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J Immunol. 2015 Oct 15;195(8):3946-58. doi: 10.4049/jimmunol.1500966. Epub 2015 Sep 4.

Attenuation of Staphylococcus aureus-Induced Bacteremia by Human Mini-Antibodies Targeting the Complement Inhibitory Protein Efb.

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Department of Biodiagnostic Sciences and Technologies, I/NRASTES, National Center for Scientific Research "Demokritos," 15310 Athens, Greece;
Department of Pathology and Laboratory Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104;
Institute of Biosciences and Applications, National Center for Scientific Research "Demokritos," 15310 Athens, Greece;
Department of Clinical Microbiology, General Hospital "Evangelismos," 10676 Athens, Greece;
Department of Biochemistry and Molecular Biophysics, Kansas State University, Manhattan, KS 66506; and.
2nd Department of Pathology, University of Athens Medical School, Attikon University Hospital, 12462 Chaidari, Greece.
Department of Biodiagnostic Sciences and Technologies, I/NRASTES, National Center for Scientific Research "Demokritos," 15310 Athens, Greece;


Staphylococcus aureus can cause a broad range of potentially fatal inflammatory complications (e.g., sepsis and endocarditis). Its emerging antibiotic resistance and formidable immune evasion arsenal have emphasized the need for more effective antimicrobial approaches. Complement is an innate immune sensor that rapidly responds to bacterial infection eliciting C3-mediated opsonophagocytic and immunomodulatory responses. Extracellular fibrinogen-binding protein (Efb) is a key immune evasion protein of S. aureus that intercepts complement at the level of C3. To date, Efb has not been explored as a target for mAb-based antimicrobial therapeutics. In this study, we have isolated donor-derived anti-Efb IgGs that attenuate S. aureus survival through enhanced neutrophil killing. A phage library screen yielded mini-Abs that selectively inhibit the interaction of Efb with C3 partly by disrupting contacts essential for complex formation. Surface plasmon resonance-based kinetic analysis enabled the selection of mini-Abs with favorable Efb-binding profiles as therapeutic leads. Mini-Ab-mediated blockade of Efb attenuated S. aureus survival in a whole blood model of bacteremia. This neutralizing effect was associated with enhanced neutrophil-mediated killing of S. aureus, increased C5a release, and modulation of IL-6 secretion. Finally, these mini-Abs afforded protection from S. aureus-induced bacteremia in a murine renal abscess model, attenuating bacterial inflammation in kidneys. Overall, these findings are anticipated to pave the way toward novel Ab-based therapeutics for S. aureus-related diseases.

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