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

Abstract

The Staphylococcus aureus fibrinogen binding MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules), ClfA (clumping factor A) is an important virulence factor in staphylococcal infections and a component of several vaccines currently under clinical evaluation. The mouse monoclonal antibody aurexis (also called 12-9), and the humanized version tefibazumab are therapeutic monoclonal antibodies targeting ClfA that in combination with conventional antibiotics were effective in animal models but showed less impressive efficacy in a limited Phase II clinical trial. We here report the crystal structure and a biochemical characterization of the ClfA/tefibazumab (Fab) complex. The epitope for tefibazumab is located to the “top” of the N3 subdomain of ClfA and partially overlaps with a previously unidentified second binding site for fibrinogen. A high-affinity binding of ClfA to fibrinogen involves both an interaction at the N3 site and the previously identified docking of the C-terminal segment of the fibrinogen γ-chain in the N2N3 trench. Although tefibazumab binds ClfA with high affinity we observe a modest IC50 value for the inhibition of fibrinogen binding to the MSCRAMM. This observation, paired with a common natural occurring variant of ClfA that is not effectively recognized by the mAb, may partly explain the modest effect tefibazumab showed in the initial clinic trail. This information will provide guidance for the design of the next generation of therapeutic anti-staphylococcal mAbs targeting ClfA.

Clumping Factor B Promotes Adherence of Staphylococcus aureus to Corneocytes in Atopic Dermatitis

“…However, it is possible that an additional, unknown ligand binding site could be located outside the trench region. For example, it has recently been shown for the related protein ClfA that high-affinity binding of ClfA to its ligand, fibrinogen, involves both the dock, lock, and latch ligand binding trench and a second interaction site at the top of the N3 subdomain (27). Thus, to investigate if sequence differences outside the trench region in CC1 and CC30 ClfB might alter the affinity for L2v, 6ϫ histidine-tagged recombinant ClfB N2N3 (rClfB N2N3) proteins were purified from Escherichia coli.…”

Staphylococcus aureus clumping factor A is a force-sensitive molecular switch that activates bacterial adhesion

“…The DLL mechanism involves dynamic conformational changes of the adhesin that result in a greatly stabilized adhesin-ligand complex. The overall affinity of the interaction of ClfA with Fg is increased through interactions at a recently described second site that lies at the top of subdomain N3 outside of the DLL ligand-binding trench ( 13 ).…”

Розвиток Імунної Відповіді При Стафілококовій Пневмонії (Частина 1)

“…Молекула SpA в N-терминальной сигнальной последовательности содержит пять повторных доменов (E, D, A, B, C), обладающих аффи-нитетом к IgG, в С-терминальном регионе -LPXTG мотив, которым протеин SpA прикре-пляется к поверхности бактериальной клетки [22,42].…”