Differences in SpeB protease activity among group A streptococci associated with superficial, invasive, and autoimmune disease

Pentamidine inhibition of streptopain attenuates Streptococcus pyogenes virulence

The obligate human pathogen Group A Streptococcus (GAS; Streptococcus pyogenes) carries high morbidity and mortality, primarily in impoverished or resource-poor regions. The failure rate of monotherapy with conventional antibiotics is high, and invasive infections by this bacterium frequently require extensive supportive care and surgical intervention. Thus, it is important to find new compounds with adjunctive therapeutic benefits. The conserved secreted protease streptopain (Streptococcal pyrogenic exotoxin B; SpeB) directly contributes to disease pathogenesis by inducing pathological inflammation, degrading tissue, and promoting the evasion of antimicrobial host defense proteins. This study screened 400 diverse off-patent drug and drug-like compounds for inhibitors of streptopain proteolysis. Lead compounds were tested for activity at lower concentrations and anti-virulence activities during in vitro infection. Significant inhibition of streptopain was seen for pentamidine, an anti-protozoal drug approved for the treatment of pneumocystis pneumonia, leishmaniasis, and trypanosomiasis. Streptopain inhibition rendered GAS susceptible to killing by human innate immune cells. These studies identify unexploited molecules as new starting points for drug discovery and a potential for repurposing existing drugs for the treatment of infections by GAS.

Proinflammatory synergy between protease and superantigen streptococcal pyogenic exotoxins

Streptococcal pyogenic exotoxins (Spe proteins) secreted by Streptococcus pyogenes (group A Streptococcus, GAS) are responsible for scarlet fever and streptococcal toxic shock syndrome. Most Spes are superantigens that cause excessive inflammation by activating large numbers of T cells. However, Streptococcal pyogenic exotoxin B (SpeB) is an exception, which is pro-inflammatory through its protease activity. Prior work shows that SpeB has the potential to cleave bacterial proteins. If cleavage of superantigens results in their inactivation, this gives the possibility that these two classes of exotoxins work at cross-purposes. We examined SpeB cleavage of the 11 major GAS superantigens and found that lability was not specific to structure, conservation, or, when compared to orthologous superantigens from Staphylococcus aureus, species of origin. We further show that rather than strictly antagonizing superantigen activity through degradation, SpeB can synergistically enhance superantigen-induced inflammation. For SpeB-labile superantigens, such as SmeZ, this is limited due to degradation, but for protease-resistant superantigens like SpeA, activity remains synergistic even at high protease concentrations. These findings suggest two modes by which proteases like SpeB may post-translationally regulate superantigens: positively, as a force amplifier that cooperatively increases inflammation, and negatively, through degradation that could act as a rheostat-like mechanism to limit excessive immune activation. Both mechanisms may contribute to the pathogenesis of GAS and other superantigen-producing pathogens.IMPORTANCEStreptococcus pyogenes produces both superantigen and protease virulence factors to subvert host immunity. However, its major protease is highly promiscuous and would potentially limit superantigen activity through its degradation. We profile the sensitivity of the streptococcal superantigens to degradation by the protease SpeB, providing evidence that many are highly resistant. Furthermore, we show that these important toxins can have synergistic proinflammatory activity. This provides insight into diseases like scarlet fever and toxic shock syndrome caused by these toxins and suggests anti-inflammatories that may be therapeutically useful.

Abrin Toxin Paradoxically Increases Protein Synthesis in Stimulated CD4+ T-Cells While Decreasing Protein Synthesis in Kidney Cells

Abrin, a toxin of the rosary pea plant (Abras precatorius), has been implicated as causing an autoimmune demyelinating disease in humans, but the exact mechanisms responsible for the induction of these demyelinating conditions are still unknown. Certain superantigen microbial toxins such as Staphylococcus enterotoxin type A, type D, type E or streptococcal pyrogenic exotoxin type C also lead to various diseases including autoimmune disorders of the nervous system. Here, the effect of abrin toxin on the immune reaction was studied in human CD4+ T-cell lines, and its inhibition of protein synthesis in kidney cells. It is shown for the first time that low concentrations of abrin toxin up to as high as 1 to 10 ng/mL amplifies superantigen activity in stimulated T-cells, leading to excessive NFAT pathway activation and secretion of cytokines, e.g., interleukin-2 (IL-2) and interferon-γ (INFγ), in a dose-dependent manner. This behavior, except at high concentration, is contrary to the effect on other cell types. Abrin's inhibition of protein synthesis was demonstrated with Vero (kidney) cells and milk was observed to competitively reduce this effect. This new concept in the behavior of abrin in amplifying superantigen activity may explain the mechanism by which abrin toxin triggers autoimmune demyelinating disease in people exposed to low doses of the toxin via the excessive secretion of cytokines which may create excessive inflammation leading to loss of immune tolerance and triggering an immune response against self-antigens.

