Murine monoclonal antibodies to PorA of Neisseria meningitidis show reduced protective activity in vivo against B:15:P1.7,16 subtype variants in an infant rat infection model.

The major outer membrane protein PorA of Neisseria meningitidis is the target for bactericidal serosubtyping antibodies and is currently considered as a potential vaccine candidate against group B meningococcal disease. Although the minor antigenic variability of the PorA has been increasingly recognized and described, its implication for vaccine design remains unclear. In this study, the protective activity of murine monoclonal PorA specific antibodies against four isogenic meningococcal P1.7,16 target strains, the prototype P1.7,16a and three loop 4 point mutation variants (designated P1.7,16b to d) constructed from reference strain H44/76 (B:15:P1.7,16a), was evaluated in the infant rat infection model. All monoclonal antibodies had been obtained by immunization of mice with outer membrane protein preparations from meningococcal serosubtype P1.7,16 reference strain H44/76. A challenge dose of 10(5)cfu/pup was given i.p. 1-2 h after the i.p. injection of 1:100 diluted antibodies, and the development of bacteremia was assessed by culturing blood samples taken 6 h after challenge. MN14C11.6, a reference monoclonal antibody for serosubtype P1.7 epitope located in predicted loop 1 (VR1) identical in all the variants, was equally protective against all loop 4 variants. The three P1.16 specific monoclonal antibodies tested (MN5C11G, MN12H2 and 62D12-8) all completely protected animals against the prototype P1.7,16a, variably against the P1.7,16b and P1.7,16c, but not against the P1.7,16d variant. Our findings therefore suggest that certain subtype variants may escape protection in vivo conferred by PorA specific antibodies.