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. 2016 Jul 13;20(1):49-59.
doi: 10.1016/j.chom.2016.06.003.

Group B Streptococcus Degrades Cyclic-di-AMP to Modulate STING-Dependent Type I Interferon Production

Warrison A Andrade  1 , Arnaud Firon  2 , Tobias Schmidt  3 , Veit Hornung  3 , Katherine A Fitzgerald  1 , Evelyn A Kurt-Jones  1 , Patrick Trieu-Cuot  4 , Douglas T Golenbock  5 , Pierre-Alexandre Kaminski  2
Affiliations

Group B Streptococcus Degrades Cyclic-di-AMP to Modulate STING-Dependent Type I Interferon Production

Warrison A Andrade et al. Cell Host Microbe. .

Abstract

Induction of type I interferon (IFN) in response to microbial pathogens depends on a conserved cGAS-STING signaling pathway. The presence of DNA in the cytoplasm activates cGAS, while STING is activated by cyclic dinucleotides (cdNs) produced by cGAS or from bacterial origins. Here, we show that Group B Streptococcus (GBS) induces IFN-β production almost exclusively through cGAS-STING-dependent recognition of bacterial DNA. However, we find that GBS expresses an ectonucleotidase, CdnP, which hydrolyzes extracellular bacterial cyclic-di-AMP. Inactivation of CdnP leads to c-di-AMP accumulation outside the bacteria and increased IFN-β production. Higher IFN-β levels in vivo increase GBS killing by the host. The IFN-β overproduction observed in the absence of CdnP is due to the cumulative effect of DNA sensing by cGAS and STING-dependent sensing of c-di-AMP. These findings describe the importance of a bacterial c-di-AMP ectonucleotidase and suggest a direct bacterial mechanism that dampens activation of the cGAS-STING axis.

Keywords: Streptococcus agalactiae; c-di-AMP; cGAS; ectonucleotidase; interferon-β.

