Abstract:
Staphylococcus aureus is a versatile and opportunistic bacterium that is the cause of a large proportion of both hospital-‐acquired and community-‐acquired infections. The wide range of immune evasion factors produced by S. aureus aid in its survival by disrupting or evading components of the host immune system. Furthermore, over the years there has been a problematic rise in multi-‐antibiotic resistant strains. Staphylococcal superantigen-‐ like proteins (SSLs) are a family of virulence factors that target various aspects of host innate immune responses. The investigations of this thesis looked at SSL4, a member of the glycan-‐binding subfamily of SSLs, which has previously been shown to bind to and be internalized by human myeloid cells. Endogenously expressed SSL4 by S. aureus Newman was shown to remain associated with the bacterial pellet as opposed to secretion into the supernatant as occurred with SSL4 expression by Lactococcus lactis pLZ12KmP23-‐TA:ssl4. This could mean that SSL4 either binds back to S. aureus or is not secreted due to its unique N-‐terminal repeat sequence. The stable full-‐length form of recombinant SSL4 was shown to exhibit the same binding and internalization properties as the previously studied truncated form of SSL4 to both human and murine leukocytes. SSL4 proteins (including a glycan-‐binding mutant) were also found to bind back to S. aureus in the presence of human plasma by utilizing plasma components such as fibrinogen in a non-‐carbohydrate-‐dependent manner. Furthermore, SSL4 was shown to bind to skin and lung epithelial cells (in a carbohydrate-‐dependent manner) that are components of tissue types and body systems commonly colonized by S. aureus. The binding of SSL4 to S. aureus through plasma components and then internalization into host cells through two different pathways could present a novel mechanism of S. aureus invasion. With the results of this study we have built on our knowledge and understanding of SSL4 as an endogenously expressed protein and its various cellular interactions. This in turn is advancing our understanding of the incredibly complex host pathogen interactions. Further research in this area could lead to identifying possible targets for novel therapeutic strategies directed against S. aureus.