Identification of potential vaccine candidates against Neisseria meningitidis

Abstract

Neisseria meningitidis is an important cause of invasive bacterial infection in children worldwide. The difficulty of inducing an effective immune response against the serogroup B N. meningitidis (NmB) capsular polysaccharide has lead to the search for vaccine candidates in different strategies. Outer membrane vesicle (OMV) based vaccines have been used to provide strain-specific protection to NmB infection. The identification of immunogenic outer membrane proteins (OMPs) may help to develop a vaccine with broader strain coverage. Ninety-three OMPs were selected, based on proteomic analysis of OMV content, and other criteria. OMPs were individually expressed, purified, and refolded before being printed onto microarray slides. IgG responses were determined using antisera from vaccinated mice, or adults who had received an OMV vaccine in a Phase I Clinical Trial (PMCID: PMC4535279). For human antisera, IgG binding was quantified pre-immunization, and at two post-immunization stages. Highly immunogenic antigens in mouse and human were determined based on the magnitude and significance of IgG responses between pre- and post-immunizations. All antigens with the highest responses to human IgG were integral OMPs (iOMPs) but, in mice, the proportion of iOMPs and soluble proteins was more evenly distributed. The repertoire of highly responding antigens in human and mouse was similar, but not identical. The dominance of IgG responses to iOMPs in humans emphasizes the importance of these antigens in the immune response to OMV vaccination. This result raises the importance of maintaining protein conformational integrity, particularly for integral membrane proteins. Additionally, this work shows that protein antigen microarray panels can be a valuable asset to study responses to vaccines which contain complex mixtures of antigens, such as OMVs. The microarray study confirmed the importance of the PorA porin as a principal antigen in eliciting immunity against meningococcal disease. In this study it was shown that recombinant PorA was able to induce differentiation of monocytic THP-1 cells, as reflected by the augmented expression of the co-stimulatory molecules CD40 and CD86, as well as MHC II molecules, by flow cytometry. In addition, PorA derived from invasive strains induced significantly increased expression of chemokines which are active against neutrophils (RANTES), monocytes (IP-10), macrophages (IL-6, IP-10, MIF), T-cells (IL-6, IP-10, RANTES), plasma cells (IL-6) and DC (IP-10), as well as the recruitment of monocyte and neutrophil cells. The variable activation ability of different PorA variants against THP-1 cells may be due to differential susceptibility to cathepsins, which affect the digestion of endocytic antigens and the binding to the MHC II molecules. In summary, this in vitro data suggests that PorA variants from invasive strains promote higher levels of inflammation, whereas PorA variants from commensal strains induce a lower magnitude of inflammatory responses which may benefit colonisation.

Date of Award	19 Jan 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jeremy Derrick (Supervisor) & James Linton (Supervisor)