Comparative evaluation of Gram-positive membrane components in activating the innate immune system

Abstract

Lipopolysaccharide (LPS) is the major immunostimulatory component of Gramnegative bacteria, but its counterpart in Gram-positive bacteria is still under discussion. Looking on the Gram-positive cell wall, three components are considered to be recognised by cells of the human innate immune system: Peptidoglycan (PGN), quantitatively the main component, lipoteichoic acid (LTA) as a similar amphiphile structure compared to LPS and lipoproteins (LP). To find out more about the immunostimulatory capacity of these membrane components, the first approach in this thesis was to screen public available literature for evidence, that cytokine release in humans is connected with one or more of the named components. This research was done systematically as a meta-analysis with the four well-known Koch-Dale (K/D) criteria with a restriction to human studies. Taken together, the results of the meta-analysis indicated that PGN and LPs might play a role in cytokine induction and therefore in immune recognition of Gram-positive bacteria in humans, but the evidence for LTA being the major immune stimulus in Gram-positive bacteria is strong as it is the only investigated molecule fulfilling all K/D criteria. The interesting findings of this meta-analysis needed to be investigated experimentally. The model organism for these investigations was Staphylococcus aureus (SA), which is a frequent human pathogen and often colonises humans asymptomatically, but is also able to induce severe infections in tissue or even spreading into the blood. In the second part of the thesis, three different SA 113 mutants, which were lacking lipoproteins (Δlgt) or wall teichoic acids (ΔTA) or possessed a reduced alanine content of the LTA (Δdlt) were compared to its corresponding wildtype with respect to their immunostimulatory capacity in human primary cells. We could finally show that the different mutants differ only marginally in their immunostilumatory capacity. Despite the strong evidence that LTA is a major immunostimulatory principle of Gram-positive bacteria, recent reports suggested that not LTA but lipoproteins are the dominant immunostimulatory structures of SA. Therefore we compared the LTA from SA 113 Δlgt and its corresponding wildtype in more detail. This study clearly shows major similarities between wt and lgt LTA, but differences in immunrecognition to synthetic lipoproteins. In summary, the results of this thesis contribute to the understanding of the innate immune response with the focus on cell wall components of Gram-positive bacteria. This may lead to new approaches to treatments against Gram-positive bacterial infections in the future.