Group A streptococcal M protein activates the NLRP3 inflammasome


Group A Streptococcus (GAS) is among the top ten causes of infection-related mortality in humans. M protein is the most abundant GAS surface protein, and M1 serotype GAS strains are associated with invasive infections, including necrotizing fasciitis and toxic shock syndrome. Here, we report that released, soluble M1 protein triggers programmed cell death in macrophages (Mϕ). M1 served as a second signal for caspase-1-dependent NLRP3 inflammasome activation, inducing maturation and release of proinflammatory cytokine interleukin-1β (IL-1β) and macrophage pyroptosis. The structurally dynamic B-repeat domain of M1 was critical for inflammasome activation, which involved K+ efflux and M1 protein internalization by clathrin-mediated endocytosis. Mouse intraperitoneal challenge showed that soluble M1 was sufficient and specific for IL-1β activation, which may represent an early warning to activate host immunity against the pathogen. Conversely, in systemic infection, hyperinflammation associated with M1-mediated pyroptosis and IL-1β release could aggravate tissue injury.

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Fig. 1: GAS M1 protein promotes cell death in macrophages.
Fig. 2: M1 protein provides a second signal that specifically triggers IL-1β signalling.
Fig. 3: M1 B-repeat region is essential for activation of the NLRP3 inflammasome and pyroptosis in a caspase-1-dependent manner requiring potassium efflux.
Fig. 4: M1 uptake is required for inflammasome activation.
Fig. 5: M1 action as natively expressed on GAS in vitro and in vivo and as soluble protein in vivo.
Fig. 6: Model of M1 protein-mediated NLRP3 inflammasome activation and pyroptosis.


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The authors thank the members of the P.G. and V.N. laboratories for their valuable input. This work was supported by NIH grants AI096837 (P.G. and V.N), AI077780 (V.N.) and AI52430 (H.M.H.) A.M.R. is a San Diego IRACDA Program fellow supported by NIGMS/NIH award K12GM068524, N.J.G. is supported by the UCSD Genetics Training Program (T32 GM008666) and the UCSD Global Health Institute and C.N.L. is a recipient of an A.P. Giannini Foundation Postdoctoral Research Fellowship.

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J.A.V., P.G. and V.N. formulated the original hypothesis, designed the study and analysed the results. J.A.V., A.M.R., N.J.G., C.N.L., N.G. and S.R.A. performed and optimized experiments. H.M.H. provided novel reagents. J.A.V., P.G. and V.N. wrote the manuscript and all authors reviewed the manuscript, data and conclusions before submission.

Correspondence to Partho Ghosh or Victor Nizet.

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