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Utilising Staphylococcus aureus immune evasion protein Sbi as a novel vaccine conjugate
Abstracts / Molecular Immunology 89 (2017) 197–199
166 Utilising Staphylococcus aureus immune evasion protein Sbi as a novel vaccine conjugate Catherine R. Back 1,∗ , Yi Yang 2 , Ayla A. Wahid 1 , Rebecca Walters 2 , Joshua Paulin 2 , Melissa A. Graewert 3 , Dmitri I. Svergun 3 , Kevin J. Marchbank 2 , Jean M.H. van den Elsen 1 1 Department of Biology & Biochemistry, University of Bath, UK 2 Institute of Cellular Medicine, University of Newcastle, UK 3 Deutsches Elektronen-Synchrotron, EMBL Hamburg, Germany
Background: Staphylococcus aureus immune evasion protein Sbi modulates both adaptive and innate immune systems of human and animal hosts by interacting with antibodies and by binding complement component C3. The protein consists of four globular domains (I–IV), a cell wall spanning region and a tyrosine-rich C-terminal region, and occurs both anchored to the cell envelope as well as extracellularly. Domains I and II can sequester IgG and thereby inhibit the classical complement pathway (CP), similar to SpA. Domains III and IV are unique among all complement evasion proteins, because of their ability to cause futile fluid-phase consumption of C3 via activation of the alternative complement pathway (AP). The latter domains are concealed by the capsule and
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only become active when secreted, acting as an ‘anti-missile flare’. Previous work from our group has revealed that Sbi-III-IV binds C3 via the C3dg domain, resulting in the formation of a covalent adduct with C3b. This interaction allows it to act as a subunit of the AP C3 convertase, C3bBbP, leading to the consumptive cleavage of many molecules of native C3. Aims: We set out to unveil the molecular mechanism by which Sbi is able to cause C3 depletion via a covalent Sbi-C3b adduct. In addition, we intend to harness the immuno-modulatory features of Sbi-III-IV as a novel vaccine adjuvant by fusing it to antigens, thereby encouraging the deposition of C3b fragments iC3b and C3dg, aiding stimulation of both B and T cell responses. Results: We show that Sbi-III-IV, via specific ester- and amidelinked C3b adducts, forms strong AP C3 convertase precursors that are protective against Factor I-mediated degradation. We also reveal that Sbi-III-IV hijacks Factor H-related protein-1 (FHR-1), resulting in the formation of a tripartite complex that blocks formation of an AP C5 convertase and accelerates C3 depletion. A crystal structure of the C3d:Sbi-III-IV complex, in combination with SAXS analyses of the Sbi-III-IV:C3d:FHR-1 tripartite complex, provides crucial molecular details for our understanding of the complement consumptive nature of Sbi and its potential use as a vaccine conjugate. We also show that administration of a Mycobacterium tuberculosis Ag85b-based conjugate pro-vaccine, utilising Sbi-III-IV, during vaccination leads to an enhanced immune response in vivo. http://dx.doi.org/10.1016/j.molimm.2017.06.209
166 Utilising Staphylococcus aureus immune evasion protein Sbi as a novel vaccine conjugate Catherine R. Back 1,∗ , Yi Yang 2 , Ayla A. Wahid 1 , Rebecca Walters 2 , Joshua Paulin 2 , Melissa A. Graewert 3 , Dmitri I. Svergun 3 , Kevin J. Marchbank 2 , Jean M.H. van den Elsen 1 1 Department of Biology & Biochemistry, University of Bath, UK 2 Institute of Cellular Medicine, University of Newcastle, UK 3 Deutsches Elektronen-Synchrotron, EMBL Hamburg, Germany
Background: Staphylococcus aureus immune evasion protein Sbi modulates both adaptive and innate immune systems of human and animal hosts by interacting with antibodies and by binding complement component C3. The protein consists of four globular domains (I–IV), a cell wall spanning region and a tyrosine-rich C-terminal region, and occurs both anchored to the cell envelope as well as extracellularly. Domains I and II can sequester IgG and thereby inhibit the classical complement pathway (CP), similar to SpA. Domains III and IV are unique among all complement evasion proteins, because of their ability to cause futile fluid-phase consumption of C3 via activation of the alternative complement pathway (AP). The latter domains are concealed by the capsule and
199
only become active when secreted, acting as an ‘anti-missile flare’. Previous work from our group has revealed that Sbi-III-IV binds C3 via the C3dg domain, resulting in the formation of a covalent adduct with C3b. This interaction allows it to act as a subunit of the AP C3 convertase, C3bBbP, leading to the consumptive cleavage of many molecules of native C3. Aims: We set out to unveil the molecular mechanism by which Sbi is able to cause C3 depletion via a covalent Sbi-C3b adduct. In addition, we intend to harness the immuno-modulatory features of Sbi-III-IV as a novel vaccine adjuvant by fusing it to antigens, thereby encouraging the deposition of C3b fragments iC3b and C3dg, aiding stimulation of both B and T cell responses. Results: We show that Sbi-III-IV, via specific ester- and amidelinked C3b adducts, forms strong AP C3 convertase precursors that are protective against Factor I-mediated degradation. We also reveal that Sbi-III-IV hijacks Factor H-related protein-1 (FHR-1), resulting in the formation of a tripartite complex that blocks formation of an AP C5 convertase and accelerates C3 depletion. A crystal structure of the C3d:Sbi-III-IV complex, in combination with SAXS analyses of the Sbi-III-IV:C3d:FHR-1 tripartite complex, provides crucial molecular details for our understanding of the complement consumptive nature of Sbi and its potential use as a vaccine conjugate. We also show that administration of a Mycobacterium tuberculosis Ag85b-based conjugate pro-vaccine, utilising Sbi-III-IV, during vaccination leads to an enhanced immune response in vivo. http://dx.doi.org/10.1016/j.molimm.2017.06.209