Mucosal-associated invariant T (MAIT) cell responses in Salmonella enterica serovar Typhi strain Ty21a oral vaccine recipients

Abstract

Mucosal-associated invariant T (MAIT) cells are unconventional innate-like T cells abundant in human mucosal tissues and are associated with protective responses to microbial infections. MAIT cells have the capacity for rapid effector functions, including the secretion of cytokines and cytotoxic molecules. In this study, we examined the longitudinal circulating MAIT cell response to the live attenuated oral vaccine Ty21a (Ty21a) against Salmonella enterica serovar Typhi (S. Typhi). We enrolled healthy adults who received a course of oral live-attenuated S. Typhi strain Ty21a vaccine and assessed peripheral blood MAIT cell longitudinal responses pre-vaccination, and at seven days and one-month post-vaccination, using flow cytometry, cell migration, and tetramer decay assays. We showed that following vaccination, circulating MAIT cells were lower in frequency, but were more activated, and had higher levels of gut-homing marker integrin α4β7 and chemokine receptors CCR9 and CCR6, suggesting the potential of MAIT cells to migrate to mucosal sites. We found no significant differences in MAIT cell functionality, cytotoxicity and T-cell receptor avidity, except in TNF expression, which was higher post-vaccination. We show that MAIT cell immune responses are modulated post-vaccination against S. Typhi. This study contributes to our understanding of MAIT cells' potential role in oral vaccination against bacterial mucosal pathogens.

Keywords: Mucosal-associated invariant T (MAIT) cells; Salmonella enterica serovar Typhi; chemokine receptors; mucosal tissue; typhoid fever; vaccination; vaccine.

Figure 1.

Changes in S. Typhi specific antibody response post-vaccination. Plasma ELISA’s against S. Typhi LPS (A–C) IgA, IgG and IgM antibody responses pre-vaccination (day 0) and post-vaccination (day 7 and 1 month). The data are representative of two independent experiments out of a total of two. *P < .05, **P < .01, in Wilcoxon signed-rank test (paired samples).

Figure 2.

MAIT cell frequency and activation post-vaccination. (A) MAIT cell gating strategy and representative flow cytometry plots from unstimulated samples. (B) MAIT cells frequency as percentage of CD³⁺ T cells and (C) as percentage of CD⁸⁺ T cells. (D) absolute MAIT cell counts (CD³⁺ MAIT) pre-vaccination and post-vaccination. (E and F) Frequency of activated MAIT cells expressing CD38 and CD69 pre- and post-vaccination. Black circles are unstimulated donors. The data are representative of two independent experiments out of a total of two. *P <0.05, **P <0.01, ***P <0.001 in Wilcoxon signed-rank test (paired samples).

Figure 3.

MAIT cells effector function post-vaccination. PBMCs obtained from vaccine recipients (n = 11 per group) were stimulated with heat-killed S. Typhi at MOI of 100 and intracellular expression of (A) TNF (B) IFN-γ, (C) IL-17, (D) granzyme B and (E) Perforin in MAIT cells (CD³⁺ 5-OP-RU tetramer+ CD8+) were analyzed using flow cytometry (blue circle shows unstimulated and red circle shows stimulated data). Data were expressed as mean ± SEM. The data are representative of two independent experiments out of a total of two. Paired comparisons were made using Wilcoxon matched-pairs signed-rank test using Prism v9 (GraphPad). *P < .05, **P < .01, ***P < .001.

Figure 4.

Increased homing markers and chemokine receptor expression in MAIT cells post vaccination. PBMCs obtained from vaccine recipients (n = 11 per group) were stimulated with heat-killed S. Typhi at MOI of 100 and surface expression of (A) Integrin α4β7, (B) CCR9, (C) CD103, (D) CCR4, (E) CCR5, (F) CCR6, (G) CXCR5 expression in MAIT cells was measured using flow cytometry. Data were expressed as mean ± SEM. The data are representative of two independent experiments out of a total of two. Paired comparisons were made using Wilcoxon matched-pairs signed-rank test.