Human B- and T-cell responses after immunization with a hexavalent PorA meningococcal outer membrane vesicle vaccine.

The PorA protein from Neisseria meningitidis, a potential vaccine candidate, induces human bactericidal antibodies which are serosubtype specific. We developed a hexavalent PorA outer membrane vesicle vaccine based on reference strain H44/76. This vaccine contains the six most prevalent PorA serosubtypes as found in many countries. We previously reported on the immune responses of 20 adult volunteers after a single immunization with this vaccine. In this study, the B- and T-cell responses in three adult volunteers were studied after three consecutive immunizations (0, 2, and 11 months). The first immunization induced a strong B-cell response resulting in high immunoglobulin G levels in an outer membrane vesicle enzyme-linked immunosorbent assay. At least a fourfold increase in bactericidal activity was observed against the majority (four to six) of the vaccine antigens compared to prevaccination titers. Immunodominance was observed for one or two of the PorAs in the bactericidal assay with a set of six isogenic H44/76-derived PorA target strains. These strains carry the individual PorAs as present in the vaccine. The second and third immunizations did not induce a further increase in the immune responses. A decline in time with respect to PorA-specific antibodies was observed after each immunization. These observations were reflected by the T-cell proliferation responses. Two additional sets of isogenic H44/76-derived mutant strains were used to study the specificity and/or cross-reactivity of the induced bactericidal antibodies. These target strains differ only in expressing mutant family variants of either PorA P1.7,16 or P1.5,10, both present in the PorA vesicle vaccine. The bactericidal antibody responses found were directed predominantly against the P1.7 (loop 1 of P1.7,16) and the P1.10 (loop 4 of P1.5,10) epitopes. This indicates that different portions of PorA were involved in the induction of bactericidal antibodies depending upon the PorA serosubtype.