In vitro, in vivo and in ovo interactions of human pathogenic fungus Aspergillus fumigatus
Aspergillus fumigatus is an opportunistic pathogenic fungus found ubiquitously in nature due to airborne conidia and is a main causative agent of human diseases in immunocompromised patients. The conidial cell wall is composed of pigmented layer of dihydroxynaphthalene (DHN)-melanin adjacent to outermost rodlet layer. The primary route of A. fumigatus infection is via inhalation of conidia which, due to small size of about 2-3 μm diameter, reach the pulmonary alveolus. The first barrier to invading conidia are alveolar epithelial cells which play an essential role in host innate immune response including alveolar macrophages. Most of the diseases caused by A. fumigatus occur in lungs, however, the fungus can disseminate to other organs causing systemic infections in immunocompromised patients. A better understanding of pathogenicity of infections caused by A. fumigatus requires rational approaches for early diagnosis and effective therapies. Positron Emission Tomography/Computed Tomography (PET/CT) is an integrated system which has found its way in diagnosis of various diseases including A. fumigatus infections and to assess host-pathogen interactions for evaluation of diagnosis process, various infection models play an important role. Mice have been regarded as gold standard animal model whereas due to ethical considerations, alternative models such as embryonated chicken eggs are taken into account. During this study, various host-pathogen interactions were evaluated to overcome the following challenges:- In vitro interactions with alveolar epithelial cells: It was revealed that A. fumigatus conidia are able to survive and reside inside epithelial cells and DHN-melanin, present on conidial cell surface, plays an essential role in its survival. Moreover, it was found that presence of melanin plays a fundamental role in the uptake of conidia by epithelial cells and that melanin is able to inhibit apoptosis or cell death which increased with increase in internalization of conidia by epithelial cells. In vivo interactions with mice: various models are used to study A. fumigatus host-pathogen interactions, however, here, mice was used as an adequate model to investigate the dissemination of fungus to other organs based on the pulmonary infection. In immunocompetent mice, the fungus was cleared within a day after pulmonary infection while as in immunocompromised mice, there was presence of fungal DNA in brain in addition to lungs but the fungus was not found viable. No clear evidence could be provided for establishment of mice as model for dissemination of infection after pulmonary infection. In ovo interactions with chicken embryos: This study revealed the characterization of radiotracers using chicken embryos for PET/CT imaging and LigandTracer® instrument for in vitro assaying of radiotracer. It was demonstrated that LigandTracer® device can be optimized for evaluation of novel radiotracers for A. fumigatus infections. Furthermore, using desferri-triacetylfusarinine C (TAFC) as proof of principle, it was shown that chicken eggs can serve as model for characterization of novel radiotracers for A. fumigatus infections. Both the models together can be used for initial characterization of radiotracers for diagnosis of A. fumigatus infections.