Multimechanistic Monoclonal Antibodies (MAbs) Targeting Staphylococcus aureus Alpha-Toxin and Clumping Factor A: Activity and Efficacy Comparisons of a MAb Combination and an Engineered Bispecific Antibody Approach

Abstract

Secreted alpha-toxin and surface-localized clumping factor A (ClfA) are key virulence determinants in Staphylococcus aureus bloodstream infections. We previously demonstrated that prophylaxis with a multimechanistic monoclonal antibody (MAb) combination against alpha-toxin (MEDI4893*) and ClfA (11H10) provided greater strain coverage and improved efficacy in an S. aureus lethal bacteremia model. Subsequently, 11H10 was found to exhibit reduced affinity and impaired inhibition of fibrinogen binding to ClfA002 expressed by members of a predominant hospital-associated methicillin-resistant S. aureus (MRSA) clone, ST5. Consequently, we identified another anti-ClfA MAb (SAR114) from human tonsillar B cells with >100-fold increased affinity for three prominent ClfA variants, including ClfA002, and potent inhibition of bacterial agglutination by 112 diverse clinical isolates. We next constructed bispecific Abs (BiSAbs) comprised of 11H10 or SAR114 as IgG scaffolds and grafted anti-alpha-toxin (MEDI4893*) single-chain variable fragment to the amino or carboxy terminus of the anti-ClfA heavy chains. Although the BiSAbs exhibited in vitro potencies similar to those of the parental MAbs, only 11H10-BiSAb, but not SAR114-BiSAb, showed protective activity in murine infection models comparable to the respective MAb combination. In vivo activity with SAR114-BiSAb was observed in infection models with S. aureus lacking ClfA. Our data suggest that high-affinity binding to ClfA sequesters the SAR114-BiSAb to the bacterial surface, thereby reducing both alpha-toxin neutralization and protection in vivo. These results indicate that a MAb combination targeting ClfA and alpha-toxin is more promising for future development than the corresponding BiSAb.

α-Toxin Induces Platelet Aggregation and Liver Injury during Staphylococcus aureus Sepsis

“…MEDI4893*prophylaxis, did not affect uptake of the S. aureus consistent with the view that AT is secreted and not displayed on the staphylococcal surface AND in line with previous reports that MEDI4893* has no opsonophagocytic properties (Tkaczyk et al, 2016; Tkaczyk et al, 2017). S. aureus required 4–8 h to secrete sufficient AT, as AT-dependent platelet aggregation was only observed at 8 h while direct administration of AT caused instantaneous aggregation.…”

Virulence gene profiles: alpha-hemolysin and clonal diversity in Staphylococcus aureus isolates from bovine clinical mastitis in China

“…There were many reports regarding contribution of α-hemolysin to the pathogenesis of S. aureus infection, including cell signaling pathways that govern cell proliferation, cytokine secretion, inflammatory responses and cell-cell interactions [ 39 – 42 ]. Although polymorphisms in the hla promoter region have been described [ 43 ], the range of genetic diversity and evolution of this toxin was only assessed in human [ 43 – 45 ], with no report of a large representative collection of S. aureus from dairy cows.…”

Protective Efficacy of Monoclonal Antibodies Neutralizing Alpha-Hemolysin and Bicomponent Leukocidins in a Rabbit Model of Staphylococcus aureus Necrotizing Pneumonia

“…The rabbit experimental pneumonia protocol was reviewed and approved by the University of California, San Francisco, Institutional Animal Care and Use Committee and was conducted in a facility accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. SAN177 and SAN481 (20) were selected from human tonsillar memory B cells, as described previously (21), for their ability to cross-neutralize HlgAB, HlgBC, LukSF-PV, and LukED by binding the F subunits of the leukocidins (C. Tkaczyk and B. R. Sellman, unpublished data). Two animal efficacy studies were performed.…”