Determination of O:4 antigen-antibody affinity level in O:5 antigen positive and negative variants of Salmonella enterica serovar Typhimurium

Genetic and Structural Variation in the O-Antigen of Salmonella enterica Serovar Typhimurium Isolates Causing Bloodstream Infections in the Democratic Republic of the Congo

Salmonella enterica serovar Typhimurium causes a devastating burden of invasive disease in sub-Saharan Africa with high levels of antimicrobial resistance. No licensed vaccine is available, but O-antigen-based candidates are in development, as the O-antigen moiety of lipopolysaccharides is the principal target of protective immunity. The vaccines under development are designed based on isolates with O-antigen O-acetylated at position C-2 of abequose, giving the O:5 antigen. Serotyping data on recent Salmonella Typhimurium clinical isolates from the Democratic Republic of the Congo (DRC), however, indicate increasing levels of isolates without O:5. The importance and distribution of this loss of O:5 antigen in the population as well as the genetic mechanism responsible for the loss and chemical characteristics of the O-antigen are poorly understood. In this study, we Illumina whole-genome sequenced 354 Salmonella Typhimurium isolates from the DRC, which were isolated between 2002 and 2017. We used genomics and phylogenetics combined with chemical approaches (¹H nuclear magnetic resonance [NMR], high-performance anion-exchange chromatography with pulsed amperometric detection [HPAEC-PAD], high-performance liquid chromatography–PAD [HPLC-PAD], and HPLC-size exclusion chromatography [HPLC-SEC]) to characterize the O-antigen features within the bacterial population. We observed convergent evolution toward the loss of the O:5 epitope predominantly caused by recombination events in a single gene, the O-acetyltransferase gene oafA. In addition, we observe further O-antigen variations, including O-acetylation of the rhamnose residue, different levels of glucosylation, and the absence of O-antigen repeating units. Large recombination events underlying O-antigen variation were resolved using long-read MinION sequencing. Our study suggests evolutionary pressure toward O-antigen variants in a region where invasive disease by Salmonella Typhimurium is highly endemic. This needs to be taken into account when developing O-antigen-based vaccines, as it might impact the breadth of coverage in such regions.

The protective capacity of anti-O4 antigen antibodies against Salmonella infection is influenced by the presence or absence of the O5 antigen

Anti-O-antigen antibodies, such as anti-O4 antigen IgG, induce protective immunity against Salmonella enterica serovar Typhimurium (S. Typhimurium) infection. S. Typhimurium belongs to the group O4, which can be classified into two serological variants, namely factor O5 antigen positive (O5+) and factor O5 antigen negative (O5-). In this study, we determined the protective immunity induced by anti-O4 antigen IgG against O5+ and O5- S. Typhimurium infection in a mouse model. Unexpectedly, anti-O4 antigen IgG induced protection against O5- of S. Typhimurium, but not against O5+ of S. Typhimurium. We suggest that the affinity of the O4 antigen with anti-O4 antigen IgG is stronger in the O5- S. Typhimurium compared to the O5+ S. Typhimurium. Although anti-O4 antigen IgG has the potential to protect against S. Typhimurium infection, the effects of anti-O4 antigen IgG in protection against Salmonella infection differ depending on the presence or absence of the O5 antigen.