Targeting Alpha Toxin and ClfA with a Multimechanistic Monoclonal-Antibody-Based Approach for Prophylaxis of Serious Staphylococcus aureus Disease

Abstract

Staphylococcus aureus produces numerous virulence factors, each contributing different mechanisms to bacterial pathogenesis in a spectrum of diseases. Alpha toxin (AT), a cytolytic pore-forming toxin, plays a key role in skin and soft tissue infections and pneumonia, and a human anti-AT monoclonal antibody (MAb), MEDI4893*, has been shown to reduce disease severity in dermonecrosis and pneumonia infection models. However, interstrain diversity and the complex pathogenesis of S. aureus bloodstream infections suggests that MEDI4893* alone may not provide adequate protection against S. aureus sepsis. Clumping factor A (ClfA), a fibrinogen binding protein, is an important virulence factor facilitating S. aureus bloodstream infections. Herein, we report on the identification of a high-affinity anti-ClfA MAb, 11H10, that inhibits ClfA binding to fibrinogen, prevents bacterial agglutination in human plasma, and promotes opsonophagocytic bacterial killing (OPK). 11H10 prophylaxis reduced disease severity in a mouse bacteremia model and was dependent on Fc effector function and OPK. Additionally, prophylaxis with 11H10 in combination with MEDI4893* provided enhanced strain coverage in this model and increased survival compared to that obtained with the individual MAbs. The MAb combination also reduced disease severity in murine dermonecrosis and pneumonia models, with activity similar to that of MEDI4893* alone. These results indicate that an MAb combination targeting multiple virulence factors provides benefit over a single MAb neutralizing one virulence mechanism by providing improved efficacy, broader strain coverage, and protection against multiple infection pathologies.

Mouse model of hematogenous implant-related Staphylococcus aureus biofilm infection reveals therapeutic targets

“…In contrast, targeting both AT and ClfA with the anti-AT/ anti-ClfA mAb combination resulted in more than a 50% reduction in the propensity for the hematogenous orthopedic implant infection, decreased bacterial burden in the (+) implant surgical legs, and reduced biofilm formation on the implants. Similar to the improved activity of the mAb combination reported in a murine model of bacteremia (32), the improved efficacy of the mAb combination in this model likely resulted from targeting complementary virulence mechanisms, including neutralization of AT-mediated tissue damage, inflammation and immune evasion, inhibition of ClfA-mediated fibrinogen binding and bacterial agglutination in the synovium (38), and anti-ClfA-mediated opsonophagocytic killing of S. aureus (32). In addition, AT has been shown to promote host cell lysis to provide a nutrient source for the bacteria in a model of S. aureus vaginal mucosal biofilm infection (47), and the inhibition of AT might have had a similar effect in our hematogenous implant-related biofilm infection.…”

“…Therefore, we first evaluated the effect of anti-α-toxin (AT) (MEDI4893*) and anti-clumping factor A (ClfA) (11H10) mAbs on in vitro biofilm formation. These mAbs are high-affinity, functional mAbs reported to reduce disease severity in various S. aureus disease models (32,33). AT is a secreted pore-forming toxin (34) found to promote biofilm formation in vitro (35,36) and to contribute to immune evasion in a murine model of orthopedic implant infection (37).…”

“…To identify potential pathogenic factors that contributed to the hematogenous orthopedic implant infection in our mouse model, we investigated S. aureus AT and ClfA because they are involved in biofilm formation in vitro (35,36) and in human synovial fluid specimens ex vivo (38) and have been implicated in septic arthritis (39, 40) and primary orthopedic implant infections in mice and humans (37,41). Neutralizing the activity of AT and ClfA with human anti-AT and anti-ClfA mAbs (32,46) inhibited biofilm formation in vitro. The anti-ClfA mAb alone was a potent inhibitor of biofilm formation at both the low and high concentrations and at 12 and 24 h of the assay, whereas the anti-AT mAb significantly inhibited biofilm formation only at the high concentration and at 24 h of the assay.…”

“…Anti-AT mAb (MEDI4893*), anti-ClfA (11H10), and control human IgG1 isotype [anti-HIV gp120 (R347)] mAbs are fully human antibodies that were generated as previously described (32,46).…”

Lessons from the Crystal Structure of the S. aureus Surface Protein Clumping Factor A in Complex With Tefibazumab, an Inhibiting Monoclonal Antibody

“…Moreover, passive immunization with polyclonal ClfA antibodies generated in rats or rabbits protected mice against S. aureus induced sepsis and arthritis (Josefsson et al, 2001). Recently, a multi-mechanistic mAb targeting ClfA and the Alpha toxin was shown to be protective against S. aureus infection in a mouse model (Tkaczyk et al, 2016). A combination therapy of vancomycin with high titers of human polyclonal Abs or a mouse monoclonal antibody (mAb) called aurexis or 12-9 against ClfA was effective in a catheter induced infective endocarditis model in rabbits where treating with vancomycin alone was less effective (Patti, 2004, Vernachio et al, 2003, Weems et al, 2006).…”