Collaborative Research: Broadening perspectives on innate immunity via the response of diverse eukaryotes to a versatile bacterial pathogen

The innate immune system is the first line of defense against pathogens, allowing cells to recognize when they are infected and mount responses. Because these immune responses can also be dangerous to the cell, it is critical for organisms to distinguish between pathogen infection and commonplace bacterial exposure (e.g. to the environmental microbes around us or to our own microbiomes). This distinction is particularly challenging for organisms that eat bacteria, as they constantly inhabit microbe-rich environments and bring microbes into their own bodies. This project brings together a cross-disciplinary team of scientists to investigate innate immune responses in three different organisms that eat bacteria: 1) sponges, one of the earliest branches on the animal tree of life; 2) choanoflagellates, the closest living relatives of animals; and 3) amoebae, a more distant relative. Because these organisms span the evolutionary transition between multicellular animals and their unicellular cousins, similarities among these organisms can illuminate the origins of animal innate immune systems, as well as reveal novel strategies for antibacterial defense. Outreach components of this project include a four-week, summer, pre-college program for students from urban, public, Pittsburgh high schools. Students will conduct hands-on experiments investigating sponge immune responses to bacteria, which they will present at their schools and to the public at the Carnegie Science Center. This project will test the hypothesis that organisms with high-levels of microbial exposure may disproportionately rely on ‘effector-triggered immunity’ (ETI) to monitor for pathogen activity. The proposed approach is to use the versatile pathogen Legionella pneumophila to elicit ETI in three species selected for their phylogenetic position and lifestyle - an amoeba, a choanoflagellate, and a sponge. L. pneumophila is a pathogen that can infect diverse protists and humans by injecting bacterial effector proteins into host cells. This pathogen therefore enables the use of the same bacterial strains and species for parallel investigations of host immune responses across diverged host species. The researchers have recently established a L. pneumophila infection model for sponges. Here, they will characterize the infection modalities, immune cell types, and cellular pathways involved in these infections. They will also attempt to extend this infection model to choanoflagellates (in addition to the well-established amoeba model) and use parallel, unbiased approaches to assess if these organisms elicit ETI in response to L. pneumophila’s global translation inhibition in host cells. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.