Conformational nature of the Borrelia burgdorferi decorin binding protein A epitopes that elicit protective antibodies

Decorin binding protein A (DbpA) has been shown by several laboratories to be a protective antigen for the prevention of experimental Borrelia burgdor feri infection in the mouse model of Lyme borreliosis. However, different r ecombinant forms of the antigen having either lipidated amino termini, appr oximating the natural secretion and posttranslational processing, or nonpro cessed cytosolic forms have elicited disparate levels of protection in the mouse model. We have now used the unique functional properties of this mole cule to investigate the structural requirements needed to elicit a protecti ve immune response. Genetic and physicochemical alterations to DbpA showed that the ability to bind to the ligand decorin is indicative of a potent im munogen but is not conclusive. By mutating the two carboxy-terminal noncons erved cysteines of DbpA from B. burgdorferi strain N40, we have determined that the stability afforded by the putative disulfide bond is essential for the generation of protective antibodies. This mutated protein was more sen sitive to thermal denaturation and proteolysis, suggesting that it is in a less ordered state. Immunization with DbpA that was thermally denatured and functionally inactivated stimulated an immune response that was not protec tive and lacked bactericidal antibodies. Antibodies against conformationall y altered forms of DbpA also failed to kill heterologous B. garinii and B. afzelii strains. Additionally, nonsecreted recombinant forms of DbpA(N40) w ere found to be inferior to secreted lipoprotein DbpA(N40) in terms of func tional activity and antigenic potency. These data suggest that elicitation of a bactericidal and protective immune response to DbpA requires a properl y folded conformation for the production of functional antibodies.