Person:

Yuen, Grace

ABSTRACT The innate immune response of the nematode Caenorhabditis elegans has been extensively studied and a variety of Toll-independent immune response pathways have been identified. Surprisingly little, however, is known about how pathogens activate the C. elegans immune response. Enterococcus faecalis and Enterococcus faecium are closely related enterococcal species that exhibit significantly different levels of virulence in C. elegans infection models. Previous work has shown that activation of the C. elegans immune response by Pseudomonas aeruginosa involves P. aeruginosa-mediated host damage. Through ultrastructural imaging, we report that infection with either E. faecalis or E. faecium causes the worm intestine to become distended with proliferating bacteria in the absence of extensive morphological changes and apparent physical damage. Genetic analysis, whole-genome transcriptional profiling, and multiplexed gene expression analysis demonstrate that both enterococcal species, whether live or dead, induce a rapid and similar transcriptional defense response dependent upon previously described immune signaling pathways. The host response to E. faecium shows a stricter dependence upon stress response signaling pathways than the response to E. faecalis. Unexpectedly, we find that E. faecium is a C. elegans pathogen and that an active wild-type host defense response is required to keep an E. faecium infection at bay. These results provide new insights into the mechanisms underlying the C. elegans immune response to pathogen infection.

The vast majority of metazoan species are invertebrates, which lack an adaptive immune system for defense against microbes. Instead, their innate immune system is sufficient for defense against pathogenic organisms, which are recognized through the perception of microbe-associated molecular patterns (MAMPs), such as peptidoglycan and flagellin. MAMPs, however, are common to all microbes, pathogenic as well as commensal, and it is becoming clear that the host differentiates between the two by sensing the impact of the infection on host cellular physiology, highlighting the crosstalk between sensing of infectious microbes and host damage in the immune system. In this thesis, I investigated these interactions using the Caenorhabditis elegans infection model with two related enterococcal species, Enterococcus faecalis, which causes a lethal infection in C. elegans, and Enterococcus faecium, which does not increase host mortality. Ultrastructural imaging revealed that during infection with either E. faecalis or E. faecium, the intestine becomes distended with proliferating bacteria, and that this occurs in the absence of obvious host cellular damage. Using genetics, whole-genome transcriptional profiling, and multiplexed gene expression analysis, I demonstrated that C. elegans mounts a transcriptionally-driven defense response to both live and heat-killed enterococcal species that is potentially triggered by MAMPs, and that pre-exposure to heat-killed Enterococcus protects C. elegans against a subsequent E. faecalis infection. I also found that the C. elegans host defense response to E. faecium shows a greater dependence upon stress response signaling pathways than the C. elegans host response to E. faecalis. These results provide evidence for the ability of C. elegans to respond to MAMPs in a manner dependent upon previously identified immune signaling pathways. Additionally, these studies show that E. faecium is a weak C. elegans pathogen, and that an active host defense response is required to keep E. faecium at bay. The data presented provide evidence for the underlying differences in the host response and, potentially, virulence mechanisms, between E. faecalis and E. faecium infection of C. elegans. The paradigm of extensive crosstalk between conserved pathways that sense MAMPs, xenobiotics, and host stress observed in this thesis is likely to apply to metazoans in general.