Nucleotide-Derived Bacterial Alarmones Attenuation of 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.