Clinical Snapshot of Group A Streptococcal Isolates from an Australian Tertiary Hospital

Streptococcus pyogenes (Group A Streptococcus, GAS) is a human-restricted pathogen that causes a wide range of diseases from pharyngitis and scarlet fever to more severe, invasive infections such as necrotising fasciitis and streptococcal toxic shock syndrome. There has been a global increase in both scarlet fever and invasive infections during the COVID-19 post-pandemic period. The aim of this study was the molecular characterisation of 17 invasive and non-invasive clinical non-emm1 GAS isolates from an Australian tertiary hospital collected between 2021 and 2022. Whole genome sequencing revealed a total of nine different GAS emm types with the most prevalent being emm22, emm12 and emm3 (each 3/17, 18%). Most isolates (14/17, 82%) carried at least one superantigen gene associated with contemporary scarlet fever outbreaks, and the carriage of these toxin genes was non-emm type specific. Several mutations within key regulatory genes were identified across the different GAS isolates, which may be linked to an increased expression of several virulence factors. This study from a single Australian centre provides a snapshot of non-emm1 GAS clinical isolates that are multiclonal and linked with distinct epidemiological markers commonly observed in high-income settings. These findings highlight the need for continual surveillance to monitor genetic markers that may drive future outbreaks.

M proteins of group A Streptococcus bind hyaluronic acid via arginine-arginine/serine-arginine motifs

Tissue injury, including extracellular matrix (ECM) degradation, is a hallmark of group A Streptococcus (GAS) skin infection and is partially mediated by M proteins which possess lectin-like properties. Hyaluronic acid is a glycosaminoglycan enriched in the cutaneous ECM, yet an interaction with M proteins has yet to be explored. This study revealed that hyaluronic acid binding was conserved across phylogenetically diverse M proteins, mediated by RR/SR motifs predominantly localized in the C repeat region. Keratinocyte wound healing was decreased through the recruitment of hyaluronic acid by M proteins in an M type-specific manner. GAS strains 5448 (M1 serotype) and ALAB49 (M53 serotype) also bound hyaluronic acid via M proteins, but hyaluronic acid could increase bacterial adherence independently of M proteins. The identification of host-pathogen mechanisms that affect ECM composition and cell repair responses may facilitate the development of nonantibiotic therapeutics that arrest GAS disease progression in the skin.

Neutrophil-derived reactive agents induce a transient SpeB negative phenotype in Streptococcus pyogenes

BACKGROUND

Streptococcus pyogenes (group A streptococci; GAS) is the main causative pathogen of monomicrobial necrotizing soft tissue infections (NSTIs). To resist immuno-clearance, GAS adapt their genetic information and/or phenotype to the surrounding environment. Hyper-virulent streptococcal pyrogenic exotoxin B (SpeB) negative variants caused by covRS mutations are enriched during infection. A key driving force for this process is the bacterial Sda1 DNase.

METHODS

Bacterial infiltration, immune cell influx, tissue necrosis and inflammation in patient´s biopsies were determined using immunohistochemistry. SpeB secretion and activity by GAS post infections or challenges with reactive agents were determined via Western blot or casein agar and proteolytic activity assays, respectively. Proteome of GAS single colonies and neutrophil secretome were profiled, using mass spectrometry.

RESULTS

Here, we identify another strategy resulting in SpeB-negative variants, namely reversible abrogation of SpeB secretion triggered by neutrophil effector molecules. Analysis of NSTI patient tissue biopsies revealed that tissue inflammation, neutrophil influx, and degranulation positively correlate with increasing frequency of SpeB-negative GAS clones. Using single colony proteomics, we show that GAS isolated directly from tissue express but do not secrete SpeB. Once the tissue pressure is lifted, GAS regain SpeB secreting function. Neutrophils were identified as the main immune cells responsible for the observed phenotype. Subsequent analyses identified hydrogen peroxide and hypochlorous acid as reactive agents driving this phenotypic GAS adaptation to the tissue environment. SpeB-negative GAS show improved survival within neutrophils and induce increased degranulation.

CONCLUSIONS

Our findings provide new information about GAS fitness and heterogeneity in the soft tissue milieu and provide new potential targets for therapeutic intervention in NSTIs.

Detection of Streptococcus pyogenes Virulence Factors

Streptococcus pyogenes encodes multiple virulence factors and their presence is often related to the severity of the disease. We designed the system of four low-volume multiplex PCR reactions to detect genes encoding 20 virulence factors: spd3, sdc, sdaB, sdaD, speB, spyCEP, scpA, mac, sic, speL, speK, speM, speC, speI, speA, speH, speG, speJ, smeZ, and ssa. Classification of strains based on the virulence factors absence or presence correlates with PFGE MLST and emm typing results. The typing/detection system is fast and cost-effective, can be used to detect GAS virulence factors and as a rapid tool to effectively differentiate between strains.

Toxins and Superantigens of Group A Streptococci

Streptococcus pyogenes (i.e., the group A Streptococcus) is a human-restricted and versatile bacterial pathogen that produces an impressive arsenal of both surface-expressed and secreted virulence factors. Although surface-expressed virulence factors are clearly vital for colonization, establishing infection, and the development of disease, the secreted virulence factors are likely the major mediators of tissue damage and toxicity seen during active infection. The collective exotoxin arsenal of S. pyogenes is rivaled by few bacterial pathogens and includes extracellular enzymes, membrane active proteins, and a variety of toxins that specifically target both the innate and adaptive arms of the immune system, including the superantigens; however, despite their role in S. pyogenes disease, each of these virulence factors has likely evolved with humans in the context of asymptomatic colonization and transmission. In this article, we focus on the biology of the true secreted exotoxins of the group A Streptococcus, as well as their roles in the pathogenesis of human disease.