Tuffs, S. W. | bioRxiv Science

bioRxiv2022

The Streptococcus pyogenes hyaluronic acid capsule promotes experimental nasal and skin infection by preventing neutrophil-mediated clearance

Streptococcus pyogenes is a globally prominent human-specific pathogen responsible for an enormous burden of human illnesses, including >600 million pharyngeal and >100 million skin infections each year. Despite intensive efforts that focus on invasive indications, much remains unknown about this bacterium in its natural state during colonization of the nasopharynx and skin. Using acute experimental infection models in HLA-transgenic mice, we evaluated how the hyaluronic acid (HA) capsule contributes to S. pyogenes MGAS8232 infection within these limited biological niches. Herein, we demonstrate that HA capsule expression promotes bacterial burden in murine nasal turbinates and skin lesions by resisting neutrophil-mediated killing. HA capsule production is encoded by the hasABC operon and compared to wildtype S. pyogenes infections, mice infected with a {Delta}hasA mutant exhibited over a 1000-fold CFU reduction at 48-hours post{-}nasal challenge, and a 10,000-fold CFU reduction from skin lesions 72-hours post-skin challenge. HA capsule expression contributed substantially to skin lesion size development following subdermal inoculations. In the absence of capsule expression, S. pyogenes revealed drastically impeded growth in whole human blood and increased susceptibility to killing by isolated neutrophils ex vivo, highlighting its important role in resisting phagocytosis. Furthermore, we establish that neutrophil depletion in mice, but not macrophage depletion, recovered the reduced burden by the {Delta}hasA mutant in both the nasopharynx and skin. Together, this work confirms that the HA capsule is a key virulence determinant during acute infections by S. pyogenes and demonstrates that its predominant function is to protect S. pyogenes against neutrophil-mediated killing. AUTHOR SUMMARYStreptococcus pyogenes is a globally disseminated and human-adapted bacterial pathogen that has evolved an arsenal of evasion strategies to overcome and escape host immune clearing mechanisms. Many strains of S. pyogenes are covered by a polysaccharide capsule composed of hyaluronic acid (HA) that is widely recognized to promote severe infections. In this study, we demonstrate using the encapsulated S. pyogenes MGAS8232 strain that the HA capsule is a key virulence factor that facilitates non-invasive infections of the nasopharynx and skin. Although bacterial adhesion and entry into host cells was impeded by HA capsule expression, we show that the key function for both nasal and skin infections is to protect S. pyogenes from neutrophil-mediated killing. Depletion of neutrophils, but not macrophages, recovered the low bacterial burden by unencapsulated S. pyogenes at both sites of infection. Our findings outline an important interaction between the HA capsule and neutrophils in the establishment of acute upper respiratory and skin infections by S. pyogenes.

microbiology↗

bioRxiv2021

Superantigens promote Staphylococcus aureus bloodstream infection by eliciting pathogenic interferon-gamma (IFNγ) production that subverts macrophage function

Staphylococcus aureus is a foremost bacterial pathogen responsible for a vast array of human diseases. Staphylococcal superantigens (SAgs) constitute a family of potent exotoxins secreted by S. aureus, and SAg genes are found ubiquitously in human isolates. SAgs bind directly to MHC class II molecules and T cell receptors, driving extensive T cell activation and cytokine release. Although these toxins have been implicated in serious disease including toxic shock syndrome, we aimed to further elucidate the mechanisms by which SAgs contribute to staphylococcal pathogenesis during septic bloodstream infections. As most conventional mouse strains respond poorly to staphylococcal SAgs, we utilized transgenic mice encoding humanized MHC class II molecules (HLA-DR4) as these animals are much more susceptible to SAg activity. Herein, we demonstrate that SAgs contribute to the severity of S. aureus bacteremia by increasing bacterial burden, most notably in the liver. We established that S. aureus bloodstream infection severity is mediated by CD4+ T cells and interferon-gamma (IFN{gamma}) is produced to very high levels during infection in a SAg-dependent manner. Bacterial burden and disease severity were reduced by antibody blocking of IFN{gamma}, phenocopying isogenic SAg deletion mutant strains. Additionally, cytokine analysis demonstrated that the immune system was skewed towards a proinflammatory response that was reduced by IFN{gamma} blocking. Infection kinetics and flow cytometry analyses suggested this was a macrophage driven mechanism, which was confirmed through macrophage depletion experiments. Further validation with human leukocytes indicated that excessive IFN{gamma} allowed S. aureus to replicate at a higher rate within macrophages. Together, this suggests that SAgs promote S. aureus survival by manipulating immune responses that would otherwise be effective at clearing S. aureus. This work implicates SAg toxins as critical targets for preventing persistent or severe S. aureus disease.

microbiology↗