Characterization of a Luminescence-Based Serum Bactericidal Activity Assay for Human Sera Against a Panel of Salmonella Strains

Salmonellosis remains a major cause of morbidity and mortality in low- and middle-income countries, despite the availability of effective vaccines against <i>Salmonella enterica</i> serovar Typhi (<i>S</i>. Typhi). In response, substantial efforts have been underway to develop vaccines against the key serovars responsible for invasive non-typhoidal <i>Salmonella</i> (iNTS) disease, such as <i>S</i>. Typhimurium and <i>S</i>. Enteritidis, as well as against <i>S</i>. Paratyphi A, which, together with <i>S</i>. Typhi, is responsible for enteric fever. The O-antigens (OAg) are considered potential protective antigens; therefore, the most advanced vaccine candidates focus on these moieties. However, no correlate of protection has been identified for either iNTS or paratyphoid fever, highlighting the importance of developing robust functional assays to assess vaccine-induced immunogenicity. In this study, we present the characterization of a high-throughput luminescence-based serum bactericidal assay (L-SBA) against multiple <i>S. enterica</i> serovars, using human sera. The assay was evaluated for repeatability, intermediate precision, linearity, and specificity against a panel of <i>Salmonella</i> strains belonging to serogroups O:4, O:9, and O:2, which were selected for their epidemiological relevance and diversity in OAg expression, quantity, and glucosylation/acetylation patterns. This assay will enable testing of clinical sera from vaccine trials to evaluate the breadth of the functional activity stimulated by current <i>Salmonella</i> vaccine candidates. L-SBA demonstrated an acceptable performance with all the tested strains, resulting in being linear, specific, and precise. This study also provided preliminary evidence that human sera containing antibodies against serogroup-specific OAg can efficiently kill <i>Salmonella</i> strains expressing OAg of the matched serovar, even in the presence of variation in OAg molecular weight, glucosylation, and acetylation. The L-SBA will enable testing of clinical sera from vaccine trials to evaluate the breadth of the functional activity stimulated by current <i>Salmonella</i> vaccine candidates.