Toll-like receptor 4 confers inflammatory response to Suilysin

ARF6 promotes Streptococcus suis suilysin induced apoptosis in HBMECs

Streptococcus suis (S. suis) is a significant zoonotic microorganism that causes a severe illness in both pigs and humans and is characterized by severe meningitis and septicemia. Suilysin (SLY), which is secreted by S. suis, plays a crucial role as a virulence factor in the disease. To date, the interaction between SLY and host cells is not fully understood. In this study, we identified the interacting proteins between SLY and human brain microvascular endothelial cells (HBMECs) using the TurboID-mediated proximity labeling method. 251 unique proteins were identified in TurboID-SLY treated group, of which six plasma membrane proteins including ARF6, GRK6, EPB41L5, DSC1, TJP2, and PNN were identified. We found that the proteins capable of interacting with SLY are ARF6 and PNN. Subsequent investigations revealed that ARF6 substantially increased the invasive ability of S. suis in HBMECs. Furthermore, ARF6 promoted SLY-induced the activation of p38 MAPK signaling pathway in HBMECs. Moreover, ARF6 promoted the apoptosis in HBMECs through the activation of p38 MAPK signaling pathway induced by SLY. Finally, we confirmed that ARF6 could increase the virulence of SLY in C57BL/6 mice. These findings offer valuable insights that contribute to a deeper understanding of the pathogenic mechanism of SLY.

Dihydroartemisinin ameliorates innate inflammatory response induced by Streptococcussuis-derived muramidase-released protein via inactivation of TLR4-dependent NF-κB signaling

Muramidase-released protein (MRP) is now being recognized as a critical indicator of the virulence and pathogenicity of Streptococcus suis (S. suis). However, the identification of viable therapeutics for S. suis infection was hindered by the absence of an explicit mechanism for MRP-actuated inflammation. Dihydroartemisinin (DhA) is an artemisinin derivative with potential anti-inflammatory activity. The modulatory effect of DhA on the inflammatory response mediated by the virulence factor MRP remains obscure. This research aimed to identify the signaling mechanism by which MRP triggers the innate immune response in mouse spleen and cultured macrophages. With the candidate mechanism in mind, we investigated DhA for its ability to dampen the pro-inflammatory response induced by MRP. The innate immune response in mice was drastically triggered by MRP, manifesting as splenic and systemic inflammation with splenomegaly, immune cell infiltration, and an elevation in pro-inflammatory cytokines. A crucial role for Toll-like receptor 4 (TLR4) in coordinating the MRP-mediated inflammatory response via nuclear factor-kappa B (NF-κB) activation was revealed by TLR4 blockade. In addition, NF-κB-dependent transducer and activator of transcription 3 (STAT3) and mitogen-activated protein kinases (MAPKs) activation was required for the inflammatory signal transduction engendered by MRP. Intriguingly, we observed an alleviation effect of DhA on the MRP-induced immune response, which referred to the suppression of TLR4-mediated actuation of NF-κB-STAT3/MAPK cascades. The inflammatory response elicited by MRP is relevant to TLR4-dependent NF-κB activation, followed by an increase in the activity of STAT3 or MAPKs. DhA mitigates the inflammation process induced by MRP via blocking the TLR4 cascade, highlighting the therapeutic potential of DhA in targeting S. suis infection diseases.

Cholesterol dependent cytolysins and the brain: Revealing a potential therapeutic avenue for bacterial meningitis

Bacterial meningitis is a catastrophic nervous system disorder with high mortality and wide range of morbidities. Some of the meningitis-causing bacteria occupy cholesterol dependent cytolysins (CDCs) to increase their pathogenicity and arrange immune-evasion strategy. Studies have observed that the relationship between CDCs and pathogenicity in these meningitides is complex and involves interactions between CDC, blood-brain barrier (BBB), glial cells and neurons. In BBB, these CDCs acts on capillary endothelium, tight junction (TJ) proteins and neurovascular unit (NVU). CDCs also observed to elicit intriguing effects on brain inflammation which involves microglia and astrocyte activations, along with neuronal damage as the end-point of pathological pathways in bacterial meningitis. As some studies mentioned potential advantage of CDC-targeted therapeutic mechanisms to combat CNS infections, it might be a fruitful avenue to deepen our understanding of CDC as a candidate for adjuvant therapy to combat bacterial meningitis.

