Rational design of a meningococcal antigen inducing broad protective immunity.

The sequence variability of protective antigens is a major challenge to the development of vaccines. For Neisseria meningitidis, the bacterial pathogen that causes meningitis, the amino acid sequence of the protective antigen factor H binding protein (fHBP) has more than 300 variations. These sequence differences can be classified into three distinct groups of antigenic variants that do not induce cross-protective immunity. Our goal was to generate a single antigen that would induce immunity against all known sequence variants of N. meningitidis. To achieve this, we rationally designed, expressed, and purified 54 different mutants of fHBP and tested them in mice for the induction of protective immunity. We identified and determined the crystal structure of a lead chimeric antigen that was able to induce high levels of cross-protective antibodies in mice against all variant strains tested. The new fHBP antigen had a conserved backbone that carried an engineered surface containing specificities for all three variant groups. We demonstrate that the structure-based design of multiple immunodominant antigenic surfaces on a single protein scaffold is possible and represents an effective way to create broadly protective vaccines.

Solution structure of the factor H-binding protein, a survival factor and protective antigen of Neisseria meningitidis.

Factor H-binding protein is a 27-kDa lipoprotein of Neisseria meningitidis discovered while screening the bacterial genome for vaccine candidates. In addition to being an important component of a vaccine against meningococcus in late stage of development, the protein is essential for pathogenesis because it allows the bacterium to survive and grow in human blood by binding the human complement factor H. We recently reported the solution structure of the C-terminal domain of factor H-binding protein, which contains the immunodominant epitopes. In the present study, we report the structure of the full-length molecule, determined by nuclear magnetic resonance spectroscopy. The protein is composed of two independent barrels connected by a short link. Mapping the residues recognized by monoclonal antibodies with bactericidal or factor H binding inhibition properties allowed us to predict the sites involved in the function of the protein. The structure therefore provides the basis for designing improved vaccine molecules.

Epitope mapping of a bactericidal monoclonal antibody against the factor H binding protein of Neisseria meningitidis.

The factor H binding protein (fHbp) is a 27-kDa membrane-anchored lipoprotein of Neisseria meningitidis that allows the survival of the bacterium in human plasma; it is also a major component of a universal vaccine against meningococcus B. In this study, we used nuclear magnetic resonance spectroscopy, mutagenesis, and in silico modeling to map the epitope recognized by MAb502, a bactericidal monoclonal antibody elicited by fHbp. The data show that the antibody recognizes a conformational epitope within a well-defined area of the immunodominant C-terminal domain of the protein that is formed by two loops connecting different beta-strands of a beta-barrel and a short alpha-helix brought in spatial proximity by the protein folding. The identification of the protective epitopes of fHbp is an important factor for understanding the mechanism(s) of an effective immune response and provides valuable guidelines for designing variants of the protein able to induce broadly protective immunity.