The role of autophagy in innate immunity to intracellular bacterial pathogens

Autophagy is a cytosolic degradation pathway responsible for delivering proteins and organelles to the lysosome for degradation. Recently, this pathway has been shown to be an important component of innate immunity. In this thesis, I study two intracellular pathogens with very different lifestyles and compare how they interact with autophagy.;Salmonella enterica serovar Typhimurium is a vacuole-adapted pathogen that replicates in the Salmonella-containing vacuole (SCV) in host cells. During in vitro infection of epithelial cells, a population of bacteria damages the SCV and replicates in the cytosol. I show here that bacteria in damaged SCVs are targeted by autophagy. This serves to protect the cytosol from bacterial colonization and restricts bacterial replication in this cell-type. Furthermore, damage to the SCV membrane appears to act as a signal to recruit autophagy.;Listeria monocytogenes is a cytosol-adapted pathogen that lyses the phagosome and replicates in the cytosol. Listeriolysin O (LLO) and bacterial phospholipases C (PLCs) form pores in the phagosomal membrane to mediate this escape. Once in the cytosol, L. monocytogenes expresses ActA to mediate actin-based motility and intercellular spread. In macrophages, a significant population of bacteria is targeted by autophagy during phagosomal escape in an LLO-dependent manner. However, once in the cytosol, L. monocytogenes is able to evade autophagy. ActA is sufficient but not necessary for autophagy evasion. Accordingly, bacterial PLCs also play a role in autophagy evasion.;During infection of macrophages, we observed that a small population of L. monocytogenes replicates in Spacious Listeria -containing Phagosomes (SLAPs). LLO is essential for SLAP formation and inhibits the maturation of these compartments by forming pores in the SLAP membrane. Furthermore, we found that impaired LLO expression allows slow bacterial replication in vacuoles in an autophagy-dependent manner. Therefore, SLAPs appear to represent a 'stalemate' between bacterial virulence (LLO activity) and host innate immunity (autophagy). SLAP-like structures were observed during persistent L. monocytogenes infection of immuno-deficient mice.;This work provides insights into signals that target autophagy to intracellular pathogens and mechanisms by which pathogens have evolved to evade this innate immune mechanism. Furthermore, this research suggests a possible mechanism for the establishment of persistent infection.