Recombinant suilysin of Streptococcus suis enhances the protective efficacy of an engineered Pasteurella multocida toxin protein

Suilysin (Sly) from Streptococcus suis has been shown to elicit strong immune responses and may act as a vaccine adjuvant. In the present study, we tested the adjuvant effect of Sly using an engineered Pasteurella multocida toxin, rPMT-NC, as the antigen. The antigen was also formulated with other conventional adjuvants (aluminum hydroxide, water-in-oil-in-water) for comparison. The efficacy of these vaccine formulations were evaluated in mice. The optimal dosage of purified rSly for enhancing immune responses in mice was first determined to be 40 μg/ml based on significantly (p < 0.05) increased serum antibody titers, expression of cytokines, including interleukin (IL)-4, IL-12, and interferon (IFN)-γ and the survival rate after challenge with P. multocida. Mice immunized with rPMT-NC + rSly had augmented antibody production and cellular immunity compare to those immunized with rPMT-NC plus other adjuvants. In addition, the survival rate of mice immunized with rPMT-NC + rSly was the highest (70% v.s. 30% of mice immunized with rPMT-NC alone) among all groups. In conclusion, rSly has the potential to be used as a biological adjuvant to enhance immune responses and protective efficacy of protein-based vaccines.

Suilyin Disrupts the Blood-Brain Barrier by Activating Group III Secretory Phospholipase A2

Serious diseases caused by Streptococcus suis serotype 2 (S. suis 2) include septicaemia and meningitis, which are associated with high morbidity and mortality. Proliferation in the blood can result in a breach of the blood-brain barrier (BBB) and provide entry into the cerebrospinal fluid (CSF), where bacteria cause inflammation of the meningeal membranes resulting in meningitis. The molecular mechanisms of how this pathogen crosses the BBB remain unclear. Suilysin (SLY) has been identified as an important secreted virulence factor of S. suis 2 and may play a vital role in provoking meningitis. In this investigation, we demonstrate that SLY can increase the paracellular permeability of BBB, both in vivo and in vitro, via the activation of group III secretory phospholipase A2 (PLA2G3). Our results indicate that at lower, sublytic concentrations, the toxin can stimulate cerebral microvascular endothelial cells to release TNF-α, thereby inducing high level expressions of PLA2G3. Abnormal elevations of PLA2G3 might further injure tissues through direct cytolytic effectors or other responses.

ATP Facilitates Staphylococcal Enterotoxin O Induced Neutrophil IL-1β Secretion via NLRP3 Inflammasome Dependent Pathways

Staphylococcus aureus (S. aureus) is an important zoonotic food-borne pathogen causing severe invasive infections, such as sepsis, pneumonia, food poisoning, toxic shock syndrome and autoimmune diseases. Staphylococcal enterotoxin O (SEO) is a new type of enterotoxins of S. aureus with superantigenic and emetic activity. However, it is still unclear about SEO-induced host inflammatory response. Therefore, the mechanism of SEO-induced interleukin-1β (IL-1β) secretion in mouse neutrophils was investigated in this study. Our results showed that recombinant SEO had superantigenic activity with high level of gamma interferon (IFN-γ) production in mouse spleen cells and induced inflammatory cytokines expression including IL-1α, IL-1β, IL-6 and TNF-α in neutrophils under the action of ATP. In addition, SEO-induced IL-1β secretion was dependent on activation of Toll like receptor 4 (TLR4), nuclear factor kappa B (NF-κB) and c-jun N-terminal kinase (JNK) signaling pathways. However, SEO-induced IL-1β secretion was abolished in the neutrophils of NLRP3^-/- mice compared with those of wild type mice, indicating that activation of NLRP3 inflammasome mediated IL-1β secretion during neutrophils stimulation with SEO under the action of ATP. Moreover, this process of SEO+ATP-induced IL-1β secretion was dependent on potassium (K⁺) efflux. Taken together, our study suggests that activation of TLR4/JNK/NLRP3 inflammasome signaling pathway mediate maturation and secretion of IL-1β and provides a new insight on S. aureus virulence factor-induced host immune response.

Acquiring high expression of suilysin enable non-epidemic Streptococccus suis to cause streptococcal toxic shock-like syndrome (STSLS) through NLRP3 inflammasome hyperactivation

