Type I interferons as modulators of macrophage responses

Macrophages contribute in an essential manner to the antibacterial properties of the innate immune system. Their interaction with bacteria is very intricate because on the one hand macrophages are equipped to combat infection and kill bacteria, on the other hand some bacteria have evolved ways of escaping the bactericidal arsenal of the macrophage. Moreover, results published in recent years have shown that bacteria can kill infected macrophages, at least in some cases by an apoptotic mechanism. The significance of macrophage death induction by bacteria for the course of infections is unclear. The bacterium Listeria monocytogenes survives inside macrophages by escaping from the phagocytic vacuole into the cytoplasm. Infected macrophages may receive an activating stimulus when encountering the cytokine IFN- gamma and thus cope with the cytoplasmic infection. The appropriate timing of the IFN-gamma stimulus is very important. We have shown that some time after the initial infection L. monocytogenes causes the synthesis of the SOCS3 protein, an intracellular inhibitor of IFN responses. In the absence of activation, the bacterium multiplies in the cytoplasm, eventually killing its host by a poorly understood mechanism that involves the pore-forming bacterial protein Listeriolysin (LLO). Our unpublished results show that Listeria-induced macrophage death (LIMD) is strongly inhibited by preventing a response to type I IFN (IFN-alpha/beta). These cytokines are produced by macrophages soon after the initial infection. How L. monocytogenes triggers type I IFN synthesis and how IFN-alpha/beta promotes the induction of LIMD is unclear. Our results show that LLO is involved both in IFN synthesis and in LIMD. We do not know at present whether the requirement for the protein reflects the need for cytoplasmic escape or whether LLO is per se an inducer of interferons and cell death. In our propsal we suggest a number of experiments to address the following questions: - What is the molecular mechanism employed by L. monocytogenes to cause IFN-alpha/beta synthesis by infected macrophages? - How do type I IFN contribute to LIMD? - What is the mode of LIMD (apoptosis versus necrosis)? - What is the role of SOCS proteins in the interaction beween macrophages and L. monocytogenes? Can synthesis of SOCS3 prior to infection, e.g. by ectopic expression or by pretreatment with the de-activating cytokine IL10 prevent LIMD? Briefly summarized our experimental approach combines biochemical analyses of candidate molecules for IFN synthesis and LIMD induction with studies using macrophages from gene-targeted mice. Furthermore, we propose to combine biochemical, genetic and ultrastructural analyses to investigate the mode of LIMD. Finally, regulated expression in macrophages will clarify the role of SOCS3 in type I IFN responses and the impact of type I IFN on the function of the bacterial LLO in LIMD.

The immune system of mammalian organisms provides a number of defense strategies against microbial pathogens. This is also true for pathogens hiding within the cells of the infected organism. A prime example for intracellular defense are the Interferons (IFN). These are produced during viral infections and cause host cells to enter an antiviral state, which successfully combats rapid multiplication of a majority of viruses. In the case of bacterial pathogens predominantly extracellular species can be distinguished from those multiplying predominantly within cells of the host organism. A well-known example for such an intracellular bacterium is Listeria monocytogenes. Infection with this microbe is contracted predominantly through spoiled food, mostly dairy and meat products. In the host organism L. monocytogens can infect a number of different cell types, including macrophages, cells capable of ingesting microbes and other antigenic material. Infection of macrophages can have different outcomes depending on the general immunological framework. In case it is immunologically activated the macrophage can kill the intracellular Listeria. By contrast, a resting macrophage will be killed by the infecting bacteria and release their progeny within 24h. In the completed project we have asked which are the determining factors for the fate of a Listeria-infected macrophage and revealed a decisive influence of interferons. Interferon-gamma (IFN-) makes a major contribution to the immunological activation of macrophages, thus to the killing of Listeria. By contrast, interferon- beta (IFN-ß) promotes macrophage death caused by Listeria infection. We could show that the macrophage itself produces large amounts of IFN-ß in the early phase of infection. To generate the relevant signals causing IFN-ß synthesis, Listeria must enter the macrophage cytoplasm. Unlike the vast majority of microbe elicited signals, those targeting the IFN-ß gene are not generated by the contact between the pathogen and the macrophage surface. The response weakening the macrophage`s ability to survive infection is part of a genetic program initiated by the infecting Listeria. This program is heavily influenced by the IFN-ß produced during infection. The results obtained in the course of the project show that unlike in the case of viral infection, IFN-ß produced in response to intracellular bacteria is detrimental for the host. In further consequence these results suggest anti-IFN-ß therapy as a suitable strategy to help patients recover from infection with intracellular bacteria.