The influence of gut-derived bacteria on systemic adaptive immune responses

Abstract

Sepsis and sepsis-associated multi-organ failure are major challenges for clinicians and scientists and are characterized by high patient morbidity and mortality. One factor thought to be important in the etiology of systemic inflammatory response syndrome (SIRS) is the breakdown of the intestinal barrier resulting in bacterial translocation and subsequent bacteremia causing sepsis. Whereas the influence of sepsis on the innate immune system is well described, the counter regulatory mechanisms and the impact on adaptive immunity are still largely unknown. In this thesis, the influences of gut-derived bacteria on the induction of systemic adaptive immune responses were investigated. <br /> The results presented demonstrate that the loss of gut barrier function led to release of vast amounts of intestinal bacteria, which rapidly disseminated in the organism. Bacteremia caused suppression of adaptive cytotoxic immune responses against subsequent infections with viral and bacterial pathogens. This suppression was characterized by impaired expansion of antigen-specific cytotoxic T cells and a lack of antigen-specific cytotoxicity. Interestingly, bacterial translocation from the gut did not necessarily result in suppression of T cell responses. If only low amounts of bacteria translocated, bacteria were retained within the liver which prevented dissemination of gut-derived bacteria, bacteremia, and subsequent immunosuppression. Importantly, the induction of local immunity at peripheral sites was not affected by bacteremia, contradicting the current opinion of a general, systemic immunosuppression following sepsis. Moreover, bacteremia exclusively inhibited the generation of subsequent adaptive immune responses, whereas already initiated antigen-specific CTL responses were further stimulated by systemic bacteria. Although <i>E.coli</i>, which was used in this study as a model organism for translocating bacteria, stimulates a broad variety of Toll-like receptors (TLRs), suppression was solely dependent on TLR4 activation. Interestingly, neither TLR4 downstream signaling via MyD88 nor expression of the potent suppressive cytokine IL-10 contributed to the observed immunosuppression after bacteremia. Instead signaling via TRIF and subsequent expression of type I interferons (IFNs) were critically involved in <i>E.coli</i>-mediated CTL suppression.<br /> We could demonstrate that adaptive immune responses towards systemic pathogens were generated in the spleen and furthermore, that suppression of systemic CTL responses strictly depended on the presence of bacteria in the spleen. Splenic macrophages and DCs, which are crucially involved in the induction of T cell responses as metallophilic marginal zone macrophages (MMMs) efficiently phagocytose blood borne antigens and transfer them to cross-presenting DCs. These cells were impaired in their ability to induce adaptive immunity to subsequent adenoviral infection after exposure to <i>E.coli</i>. These findings underline a central role of splenic macrophages and DCs in immunosuppression after sepsis. <br /> The observed suppression of adaptive immunity was surprising as pathogens are known to be potent activators of the innate immune system by stimulation of pattern-recognition receptors, such as TLRs. Activation of the innate immune system is an essential prerequisite for the initiation of effective adaptive immune response. However, in this study we showed that activation of innate immunity indeed strictly depended on TLR signaling, whereas TLR signaling was dispensable for the generation of effective adaptive immune responses. Activation of TLRs led to effective induction of innate immunity but also exerts mechanisms, namely secretion of type I IFNs, which regulated adaptive immune responses in a paracrine manner.<br /> Taken together, the findings presented in this study, demonstrate a dual role of TLR signaling. Depending on the anatomical site, dose, and time point of TLR ligand application systemic adaptive immune response are either stimulated or suppressed. Moreover, these data may provide further insights for the development of new therapeutic approaches to circumvent suppression of systemic adaptive immune response in septic patients.