The epidemic Streptococcus suis (S. suis) strain [Sequence type (ST) 7] was gradually evolving from the non-epidemic ST1 strain and got the ability for high expressing of suilysin (SLY). And the high expression of SLY was required for the epidemic strain to cause NLRP3 hyperactivation, which is essential for the induction of cytokines storm, dysfunction of multiple organs, and a high incidence of mortality, the characters of streptococcal toxic shock-like syndrome (STSLS). However, it remains to be elucidated whether acquiring high SLY expression due to genome evolution was sufficient for the non-epidemic strain to cause STSLS. Here, we found that the overexpression of SLY in ST1 strain (P1/7-SLY) could obviously increase the inflammasome activation, which was dependent on NLRP3 signalling. In contrast, the strain (P1/7-mSLY) overexpressing the mutant SLY (protein without hemolytic activity) could not significantly increase the inflammasome activation. Furthermore, similar to the epidemic strain, P1/7-SLY could cause STSLS in nlrp3^+/+ mice but not in nlrp3^−/− mice. In contrast, P1/7-mSLY could not cause STSLS in both nlrp3^+/+ mice and nlrp3^−/− mice. In summary, we demonstrate that genetic evolution enabling S. suis strain to express high level of SLY may be an essential and sufficient condition for NLRP3 inflammasome hyperactivation, which could further cause cytokines storm and STSLS.

Mode of action and structural modelling of the interaction of formononetin with suilysin

AIMS

Suilysin is a critical pore-forming virulence factor of Streptococcus suis that has been demonstrated to substantially contribute to its pathogenicity. We have demonstrated that formononetin alleviates S. suis infection both in vivo and in vitro by targeting suilysin. However, the molecular mechanism of the effect is unclear. Our aim was to determine the molecular mechanism of the effect of formononetin on suilysin.

METHODS AND RESULTS

The mechanism of interaction between formononetin and suilysin was investigated by molecular modelling. The results indicated that formononetin was bound at the junction of domain two and domain four of suilysin. The binding free energy values indicated that the A415, Y412, E414, N413, T61, T62 and G416 residues are critical for this binding, this observation was confirmed by the changes in the flexibility of these residues and the distances between these residues and formononetin. The inhibitory effect of formononetin on the pore-forming activity of suilysin, binding constant and binding free energy were significantly decreased by site-specific mutagenesis of Y412 and N413. Finally, we analysed the spatial configuration of suilysin before and after formononetin binding, the results indicated that the binding changed the conformation of suilysin, especially the angle between domain two and domain four, resulting in the disruption of cholesterol binding to suilysin and in the loss of pore-forming activity.

CONCLUSIONS

Formononetin is located at the junction of domain two and domain four of suilysin, and Y412 and N413 play critical roles in the binding. Formononetin binding changes the angle between domain two and domain four of suilysin, resulting in the loss of the pore-inducing activity of suilysin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work will promote the application of formononetin to combat S. suis infections and may contribute to the development of new inhibitors or modification of existing inhibitors.

Inhibitory Effect of Piceatannol on Streptococcus suis Infection Both in vitro and in vivo

Suilysin (SLY) plays a critical role in Streptococcus suis infections making it an ideal target to the combat infection caused by this pathogen. In the present study, we found that piceatannol (PN), a natural compound, inhibits pore-formation by blocking the oligomerization of SLY without affecting the growth of S. suis and the expression of SLY. Furthermore, PN alleviated the J774 cell damage and the expression of the inflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-1α (IL-1β) induced by S. suis in vitro. The computational biology and biochemistry results indicated that PN binds to the joint region of D2 and D4 in SLY, and Asn57, Pro58, Pro59, Glu76, Ile379, Glu380, and Glu418 were critical residues involved in the binding. The binding effect between PN and SLY hindered the SLY monomers from forming the oligomers, thereby weakening the hemolytic activity of SLY. This mechanism was also verified by hemolysis analysis and analysis of K_A formation after site-specific mutagenesis. Furthermore, PN protected mice from S. suis infections by reducing bacterial colony formation and the inflammatory response in target organs in vivo. These results indicate that PN is a feasible drug candidate to combat S. suis infections.

In vitro characterization of granulocyte-colony stimulating factor (G-CSF) production by dendritic cells and macrophages during Streptococcus suis infection

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and emerging zoonotic agent. Infections induce an exacerbated inflammation that can result in sudden death (septic shock) and meningitis. Though neutrophilic leukocytosis characterizes S. suis infection, the mediators involved are poorly understood. Among them, granulocyte-colony stimulating factor (G-CSF), a pro-inflammatory cytokine, triggers proliferation of neutrophil progenitors and neutrophil mobilization. However, the systemic production of G-CSF induced during S. suis infection, the cell types involved, and the underlying mechanisms remain unknown. In a S. suis serotype 2 mouse model of systemic infection, plasma levels of G-CSF rapidly increased after infection. S. suis activation of DCs and macrophages resulted in high (> 1000 pg/mL) and comparable production levels of G-CSF, as measured by ELISA. By using mutant strains deficient in capsular polysaccharide (CPS) or lipoprotein maturation in combination with purified lipoteichoic acid (LTA) from the latter mutant strain, it was showed that G-CSF production is mainly mediated by S. suis lipoproteins. The Toll-like receptor (TLR) pathway via myeloid differentiation primary response 88 (MyD88) is required for G-CSF production by DCs and macrophages following S. suis activation, with a partial involvement of TLR2. On the other hand, TLR2-independant G-CSF production induced by S. suis requires internalization and bacterial DNA might play a role in this pathway. Finally, these signals activated nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways leading to G-CSF production. In conclusion, this study demonstrated for the first time that S. suis induces G-CSF production in vivo and DCs and macrophages are key cellular sources of this cytokine mediator, mainly via the binding of lipoproteins to TLR2. The CPS significantly reduced this activation, confirming the powerful role of this component in S. suis virulence. As such, this study contributes to better understand how DCs and macrophages produce G-CSF in response to S. suis, and potentially to other streptococci.

