Targeting Drugs to Larval Zebrafish Macrophages by Injecting Drug-Loaded Liposomes

Zebrafish (Danio rerio) larvae have developed into a popular model to investigate host-pathogen interactions and the contribution of innate immune cells to inflammatory disease due to their functionally conserved innate immune system. They are also widely used to examine how innate immune cells help guide developmental processes. By taking advantage of the optical transparency and genetic tractability of larval zebrafish, these studies often focus on live imaging approaches to functionally characterize fluorescently marked macrophages and neutrophils within intact animals. Due to their diverse functional heterogeneity and ever-expanding roles in disease pathogenesis, macrophages have received significant attention. In addition to genetic manipulations, chemical interventions are now routinely used to manipulate and examine macrophage behavior in larval zebrafish. Delivery of these drugs is typically limited to passive targeting of free drug through direct immersion or microinjection. These approaches rely on the assumption that any changes to macrophage behavior are the result of a direct effect of the drug on the macrophages themselves, and not a downstream consequence of a direct effect on another cell type. Here, we present our protocols for targeting drugs specifically to larval zebrafish macrophages by microinjecting drug-loaded fluorescent liposomes. We reveal that poloxamer 188-modified drug-loaded blue fluorescent liposomes are readily taken up by macrophages, and not by neutrophils. We also provide evidence that drugs delivered in this way can impact macrophage activity in a manner consistent with the mechanism of action of the drug. This technique will be of value to researchers wanting to ensure targeting of drugs to macrophages and when drugs are too toxic to be delivered by traditional methods like immersion.