A zebrafish larval model reveals early tissue-specific innate immune responses to Mucor circinelloides: In vivo innate immunity to mucormycetes

Mucormycosis is an emerging fungal infection that is clinically difficult to manage, with increasing incidence and extremely high mortality rates. Individuals with diabetes, suppressed immunity and traumatic injury are at increased risk of developing disease. These patients often present with defects in phagocytic effector cell function. Research utilising mammalian models and phagocytic effector cell lines has attempted to decipher the importance of the innate immune system in host defence against mucormycosis. However, these model systems have not been satisfactory to directly analyse the interaction between innate immune effector cells and infectious sporangiospores in vivo.
Here, we report the first real-time in vivo analysis of the early innate immune response to mucormycete infection using a whole animal zebrafish larval model system. We identified differential host susceptibility, dependent on the site of infection (hindbrain ventricle and swimbladder), as well as differential functions of the two major phagocyte effector cell types in response to viable and non-viable spores. Larval susceptibility to mucormycete spore infection was increased upon immunosuppressant treatment. We show for the first time that macrophages and neutrophils were readily recruited in vivo to the site of infection in an intact host and spore phagocytosis can be observed in real-time in vivo. Whilst exploring innate immune effector recruitment dynamics, we discovered the formation of phagocyte clusters in response to fungal spores that potentially play a role in fungal spore dissemination. Spores failed to activate pro-inflammatory gene expression by six hours post infection in both infection models. After 24 hours, induction of a pro-inflammatory response was only observed in hindbrain ventricle infections. Only a weak pro-inflammatory response was initiated after spore injection into the swimbladder during the same time frame.
In the future, the zebrafish larva as a live whole animal model system will contribute greatly to the study of molecular mechanisms involved in the interaction of the host innate immune system with fungal spores during mucormycosis.