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

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.
Highlights • The crystal structure of the monoclonal antibody tefibazumab in complex with ClfA (Clumping factor A) of S. aureus • Biochemical studies reveal a second binding site for fibrinogen in ClfA partially overlapping the tefibazumab epitope. • An explanation for tefibazumab modest effect in a clinical trial compared to the monoclonal's efficacy in animal models The S. aureus fibrinogen binding surface protein ClfA (Clumping factor A) is an important virulence factor and a target in several staphylococcal experimental vaccines and immunotherapeutic. One monoclonal antibody to ClfA called aurexis showed great promise in animal models for the treatment of staphylococcal infections. However, in a limited phase II clinical trial the efficacy was less impressive. We have solved the structure of the Fab fragment of tefibazumab (the humanized version of aurexis) in complex with the fibrinogen binding region of ClfA. The tefibazumab epitope partially overlaps with a previously unknown second fibrinogen binding site on ClfA. Biochemical analysis show that although tefibazumab binds to ClfA with high affinity the IC50 concentration for fibrinogen binding is modest. Furthermore, analysis of sequence variations in ClfA shows that some clinically important strains are poorly recognized by tefibazumab. These observations may partially explain the modest efficacy observed for tefibazumab in the clinical trial but provide a structural base for the design of more potent inhibitors.
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