Hiding in the yolk: A unique feature of Legionella pneumophila infection of zebrafish

The zebrafish has become a powerful model organism to study host-pathogen interactions. Here, we developed a zebrafish model to dissect the innate immune response to Legionella pneumophila during infection. We show that L. pneumophila cause zebrafish larvae death in a dose dependent manner. Additionally, we show that macrophages are the first line of defence and cooperate with neutrophils to clear the infection. Immunocompromised humans have an increased propensity to develop pneumonia, when either macrophages or neutrophils are depleted, these immunocompromised larvae become lethally sensitive to L. pneumophila. Also, as observed in human infections, the adaptor signalling molecule Myd88 is not required to control disease in the larvae. Furthermore, proinflammatory cytokine genes il1 beta and tnf-alpha were 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 type L. pneumophila invade 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 innovative L. pneumophila infection model that mimics important aspects of the human immune response to L. pneumophila infection and will allow the elucidation of mechanisms by which type IV secretion effectors allow L. pneumophila to cross host cell membranes and obtain nutrients from nutrient rich environments. Author summaryL. pneumophila is an intracellular pathogen that has co-evolved with aquatic protozoa but can also infect humans to cause a severe pneumonia. L. pneumophila infections are increasingly recognized worldwide, with outbreaks affecting hundreds of people commonly reported. Although Legionella infection has been widely studied in various model organisms, key aspects of human disease and host response to infection are not well reflected in these organisms. Here we have established the zebrafish as a new model for Legionella infection and use cutting-edge intravital imaging to reveal: the temporal dynamics of bacterial dissemination, the interactions between bacteria and macrophages/neutrophils, and within a whole-of-organism spatial context. We show that as in humans, macrophages are the main drivers of host defense and immunocompromised fish are highly susceptible to infection. In addition, we identified a previously undescribed infection phenotype in which Legionella enter and replicate in the yolk region of zebrafish larvae, an aspect which ultimately drives the outcome of infection. The new zebrafish model should prove useful as a platform to study host and bacterial factors underlying Legionella infection.

Automated summary

By developing a zebrafish model, we discovered the association between host immune response and Legionella pneumophila infection, with macrophages being the first line of defense and cooperating with neutrophils to clear the infection. Additionally, we found that immunocompromised zebrafish larvae are exceptionally sensitive to L. pneumophila. The zebrafish model can mimic the immune response to L. pneumophila infection in humans and help elucidate the mechanisms by which L. pneumophila crosses host cell membranes and obtains nutrients.