia
viability after 6 hours of phagocytosis. Immunosuppression by corticoids is associated to a reduction in ROI
production that is correlated to the intramacrophagic germination seen in these patients. Our working
hypothesis was that molecules produced by the fungus to detoxify ROIs of the host or promote growth in
the lung of the host could be virulence factors. To investigate the virulence of A. fumigatus, mutants are
constructed following gene disruption and their virulence tested in an experimental animal model of IA. No
true virulence factors have been identified to date in A. fumigatus. In particular, the catalase, proteases and
toxin secreted by the mycelium of this fungus do not play a role in the pathogenesis of A. fumigatus in
experimentally induced infections. In contrast, the melanin layer of the conidial cell wall efficiently scavenges reactive oxygen species and plays a role in protecting the fungus against the phagocytic reactions.
Accordingly, mutants with white conidia resulting from the inactivation of ALB1, a polyketide synthase
essential for the synthesis of dihydroxynaphtalene melanin present on the surface of the A. fumigatus conidia, were more efficiently damaged by phagocytes than wild-type. The polygenic nature of the fungal virulence will be now investigated using post genomic approaches made posssible because of the recent
sequencing of the A. fumigatus genome.
Mycelial growth in the lung of the patients is associated to the release of antigens that circulate in the
biological fluids of the patients. One of these antigens is the galactomannan that is both a major component
of the fungal cell wall but is also secreted by the fungus. Galactomannan detection by an enzyme
immunoassay developed in collaboration with our laboratory, has proven to be a potentially promising tool
for the early diagnosis of IA. Sensitivity and specificity of the test appear to be fairly good. False positive
tests however have been reported to increase recently. Two sources of false positives have been identified :
some of them are now known to be associated to the treatment of patients with Piperacillin-Tazobactam and
others are due to food products contaminated with galactomannan and crossing the intestinal mucosa damaged by the toxic anti-cancer treatments. The serological diagnosis of IA will be discussed as well as new
method to diagnose aspergillosis using recombinant antigens will be presented.
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