Differential role of MyD88 signaling in Streptococcus suis serotype 2-induced systemic and central nervous system diseases

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and a zoonotic agent responsible for sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the systemic and central nervous system (CNS) infections. However, S. suis serotype 2 strains are genetically and phenotypically heterogeneous, being composed of a multitude of sequence types (STs) whose virulence greatly varies. Yet, most studies have used 'classical' virulent Eurasian ST1 or ST7 strains, even though ST25 and ST28 strains account for most isolates in North America. While recognition of S. suis by innate immune cells has been associated with the myeloid differentiation primary response 88 (MyD88)-dependent Toll-like receptor (TLR) pathway in vitro, particularly surface-associated TLR2, little information is available regarding its role in vivo. This study demonstrates for the first time a differential role of MyD88 signaling in S. suis-induced systemic and CNS diseases, regardless of strain background diversity. The MyD88-dependent pathway is critical for the development of systemic disease via its role in inflammation, which subsequently controls bacterial burden. However, and differently from what has been described in vitro, TLR2 and TLR4 individually do not contribute to systemic disease, suggesting possible compensation in their absence and/or a collaborative role with other MyD88-dependent TLRs. On the other hand, CNS disease does not necessarily require MyD88 signaling and, consequently, neither TLR2 nor TLR4, suggesting a partial implication of other pathways. Finally, regardless of its notable heterogeneity, recognition of S. suis serotype 2 appears to be similar, indicating that recognized components are conserved motifs.

TRIM32 Drives Pathogenesis in Streptococcal Toxic Shock-Like Syndrome and Streptococcus suis Meningitis by Regulating Innate Immune Responses

Streptococcus suis is an emerging zoonotic agent that causes streptococcal toxic shock-like syndrome (STSLS) and meningitis in humans, with high mortality and morbidity. The pathogenesis of both STSLS and central nervous system (CNS) infections caused by S. suis is not well understood. TRIM32, a member of the tripartite motif (TRIM) protein family, has been reported to regulate host inflammatory responses. In this study, we showed that TRIM32 deficiency significantly reduced the level of bacteremia and the production of proinflammatory cytokines following severe S. suis infection, protecting infected mice from STSLS. The influence of TRIM32 gene deletion on a range of processes known to be involved in S. suis meningitis was also examined. Both levels of bacterial loads and indications of brain hemorrhage were reduced in infected Trim32^{- / -} mice compared with infected wild-type (WT) controls. We also found that TRIM32 deficiency increased the permeability of the blood-brain barrier (BBB) and the recruitment of inflammatory monocytes during the early course of S. suis infection, potentially limiting the development of S. suis meningitis. Our results suggest that TRIM32 sensitizes S. suis-induced infection via innate immune response regulation.

Coinfection with porcine circovirus type 2 and Streptococcus suis serotype 2 enhances pathogenicity by dysregulation of the immune responses in piglets

Porcine circovirus type 2 (PCV-2) and Streptococcus suis (S. suis) are common pathogens in pigs. Both pathogens are associated with the porcine respiratory disease complex. Clinically, coinfection of PCV-2 and S. suis are often detected in pigs with respiratory symptoms, while interactions between the two pathogens during coinfection and the coinfection pathogenesis are poorly understood. In this study, a piglet model coinfected with PCV-2 and Streptococcus suis serotype 2 (SS2) was established; coinfection of piglets increased the contents of SS2 in blood, and piglets showed more severe pneumonia, myocarditis and arthritis. Peripheral blood mononuclear cells (PBMCs) were collected and coinfected piglets showed high expression levels of inflammatory cytokines and TLR2, TLR4, while levels of CD4, CD8 and MHC II were reduced. In addition, in order to further explore the mechanisms of coinfection induced cytokine overexpression, an in vitro model of coinfection with PCV-2 and SS2 was established using cells of the porcine monocytic line 3D4/21. Similar to the in vivo results,coinfected cells exhibited increased expression of the cytokines IL-6, IL-8, TNF-α and the receptors TLR2, TLR4, while they showed a lower expression of MHC II than cells infected with SS2 alone. Furthermore, in coinfected 3D4/21 cells, both MAPK and NF-κB signaling pathways were activated, and the increased expression of IL-8 was related to TLR4. In general, coinfection with PCV-2 and SS2 exacerbated the inflammatory response and probably impaired macrophage antigen presentation, resulting in immune dysregulation and increasing the severity of host infection.

