Methicillin-resistant Staphylococcus aureus (MRSA)are able to cause skin-infections but also life-threatening necrotizing pneumonia in otherwise healthy individuals. Nearly all of these community-acquired MRSA (CA-MRSA) strains express the secreted pore-forming leukotoxin Panton-Valentine leukocidin (PVL), however its role in pathogenesis remains controversial. To further probe its function, we have generated recombinant PVL as well as its subunits, LukS-PV and LukF-PV. We show that PVL is highly cytotoxic to human macrophages and that this depends on a functional PVL complex, as the subunits LukS-PV or LukF-PV show little affect. Intriguingly, murine macrophages remain refractory to PVL toxicity, as well as human THP-1 monocytes and HeLa cells, suggesting that PVL requires specific host factors. To gain a more dynamic view how PVL kills, we followed the interaction of PVL with macrophages using live-cell imaging. This shows that PVL can target mitochondria to induce loss of bioenergetics. More significantly, PVL toxicity can be blocked by inhibiting intracellular potassium efflux and cathepsin B activity in human macrophages. This suggests that PVL kill macrophages by activating host cell death factors. We are now using genetic approaches to delineate the host death signalling pathways and to determine their role in PVL positive MRSA infections.