In bony fish, the gill filaments are essential for gas exchanges, but also are vulnerable to infection by water-borne microorganisms. Omnipresent across fish, gill-associated lymphoid tissues (GIALT) regulate interactions with local microbiota and halt infection by pathogens. A special GIALT structure has recently been found in Salmonids, the interbranchial lymphoid tissue (ILT). However, the structural variation of GIALT across bony fish remains largely unknown. Here, we show how this critical zone of interaction evolved across fishes. By labeling a conserved T-cell epitope on tissue sections, we find that several basal groups of teleosts possess typical ILT, while modern teleosts have lymphoepithelium of different shape and size at the base of primary gill filaments. Within Cypriniformes, neither body size variation between two related species, zebrafish and common carp, nor morphotype variation, did have a drastic effect on the structure of ILT. Thereby this study is the first to describe the presence of ILT in zebrafish. The ILT variability across fish orders seems to represent different evolutionary solutions to balancing trade-offs between multiple adaptations of jaws and pharyngeal region, and immune responses. Our data point to a wide structural variation in gill immunity between basal groups and modern teleosts.

In the development of therapeutic nanoparticles (NP), there is a large gap between in vitro testing and in vivo experimentation. Despite its prominence as a model, the mouse shows severe limitations for imaging NP and the cells with which they interact. Recently, the transparent zebrafish larva, which is well suited for high-resolution live-imaging, has emerged as a powerful alternative model to investigate the in vivo behavior of NP. Poly(D,L lactic acid) (PLA) is widely accepted as a safe polymer to prepare therapeutic NP. However, to prevent aggregation, many NP require surfactants, which may have undesirable biological effects. Here, we evaluate 'safe-by-design', surfactant-free PLA-NP that were injected intravenously into zebrafish larvae. Interaction of fluorescent NPs with different cell types labelled in reporter animals could be followed in real-time at high resolution; furthermore, by encapsulating colloidal gold into the matrix of PLA-NP we could follow their fate in more detail by electron microscopy, from uptake to degradation. The rapid clearance of fluorescent PLA-NP from the circulation coincided with internalization by endothelial cells lining the whole vasculature and macrophages. After 30 min, when no NP remained in circulation, we observed that macrophages continued to internalize significant amounts of NP. More detailed video-imaging revealed a new mechanism of NP transfer where NP are transmitted along with parts of the cytoplasm from endothelial cells to macrophages.