Complement C3aR/C5aR-binding protein Suilysin of Streptococcus suis contributes to monocyte chemotaxis

Streptococcus suis is an emerging swine and human pathogen causing severe infections and sudden death. During infection, complement C3a and C5a were reported to induce immune cells towards infection and injury sites via their corresponding receptors C3aR and C5aR. However, how S. suis evade immune surveillance mediated by C3aR and C5aR remains unclear. In this study, we analyze and construct an S. suis bacterial two-hybrid prey library containing 39 LPXTG motif anchored proteins and 18 secreted proteins. Two highly possible C3aR-binding proteins: thiol-activated toxin Suilysin, putative RTX family exoprotein A gene and three highly possible C5aR-binding proteins: thiol-activated toxin Suilysin, putative 5'-nucleotidase and subtilisin-like serine protease are identified through bacterial two-hybrid assay. Far-western blot assay confirms that a cholesterol-binding cytolysin Suilysin can interact with both C3aR and C5aR. Chemotaxis assays demonstrate that recombinant and natural Suilysin can inhibit monocyte chemotaxis mediated by C3a and C5a. These findings enlarge our knowledge of suilysin biological significance and provide a new perspective on S. suis complement evasion.

The Impact of SsPI-1 Deletion on Streptococcus suis Virulence

(1) Background: Streptococcus suis is an important zoonotic pathogen that infects pigs and can occasionally cause life-threatening systemic infections in humans. Two large-scale outbreaks of streptococcal toxic shock-like syndrome in China suggest that the pathogenicity of S. suis has been changing in recent years. Genetic analysis revealed the presence of a chromosomal pathogenicity island (PAI) designated SsPI-1 in Chinese epidemic S. suis strains. The purpose of this study is to define the role of SsPI-1 in the virulence of S. suis. (2) Methods: A SsPI-1 deletion mutant was compared to the wild-type strain regarding the ability to attach to epithelial cells, to cause host disease and mortality, and to stimulate host immune response in experimental infection of piglets. (3) Results: Deletion of SsPI-1 significantly reduces adherence of S. suis to epithelial cells and abolishes the lethality of the wild-type strain in piglets. The SsPI-1 mutant causes no significant pathological lesions and exhibits an impaired ability to induce proinflammatory cytokine production. (4) Conclusions: Deletion of the SsPI-1 PAI attenuates the virulence of this pathogen. We conclude that SsPI-1 is a critical contributor to the evolution of virulence in epidemic S. suis.

Interactions of Streptococcus suis serotype 9 with host cells and role of the capsular polysaccharide: Comparison with serotypes 2 and 14

Streptococcus suis is an important porcine bacterial pathogen and a zoonotic agent responsible for sudden death, septic shock and meningitis, of which serotype 2 is the most widespread, with serotype 14 also causing infections in humans in South-East Asia. Knowledge of its pathogenesis and virulence are almost exclusively based on these two serotypes. Though serotype 9 is responsible for the greatest number of porcine cases in Spain, the Netherlands and Germany, very little information is currently available regarding this serotype. Of the different virulence factors, the capsular polysaccharide (CPS) is required for S. suis virulence as it promotes resistance to phagocytosis and killing and masks surface components responsible for host cell activation. However, these roles have been described for serotypes 2 and 14, whose CPSs are structurally and compositionally similar, both containing sialic acid. Consequently, we evaluated herein the interactions of serotype 9 with host cells and the role of its CPS, which greatly differs from those of serotypes 2 and 14. Results demonstrated that serotype 9 adhesion to but not invasion of respiratory epithelial cells was greater than that of serotypes 2 and 14. Furthermore serotype 9 was more internalized by macrophages but equally resistant to whole blood killing. Though recognition of serotypes 2, 9 and 14 by DCs required MyD88-dependent signaling, in vitro pro-inflammatory mediator production induced by serotype 9 was much lower. In vivo, however, serotype 9 causes an exacerbated inflammatory response, which combined with persistent bacterial presence, is probably responsible for host death during the systemic infection. Though presence of the serotype 9 CPS masks surface components less efficiently than those of serotypes 2 and 14, the serotype 9 CPS remains critical for virulence as it is required for survival in blood and development of clinical disease, and this regardless of its unique composition and structure.

