Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), especially the USA300 pulsotype, is a frequent cause of skin and soft-tissue infections and severe pneumonia. Despite appropriate antibiotic treatment, complications are common and pneumonia is associated with high mortality. S. aureus strains express multiple cytotoxins, including alpha-hemolysin (Hla) and up to five bi-component leukocidins that specifically target phagocytic cells for lysis. CA-MRSA USA300 strains carry the genes for all six cytotoxins. Species specificity of the leukocidins greatly contributes to the ambiguity regarding their role in S. aureus pathogenesis.We performed a comparative analysis of leukocodin susceptibility of human, rabbit and mouse polymorphonuclear leukocytes (PMNs) to assess the translational value of mouse and rabbit S. aureus models. We found that mouse PMNs were largely resistant to LukSF-PV, HlgAB and HlgCB and only susceptible to LukED, whereas rabbit and human PMNs were highly sensitive to all these cytotoxins. In the rabbit pneumonia model with a USA300 CA-MRSA strain, passive immunization with a previously identified human monoclonal antibody, Hla-F#5, that cross-neutralizes Hla, LukSF-PV, HlgAB, HlgCB, and LukED provided full protection, whereas an Hla-specific mAb was only partially protective. In the mouse USA300 CA-MRSA pneumonia model, both types of antibodies demonstrated full protection, suggesting that Hla, but not leukocidins, is the principal virulence determinant in mice. As the rabbit recapitulates the high susceptibility to leukocidins characteristic of humans, this species represents a valuable model to assess novel, cytotoxin-targeting anti-S. aureus therapeutic approaches.
Improved protection in a rabbit model of CA-MRSA necrotizing pneumonia upon neutralization of leukocidins in addition to alpha-hemolysin.