A pan genome reverse vaccinology approach to prevent Streptococcus agalactiae infection in farmed tilapia

Abstract

Streptococcus agalactiae (Group B Streptococcus; GBS) is a Gram-positive pathogenic bacterium that causes disease in a wide range of animals including fish. Outbreaks of GBS in farmed fish have substantial economic impacts on the aquaculture industry, especially in tilapia production, a staple food product in tropical and subtropical low-and middle-income countries and the second most widely farmed fish after carp. Although vaccination can effectively prevent disease caused by GBS in fish it often fails after relatively short periods of use. Capsular polysaccharide (CPS) is the major protective antigen in GBS vaccines and a critical virulence factor. There are currently ten CPS serotypes of GBS and, whilst vaccination against one serotype confers protection in fish, it is not cross-protective against other serotypes. In addition, under the strong selective pressure of vaccination-driven adaptive immunity, evolution of novel serotypes can occur. To improve the serotype crossprotective efficacy of GBS vaccines for aquaculture, I employed a pan-genome reverse vaccinology approach to design candidate serotype-independent protective vaccines based on conserved proteins in fish-pathogenic GBS serotypes. Reverse vaccinology exploits recent advances in genomics to predict antigens in the genome sequence. Reverse vaccinology combined with pan-genome can identify the antigens that are conserved across a diversity of strains of a pathogen.To identify potential vaccine candidate antigens, a pan-genome was constructed for GBS comprising 82 genomes including representative isolates from wild marine fish and stingrays, farm and companion animals and from human clinical cases in Australia, along with isolates from farmed tilapia in Honduras and representative genomes from all current serotypes available via the NCBI Genbank database. Core genome single nucleotide polymorphism 3 (SNP) analysis indicated dissemination of the aquatic host-adapted ST-261 lineage in wild Australian fish from an ancestral tilapia strain, which is congruent with several introductions of tilapia into northern Australia from Israel and Malaysia during the 1970s and 1980s.Aquatic serotype Ib GBS strains have lost many virulence factors during adaptation, but six adhesins, named ST-261 adhesins, were well-conserved across the aquatic isolates and might be critical for virulence in fish and good targets for vaccine development. Three of six ST-261 adhesins were also conserved across all strains including terrestrial isolates and were chosen as vaccine candidates. Expressions of ST-261 adhesin genes were inversely co-regulated with CPS expression dependent upon carbon source and metal ions in the medium. However, gene transcription was not reflective of surface expression as serum antibodies derived in tilapia against recombinant adhesins did not detect similar variation in the level of ST-261 adhesins in whole cell ELISA or Western blot. This may be due to posttranscriptional processing causing a disconnect between gene and protein expression.To evaluate th...

Genomics‐driven prophylactic measures to increase streptococcosis resistance in tilapia

“…For example, GBS may not be recognized by vaccine‐induced antibodies if the respective antigens, particularly surface‐anchored proteins, are masked by bacterial capsular polysaccharides or not expressed properly at the stage of bacterial colonization/proliferation. Kawasaki (2019) reported that, despite the strong antibody responses in Nile tilapia, the recombinant adhesins (discovered by the RV approach) were unable to induce protection from a lethal challenge, which is likely due to the post‐transcriptional processing of adhesins and antigen masking by the bacterial capsule. Therefore, accurate prediction of genuine protective antigens, in silico or in vitro, is undoubtedly crucial to leverage the efficacy of the antigen discovery framework.…”

“…Furthermore, antigenic proteins detected by 'traditional RV' do not necessarily convey protective immune responses in vivo for various reasons. For example, GBS may not be recognized by vaccine-induced antibodies if the respective antigens, particularly surface-anchored proteins, are masked by bacterial capsular polysaccharides or not expressed properly at the stage of bacterial colonization/proliferation Kawasaki (2019). reported that, despite the strong antibody responses in Nile tilapia, the recombinant adhesins (discovered by the RV approach) were unable to induce protection from a lethal challenge, which is likely due to the post-transcriptional processing of adhesins and antigen masking by the bacterial capsule.…”