Western Australia experienced an outbreak of invasive meningococcal disease (IMD) caused by serogroup W clonal complex 11 (MenW:cc11) between 2013-2020. Successive vaccination campaigns of the ACYW conjugate vaccine in young adults and children were conducted in 2017 and 2019. In 2019-2020, five IMD cases were caused by serogroup C (MenC) isolates. We investigated whether capsule switching to serogroup C from serogroup W impacted meningococcal virulence.
A recombination-adjusted timed phylogeny of cc11 outbreak isolates from 83 IMD cases isolated between January 2013 and June 2020 was produced using Illumina paired-read sequencing aligned to a reference genome produced from an early outbreak isolate. Recombination was assessed using the Genome Comparator tool from the Neisseria PubMLST website and manual curation. To assess the impact of capsule switching on virulence, isolates were tested for colonization of Detroit 562 epithelial cells and survival in differentiated THP-1 macrophages. Mutants in sialic acid biosynthesis (cssB), capsule polysaccharide synthesis (csc/csw), and lipopolysaccharide (LOS) sialylation (lst) were compared between three MenW and three MenC isolates.
Phylogenetic analysis demonstrated that the five MenC:cc11 isolates were clonally related to the MenW:cc11 outbreak lineage and therefore were likely true capsule-switched variants. Including the capsule synthesis locus, six recombination regions were observed in all five MenC:cc11 isolates and were therefore predicted to have occurred either before or during the capsule switching event. These regions encoded both metabolic and known virulence genes including lst, opaD, and opaJ. Although adherence to epithelial cells was not significantly different between MenC and MenW isolates, invasivity changed in association with each sequential recombination event. MenC capsule expression inhibited epithelial invasion more than the MenW capsule (5.78-13.01-fold vs 0-3.01-fold reduction, respectively, p<0.05), while no effect was seen for LOS sialylation (3/4 isolates). While survival in macrophages remained similar, MenW isolates were internalised more readily than MenC isolates (p<0.001).
This study examined the impact of capsule switching on meningococcal virulence in the context of a meningococcal outbreak. We find support for the metabolic hypothesis of meningococcal virulence and show that compatibility of the meningococcal genome with a given capsule type is important for the maintenance of meningococcal virulence.