Mechanisms of Salmonella internalization into host cells

The Gram-negative bacterial pathogen Salmonella enterica is responsible for over 1.5 million cases of food-related illnesses in the US annually (Meads et al., 1999). These facultative intracellular pathogens readily infect various host cells, ranging from intestinal epithelial cells to dendritic cells and macrophages. S. enterica uptake into these cells involves coordinate regulation of different bacterial and host factors to induce local cytoskeletal rearrangements.;Previous work has shown that infection of epithelial cells with S. typhimurium induces tyrosine phosphorylation of multiple host proteins. However, the identity of these proteins and their associated kinases are currently unknown. Therefore, the main purpose of my research is to examine whether the tyrosine kinase c-Abl is necessary for S. typhimurium entry into host cells. My findings reveal that epithelial cells depleted of c-Abl are more resistant to Salmonella infection. Additionally, the c-Abl kinase activity is needed for entry into cells since pretreatment with a c-Abl specific inhibitor STI571 significantly reduces S. typhimurium internalization. This is the first report that demonstrates the phosphorylation of the adaptor molecules, Abi-1 and CrkII, by c-Abl kinase in response to Salmonella infection.;Clearance of bacteria by phagocytic cells plays an essential role in the host immune response. However, in contrast to invasion in epithelial cells, the mechanism of S. typhimurium entry into macrophages is less well characterized. BAI1 is a recently described receptor that mediates the binding and phagocytosis of apoptotic cells. Engagement of BAI1 triggers phagocytosis through a mechanism that involves the ELMO/Dock180 complex, a Rac activating protein which has a pivotal role in the actin reorganization. Here, I collaborated with Dr. Soumita Das to determine if the ELMO protein is involved in S. typhimurium internalization. We found that Salmonella invasion is impaired in macrophages depleted of ELMO1 compared to wild-type macrophages. Furthermore, ELMO1-depleted macrophages showed decreased Rac1 activity compared to control cells after infection. Moreover, disruption of ELMO-mediated signaling leads to altered TNF-alpha production. As BAI1 is responsible for apoptotic cell recognition, we also investigated whether BAI1 may function in Salmonella recognition and binding. Our preliminary results suggest that BAI1 facilitates S. typhimurium attachment and internalization.