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
