Nucleotide-derived bacterial alarmones attenuate the induction of type-I interferon responses in a murine macrophage reporter cell line

The stringent response is a well-studied phenomenon in many bacterial systems and regulates resource-consuming activities such as transcription, translation, and replication. The stringent response is a well-conserved signaling framework, as are the nucleotide-derived signaling mediators, collectively referred to as (p)ppGpp or as alarmones. There is a wealth of research evaluating nucleotide-derived alarmone signaling in bacterial models, however, their potential to modulate innate immune signaling has not yet been evaluated. Several common pathogen-synthesized molecules, such as lipopolysaccharide (LPS) and cyclic-di-AMP (c-di-AMP), act as pathogen-associated molecular patterns (PAMPs), which are common patterns that alert the innate immune system of bacterial infection. The goal of this work is to elucidate the impact of (p)ppGpp on innate immune signaling. To explore this, RAW-Dual cells were incubated with guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), two well-studied nucleotide-derived alarmones found in many different pathogenic bacteria, as well as with GTP. Both ppGpp and pppGpp were able to significantly reduce the expression of secreted luciferase in RAW-Dual cells in a dose-dependent manner, indicating a reduction of the interferon-stimulated regulatory elements (ISREs). Neither alarmone impacted secreted embryonic alkaline phosphatase (SEAP) secretion, which reports for NF-kB activation. This is the first work to suggest that nucleotide-derived alarmones produced by bacteria may impact an arm of innate immunity responsible for type-I interferon secretion. Author SummaryMany different disease-causing bacteria use a similar signaling network to help them survive in environments with low resources; this network is called "the stringent response." All bacteria that use this network also produce signaling molecules, called alarmones, that help coordinate their response to resource deprivation. Interestingly, the human immune system recognizes molecules that are made by many pathogens. As such, our team decided to evaluate whether these bacterially-produced alarmones are able to affect our immune system. We discovered, rather than stimulating our immune system, that high concentrations of these alarmone compounds are able to turn off an important arm of the immune system that is essential in combating viral infections. This suggests another mechanism that bacteria may use to hide from the immune system during infection. In addition, there are potential clinical uses for a molecule that is selectively turn off arms of the immune system, and these molecules may have potential as future anti-inflammatory drugs after further research is able to explore their mechanism of action.