| Staphylococcus aureus is an important human pathogen responsible for tremendous morbidity and mortality worldwide. The prevalence of antibiotic resistance among S. aureus strains has complicated the treatment of staphylococcal infections for decades. Methicillin-resistant S. aureus (MRSA) is strongly associated with skin and soft tissue infections, but can also cause more severe diseases such as osteomyelitis, endocarditis, pneumonia, necrotizing fasciitis, and sepsis. The pathogen relies on an arsenal of virulence factors to overcome the host immune system. Of these factors, the staphylococcal beta-barrel pore-forming toxins are employed by S. aureus to target and kill neutrophils, which are crucial for bacterial clearance. Interestingly, a single S. aureus strain can produce several pore-forming toxins, all of which possess the ability to kill host neutrophils through osmotic lysis. Understanding why S. aureus produces such a large repertoire of seemingly redundant leukotoxins has been a major goal in the staphylococcal toxin field. Through attempts to elucidate the roles of each of these toxins in S. aureus -mediated cytotoxicity, we identified a novel, highly divergent bi-component pore-forming cytotoxin denoted leukocidin A/B (LukAB). In our first study, we established that LukAB contributes to the cytotoxicity of methicillin-sensitive and methicillin-resistant S. aureus clinical isolates, potently kills human phagocytes, and plays a role in S. aureus pathogenesis. We next defined the role of LukAB in the context of S. aureus-neutrophil interactions. We discovered that LukAB, in addition to being an extracellular weapon, is utilized by S. aureus to escape from within neutrophils post-phagocytosis. Importantly, we were able to further these findings by identifying the CD11b subunit of the Mac-1 integrin as a critical cellular factor utilized by both extracellular and phagocytosed S. aureus to target and kill neutrophils via LukAB. CD11b is specifically utilized by LukAB and not the other leukotoxins, as we mapped the interaction between LukAB and CD11b to the distinctive C-terminal region of the LukA subunit and the I domain of CD11b. Overall, the findings presented in this thesis provide an in-depth characterization of LukAB, highlighting the unique properties of this toxin and identifying a novel therapeutic target for the treatment of S. aureus infections. |
