Qualification of a high-throughput luminescence-based Serum Bactericidal Activity assay to assess the functionality of human sera against five Shigella serotypes.

Shigellosis is one of the leading causes of diarrhoeal disease worldwide. It mostly affects children under 5 years and elderly persons over 70 years in low- and middle-income countries (LMICs). The high disease incidence in LMICs and the rise of antimicrobial resistance have prompted efforts to develop novel vaccines against shigellosis. Several multicomponent vaccines are currently under development; to date, however, no correlate of protection against shigellosis has been defined. The Serum Bactericidal Activity assay (SBA) is regularly applied to measure the complement-mediated functional activity of vaccine-elicited antibodies for other bacterial species. To facilitate the evaluation of Shigella candidate vaccines and to support licensing, a high-throughput SBA, based on luminescence readout (L-SBA), has been developed to test antibody functionality. In this study, the assay has been transferred to a Clinical Research Organization (CRO) and qualified for five epidemiologically relevant Shigella serotypes (S. flexneri 2a, 1b, 3a, 6 and S. sonnei) using pooled human serum samples. The repeatability, intermediate precision and linearity of the assay were confirmed, together with the ability of the assay to detect serotype specific Shigella antibody activity. This study confirms that the L-SBA method can be performed by different laboratories and is suitable for testing multicomponent vaccine candidates against shigellosis.