Staphylococcus aureus can grow as free-floating, planktonic cocci or readily form biofilms. In fact, it is believed that more than 80% of staphylococcal infections are caused by biofilms. A biofilm is defined as a sessile community of microorganisms anchored to abiotic or biotic surfaces using a slime-like glycocalyx. S. aureus can readily form biofilms on a variety of living tissues as well as on several types of abiotic implants such as cardiac valves, catheters and prosthetic joints in vivo. Staphylococcal biofilms (SB) are refractory to antibacterial therapy and resistant to the host's immune system. It is believed that numerous secreted and cell-bound virulence factors contribute to persistence/immune evasion by SB in vivo. However, whether staphylococcal superantigen exotoxins (SSAgs) contribute to this process is not known. SSAgs are the most potent T cell activators known. SSAgs bind directly to MHC class II molecules without undergoing any processing. Subsequently, they cause polyclonal activation of 20-50% of CD4+ and CD8+ ?? TCR+ T cells expressing certain TCR V? families irrespective of their antigen specificities. Considering the unique immune functions of SSAgs and the known fact that chronic activation of conventional ?? TCR+ T cells can lead to their exhaustion and ultimately deletion, we propose that continued polyclonal activation of T cells by SSAgs produced by chronic Staphylococcal biofilm infections (SBI) would lead to T cell exhaustion/deletion, which will facilitate immune evasion and persistence of SBs. Unlike planktonic S. aureus, the production of SSAgs by S. aureus biofilms in vivo, has not been adequately investigated. Therefore, we propose to explore the many unknown roles of SSAgs in SBI using our humanized mouse model.
In Specific Aim 1, we will ?Establish the temporal kinetics of production of SSAg by SB in vivo?. Subsequently, in Specific Aim 2, we will ?Establish that SSAgs contribute to immune evasion, persistence and immunopathogenesis of chronic staphylococcal biofilm infections in vivo?. These exploratory studies will be conducted using a panel of immunocompetent, immunodeficient and reporter mice transgenically expressing HLA-DR3 using the catheter-associated biofilm infection model. The reasons being, (i) Conventional mice are 1011 times more resistant to SSAg due to poor binding of SSAg to murine compared to human MHC (HLA) class II molecules. Therefore, our humanized mice transgenically expressing HLA-DR3 molecules respond robustly to SSAg. (ii) Catheters are one of the most widely implants and infection of catheters with S. aureus biofilms is very common. Overall, the teleological reasons as to why S. aureus would produce so many different SSAg with similar biological functions have been debated. Given that 80% of staphylococcal infections involve biofilms formation, either on artificial implants or living tissue, and a significant percentage of these isolates produce one or more SSAg, our hypothesis that SSAg may contribute to the growth and survival of staphylococcal biofilms is innovative and the knowledge gained from our study could have a profound impact on treatment/management of SBI and hence, highly significant.
Infection of living tissues and artificial implants with Staphylococcus aureus biofilms is extremely common. However, S. aureus has evolved several strategies to escape from immune system and persist for prolonged periods of time inside the body. We will investigate using humanized mice the mechanisms by which superantigens, a family of potent staphylococcal toxins, contribute to the persistence of S. aureus biofilms.