Interleukin-1 signaling induced by Streptococcus suis serotype 2 is strain-dependent and contributes to bacterial clearance and inflammation during systemic disease in a mouse model of infection

Streptococcus suis serotype 2 is an important porcine pathogen and zoonotic agent causing sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the infection. A rapid, effective and balanced innate immune response against S. suis is critical to control bacterial growth without causing excessive inflammation. Even though interleukin (IL)-1 is one of the most potent and earliest pro-inflammatory mediators produced, its role in the S. suis pathogenesis has not been studied. We demonstrated that a classical virulent European sequence type (ST) 1 strain and the highly virulent ST7 strain induce important levels of IL-1 in systemic organs. Moreover, bone marrow-derived dendritic cells and macrophages contribute to its production, with the ST7 strain inducing higher levels. To better understand the underlying mechanisms involved, different cellular pathways were studied. Independently of the strain, IL-1β production required MyD88 and involved recognition via TLR2 and possibly TLR7 and TLR9. This suggests that the recognized bacterial components are similar and conserved between strains. However, very high levels of the pore-forming toxin suilysin, produced only by the ST7 strain, are required for efficient maturation of pro-IL-1β via activation of different inflammasomes resulting from pore formation and ion efflux. Using IL-1R^−/− mice, we demonstrated that IL-1 signaling plays a beneficial role during S. suis systemic infection by modulating the inflammation required to control and clear bacterial burden, thus promoting host survival. Beyond a certain threshold, however, S. suis-induced inflammation cannot be counterbalanced by this signaling, making it difficult to discriminate its role.

An NLRP3 inflammasome-triggered cytokine storm contributes to Streptococcal toxic shock-like syndrome (STSLS)

Infection with the Streptococcus suis (S. suis) epidemic strain can cause Streptococcal toxic shock-like syndrome (STSLS), which is characterized by a cytokine storm, dysfunction of multiple organs and a high incidence of mortality despite adequate treatment. Despite some progress concerning the contribution of the inflammatory response to STSLS, the precise mechanism underlying STSLS development remains elusive. Here, we use a murine model to demonstrate that caspase-1 activity is critical for STSLS development. Furthermore, we show that inflammasome activation by S. suis is mainly dependent on NLRP3 but not on NLRP1, AIM2 or NLRC4. The important role of NLRP3 activation in STSLS is further confirmed in vivo with the NLRP3 inhibitor MCC950 and nlrp3-knockout mice. By comparison of WT strain with isogenic strains with mutation of various virulence genes for inflammasome activation, Suilysin is essential for inflammasome activation, which is dependent on the membrane perforation activity to cause cytosolic K⁺ efflux. Moreover, the mutant strain msly (P353L) expressing mutagenic SLY without hemolytic activity was unable to activate the inflammasome and does not cause STSLS. In summary, we demonstrate that the high membrane perforation activity of the epidemic strain induces a high level of NLRP3 inflammasome activation, which is essential for the development of the cytokine storm and multi-organ dysfunction in STSLS and suggests NLRP3 inflammasome as an attractive target for the treatment of STSLS.

The two large-scale human Streptococcus suis epidemics have caused unusual development of Streptococcal Toxic-Shock-like Syndrome (STSLS) and high incidence of mortality despite adequate treatments. However, how the epidemic strain causes STSLS remained to be elucidated. Because an excessive high level of inflammasome-regulated cytokine was detected in the blood of STSLS patients, we used a murine model to identify the role of inflammasome activation on the development of STSLS. We found that NLRP3 activation contributed to STSLS with the pharmacological inhibition and NLRP3^-/- mice. We identified a novel mechanism of STSLS in that increased suilysin expression in S. suis highly virulent strain could induce high level of cytosolic K⁺ efflux, an essential event for NLRP3 inflammasome activation, and then further cause a cytokine storm, dysfunction of multiple organs and a high incidence of mortality, the characters of STSLS. Therefore, our study provides insights for STSLS development and highlights NLRP3 inflammasome as an attractive target for the treatment of STSLS.