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Figures

Figure 1
Figure 1. cGAS is the main mediator of IFN-β production in response to WT GBS
(A–C) Quantification of IFN-β induction by qRT-PCR in human THP-1 cells and cGAS−/− (A), IFI16−/− (B), and STING −/− (C) inactivated cell lines following 6 hrs of infection with GBS WT or the isogenic ΔcylE mutant (MOI-6). Transfection with DNA (3 μg/ml) was added as a control. (D–E) Quantification of IP-10 production in THP-1 cGAS−/− (D) and STING −/− (E) by ELISA after 18 hrs of infection. (F–G) Quantification of IFN-β induction using WT, cGAS−/− and STING−/− BMDMs at the mRNA level after 6 hrs of infection (F) or at the protein level after 18 hrs of infection (G) with GBS WT and ΔcylE mutant (MOI-6). An additional LPS control (100 ng/ml), independent of the cGAS-STING axis, was added. (H–I) Same experiments as in (F–G) to quantify levels of TNF-α mRNA (H) and protein (I). Data are represented as mean ± SD of at least three independent experiments. Asterisks indicate statistically significant differences (*,p < 0.05; **, p < 0.01 and ***, p < 0.001). NS, not significant. See also Figure S1.
Figure 2
Figure 2. CdnP degrades extracellular c-di-AMP
(A) GBS synthesizes only c-di-AMP. The intracellular concentration of cyclic di-nucleotides was quantified by LC-MS/MS in total WT GBS extracts. (B) GBS degrades extracellular c-di-AMP. At t = 0, 100 μM c-di-AMP was added to early stationary phase cultures of GBS previously washed and resuspend in Tris-HCl (pH 7.5) containing 5 mM MnCl2. Kinetics of extracellular c-di-AMP degradation (mean and S.D. from at least three independent bacterial cultures) were followed by RR-HPLC for the WT (black dots), the ΔnudP (blue triangles), and the ΔcdnP* (red squares) ectonucleotidase mutants. (C) Extracellular c-di-AMP is degraded into adenosine by the sequential activity of the NudP and CdnP ectonucleotidases. Same experiments as in (B) monitoring the formation of the reaction products at the end of the experiment (t = 100 min) for the WT (black), the ΔnudP (blue), and the ΔcdnP* (red) ectonucleotidase mutants. (D) Schematic representations of NudP and CdnP. The archetypal metallophosphatase (Metallophos, pfam domain 00149) and substrate-binding (5-Nucleotid_C, pfam domain 02872) domains of ectonucleotidases are colored (blue for NudP, red for CdnP). Numbers indicate the amino acid positions of the domains in the proteins. Transmembrane domains are indicated as grey boxes and are necessary for secretion (SP: signal peptide) and cell wall anchoring through the LPxTG motif (black rhombus). The black asterisk indicates the position of the conserved histidine residue at position 199 of CdnP essential for metallophosphodiesterase activity. The corresponding mutation to alanine in the catalytically inactive (cat−) mutant CdnP* is shown. (E) CdnP activity limits extracellular c-di-AMP accumulation by GBS. Extracellular c-di-AMP in the GBS culture supernatants was quantified by an enzyme-linked assay followed by RR-HPLC. The two control GBS strains (black: WT and white: WTbk) are compared in parallel with the ΔcdnP* mutant (red). Mean and S.D. in arbitrary units (AU) are calculated from at least three independent experiments. See also Figure S2.
Figure 3
Figure 3. The ectonucleotidase CdnP is a manganese-dependent c-di-AMP phosphodiesterase
(A) Analytical ultracentrifugation analysis of rCdnP. The purified recombinant CdnP protein (residues 29–768) is a monomer with an elongated shape. Sedimentation coefficients are expressed in Svedberg units where 1 S =10−13 S. (B and C) Influence of the pH and cations on the activity of CdnP. Quantification of inorganic phosphate release (Pi) was done with Biomol Green reagents after incubation of 3 nM rCdnP with 2 mM of 2′,3′ cUMP in the presence of 2 mM Mn2+ (B) or different cations at pH 7.5 (C). Samples were taken every 10 min and activities are expressed as μmol of Pi per min per mg of proteins. (D) CdnP degrades c-di-AMP into AMP. rCdnP was incubated with c-di-AMP at 37°C. Kinetics of substrate degradation and product formation was followed by RR-HPLC. Representative chromatograms in arbitrary units (mAU) are shown. (E) Same experiment as in (D) with the quantification of AMP formation by the rCdnP protein and the inactive rCdnP* mutant with the H199A substitution. See also Figure S3.
Figure 4
Figure 4. Increased type I interferon response by GBS ΔcdnP* mutant is dependent on STING
(A) THP-1 cells were infected with GBS WT, WTbk or ΔcdnP* mutant for 4 hrs at different MOIs and IFN-β mRNA levels were quantified by qRT-PCR and normalized to GAPDH levels. Uninfected cells (media) served as controls. (B) Quantification of IFN-β mRNA in WT and cGAS−/− THP-1 cells after 4 hrs of infection (MOI-6) with GBS WT, WTbk, or ΔcdnP* mutant or after transfection with c-di-AMP (100 nM). (C) Quantification of IP-10 by ELISA in WT and cGAS−/− THP-1 cells supernatants after 18 hrs of infection. (D) Same experiment as in (B) with STING−/− THP-1 cells. (E) Same experiment as in (C) with STING−/− THP-1 cells. (F) Same experiment as in (B) with WT, cGAS−/− and STING−/− BMDMs. (G) Same experiment as in (F) with the quantification of mouse IFN-β production by ELISA after 18 hrs of infection. All data are represented as mean ± SD of three experiments and asterisks indicate statistically significant differences (*, p < 0.05; **, p < 0.01 and ***, p < 0.001). ND, not detected. See also Figure S2 and S4.
Figure 5
Figure 5. GBS ΔcdnP* mutant induces higher type I interferon response in vivo in both WT and cGAS−/−, but not in STING−/− mice
(A–C) WT, cGAS−/−, and STING−/− mice (n = 10 for each genotype) were infected by intravenous injection with 1.5 × 107 CFUs of GBS WT, WTbk, or ΔcdnP* mutant. Mice were bled 12 and 24 post-infection. Serum IFN-β (A) and TNF-α (B) levels were measured by ELISA, and blood bacteremia was evaluated by plating serial dilutions (C). (D) Same experimental condition as in (A–C) evaluating spleen bacteremia after 24 of infection (n = 8 for each time point). Data are presented as mean ± SD of one of the two independent experiments that yielded similar results. Asterisks indicate that differences are statistically significant (*,p < 0.05; **, p < 0.01 and ***, p < 0.001).
Figure 6
Figure 6. Model of cGAS/STING activation by WT GBS and ΔcdnP* mutant
Top panel: WT GBS induces type I IFN by a cGAS-dependent pathway following bacterial DNA sensing, synthesis of 2′3′ cGAMP, and activation of the STING/TBK1/IRF3 axis. The c-di-AMP released by GBS is degraded by the CdnP cell wall-anchored ectonucleotidase into AMP, which is further degraded by the second ectonucleotidase NudP into adenosine (Ado). Bottom panel: in the absence of the CdnP ectonucleotidase (GBS ΔcdnP* mutant), c-di-AMP accumulation outside the bacteria activates STING independently of cGAS, and, together with GBS DNA sensing by cGAS, leads to type I IFN over-production.
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