Type I interferons in infection with Streptococcus pyogenes

Recognition of pathogens by cells of the innate immune system is fundamental for launching a successful defence against infection. The encounter of pathogen-associated molecular patterns (PAMPs) with appropriate pattern recognition receptors (PRRs) results in activation of protective mechanisms. We and others recently reported that the major Gram-positive human pathogen Streptococcus pyogenes causes innate immune cells to produce inflammatory cytokines (e.g. TNF) and type I interferons (IFNs) by employing so far unknown PRRs that are distinct from those usually involved in recognition of bacteria. Our last study revealed that type I IFN signaling is required for successful defence of mice against invasive S. pyogenes infection. Furthermore, the type I IFN- inducing PAMPs were defined as DNA in macrophages and RNA in conventional dendritic cells (cDCs). PAMP signaling was entirely dependent on the adaptor MyD88 in cDCs but not in macrophages. The precise nature of these PAMPs remains to be characterized. The proposed project aims at identifying the PRRs and PAMPs involved in type I IFN induction by S. pyogenes and at describing the mechanism of the beneficial role of type I IFNs including the most relevant type I IFN-producing and -responding cells in vivo. For the PRR and PAMP characterization we will focus on cDCs since these cells produce higher type I IFN amounts than macrophages, and the PAMP in cDCs (i.e. bacterial RNA) is specific in contrast to the rather unspecific activity of DNA in macrophages. As a main strategy, we will purify host cell proteins specifically binding to S. pyogenes-derived RNA. Subsequent functional characterisation of these proteins will allow identification of the genuine PRR. The PRR will in turn allow a precise analysis of the IFN-inducing RNA properties which, as revealed by our preliminary data, do not depend on 5` triphosphate commonly found in bacterial RNA. To find cells that are the critical type I IFN producers and effectors in vivo we will employ mice allowing detection of IFN-ß production in vivo (IFN-ß reporter mice) and mice with cell type-specific deletion of type I IFN receptor and the IFN-ß gene. Our preliminary data suggest type I IFNs exert their beneficial effects by limiting neutrophil activity. We will examine the effect of type I IFNs on neutrophils in context of S. pyogenes infection by in vivo (e.g. adoptive transfer, cell type-specific IFNAR1 ablation etc.) and in vitro (phagocytosis, NO production, extracellular traps formation, migration etc.) approaches. The results of the project will improve our understanding of the so far not well defined role of type I IFNs in bacterial infections. Importantly, the identified PRRs and PAMPs involved in S. pyogenes infection will highly likely represent novel key players in recognition of other bacterial species so that they will be of a general interest.

Not too little, not too muchjust right: The findings of this study reveal how the immune response against the important pathogenic bacterium Streptococcus pyogenes is balanced such that a strong but not destructive immune reaction is generated. Successful defense against infectious agents fundamentally depends on a precise balance between the activation and resolution of inflammation. Failure to launch robust inflammation may result in an unrestricted dissemination of the pathogen whereas non-resolving inflammation is associated with exaggerated tissue damage. Both situations represent realistic and serious health risks. Mechanisms that keep immune responses against infections in balance remain in most cases elusive. The results of the project show on one hand molecular interactions causing the activation of the immune response against S. pyogenes, and on the other hand, how the immune response is kept in the right range. Our findings establish that S. pyogenes is recognized by the immune system in two main ways. First, S. pyogenes products localized on the bacterial cell surface interact with the immune cell receptor Tlr2. Second, S. pyogenes is lysed upon phagocytosis and its RNA is recognized by Tlr8 in humans and Tlr13 in mice. Activation of these pathways causes the immune cells to secrete cytokines, which are secondary messengers communicating to the infected organism to launch immune responses. Activation of both of these recognition pathways is required for protection against the infection. Thus, the pathways are not redundant. One important group of the secreted cytokines are type I interferons (IFN). They have been known for more than half a century as key cytokines for resistance against viral infections. Their role in bacterial infections is less clear since they can be beneficial or harmful, depending on the infecting bacterial pathogen. Our study shows type I IFNs are essential for preventing the immune system to overreact. In the absence of type I IFN responses, infection with S. pyogenes causes an exaggerated immune response leading to severe damage of various organs with fatal consequences for the infected host. These lethal effects result mainly from overproduction of the strong proinflammatory cytokine IL-1b, since by reducing the function of this cytokines the lethal consequences of the absence of type I IFNs are rescued. However, a too low level of IL-1b responses is detrimental as well since it causes insufficient immune response. In conclusion, our study demonstrates that protection against S. pyogenes infection requires a robust multilayered immune response. This response is controlled type I IFNs in order to prevent lethal inflammation to occur. The findings suggest that modulation of immune responses might help in the therapy of severe life-threatening S. pyogenes infections, which are difficult to treat using currently available medical approaches.