Evaluation of a standardised Vi poly-l-lysine ELISA for serology of Vi capsular polysaccharide antibodies

Abstract

Typhoid vaccines based on protein-conjugated capsular Vi polysaccharide (TCVs) prevent typhoid in infants and young children. Analysis of the serum anti-Vi IgG response following immunisation against typhoid confirms the immunogenicity of TCVs and forms an important part of the pathway to licensing. Comparative studies could expedite the licencing process, and the availability of a standardised ELISA method alongside the 1st International Standard (IS) 16/138 for anti-typhoid capsular Vi polysaccharide IgG (human) will facilitate this process. To this end, a non-commercial ELISA based on a coat of Vi and poly- l -lysine (Vi-PLL ELISA) was evaluated by 10 laboratories. Eight serum samples, including IS 16/138, were tested in the standardised Vi-PLL ELISA ( n = 10), a commercial Vi ELISA ( n = 3) and a biotinylated Vi ELISA ( n = 1). Valid estimates of potencies relative to IS 16/138 were obtained for all samples in the Vi-PLL ELISA and the commercial ELISA, with good repeatability and reproducibility evident from the study results and concordant estimates obtained by the two ELISA methods. The study demonstrates that the Vi-PLL ELISA can be used in clinical trial studies to determine the immunogenicity of TCVs.

Glycoconjugate vaccines against Salmonella enterica serovars and Shigella species: existing and emerging methods for their analysis

“…Following the request of the WHO Expert Committee on Biological Standardization (ECBS), Vi-PLL ELISA has been finally assessed in 2020 as a suitable substitute to the VaccZyme ELISA, allowing to use this uniform non-commercial, and thus more available, method to use at a lower cost. The Vi-PLL procedure is publicly available and uses standard biological agents, as opposed to VaccZyme (WHO 2018 ; Rigsby et al 2020 ). Key considerations for implementation and smooth performance are the level of reagent standardisation (reference standard), validity criteria (system and sample suitability) and the inclusion of assay-specific run controls which may be study samples representative of Vi PS vaccines (Rigsby et al 2020 ).…”

“…The Vi-PLL procedure is publicly available and uses standard biological agents, as opposed to VaccZyme (WHO 2018 ; Rigsby et al 2020 ). Key considerations for implementation and smooth performance are the level of reagent standardisation (reference standard), validity criteria (system and sample suitability) and the inclusion of assay-specific run controls which may be study samples representative of Vi PS vaccines (Rigsby et al 2020 ).…”

“…Standardised ELISA methods are available for immunogenicity determination. Currently, two methods have been accredited as repeatable and reliable for serological ELISAs of serum responses against Vi-based conjugates from clinical studies: commercial VaccZyme and in-house Vi-poly-L-lysine (PLL)-based immunoassays, where Vi is pre-coated with PLL to give superior results to native coating, as PS is negatively charged allowing PLL to adhere and improve Vi interaction with solid phase (Rijpkema et al 2018;Rigsby et al 2020). The biotinylated Vi ELISA can be used for control comparison of potencies but not for the geometric mean potencies, or (Giuntini et al 2017) potency estimates, as it gives alternative reads to the VaccZyme ELISA (Rijpkema et al 2018).…”

Doehlert Matrix-assisted optimization of Salmonella typhi Vi polysaccharide purification parameters

“…It was reported that due to tourism also the bacteria spreads across the various European countries and USA [3]. The rate of risk of infection is severe in countries in the continents of Asia, Africa, and Latin America where poor sanitation is observed [7]. Typhoid fever is mostly observed in underaged children and school going children.…”

“…Currently, there are two quite distinct vaccines that are available in the market. One oral live attenuated vaccine that contain strain Ty21a and Vi polysaccharide vaccine administered parenterally [7]. The oral typhoid vaccine is given in the form of enteric capsules as three to four doses [8].…”