Fig. 6

Phagocytes control Mucor circinelloides spores in innate granulomas in vivo. (A and B) We tested the mathematical prediction that proinflammatory cytokine expression supports phagocyte recruitment by evaluating the early transcription response (1 hpi) of two proinflammatory cytokine genes (tnf-α and il-8, coding for tumor necrosis factor alpha and interleukin-8, respectively) after spore exposure in control- and dexamethasone-treated fish. Dexamethasone-treated fish showed significantly lower levels of transcript for both cytokines (P < 0.01 [n = 6 with 15 fish each], Mann-Whitney U test). (C) While phagocyte-induced spore death was identified by our in silico modeling as an efficient mechanism to protect from onset of mucormycosis, and we showed rapid and robust recruitment of phagocytes to the site of infection in vivo, spores are not cleared from zebrafish larvae over a time course of 96 h after infection, as demonstrated by CFU countings indicating live spores (P = 0.229 [n = 5], Kruskal-Wallis test). (D) We were able to show a lack of induction of reactive oxygen species (ROS) in response to fungal spores compared to the ROS-inducing stimulus PMA (P < 0.0001 [n = 9], Mann-Whitney U test), indicating a lack of antimicrobial activity. (E) Representative image of an innate granuloma formed of fungal spores and macrophages (maximum-intensity projection of fluorescent z-stack with 43 sections every 4 μm; scale bar, 100 μm). (F) Dexamethasone treatment decreases the formation of innate granulomas (P < 0.0001 [n = 61], Fisher’s exact test) while increasing hyphal formation (P < 0.0001 [n = 61], Fisher’s exact test) and the occurrence of spore dissemination.