Antigen I/II Participates in the Interactions of Streptococcus suis Serotype 9 With Phagocytes and the Development of Systemic Disease

Streptococcus suis is an important porcine bacterial pathogen and a zoonotic agent causing a variety of pathologies including sudden death, septic shock, and meningitis. Though serotype 2 is the most studied serotype due to its presence worldwide, serotype 9 is responsible for the greatest number of porcine cases in Spain, the Netherlands, and Germany. Regardless of its increasing importance, very few studies have investigated S. suis serotype 9 virulence factors and pathogenesis. Antigens I/II (AgI/II) are multimodal adhesion proteins implicated in host respiratory tract and oral cavity persistence of various pathogenic human streptococci. It was recently demonstrated that AgI/II is involved in various bacterial functions for serotype 9, participating in the initial steps of the pathogenesis of the infection. However, its contribution to the systemic infection remains unknown. As such, we evaluated herein the role of the S. suis serotype 9 AgI/II in the interactions with phagocytes and the development of systemic disease in a mouse model of infection. Results demonstrated that the presence of AgI/II is important for the development of clinical systemic disease by promoting bacterial survival in blood possibly due to its effect on S. suis phagocytosis, as shown with macrophages and dendritic cells. Furthermore, AgI/II directly participates in dendritic cell activation and pro-inflammatory mediator production following recognition by the Toll-like receptor pathway, which may contribute to the exacerbated systemic inflammation responsible for host death. Taken together, this study demonstrates that the S. suis serotype 9 AgI/II is important for virulence during systemic infection and development of disease. In fact, this is the first study to describe a role of an AgI/II family member in systemic bacterial disease.

Listeriolysin O: A phagosome-specific cytolysin revisited

Listeriolysin O (LLO) is an essential determinant of Listeria monocytogenes pathogenesis that mediates the escape of L. monocytogenes from host cell vacuoles, thereby allowing replication in the cytosol without causing appreciable cell death. As a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins, LLO is unique in that it is secreted by a facultative intracellular pathogen, whereas all other CDCs are produced by pathogens that are largely extracellular. Replacement of LLO with other CDCs results in strains that are extremely cytotoxic and 10,000-fold less virulent in mice. LLO has structural and regulatory features that allow it to function intracellularly without causing cell death, most of which map to a unique N-terminal region of LLO referred to as the proline, glutamic acid, serine, threonine (PEST)-like sequence. Yet, while LLO has unique properties required for its intracellular site of action, extracellular LLO, like other CDCs, affects cells in a myriad of ways. Because all CDCs form pores in cholesterol-containing membranes that lead to rapid Ca²⁺ influx and K⁺ efflux, they consequently trigger a wide range of host cell responses, including mitogen-activated protein kinase activation, histone modification, and caspase-1 activation. There is no debate that extracellular LLO, like all other CDCs, can stimulate multiple cellular activities, but the primary question we wish to address in this perspective is whether these activities contribute to L. monocytogenes pathogenesis.

Triggering Receptor Expressed on Myeloid Cells-1 Signaling: Protective and Pathogenic Roles on Streptococcal Toxic-Shock-Like Syndrome Caused by Streptococcus suis

Streptococcus suis infections can cause septic shock, which is referred to as streptococcal toxic-shock-like syndrome (STSLS). The disease is characterized by a severe inflammatory response, multiple organ failure, and high mortality. However, no superantigen that is responsible for toxic shock syndrome was detected in S. suis, indicating that the mechanism underlying STSLS is different and remains to be elucidated. Triggering receptor expressed on myeloid cells-1 (TREM-1), belonging to the Ig superfamily, is an activating receptor expressed on myeloid cells, and has been recognized as a critical immunomodulator in several inflammatory diseases of both infectious and non-infectious etiologies. In this review, we discuss the current understanding of the immunoregulatory functions of TREM-1 on acute infectious diseases and then highlight the crucial roles of TREM-1 on the development of STSLS.

Type I Interferon Induced by Streptococcus suis Serotype 2 is Strain-Dependent and May Be Beneficial for Host Survival

