Functional characterisation of superantigens in Staphylococcus aureus disease pathogenesis
Nutbeam-Tuffs, Stephen William
Bacterial superantigens (SAgs) are virulence factors that induce nonspecific T-cell proliferation contributing to host immune avoidance, and occasionally severe life-threatening toxinoses such as toxic shock syndrome. In the current study, the multiple functions of 3 superantigens named staphylococcal enterotoxin-like toxins X, Y and Z are investigated. SElX and SElZ were non-emetic in a musk shrew model of emesis. SElX is structurally and phylogenetically related to staphylococcal superantigen-like proteins (SSls) which are non-mitogenic but exhibit a variety of immune modulatory properties. We carried out protein and gene expression analysis of mutants of different S. aureus gene regulators and demonstrated that selx expression is controlled by saeRS, a two-component regulator linked to the bacterial response to phagocytic signals. Considering the co-regulation of SElX with known mediators of innate immune evasion we investigated a potential role for SElX in both humoral and cellular innate immune modulation and discovered that SElX strongly binds to human, bovine, murine, and laprine neutrophils and interferes with IgG-mediated phagocytosis, independently of Fcγ receptor signalling. Bacterial survival assays with neutrophils demonstrated that the deletion of selx significantly reduced the ability of S. aureus to resist neutrophil killing. Site-directed mutagenesis in the conserved sialic acid-binding motif of SElX abolished its neutrophil binding capacity, which is consistent with a critical role for glycosylated receptors in this interaction. Importantly, the sialic-acid binding mutants of SElX retained the ability to induce T-cell proliferation demonstrating that the distinct functions of SElX are mechanistically independent. Affinity precipitation experiments identified potential glycoprotein receptors for SElX and the interaction with protein ICAM-3, an important ligand for MAC-1 integrins, was validated suggesting SElX may interfere with cell signalling. Taken together, we present the first example of a bi-functional SAg that can manipulate two distinct arms of the human immune system and contribute to S. aureus survival during infection.