title: High resolution mapping of the B cell epitopes of staphylokinase in humans using negative selection of a phage-displayed antigen library.
abstract: Staphylokinase (Sak), a 16-kDa protein secreted by Staphylococcus aureus, induces fibrin-specific thrombolysis in patients with thrombotic disorders. However, Sak also elicits high titers of neutralizing Abs that persist for several months and preclude its repeated use in humans. To identify the antigenic determinants of Sak recognized by humans, a phage-displayed library of Sak variants was selected for mutants that escape binding to an affinity matrix derivatized with patient-specific polyclonal anti-Sak Abs. Fifty-six escape Sak variants were identified after three selection cycles using human polyclonal anti-Sak IgGs obtained from four different patients. DNA sequencing revealed 213 amino acid substitutions, of which 73% were found at 25 positions clustered in eight discontinuous Sak antigenic segments. Although each antigenic segment was recognized to a variable extent by each patient antiserum, the main epitopes of Sak in all patients were roughly targeted to two large discontinuous areas covering 35% of the solvent-accessible surface of Sak. The antigenic area I comprises three segments centered on residues 66, 73, and 135, while the antigenic area II consists of four segments centered on positions 20, 95, 102, and 121. These results suggest that a secondary immune response against Sak can occur in patients, and confirm an initial site-directed mutagenesis study wherein amino acid Lys74 was shown to play a prominent antigenic role. Comprehensive mapping of the most relevant sites of Sak that are antigenic for humans will guide efforts to modulate the immunogenicity of this therapeutically important molecule.
authors.affiliation: Center for Transgene Technology and Gene Therapy, Flanders Interuniversity Institute for Biotechnology, Katholieke Universiteit Leuven, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Jenné
authors.forename: K
authors.initials: K
authors.lastname: Brepoels
authors.forename: D
authors.initials: D
authors.lastname: Collen
authors.forename: L
authors.initials: L
authors.lastname: Jespers
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics / Q000276:immunology; D000907:Antibodies, Bacterial / Q000737:chemistry; D000942:Antigens, Bacterial / Q000737:chemistry* / Q000235:genetics / Q000276:immunology; D018604:Epitope Mapping / Q000379:methods*; D018985:Epitopes, B-Lymphocyte / Q000737:chemistry* / Q000276:immunology; D014644:Genetic Variation / Q000276:immunology; D006801:Humans; D016056:Immunodominant Epitopes / Q000737:chemistry* / Q000276:immunology; D007074:Immunoglobulin G / Q000737:chemistry; D017911:Inoviridae / Q000235:genetics / Q000276:immunology; D008666:Metalloendopeptidases / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D008958:Models, Molecular; D009154:Mutation; D019151:Peptide Library; D013211:Staphylococcus aureus / Q000201:enzymology / Q000235:genetics / Q000276:immunology*
chemical_list: D000907:Antibodies, Bacterial; D000942:Antigens, Bacterial; D018985:Epitopes, B-Lymphocyte; D016056:Immunodominant Epitopes; D007074:Immunoglobulin G; D019151:Peptide Library; D008666:Metalloendopeptidases; C022653:auR protein, Staphylococcus aureus