Mechanisms of mast cell activation by bacterial virulence factors : Implications for mucosal infection

Mast cells are innate immune cells, which can be found in nearly all tissues of the human body. Activated by receptors for IgE or bacterial compounds, mast cells are complex effector cells which respond in many ways to bacterial infection. However, decades of research did not lead to a general model of how mast cells are activated by bacteria. Examples of pathogenic bacteria include the extracellular Streptococcus equi subspecies equi (S. equi) that colonizes the airways in horses by secretion of powerful toxins, and the invasive enterobacterium Salmonella enterica subspecies Typhimurium (S.Tm). The latter is taken up with contaminated food and invades the epithelium in the distal small and proximal large intestine to reach the deeper layers of the tissue. Mast cells are present as early responders towards both bacteria. In this thesis, I elucidated how pathogenic bacteria activate these versatile immune cells. In Paper I, I demonstrated which virulence factors of S. equi that activate mast cells, by utilizing bacterial knockout mutants of several virulence factors or a combination of those. While superantigens or the protective capsule did not lead to mast cell activation, removal of the pore-forming toxin streptolysin S led to complete ablation of the mast cell response to S. equi, establishing that sublytic pore formation leads to membrane stress, which results in inflammation. To compare our findings from extracellular bacteria with the invasive S.Tm, we explored in Paper II, if its type-three-secretion system induces a similar sublytic pore-mediated immune activation. We found however, that S.Tm triggers mast cell activation by a two-step activation process, involving 1) priming by toll-like receptor (TLR) 4 and 2) effector-induced immunity. While we mainly focused on classical mouse mast cells, we extended our findings further in Paper III. In this study, the use of connective tissue-type and mucosal-type mouse mast cells, as well as a human mast cell line broadened the validity of our two-step activation model. We discovered, moreover, that mucosal-like mast cells lack TLR2 and TLR4, making them only responsive to invasive bacteria. Overall, this thesis contributes to our understanding of the general mechanisms for how mast cells are activated by bacteria.