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Aspergillus fumigatus is an important pathogen of the immunocompromised host causing pneumonia and invasive disseminated disease with high mortality. Previously, we identified a mutant strain (white, W) lacking conidial pigmentation and, in addition, the conidia showed a smooth surface morphology, whereas wild-type (WT) conidia are grey-green and have a typical ornamentation. W conidia appeared to be less protected against killing by the host defence, e.g., were more susceptible to oxidants in vitro and more efficiently damaged by human monocytes in vitro than WT conidia. When compared to the WT, the W mutant strain showed reduced virulence in a murine animal model. Genetic analysis suggested that the W mutant carried a single mutation which caused all of the observed phenotypes. Here. we report the construction of a genomic cosmid library of A. fumigatus and its use for complementation of the W mutant. Transformation of the W mutant was facilitated by co-transformation with plasmid pHELP1 carrying the autonomously replicating ama1 sequence of A. nidulans which also increased the transformation efficiency of A. fumigatus by a factor of 10. Using this cosmid library a putative polyketide synthase gene, designated pksP (polyketide synthase involved in pigment biosynthesis) was isolated. The pksP gene has a size of 6660 bp. pksP consists of five exons separated by short (47-73 bp) introns. Its deduced open reading frame is composed of 2146 amino acids. The pksP gene complemented both the white phenotype and the surface morphology of the W mutant conidia to wild type. Whereas W mutant conidia caused a strong reactive oxygen species (ROS) release by polymorphonuclear leukocytes, the ability of pksP-complemented W mutant conidia to stimulate ROS release was significantly reduced and comparable to that of WT conidia. In addition, the complemented strains showed restored virulence in a mouse model.

Infections with mold pathogens have emerged as an increasing risk faced by patients under sustained immunosuppression. Species of the Aspergillus family account for most of these infections, and in particular Aspergillus fumigatus may be regarded as the most important airborne pathogenic fungus. The improvement in transplant medicine and the therapy of hematological malignancies is often complicated by the threat of invasive aspergillosis. Specific diagnostic methods are still limited as are the possibilities of therapeutic intervention, leading to the disappointing fact that invasive aspergillosis is still associated with a high mortality rate that ranges from 30% to 90%. In recent years considerable progress has been made in understanding the genetics of A. fumigatus, and molecular techniques for the manipulation of the fungus have been developed. Molecular genetics offers not only approaches for the detailed characterization of gene products that appear to be key components of the infection process but also selection strategies that combine classical genetics and molecular biology to identify virulence determinants of A. fumigatus. Moreover, these methods have a major impact on the development of novel strategies leading to the identification of antimycotic drugs. This review summarizes the current knowledge on the biology, molecular genetics, and genomics of A. fumigatus.

SummaryPreviously, we described the isolation of an Aspergillus fumigatus mutant producing non-pigmented conidia, as a result of a defective polyketide synthase gene, pksP ( polyketide synthase involved in pigment biosynthesis). The virulence of the pksP mutant was attenuated in a murine animal infection model and its conidia showed enhanced susceptibility towards damage by monocytes in vitro. Because macrophagemediated killing is critical for host resistance to aspergillosis, the interaction of both grey-green wildtype conidia and white pksP mutant conidia with human monocyte-derived macrophages (MDM) was studied with respect to intracellular processing of ingested conidia. After phagocytosis, the percentage of wild-type conidia residing in an acidic environment was approximately fivefold lower than that observed for non-pigmented pksP mutant conidia. The phagolysosome formation, as assessed by co-localization of LAMP-1 and cathepsin D with ingested conidia, was significantly lower for wild-type conidia compared with pksP mutant conidia. Furthermore, the intracellular kill of pksP mutant conidia was significantly higher than of wild-type conidia, which was markedly increased by chloroquine, a known enhancer of phagolysosome fusion. Taken together, these findings suggest that the presence of a functional pksP gene in A. fumigatus conidia is associated with an inhibition of phagolysosome fusion in human MDM. These data show for the first time that a fungus has the capability to inhibit the fusion of the phagosome with the lysosome. This finding might help explain the attenuated virulence of pksP mutant strains in a murine animal model and provides a conceptual frame to understand the virulence of A. fumigatus .

A defect in the pksP gene of Aspergillus fumigatus is associated with the loss of conidial pigmentation, a profound change of the conidial surface structure, and reduced virulence. The structural change of the conidial surface structure was not observed in similar A. nidulans wA mutants. Our data indicate that the pigment of both species is important for scavenging reactive oxygen species and for protection of conidia against oxidative damage.Aspergillus spp. are the predominant causative agents of invasive pulmonary aspergillosis (IPA), an often lethal infection of the immunocompromised host (4,10,13,16). Since conidia are the infectious agent in IPA, recent studies focused on the elucidation of conidial factors contributing to pathogenicity (8,17). Previously, we and others have shown that conidia lacking pigmentation due to the defective polyketide synthase gene pksP were less resistant to the attack by monocytes in vitro and showed reduced virulence in a murine animal model (8,9,17,18). During these studies, it became apparent that coincubation of human phagocytes with pksP mutant conidia resulted in a marked increase in the release of reactive oxygen species (ROS) compared with wild-type (wt) conidia (8). Since a defective pksP gene not only impaired conidial pigmentation but concomitantly resulted in profound alterations of the conidial surface (8, 9), the question arose as to whether the large amounts of ROS detected after incubation of phagocytes with pksP mutant conidia were due to a change in the activation pattern of the cells or, alternatively, reflected the lack of ROS quenching capacity caused by the loss of conidial pigment. To address this question, conidia of wt strains of both Aspergillus fumigatus and the nonpathogenic fungus Aspergillus nidulans were compared with their respective pigmentless mutant strains.The WA mutant of A. nidulans (strain WG370; wA3 bgaO biA1) lacking the conidial pigment due to a defective polyketide synthase gene (wA) was constructed by a sexual cross of appropriate parental strains (12) using standard genetic techniques (14). The wA gene product might have a function similar to that of the pksP gene product of A. fumigatus, although major differences between the pigment biosyntheses of the two Aspergillus species exist (2, 18; this study).As previously reported, conidia of the A. fumigatus wt strain showed a rough surface; i.e., they had an ornamentation which was lacking in the pksP mutant strain (8, 9) ( Fig. 1A and B). The A. nidulans wt conidia showed a similar ornamentation (Fig. 1C). In contrast to the pigmentless pksP mutant strain of A. fumigatus, however, similar pigmentless conidia of A. nidulans (wA) still exhibited the ornamentation characteristic of wild-type conidia (Fig. 1D). Taken together, the difference in surface structure between the A. fumigatus pksP mutant and the A. nidulans wA mutant further supports the assumption that different pathways exist for either conidial pigment biosynthesis or pigment deposition in the two species (1). As was noted p...