Antibody Responses to Salmonella Typhi Antigens among Typhoid Recovered Individuals in an Endemic Region

PurposeThis study focuses on evaluating the immune responses, specifically levels of IgA, IgG, and IgM antibodies against five distinct Salmonella Typhi antigens (Vi polysaccharide, H-protein, Lipopolysaccharide, Hemolysin E, and OMP) in the plasma samples collected from participants 3 months after typhoid. We also examined if the anti-Vi polysaccharide antibodies present in the typhoid-recovered participants plasma had an activating effect on NK cells isolated from healthy individuals. MethodsELISA was used to measure the antibody responses to the five S. Typhi antigens in plasma samples from typhoid-recovered individuals and healthy participants. Vi polysaccharide specific plasma antibodies were tested for activation of the NK cells isolated from the healthy participants. ResultsPolysaccharide antigens mounted better IgA responses among typhoid recovered individuals as compared to healthy individuals. Anti-S. Typhi IgM was significantly elevated in the typhoid group for all five antigens (p<0.05). No significant differences were seen in IgG responses. Notably, healthy participants also exhibited considerable levels of antibodies to all the antigens, indicating background seroreactivity. ConclusionPolysaccharide antigens elicited significantly better IgA responses in recovered typhoid participants, possibly reflecting mucosal immune priming from the gut. Presence of IgM antibodies in recovered participants suggest a durable IgM response after Typhoid. Similar levels of IgG in both the groups may indicate waning IgG levels towards the baseline post-infection. These findings highlight the complexity of the immune response against S. Typhi in endemic settings.