Development of vaccines to prevent Neisseria gonorrhoeae infections

Abstract

Gonorrhoea, caused by the Gram-negative bacterium Neisseria gonorrhoeae, remains a prevalent sexually transmitted infection with an estimated ∼ 80 million new cases annually. The World Health Organization has designated N. gonorrhoeae as a high-priority pathogen due to its increasing resistance to all classes of antibiotics. This adaptability threatens the effectiveness of current treatments and raises concerns about the emergence of an era of untreatable gonorrhoea. An effective gonorrhoea vaccine is the most reliable and sustainable intervention to curb gonococcal transmission, prevent serious health complications, and protect vulnerable populations. However, developing a gonorrhoea vaccine presents significant scientific hurdles, include the bacterium’s high antigenic variability, lack of a clear immune correlate of protection and the absence of an animal model that fully replicates the complexities of infection in human. This study focused on MafA2/3 adhesin, an outer membrane adhesin protein, restricted to pathogenic Neisseria species and involved in initial colonization. MafA2/3 was identified in all studied N. gonorrhoeae isolates and recognized by sera from patients with uncomplicated gonorrhoea or individuals vaccinated with MenB-OMV vaccines. In this study, the analysis of 17,947 N. gonorrhoeae isolates showed that MafA2/3 adhesin protein is conserved with 98% sequence similarities of the most common 7 Alleles covering ∼91% of all N. gonorrhoeae isolates identified globally. The mafA-Allele 193 (strain P9-17, NEIS 0596, NGO 1393/1584) encoding sequence fused with His-SUMO (histidine-small ubiquitin-like modifier) and successfully expressed and purified as a recombinant protein in high yield in both E. coli BL21 (DE3) pLysS and E.coli Rosetta (DE3) pLysS. The BALB/c mice immunized with this recombinant protein produced antibodies that recognized MafA2/3 adhesin in ELISA, western blot and flow cytometry on the outer membrane preparations or the surface of live cells for homologous P9-17 and heterologous FA1090 gonococcal strains. The mice anti-MafA2/3 adhesin sera showed also cross-protective bactericidal activity against the homologous P9-17 and heterologous FA1090 for a titre of up to 256 and 64, respectively. Additionally, serum from a NZW rabbit immunized with MafA2/3 confirmed the protein’s presence across 50 CDC/FDA gonococcal isolates, although expression levels varied. Further, synthesized multiple antigen peptides vaccine based on MafA2/3 B-cell and T-cell epitope sequences, showed modest antigenicity and cross-bactericidal activity with a significant role of adjuvants. To enhance immune response breadth, a recombinant chimeric protein (renamed B-13), incorporating B-cell epitopes from 13 outer membrane proteins, was developed and expressed in E. coli. However, this chimeric vaccine showed no significant immunogenicity or bactericidal activity, highlighting the challenges of this vaccine approach. This study demonstrates that MafA2/3 adhesin protein can be produced cost-effectively at large scale using SUMO tag expression system and shows potential as a cross-protective gonorrhoea vaccine candidate. This study also advocates for using different vaccine approaches to tackle gonorrhoea infection as a polytheistic, not monotheistic vaccine.

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Identifiers

ORCID for Myron Christodoulides: orcid.org/0000-0002-9663-4731

ORCID for Chris McCormick: orcid.org/0000-0002-6155-9161

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