Nanoparticles as drug delivery system against tuberculosis in zebrafish embryos: direct visualization and treatment

Nanoparticles (NPs) enclosing antibiotics have provided promising therapy against Mycobacterium tuberculosis (Mtb) in different mammalian models. However, the NPs were not visualized in any of these animal studies. Here, we introduce the transparent zebrafish embryo as a system for noninvasive, simultaneous imaging of fluorescent NPs and the fish tuberculosis (TB) agent Mycobacterium marinum (Mm). The study was facilitated by the use of transgenic lines of macrophages, neutrophils, and endothelial cells expressing fluorescent markers readily visible in the live vertebrate. Intravenous injection of Mm led to phagocytosis by blood macrophages. These remained within the vasculature until 3 days postinfection where they started to extravasate and form aggregates of infected cells. Correlative light/electron microscopy revealed that these granuloma-like structures had significant access to the vasculature. Injection of NPs induced rapid uptake by both infected and uninfected macrophages, the latter being actively recruited to the site of infection, thereby providing an efficient targeting into granulomas. Rifampicin-loaded NPs significantly improved embryo survival and lowered bacterial load, as shown by quantitative fluorescence analysis. Our results argue that zebrafish embryos offer a powerful system for monitoring NPs in vivo and rationalize why NP therapy was so effective against Mtb in earlier studies; bacteria and NPs share the same cellular niche.