Protein A-specific monoclonal antibodies and prevention of Staphylococcus aureus disease in mice

Staphylococcus aureus is a leading cause of human soft tissue infections and bacterial sepsis. The emergence of antibiotic-resistant strains (methicillin-resistant S. aureus [MRSA]) has prompted research into staphylococcal vaccines and preventive measures. The envelope of S. aureus is decorated with staphylococcal protein A (SpA), which captures the Fcγ portion of immunoglobulins to prevent opsonophagocytosis and associates with the Fab portion of V(H)3-type B cell receptors to trigger B cell superantigen activity. Nontoxigenic protein A (SpA(KKAA)), when used as an immunogen in mice, stimulates humoral immune responses that neutralize the Fcγ and the V(H)3(+) Fab binding activities of SpA and provide protection from staphylococcal abscess formation in mice. Here, we isolated monoclonal antibodies (MAbs) against SpA(KKAA) that, by binding to the triple-helical bundle fold of its immunoglobulin binding domains (IgBDs), neutralize the Fcγ and Fab binding activities of SpA. SpA(KKAA) MAbs promoted opsonophagocytic killing of MRSA in mouse and human blood, provided protection from abscess formation, and stimulated pathogen-specific immune responses in a mouse model of staphylococcal disease. Thus, SpA(KKAA) MAbs may be useful for the prevention and therapy of staphylococcal disease in humans.