Live oral typhoid vaccine Ty21a induces cross-reactive humoral immune responses against Salmonella enterica serovar Paratyphi A and S. Paratyphi B in humans

Abstract

Enteric fever caused by Salmonella enterica serovar Paratyphi A infection has emerged as an important public health problem. Recognizing that in randomized controlled field trials oral immunization with attenuated S. enterica serovar Typhi live vaccine Ty21a conferred significant cross-protection against S. Paratyphi B but not S. Paratyphi A disease, we undertook a clinical study to ascertain whether humoral immune responses could explain the field trial results. Ty21a immunization of adult residents of Maryland elicited predominantly IgA antibody-secreting cells (ASC) that recognize S. Typhi lipopolysaccharide (LPS). Cross-reactivity to S. Paratyphi A LPS was significantly lower than that to S. Paratyphi B LPS. ASC producing IgG and IgA that bind LPS from each of these Salmonella serovars expressed CD27 and integrin α4β7 (gut homing), with a significant proportion coexpressing CD62L (secondary lymphoid tissue homing). No significant differences were observed in serum antibody against LPS of the different serovars. Levels of IgA B memory (B(M)) cells to S. Typhi LPS were significantly higher than those against S. Paratyphi A or B LPS, with no differences observed between S. Paratyphi A and B. The response of IgA B(M) to outer membrane proteins (OMP) from S. Typhi was significantly stronger than that to OMP of S. Paratyphi A but similar to that to OMP of S. Paratyphi B. The percentages of IgG or IgA B(M) responders to LPS or OMP from these Salmonella strains were similar. Whereas cross-reactive humoral immune responses to S. Paratyphi A or B antigens are demonstrable following Ty21a immunization, they cannot explain the efficacy data gleaned from controlled field trials.

Fig 1

Specific antibody-secreting cells (ASC) in Ty21a vaccinees. Levels of IgA (A) and IgG (B) ASC specific to lipopolysaccharide (LPS) from S. Typhi, S. Paratyphi A (Para A), and S. Paratyphi B (Para B) in Ty21a vaccinees (n = 16) before and 7 days after immunization are shown. Data are expressed as mean ± standard error of the mean (SEM). #, P < 0.05 compared to the respective day 0 value. **, P < 0.001; *, P < 0.05 (Wilcoxon matched-pair test).

Fig 2

Homing characteristics of ASC in Ty21a vaccinees. Shown are IgA (A, B, and C) and IgG (D, E, and F) ASC specific for LPS from S. Typhi (A and D), S. Paratyphi A (B and E), and S. Paratyphi B (C and F) in sorted subsets from 4 individual Ty21a vaccinees. Data for each sorted subset as defined in the legend box are shown as the percentages of total LPS-specific ASC observed in the corresponding subject (ASC in each sorted subset/total ASC detected in the corresponding subject × 100). Error bars indicate SEMs. **, P < 0.01; ***, P < 0.001. #, P < 0.01 compared to the corresponding B_M LN subset (Mann-Whitney test, 2 tailed).

Fig 3

LPS-specific serum antibody responses in Ty21a vaccinees. Shown are postvaccination fold increases in serum anti-LPS antibodies to S. Typhi-, S. Paratyphi A (Para A)-, and S. Paratyphi B (Para B)-specific IgA (A) and IgG (Panel B) in Ty21a vaccinees (n = 17). The dashed horizontal lines represent 4-fold increases, the cutoff for seroconversion. Error bars indicate SEMs. **, P < 0.01; *, P < 0.05 (by Wilcoxon matched-pair test, two tailed).

Fig 4

B memory (B_M) responses in Ty21a vaccinees, showing specific IgA B_M responses for LPS from S. Typhi, S. Paratyphi A (Para A), and S. Paratyphi B (Para B) following vaccination with Ty21a (n = 15). Shown are prevaccination (day 0; open bars) and postvaccination (at either day 42, 84, or 118; closed bars) peak levels (A) and postvaccination peak net increases in B_M frequency (net = postvaccination peak minus prevaccination level) (B). The dashed horizontal line represents the cutoff for postvaccination responders, defined as described in Materials and Methods. Error bars indicate SEs. P values that are written out are for postvaccination peaks, compared to the corresponding levels at day 0. **, P < 0.001; *, P < 0.01 (Wilcoxon matched-pair test, 2 tailed).

Fig 5

B memory (B_M) responses in Ty21a vaccinees, showing specific IgA B_M responses for outer membrane protein (OMP) from S. Typhi, S. Paratyphi A (Para A), and S. Paratyphi B (Para B) following vaccination with Ty21a (n = 16). Shown are prevaccination (day 0; open bars) and postvaccination (at either day 42, 84, or 118; closed bars) peak levels (A) and postvaccination peak net increases in B_M frequency (net = postvaccination peak minus prevaccination level) (B). The dashed horizontal line represents the cutoff for postvaccination responders, defined as described in Materials and Methods. Error bars indicate SEs. P values that are written out are for postvaccination peaks, comparing the corresponding levels at day 0. **, P < 0.001; *, P < 0.05 (Wilcoxon matched-pair test, two tailed).

Fig 6

Percentages of responders for LPS (A) and OMP (B) from S. Typhi-, S. Paratyphi A-, and S. Paratyphi B-specific antibody in culture supernatants (ALS) among Ty21a vaccinees (n = 15). Responders were defined as those with at least a 2-fold rise in antibody titer at any postimmunization time (days 42, 84, and 180) compared to the corresponding preimmunization (day 0) level.