Streptococcus pyogenes Uses Quorum Sensing to Attenuate Innate Immune Signaling

In response to the human-restricted pathosymbiont, Streptococcus pyogenes (Group A Streptococcus, GAS), innate immune cells activate NFkappaB-mediated pro-inflammatory cytokine production. This helps host cells coordinate an effort to control the bacteria. However, bacteria also use clever mechanisms to interfere with host cells. Like many bacteria, GAS senses and responds to extracellular signals. This mechanism, called quorum sensing (QS), enables synchronization of genetic programs at the population level. We previously determined that activation of the Rgg2/3 QS system in GAS results in surface alterations, leading us to hypothesize that QS might alter interactions with host immune cells. Using mutant strains of GAS with either constitutively activated (QS-ON) or repressed (QS-OFF) Rgg2/3, host cells were infected and NFB and pro-inflammatory responses were measured. Macrophage pro-inflammatory responses to GAS were attenuated when QS was active. When macrophages were inoculated with both QS-ON and OFF mutants together, the cytokine response remained attenuated, indicating an active mechanism of suppression. QS-ON GAS also suppressed stimulation by various TLR agonists. The suppressive capacity was eliminated when QS-ON GAS was killed, separated from macrophages by a membrane, or treated with a QS-inhibitor. Examination of isogenic mutants of Rgg2/3-regulated genes revealed the requirement of a putative biosynthetic gene cluster (BGC) for QS-mediated immunosuppression. A closer examination of NFkappaB signaling components revealed no defect in IKK phosphorylation, IkappaB degradation, or NFkappaB p65 nuclear translocation, but NFkappaB activity and gene expression and was suppressed. These data support a model whereby live QS-ON GAS contacts macrophages and presents or delivers a BGC-produced signal that interferes with late steps in pro-inflammatory signaling pathways. The effect of QS-mediated immunosuppression was also examined in vivo using a mouse intraperitoneal model; however, this model revealed only subtle effects on host responses. Future experiments are critical to better understanding the potential physiological relevance of this QS-mediated immunosuppressive property of GAS. A deeper mechanistic understanding that elucidates host targets and bacterial effectors could provide a compelling rationale to target the GAS Rgg2/3 pathway as a new approach to treat infection.