Mechanisms behind pathogen activation and evasion of Toll-like receptor signaling

Toll-like receptors (TLRs) are signaling receptors of the innate immune system, sensing a variety of bacterial and viral components. Our central hypothesis is that immune cell recognition of invading pathogens starts with an interaction between microbial structures and TLRs. The following immune activation will then initiate the clearing of infection, however, an excessive systemic activation may also lead to the development of septic shock. Our goals are to define the signaling pathways leading to pathog en-induced activation of the immune system via TLRs, and to define microbial mechanisms evolved in order to evade recognition by the same sensor systems. We will study the relative contribution to immune response to bacteria by TLRs and components of the signaling cascade, in primary human cells. This will be done by using specific downregulation of signaling molecule expression by means of RNA interference, and by other methods of inhibition. In addition, we will define which regions of TLRs that are ne cessary for the induction of signaling, including identify regions important for ligand binding and signal initiation. The model system for this study will be activation of cells via TLR9 by bacterial DNA or CpG oligodeoxynucleotides. Finally, we will stu dy the evasion of TLR recognition by the Gram-negative bacteria Yersinia pestis, the causative agent of plague. Y. pestis appears to avoid recognition by TLR4, by altering its lipopolysaccharide in the host. We hypothesize that this may contribute to mini mize the host response to the bacteria, thereby making it possible for the microbe to multiply without a proper control of the infection. The completion of these studies should lead to an increased understanding of the mechanisms behind onset of immune re sponses to microbial pathogens. This knowledge will provide a basis for developing new therapies for infectious diseases, and novel immune modulators and adjuvants.