A protein A based Staphylococcus aureus vaccine with improved safety.

Exposure to Staphylococcus aureus does not lead to immunity as evidenced by the persistent colonization of one third of the human population. S. aureus immune escape is mediated by factors that preempt complement activation, destroy phagocytes, and modify B and T cell responses. One such factor, Staphylococcal protein A (SpA) encompasses five Immunoglobulin binding domains (IgBDs) that associate with the Fcγ domain to block phagocytosis. IgBDs also associate with Fab encoded by VH3 clan related genes. SpA binding to VH3-IgM that serves as a B cell receptor results in B cell expansion and secretion of antibodies with no specificity for S. aureus. SpA crosslinking of VH3-IgG and VH3-IgE bound to cognate receptors of mast cells and basophils promotes histamine release and anaphylaxis. Earlier work developed a prototype variant SpAKKAA with four amino acid substitutions in each IgBD. When tested in animal models, SpAKKAA elicited neutralizing antibodies and protection against infection. We show here that SpAKKAA retains crosslinking activity for VH3-IgG and VH3-IgE. We use a rational approach to design and test 67 new SpA variants for loss of VH3 binding and anaphylactic activities. We identify two detoxified candidates that elicit SpA-neutralizing antibodies and protect animals from S. aureus colonization and bloodstream infection. The new detoxified SpA candidates bear three instead of four amino acid substitutions thus increasing the development of SpA-specific antibodies. We propose that detoxified SpA variants unable to crosslink VH3-idiotypic immunoglobulin may be suitably developed as clinical-grade vaccines for safety and efficacy testing in humans.