Studies on the host response to infection in the circulatory system

Abstract: Sepsis is a heterogeneous, dynamic, and complex syndrome caused by interaction between the host and the invading pathogen. It is associated with an overwhelmingly dysregulated immune response, and also immunosuppression. In spite of our improved understanding of the underlying molecular pathology, sepsis still remains a common cause of morbidity and mortality worldwide. This is reflected in the unsatisfying results obtained from numerous clinical sepsis trails and in the fact that there is no sepsis-specific treatment available today. It has been proposed that it is time to reconsider the concept of sepsis and to develop novel approaches to the study of severe infectious diseases. The main goal of this thesis was to study host-pathogen interactions in the circulatory system. The research strategy pursued consisted of two distinct parts. Firstly, I worked on elucidating individual host-pathogen interaction mechanisms regarding the close crosstalk between coagulation and the innate immune system. Secondly, I used state-of-the art mass spectrometry analysis in order to get a holistic overview of the molecular processes associated with the pathogenesis of sepsis. In the first two papers, we investigated mechanisms of host-pathogen interaction while focusing on two proteins: tissue factor (TF) and protein C inhibitor (PCI). Paper I showed that soluble M1 protein from Streptococcus pyogenes induces TF expression on monocytes and pro-coagulant activity in whole blood. Paper II demonstrated a novel antimicrobial agent, PCI, and revealed a novel mechanism for how the coagulation system participates in the host defense against bacteria. In the second part of my thesis work, I used mass spectrometry (MS) to study three different biological systems: an individual cell type, clinical plasma samples, and a mouse infection model. The aim here was to develop a testable system that would enable us to study, modify, and evaluate putative effector molecules involved in the pathogenesis of sepsis. In manuscript III, I used several mass spectrometry-based techniques to study the neutrophil proteome under conditions of health and disease, and thereby identified a neutrophil-derived protein abundance pattern in plasma samples from sepsis patients. In manuscript IV, I used SWATH-MS to generate a digital compendium of data from blood plasma samples at different stages of sepsis. This study had two important novel contributions: a deep proteome map of plasma protein dynamics during severe infections and a valuable digital data resource for future sepsis research. In manuscript V, we set out to determine the dynamic changes in protein expression in mouse plasma during severe infection and compared the results to those in human sepsis plasma, with a view to facilitating transfer and interpretation of experimental observations between the two biological systems. The two research strategies used in this thesis were complementary, since the first part provided molecular details of individual biological mechanisms and the other part gave an integrated molecular overview of many separate biological events. The use of mass spectrometry-based techniques allows us to study and integrate information from in-vitro experiments, in-vivo model systems, and clinical observations. The work described in this thesis has hopefully laid the foundation for a resource that may facilitate and improve future research on sepsis.