Opportunistic fungal infection is a serious life-threatening complication of immune suppression, both due to impaired adaptive immunity as in HIV/AIDS, and from prolonged myelosuppression such as occurs after high dose anticancer chemotherapy. In south-east Asia, penicilliosis due to Penicillium marneffei infection is a common infection of HIV/AIDS patients. We have developed a zebrafish embryo model of penicilliosis that exploits the experimental flexibility of this animal model and its opportunity to image the host/pathogen interaction in vivo. This model provides a host with intact innate immunity but absent adaptive immunity. As zebrafish are ectothermic, they provide opportunity to explore the consequences of P. marneffei’s thermal dimorphism on the host/pathogen interaction in a manner not possible in mammalian models.
Zebrafish embryos mount a protective host response to inoculation with P. marneffei spores, accompanied by an emergency myelopoietic response boosting both neutrophil and macrophage numbers. P. marneffei spores are actively and predominantly phagocytosed by macrophages in this model, as in mammalian hosts. We used genetic approaches to manipulate host leukocyte numbers, relative proportions of neutrophils and macrophages, and leukocyte functionality to show that during the period of infection establishment, neutrophils are fungicidal to P. marneffei in a myeloperoxidase-dependent manner, whereas macrophages provide a protective niche. Exploiting the optical transparency of zebrafish, interactions between spores, neutrophils and macrophages have been imaged in vivo, using transgenic lines that mark macrophages and neutrophils with red and green fluorophores, and calcofluor (blue) staining of fungal spores. Novel host/pathogen interactions have been observed. Antifungal agents suppress P. marneffei infection, a proof-of-principle demonstration of this model’s utility for testing antifungal agents for bioavailability, toxicity and efficacy in vivo.
This model provides a new approach for studying the pathogenesis of fungal infection, for investigating the host/pathogen interaction in vivo, and for testing novel mechanisms and approaches for treating fungal infection.