IgA and IgG1 Specific to Vi Polysaccharide of Salmonella Typhi Correlate With Protection Status in a Typhoid Fever Controlled Human Infection Model

Abstract

Vaccination against Salmonella Typhi using the Vi capsular polysaccharide, a T-cell independent antigen, can protect from the development of typhoid fever. This implies that antibodies to Vi alone can protect in the absence of a T cell-mediated immune response; however, protective Vi antibodies have not been well-characterized. We hypothesized that variability in the biophysical properties of vaccine-elicited antibodies, including subclass distribution and avidity, may impact protective outcomes. To interrogate the relationship between antibody properties and protection against typhoid fever, we analyzed humoral responses from participants in a vaccine efficacy (VE) trial using a controlled human infection model (CHIM) who received either a purified Vi polysaccharide (Vi-PS) or Vi tetanus toxoid conjugate (Vi-TT) vaccine followed by oral challenge with live S. Typhi. We determined the avidity, overall magnitude, and vaccine-induced fold-change in magnitude from before immunization to day of challenge of Vi IgA and IgG subclass antibodies. Amongst those who received the Vi-PS vaccine, Vi IgA magnitude (FDR p = 0.01) and fold-change (FDR p = 0.02) were significantly higher in protected individuals compared with those individuals who developed disease ("diagnosed"). In the Vi-TT vaccine group, the responses of protected individuals had higher fold-change in Vi IgA (FDR p = 0.06) and higher Vi IgG1 avidity (FDR p = 0.058) than the diagnosed Vi-TT vaccinees, though these findings were not significant at p < 0.05. Overall, protective antibody signatures differed between the Vi-PS and Vi-TT vaccines, thus, we conclude that although the Vi-PS and Vi-TT vaccines were observed to have similar efficacies, these vaccines may protect through different mechanisms. These data will inform studies on mechanisms of protection against typhoid fever, including identification of antibody effector functions, as well as informing future vaccination strategies.

Keywords: Salmonella Typhi (S. Typhi); Vi polysaccharide; antibody; avidity; correlates of protection (CoP); humoral immunity; typhoid fever; vaccination.

Figure 1

IgA dominates the vaccine-elicited antibody response to Vi polysaccharide. Concentration of antigen-specific, vaccine-induced IgA, IgG1, IgG2, and IgG3 to ViBIOT (A) and tetanus toxoid (B) by vaccine group, Vi-PS in black and Vi-TT in green, of positive vaccine responders only. Percent positive responders indicated post-vaccination at D0. Vi-PS vaccinees exhibited no vaccine-induced tetanus toxoid response. Data points are representative of n = 2 independent experiments (each with n = 2 technical replicates). Fold-change in magnitude of the response to Vi from Baseline to Day of Challenge across subclasses by vaccine group (C). A principal components analysis with all tetanus and Vi responses included (D) with a scatter plot of the first (PC1) and second (PC2) principal components is shown. Each measurement from a Vi-PS (n = 35 participants) or a Vi-TT (n = 37 participants) vaccinee is represented by a black or green dot, respectively. Ellipses represent 95% confidence regions.

Figure 2

ViBIOT-specific total IgA magnitude, fold change, and avidity higher in protected individuals of both vaccine groups. ViBIOT-specific IgA magnitude (A) and fold change (B) by vaccine group over time. ViBIOT-specific IgA magnitude (C) and fold change (D) by diagnosed/protected outcome at day of challenge. ViBIOT IgA avidity index (E) by diagnosed/protected outcome at day of challenge. Diagnosed and protected individuals are represented by open and closed circles, respectively. Data points are representative of n = 2 independent experiments (each with n = 2 technical replicates). n.s indicates non-significant FDR-corrected p values.

Figure 3

nViPS-specific total IgA magnitude, fold change, and avidity higher in protected individuals of both vaccine groups. nViPS-specific IgA magnitude (A) fold change (B) μg/mL equivalents of WHO typhoid IS 16/138 (C) and avidity index (D) by diagnosed/protected outcome at day of challenge. Data points are representative of n = 2 independent experiments (each with n = 2 technical replicates). n.s indicates non-significant FDR-corrected p values. nt indicates not tested for statistical significance.

Figure 4

nViPS IgA1 and IgA2 fold change higher in protected individuals in both vaccine groups. nViPS-specific IgA1 magnitude and fold change (A) and nViPS-specific IgA2 magnitude and fold change (B) by vaccine group from baseline to day of challenge. nViPS IgA1 and IgA2 avidity index (C) by vaccine group at day of challenge. IgA1 and IgA2 fold change from baseline to day of challenge by protected/diagnosed status (D) and IgA1 and IgA2 avidity index at day of challenge (E) by protected/diagnosed status. Data points are representative of n = 2 independent experiments (each with n = 2 technical replicates). Spearman correlation between nViPS IgA1 and IgA2 magnitude (F) at day of challenge. n.s indicates non-significant FDR-corrected p values.

Figure 5

Vi polysaccharide IgG1 avidity higher in protected individuals. ViBIOT IgG1 avidity index at pH 3.0 in positive vaccine responders by vaccine group over time (A) and by diagnosed/protected outcome at day of challenge (B). Avidity index in positive vaccine responses of anti-ViBIOT IgG1 over time (C) from 4 weeks post-vaccination (D0) to 6 months post-challenge (D180) by vaccine group and protection status. Bolded lines indicate median avidity index and faint lines indicate individual level data. Data points are representative of n = 2 independent experiments (each with n = 2 technical replicates). nt indicates not tested for statistical significance.

Figure 6

Different immune signatures of protection status for Vi-PS and Vi-TT. Variables in analysis of immune signatures between vaccine groups include magnitude, fold-change, and avidity of IgA1, IgA2, IgG1, IgG2, and IgG3 to Vi-Biot and nViPS. Volcano plot of FDR-corrected p-value and fold difference by diagnosed/protected outcome for Vi-PS vaccinees (A) and Vi-TT vaccinees (B). Red dots represent FDR-corrected p-values < 0.05, blue dots represent raw p-values <0.05 or fold difference >3 (trending).