1 Center for Biological Sequence Analysis, Department of Systems Biology, Technical University of Denmark2 Department of Systems Biology, Technical University of Denmark
Salmonellae are food borne pathogens, typically acquired by the oral ingestion of contaminated food or water, causing disease in both healthy and immunocompromised individuals. To gain insight into early immune regulation events caused by Salmonella as well as inflammatory signatures induced by Salmonella and other bacteria in human monocyte-derived dendritic cells (DC), we examined these properties using in vivo and in vitro experimental settings. The outcome of infection with Salmonella depends on the host as well as the infecting serovar. Understanding the relative risks associated with and within different serovars is of major importance for public health. Using an established mouse model, we compared the pathogenicity of two S. Typhimurium strains (SL1344 and DT120) and found that the passage through and the ability to proliferate within the host gastrointestinal system determined the pathogenicity of these strains. Salmonella is a mucosal pathogen, gaining access to host systemic circulation by crossing the gut epithelial barrier and residing intracellularly in DC and Mφ. Until recently focus has been centred on the involvement of Mφ and the conventional antigen-presenting DC (mDC) in bacterial infections, whereas the other major dendritic cell subset, plasmacytoid DC (pDC), plays an important part in antiviral responses, and is less well characterised in regard to antibacterial immunity. Using multi-parametric flow cytometry, we were able to show for the first time that pDC accumulated in Peyer’s patches 24 hours after murine oral Salmonella challenge and while Mφ and mDC exhibited dose-related cellular atrophy, pDC were less susceptible to bacteria-induced cell death, suggesting a role for pDC in early stage Salmonella containment. Furthermore, we identified a number of both DC and Mφ subsets, two of which following infection, accumulated in Peyer’s patches and lamina propria, respectively. Generally, we tend to set apart pathogenic bacteria from opportunistic pathogens and commensal bacteria based on their abilities to induce disease in different hosts, however, the nature of the inflammatory response they induce in DC that set them apart from commensal bacteria remains largely unclear. In the present study, we developed a system by which we were able to compare the bacteria-induced imprint of important regulatory proteins in DC to bacterial-encoded ligands. We observed that DC responded to six different bacteria in a phyla-specific manner giving rise to similar inflammatory signatures within the groups of proteobacteria, firmicutes and actinobacteria, hence being independent on pathogenic versus non-pathogenic properties, and also on the bacteria-to-cell ratio for most bacteria. The results presented in this thesis add to the current knowledge about innate immunity to Salmonella, suggest new host immune cell subsets important for bacterial containment and provide a basic understanding of bacteria-induced DC inflammatory programs. The two latter could prove important in regard to treatment regimes, as targeted modulation of DC profiles for instance by probiotics, could lead to improved therapy for a number of gut related diseases.