Abstract:
Staphylococcus aureus can cause severe diseases with symptoms ranging from superficial skin infections to infective endocarditis and toxic shock syndrome. A family of 14 virulence factors known as Staphylococcal Superantigen-Like proteins (SSLs) have been identified that have affinity for molecules of the innate immune system and aid with immune evasion. A clade within the SSL family has been identified to bind carbohydrates. SSL11, a member of this clade, is able to bind sialyllactosamine (sLacNAc) with high affinity. Sialylation is an important glycosylation of immune receptors as the carbohydrate is directly involved in immune recognition and signalling. It is therefore hypothesised that SSL11 helps the bacterium evade immune surveillance by interfering with the immune recognition of sialylated receptors and their subsequent responses. This thesis sought to further investigate the influence SSL11 has on immune function. SSL11 binding to Sialyl Lewis X (sLex) exhibited no differences in affinity when compared to SSL11 binding sLacNAc. This lack of specificity for additional sugars explains why SSL11 does not compete or saturate receptors on the surface of cells, as it can promiscuously bind receptors exposing α2-3 linked sialylated galactose. SSL11 was observed to dimerise in solution as well as on the cell surface. The dimer exhibited prolonged binding to sLex with incomplete dissociation from the sialylated carbohydrate. The SSL11 dimer caused the aggregation of myeloid cells by cross-linking receptors on adjacent cell surfaces. While SSL11 exhibited no effect on complement activation, apoptosis, platelet aggregation or phagocytosis, it did halt the migration of neutrophils to opsonised bacteria. This is not due to a loss in sensing the chemotactic anaphylatoxin C5a, but more likely due to the activity of SSL11 at the uropod which appears to tether the cells down at this location. This may relate to observed changes in actin in myeloid cells that have internalised SSL11. It is currently unknown if SSL11 perturbs receptor functions and subsequent cellular responses at the uropod or if it directly prevents the detachment of the neutrophil. SSL11 internalisation into neutrophils and macrophages is via clathrin-dependent receptor-endocytosis. Following endocytosis, SSL11 travels in endosomes along the microtubule network to an unknown location. The most likely targets are the Golgi or lysosomal organelles. Internalisation of SSL11 appears to be associated with phenotypic cell changes and is completely absent in THP-1 monocytes. It is hypothesised that SSL11 is binding Mac-1 and using its activity to shuttle SSL11 into the cell. The observed influences SSL11 has on neutrophil migration, in conjunction with aggregation, indicate that it could interfere with several steps in myeloid recruitment. This would be important in aiding the survival of the bacterium which is why SSL11 has been conserved in all observed isolates.