Aspergillus fumigatus causes invasive disease in severely immunocompromised hosts but is readily cleared when host innate defenses are intact. Animal models for evaluation of therapeutic strategies to combat invasive aspergillosis that closely mimic human disease are desirable. We determined optimal dosing regimens for neutrophil depletion and evaluated the course of infection following aerosol infection in mice by determining survival, organ fungal burden, and histopathology in mice in which neutropenia was induced by three methods, administration of granulocyte-depleting monoclonal antibody RB6-8C5 (MAb RB6), administration of cyclophosphamide, and administration of both agents. Administration of either individual agent resulted in a requirement for relatively high conidial inocula to achieve 100% mortality in both BALB/c and C57BL/6 mice, although the infection appeared to be somewhat more lethal in C57BL/6 mice. Death following induction of neutropenia with MAb RB6 occurred when a relatively low fungal burden was present in the lung and may have been related to the inflammatory response associated with neutrophil recovery. In contrast, administration of both agents reduced the lethal inoculum in each mouse strain by approximately 1 log 10 , and C57BL/6 mice that received both agents had a higher fungal burden and less inflammation in the lung at the time of death than BALB/c mice or mice of either strain that received MAb RB6 alone. Our data suggest that the relationship among fungal burden, inflammation, and death is complex and can be influenced by the immunosuppression regimen, the mouse strain, and the inoculum.

Fungal pathogens are notorious for causing chronic and latent infections, but the mechanism by which they evade the immune response is poorly understood. A major limitation in the study of chronic fungal infection has been the lack of suitable animal models where the infection is controlled and yet persists. Pulmonary Cryptococcus neoformans infection in rats results in a diffuse pneumonitis that resolves without dissemination or scarring except for the persistence of interstitial and subpleural granulomas that harbor viable cryptococci inside macrophages and epithelioid cells. Infected rats are asymptomatic but remain infected for as long as 18 months after inoculation with C. neoformans. Containment of infection is associated with granuloma formation that can be partially abrogated by glucocorticoid administration. Using this model, we identified several features associated with persistent infection in the rat lung, including (i) localization of C. neoformans to discrete, well-organized granulomas; (ii) intracellular persistence of C. neoformans within macrophages and epithelioid cells; (iii) reduced inducible nitric oxide synthase expression by granulomas harboring C. neoformans; and (iv) reduced antibody responses to cryptococcal polysaccharide. The results show that maintenance of persistent infection is associated with downregulation of both cellular and humoral immune responses.Cryptococcus neoformans is a fungal pathogen that causes meningoencephalitis in immunocompromised individuals. Infection is believed to be acquired through the respiratory tract, although the precise relationship between pulmonary and central nervous system infection is not understood. Several lines of evidence suggest that C. neoformans causes persistent, primary lung infection in immunocompetent individuals that is similar to infections caused by Mycobacterium tuberculosis and Histoplasma capsulatum. Primary cryptococcal infection is likely to be associated with few or minimal symptoms, but it may disseminate in the context of an acquired immunodeficiency, such as AIDS, or corticosteroid therapy. Persistent, pulmonary cryptococcosis has been described in humans. In one study, approximately 47% of individuals with pulmonary cryptococcosis had abnormal radiographic findings for at least 3 months before diagnosis, and an additional 17% of patients had radiographic findings for more than 18 months before diagnosis (2). Primary cryptococcal pneumonia has also been described as an incidental finding of autopsy studies of immunocompetent individuals. In these cases, infection was associated with small subpleural granulomas containing C. neoformans (16). A primary cryptococcal complex consisting of circumscribed granulomas with hilar lymphadenitis without calcifications has also been described (24).Current animal models are inadequate for studying the pathogenesis of persistent cryptococcosis. The two species that have been most extensively studied are mice and rabbits. Mice are extremely susceptible to pulmonary infection, which is inva...

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Neutrophils are generally considered to contribute to host defense through their potent microbicidal activity. However, there is accumulating evidence that neutrophils also have an important regulatory role in establishing the balance of Th1 and Th2 responses. This study investigated the role of neutrophils in defense against pulmonary Cryptococcus neoformans infection using neutrophil-depleted BALB/c mice generated by administering mAb RB6-8C5. Neutropenic mice with pulmonary infection survived significantly longer than control mice, but there was no difference between groups infected intravenously. On day 1 of infection, neutropenic mice had significantly smaller fungal burdens than control mice. On day 7, neutropenic mice had significantly higher lung concentrations of IL-10, TNF- § , IL-4, and IL-12 than control mice, but there was no difference in IFN-+ and MCP-1 levels. Neutrophils influenced the outcome of cryptococcal infection in mice through mechanisms that did not involve a reduction in early fungal burden. The absence of neutrophils in lung tissue during the initial stages of infection appeared to alter the inflammatory response in a manner that was subsequently beneficial to the host. Higher levels of Th1-and Th2-associated cytokines in neutropenic mice could have simultaneously promoted a strong cellular response while reducing inflammatory damage to the lung. Our results support the emerging concept that neutrophils play an important function in modulating the development of the immune response.

The echinocandin derivative caspofungin (MK-0991, L-743,872) inhibits 1,3-beta-d-glucan synthesis and is active against several medically important fungi but is relatively ineffective against Cryptococcus neoformans. To investigate the mechanism of C. neoformans resistance, the prevalence of 1,3- and 1,6-beta-d-glucan linkages was determined in cells grown with and without caspofungin, using affinity-purified antisera and gold particle immunoelectron microscopy. Cryptococcal strains ATCC 24067 (serotype D) and MY2061 (serotype A) were studied. Growth at 4 microg/mL of caspofungin reduced both glucan linkages in both strains. However, growth at 2 microg/mL resulted in reduced 1,6-beta-d-glucan linkage only for MY2061. Inhibition of 1,6-beta-d-glucan synthesis may be an additional mechanism of action for pneumocandins. The relatively low efficacy of caspofungin against C. neoformans may result from reduced activity against C. neoformans glucan synthase or from yet undiscovered mechanisms of action operative in other fungal pathogens but not in C. neoformans.