PROJECT SUMMARY/ABSTRACT Staphylococcus aureus is responsible for many hospital- and community-associated infections worldwide. However, therapeutic options to combat these infections are limited due to the high level of antibiotic resistance and lack of effective vaccines. Thus, there is a significant need for the development of effective therapeutics against this bacterium. Critical to the pathogenesis of S. aureus is the killing of phagocytes, innate immune cells integral to the control of Staphylococcal infections. The long-term goal of this research program is to understand the mechanism employed by S. aureus to injure these critical immune cells. We have discovered that the leukocidin A/B (LukAB) plays an essential role in protecting S. aureus from phagocyte-mediated killing by targeting and eliminating these cells. LukAB is chromosomally encoded and found in most clinical strains yet is the most divergent pore-forming toxin. LukAB: is responsible for S. aureus-mediated demise of primary human phagocytes during ex vivo infections; binds host proteins CD11b and HVCN1; exhibits species specificity by preferentially targeting the human CD11b and HVCN1 over the murine proteins; is produced in vivo during both murine and human infections; and is responsible for killing human leukocytes from both the extracellular milieu as well as intracellularly in the phagosome when produced by S. aureus. The primary goals of this competitive renewal application are to: define the molecular mechanism(s) by which LukAB kills human phagocytes (Aim 1), disentangle the regulatory network involved in lukAB expression (Aim 2), and delineate the contribution of LukAB to S. aureus pathogenesis in vivo (Aim 3). To accomplish these Aims, we will employ a multidisciplinary approach that combines techniques from bacteriology, biochemistry, cell biology, immunology, and pathogenesis, together with ex vivo and in vivo infection models. Completion of these studies will provide much needed fundamental insight into how LukAB contributes to the pathogenesis of S. aureus.