Validation and use of a serum bactericidal antibody assay for Neisseria meningitidis serogroup X in a seroprevalence study in Niger, West Africa

Invasive meningococcal disease (IMD) affects approximately 1.2 million people worldwide annually. Prevention of IMD is mostly provided through vaccination; however, no licensed vaccine is currently available to protect against meningococcal serogroup X associated infection. Limited data are available on the natural immunity to Neisseria meningitidis serogroup X within the African sub-Saharan meningitis belt. The objective of the study was to provide an overview of natural immunity to serogroup X within a community in the African meningitis belt prior to the introduction of a pentavalent conjugate vaccine (NmCV-5). Prior to its introduction, a validated assay to assess vaccine efficacy was also required. This study therefore incorporated two objectives: a seroprevalence study to assess natural immunity in serum samples (n = 377) collected from Niger, West Africa in 2012, and the validation of a serogroup X serum bactericidal antibody (SBA) assay. Seroprevalence data obtained found that natural immunity to N. meningitidis serogroup X were present in 52.3% of study participants. The highest putative protective titres (≥8) to serogroup X were seen in age group 5-14 years-old (73.9%) and lowest in ages < 1 year old (0%). The SBA assay was successfully validated for selectivity/specificity, precision/reproducibility, linearity, and stability. This study demonstrated the suitability of the serogroup X SBA assay in clinical trials for future meningococcal conjugate vaccines containing serogroup X polysaccharides.

Meningococci of Serogroup X Clonal Complex 181 in Refugee Camps, Italy

Four cases of infection with serogroup X meningococci (MenX) (1 in 2015 and 3 in 2016) occurred in migrants living in refugee camps or reception centers in Italy. All MenX isolates were identified as clonal complex 181. Our report suggests that serogroup X represents an emerging health threat for persons arriving from African countries.

Genetic Analysis of Neisseria meningitidis Sequence Type 7 Serogroup X Originating from Serogroup A

Neisseria meningitidis causes meningococcal disease, often resulting in fulminant meningitis, sepsis, and death. Vaccination programs have been developed to prevent infection of this pathogen, but serogroup replacement is a problem. Capsular switching has been an important survival mechanism for N. meningitidis, allowing the organism to evolve in the present vaccine era. However, related mechanisms have not been completely elucidated. Genetic analysis of capsular switching between diverse serogroups would help further our understanding of this pathogen. In this study, we analyzed the genetic characteristics of the sequence type 7 (ST-7) serogroup X strain that was predicted to arise from ST-7 serogroup A at the genomic level. By comparing the genomic structures and sequences, ST-7 serogroup X was closest to ST-7 serogroup A, whereas eight probable recombination regions, including the capsular gene locus, were identified. This indicated that serogroup X originated from serogroup A by recombination leading to capsular switching. The recombination involved approximately 8,540 bp from the end of the ctrC gene to the middle of the galE gene. There were more recombination regions and strain-specific single-nucleotide polymorphisms in serogroup X than in serogroup A genomes. However, no specific gene was found for each serogroup except those in the capsule gene locus.

Neisseria meningitidis Serogroup X in Sub-Saharan Africa

The epidemiology of meningococcal disease varies by geography and time. Whole-genome sequencing of Neisseria meningitidis serogroup X isolates from sub-Saharan Africa and Europe showed that serogroup X emergence in sub-Saharan Africa resulted from expansion of particular variants within clonal complex 181. Virulence of these isolates in experimental mouse models was high.