Hiding in the yolk: a unique feature ofLegionella pneumophilainfection of zebrafish

The zebrafish has become a powerful model organism to study host-pathogen interactions. Here, we developed a zebrafish model ofLegionella pneumophilainfection to dissect innate immune responses. We show thatL. pneumophilacause zebrafish larvae death in a dose dependent manner, and that macrophages are the first line of defence, with neutrophils cooperating to clear the infection. When either macrophages or neutrophils are depleted, these “immunocompromised” larvae become lethally sensitive toL. pneumophilasimilar to what is known for humans that develop pneumonia. Also as observed in human infections, the adaptor signalling molecule Myd88 is not required to control disease in the larvae. Furthermore, proinflammatory cytokine genesil1ßandtnfαwere upregulated during infection, recapitulating key immune responses seen in human infection. Strikingly, we uncovered a previously undescribed infection phenotype in zebrafish larvae, whereby bloodborne, wild typeL. pneumophilainvade and grow in the larval yolk region, a phenotype not observed with a type IV secretion system deficient mutant that cannot translocate effectors into its host cell. Thus, zebrafish larva represents an innovativeL. pneumophilainfection model that that on one hand mimics important aspects of the human immune response toL. pneumophilainfection and that on the other hand will allow to elucidate the mechanisms by which type IV secretion effectors allowL. pneumophilato cross membranes and to obtain nutrients from nutrient rich environments.