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and emerging zoonotic agent mainly responsible for sudden death, septic shock, and meningitis, with exacerbated inflammation being a hallmark of the infection. However, serotype 2 strains are genotypically and phenotypically heterogeneous, being composed of a multitude of sequence types (STs) whose virulence greatly varies: the virulent ST1 (Eurasia), highly virulent ST7 (responsible for the human outbreaks in China), and intermediate virulent ST25 (North America) are the most important worldwide. Even though type I interferons (IFNs) are traditionally associated with important antiviral functions, recent studies have demonstrated that they may also play an important role during infections with extracellular bacteria. Upregulation of IFN-β levels was previously observed in mice following infection with this pathogen. Consequently, the implication of IFN-β in the S. suis serotype 2 pathogenesis, which has always been considered a strict extracellular bacterium, was evaluated using strains of varying virulence. This study demonstrates that intermediate virulent strains are significantly more susceptible to phagocytosis than virulent strains. Hence, subsequent localization of these strains within the phagosome results in recognition of bacterial nucleic acids by Toll-like receptors 7 and 9, leading to activation of the interferon regulatory factors 1, 3, and 7 and production of IFN-β. Type I IFN, whose implication depends on the virulence level of the S. suis strain, is involved in host defense by participating in the modulation of systemic inflammation, which is responsible for the clearance of blood bacterial burden. As such, when induced by intermediate, and to a lesser extent, virulent S. suis strains, type I IFN plays a beneficial role in host survival. The highly virulent ST7 strain, however, hastily induces a septic shock that cannot be controlled by type I IFN, leading to rapid death of the host. A better understanding of the underlying mechanisms involved in the control of inflammation and subsequent bacterial burden could help to develop control measures for this important porcine and zoonotic agent.

Interaction of factor H-binding protein of Streptococcus suis with globotriaosylceramide promotes the development of meningitis

Streptococcus suis is an important emerging zoonotic agent that causes acute bacterial meningitis in humans with high mortality and morbidity. Our previous work showed that factor H-binding protein (Fhb) contributed to virulence of S. suis, but the role of Fhb in the development of S. suis meningitis remained unclear. In this study, we demonstrated for the first time that Fhb contributed to the traversal of S. suis across the human blood-brain barrier by allelic-exchange mutagenesis, complementation and specific antibody blocking studies. We also showed that globotriaosylceramide (Gb3), the receptor of Fhb, was involved in this process and affected S. suis infection-induced activation of myosin light chain 2 through Rho/ROCK signaling in hCMEC/D3 cells. Using a murine model of S. suis meningitis, we further demonstrated that Gb3-deficiency prevented the mice from developing severe brain inflammation or injury. Our results demonstrate that the Fhb-Gb3 interaction plays an important role in the development of S. suis meningitis and might be a potential therapeutic target against S. suis infection.

Effects of Suilysin on Streptococcus suis-Induced Platelet Aggregation

Blood platelets play important roles during pathological thrombocytopenia in streptococcal toxic shock syndrome (STSS). Streptococcus suis (S. suis) an emerging human pathogen, can cause STSS similarly to S. pyogenes. However, S. suis interactions with platelets are poorly understood. Here, we found that suilysin (SLY), different from other bacterial cholesterol-dependent cytolysins (CDCs), was the sole stimulus that induced platelet aggregation. Furthermore, the inside-out activation of GPIIb/IIIa of platelets mediated SLY-induced platelet aggregation. This process was triggered by Ca²⁺ influx that depend on the pore forming on platelets by SLY. Additionally, although SLY induced α-granule release occurred via the MLCK-dependent pathway, PLC-β-IP3/DAG-MLCK and Rho-ROCK-MLCK signaling were not involved in SLY-induced platelet aggregation. Interestingly, the pore dependent Ca²⁺ influx was also found to participate in the induction of platelet aggregation with pneumolysin (PLY) and streptolysin O (SLO), two other CDCs. It is possible that the CDC-mediated platelet aggregation we observed in S. suis is a similar response mechanism to that used by a wide range of bacteria. These findings might lead to the discovery of potential therapeutic targets for S. suis-associated STSS.

Suilysin Stimulates the Release of Heparin Binding Protein from Neutrophils and Increases Vascular Permeability in Mice

Most of the deaths that occurred during two large outbreaks of Streptococcus suis infections in 1998 and 2005 in China were caused by streptococcal toxic shock syndrome (STSS), which is characterized by increased vascular permeability. Heparin-binding protein (HBP) is thought to mediate the vascular leakage. The purpose of this study was to investigate the detailed mechanism underlying the release of HBP and the vascular leakage induced by S. suis. Significantly higher serum levels of HBP were detected in Chinese patients with STSS than in patients with meningitis or healthy controls. Suilysin (SLY) is an exotoxin secreted by the highly virulent strain 05ZYH33, and it stimulated the release of HBP from the polymorphonuclear neutrophils and mediated vascular leakage in mice. The release of HBP induced by SLY was caused by a calcium influx-dependent degranulation. Analyses using a pharmacological approach revealed that the release of HBP induced by SLY was related to Toll-like receptor 4, p38 mitogen-activated protein kinase, and the 1-phosphatidylinositol 3-kinase pathway. It was also dependent on a G protein-coupled seven-membrane spanning receptor. The results of this study provide new insights into the vascular leakage in STSS associated with non-Group A streptococci, which could lead to the discovery of potential therapeutic targets for STSS associated with S. suis.