[Role of the innate immune response in sepsis]

The innate immune system succeeds against the majority of infections before the adaptive immune system is activated. New findings contribute to a better understanding of the pathophysiology of sepsis and lead to the development of new therapeutic strategies. The innate immune system, being responsible for the first response to infections, can trigger adaptive immune responses in case the initial response is ineffective. Both arms of the immune system interact with each other, mainly via cell-cell-interactions but also by soluble factors, such as cytokines and chemokines. Two sub-populations of helper T-cells direct both balanced activation and inhibition of the two arms of the immune systems using specific patterns of cytokine release. Results obtained in new animal models of sepsis, taking a progressive growth of bacteria into account, have implied that existing knowledge has to be reanalyzed. The idea of sepsis as a mere "over-reaction to inflammation" has to be abandoned. Various so-called pattern recognition receptors (e.g. toll-like receptors, TLRs, NOD proteins) are located intracellularly or in the plasma membrane of innate immune cells and recognize certain patterns expressed exclusively by extracellular pathogens. Upon receptor engagement, intracellular signaling pathways lead to cellular activation, followed by release of various cytokines and anti-microbial substances. During the course of sepsis a cytokine shift towards increasing immune suppression occurs. The innate immune system also contributes to the migration of leukocytes in inflammed tissue, involving chemokines and adhesion molecules. Leukocytes also secrete the tissue factor leading to formation of thrombin. The environment in sepsis can cause disseminated intravascular coagulation (DIC), but at the same time thrombin triggers the release of chemokines and adhesion molecules through endothelial cells, which represents a positive feedback mechanism for innate immune responses. New therapeutic strategies for sepsis try to establish a well-balanced immune response. Intervention is accomplished through inhibition of inflammatory cytokines, their receptors or through activation of immunostimulatory responses.