No abstract

In the complement system, the opsonin C3b binds to the bacterial cell surface and mediates the opsonophagocytosis. However, the cell-wall protein SdrE of Staphylococcus aureus inhibits the C3b activity by recruiting the complement regulatory protein factor H (fH). SdrE binds to fH via its N-terminal N2N3 domain, which are also found in six other staphylococcal cell-wall proteins. In this study, we report that not only the N2N3 domain of SdrE but also those of ClfA, FnbpA and FnbpB can bind to fH. When immobilized on a microplate, the N2N3 domains recruited fH and enhanced the factor I (fI)-mediated cleavage of C3b. When mixed with fH and S. aureus cells, the N2N3 domains inhibited the fH binding to S. aureus cells and reduced the fI-mediated C3b cleavage on the bacterial cell surface. The F(ab)′2 fragments of the rabbit N2N3 antibodies also inhibited the fH binding to the S. aureus cell surface. When added to human blood, the N2N3 antibodies or the N2N3 domain proteins significantly increased the bactericidal activity. Based on these results, we conclude that, in S. aureus, not only SdrE but also ClfA, FnbpA and FnbpB can contribute to the inhibition of C3b-mediated opsonophagocytosis.

No abstract

In Gram-positive pathogen Staphylococcus aureus , pore-forming toxins (PFTs) such as leukocidins and hemolysins play prominent roles in staphylococcal pathogenesis by killing host immune cells and red blood cells (RBCs). However, it remains unknown which combination of toxin antigens would induce the broadest protective immune response against those toxins. In this study, by targeting six major staphylococcal PFTs (i.e., HlgAB, HlgCB, LukAB, LukED, LukSF-PV, and Hla), we generated ten recombinant toxins or toxin-subunits, three toxoids, and their rabbit antibodies. Using the cytolytic assay for RBCs and polymorphonuclear cells (PMNs), we determined the best combination of toxin antibodies conferring the broadest protection against those staphylococcal PFTs. Although anti-HlgA IgG (HlgA-IgG) showed low cross-reactivity to other toxin components, it was essential to protect rabbit and human RBCs and human PMNs. For the protection of rabbit RBCs, Hla H35L toxoid-IgG was also required, whereas, for human PMNs, LukS-IgG and LukA E323A B-IgG were essential too. When the toxin/toxoid antigens HlgA, LukS-PV, Hla H35L , and LukA E323A B were used to immunize rabbits, they increased rabbit survival; however, they did not block staphylococcal abscess formation in kidneys. Based on these results, we proposed that the combination of HlgA, LukS, Hla H35L , and LukA E323A B is the optimal vaccine component to protect human RBCs and PMNs from staphylococcal PFTs. We also concluded that a successful S. aureus vaccine requires not only those toxin antigens but also other antigens that can induce immune response blocking staphylococcal colonization.