Protective and Immunochemical Activities of Monoclonal Antibodies Reactive with theBacillus anthracisPolypeptide Capsule

Abstract

Bacillus anthracis is surrounded by a polypeptide capsule composed of poly-gamma-D-glutamic acid (␥DPGA). In a previous study, we reported that a monoclonal antibody (MAb F26G3) reactive with the capsular polypeptide is protective in a murine model of pulmonary anthrax. The present study examined a library of six MAbs generated from mice immunized with ␥DPGA. Evaluation of MAb binding to the capsule by a capsular "quellung" type reaction showed a striking diversity in capsular effects. Most MAbs produced a rim type reaction that was characterized by a sharp increase followed directly by a decrease in refractive index at the capsular edge. Some MAbs produced a second capsular reaction well beneath the capsular edge, suggesting complexity in capsular architecture. Binding of MAbs to soluble ␥DPGA was assessed by a fluorescence perturbation assay in which a change in the MAb intrinsic fluorescence produced by ligand binding was used as a reporter for antigen-antibody interaction. The MAbs differed considerably in the complexity of the binding curves. MAbs producing rim type capsule reactions typically produced the more complex binding isotherms. Finally, the protective activity of the MAbs was compared in a murine model of pulmonary anthrax. One MAb was markedly less protective than the remaining five MAbs. Characteristics of the more protective MAbs included a relatively high affinity, an immunoglobulin G3 isotype, and a complex binding isotherm in the fluorescence perturbation assay. Given the relatively monotonous structure of ␥DPGA, the results demonstrate a striking diversity in the antigen binding behavior of ␥DPGA antibodies.

Treatment of Experimental Anthrax with Recombinant Capsule Depolymerase

“…CapD, while not directly bactericidal, facilitates host cell phagocytic killing of encapsulated bacilli, possibly by exposing pathogen-associated molecular patterns and promoting complement deposition on the bacterial surface. In this respect, the use of enzymes to target bacteria for phagocytic killing is similar to passive immunization with anticapsule antibodies, an approach that has recently been reported to afford protection in animal models of anthrax (23,24).…”

“…Recent reports have demonstrated that anticapsule antibodies can opsonize B. anthracis (6,37,40) and that active and passive immunization protect mice against experimental anthrax (6,19,23,24), suggesting that the capsule is an attractive vaccine and therapeutic target. The strategy of enzymatically removing capsule from the bacterial surface as an approach to treat infections dates back to the work of Avery and Dubos in 1931 (2).…”

Constant domains influence binding of mouse–human chimeric antibodies to the capsular polypeptide ofBacillus anthracis

“…Previous reports found that the binding pocket of F26G3 accommodates 5 PGA residues; thus, we predict that inter-and intra-antigen binding may collectively contribute to the overall strength of the interaction. 11,16 Figure 1. Binding of anti-capsular antibodies to pGA-coated sensor surface.…”

“…13 Our laboratory has previously reported isolation of the murine monoclonal antibody (MuAb) F26G3 IgG3 that binds to PGA. 11 More recently, we reported the construction of a murine variabledomain identical IgG subclass-switch family of F26G3 (IgG3, IgG1, IgG2b, and IgG2a). 14 These antibodies had a diverse array of heavy chain (C H )-dependent immunochemical and protective activities.…”

“…[6][7][8][9] Previous studies identified the capsule as a potential therapeutic target in both active and passive immunization strategies. [10][11][12] Capsule-binding antibodies increase opsonization and phagocytosis of vegetative bacteria. 13 Our laboratory has previously reported isolation of the murine monoclonal antibody (MuAb) F26G3 IgG3 that binds to PGA.…”