Bactericidal activity of antibodies elicited against the Neisseria meningitidis 37-kDa ferric binding protein (FbpA) with different adjuvants.

The 37-kDa ferric binding protein, FbpA, from three Neisseria meningitidis strains was purified to homogeneity with iron-affinity chromatography and used for immunisation of mice employing four different adjuvants: aluminium hydroxide, Freund's, the saponin Quil-A, and a Ribi adjuvant system (RAS). Controls immunised without adjuvant were also included. All sera obtained were monospecific for the meningococcal FbpA, with antibody titres higher when RAS and Quil-A were used (256), PBS resulting in titres similar to those of Freund's (64), and, surprisingly, with no antibodies elicited when aluminium hydroxide, the only approved adjuvant for use in humans, was used. All anti-FbpA sera bound to intact meningococcal cells, showing a complete cross-reactivity, but the bactericidal activity of anti-FbpA antibodies, demonstrated for the first time in this work, was low (32% of killing with the homologous strain), and the analysis of immunoglobulin isotypes showed that the non-bactericidal IgG1 was predominant. The results confirm that the FbpA is surface-exposed, antigenic, and able to elicit bactericidal antibodies, although, in the conditions and with the adjuvants tested, killing efficacy was low and cross-killing was very variable, not supporting the inclusion of this protein in vaccine formulations. Nevertheless, given the high conservation of the FbpA in the genus Neisseria, its surface exposure and its antigenicity, studies on immunisation with peptides corresponding to the exposed epitopes and/or new adjuvant systems could improve the bactericidal response to this protein, making it suitable for vaccine development.