Structural basis for neutralizing antibody binding to pertussis toxin

Pertussis toxin (PT) is a key protective antigen in vaccine- and natural immunity-mediated protection from <i>Bordetella pertussis</i> infection. Despite its importance, no PT-neutralizing epitopes have been characterized structurally. To define neutralizing epitopes and identify key structural elements to preserve during PT antigen design, we determined a 3.6 Å cryoelectron microscopy structure of genetically detoxified PT (PTg) bound to hu11E6 and hu1B7, two potently neutralizing anti-PT antibodies with complementary mechanisms: disruption of toxin adhesion to cells and intracellular activities, respectively. Hu11E6 binds the paralogous S2 and S3 subunits of PTg via a conserved epitope but surprisingly did not span the previously identified sialic acid-binding site implicated in toxin adhesion. Hu11E6 specifically prevented PTg binding to sialylated N-glycans and a sialylated model receptor, as demonstrated by high-throughput glycan array analysis and ELISA, while a T cell activation assay showed that it blocks PTg mitogenic activities to define its neutralizing mechanism. Hu1B7 bound a quaternary epitope spanning the S1 and S5 subunits, although functional studies of hu1B7 variants suggested that S5 binding is not involved in its PT neutralization mechanism. These results structurally define neutralizing epitopes on PT, improving our molecular understanding of immune protection from <i>B. pertussis</i> and providing key information for the future development of PT immunogens.