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Background: The impact of aging on the immune system is unequivocal and results in an altered immune status termed immunosenescence. In humans, the mechanisms of immunosenescence have been examined almost exclusively in blood. However, most immune cells are present in tissue compartments and exhibit differential cell (e.g., memory T cells -T M ) subset distributions. Thus, it is crucial to understand immunosenescence in tissues, especially those that are exposed to pathogens (e.g., intestine). Using a human model of oral live attenuated typhoid vaccine, Ty21a, we investigated the effect of aging on terminal ileum (TI) tissue resident memory T (T RM ) cells. T RM provide immediate adaptive effector immune responsiveness at the infection site. However, it is unknown whether aging impacts T RM S. Typhi-responsive cells at the site of infection (e.g., TI). Here, we determined the effect of aging on the induction of TI S. Typhi-responsive T RM subsets elicited by Ty21a immunization., Results: We observed that aging impacts the frequencies of TI-lamina propria mononuclear cells (LPMC) T M and T RM in both Ty21a-vaccinated and control groups. In unvaccinated volunteers, the frequencies of LPMC CD103- CD4+ T RM displayed a positive correlation with age whilst the CD4/CD8 ratio in LPMC displayed a negative correlation with age. We observed that elderly volunteers have weaker S. Typhi-specific mucosal immune responses following Ty21a immunization compared to adults. For example, CD103+ CD4+ T RM showed reduced IL-17A production, while CD103- CD4+ T RM exhibited lower levels of IL-17A and IL-2 in the elderly than in adults following Ty21a immunization. Similar results were observed in LPMC CD8+ T RM and CD103- CD8+ T cell subsets. A comparison of multifunctional (MF) profiles of both CD4+ and CD8+ T RM subsets between elderly and adults also showed significant differences in the quality and quantity of elicited single (S) and MF responses., Conclusions: Aging influences tissue resident T M S. Typhi-specific responses in the terminal ileum following oral Ty21a-immunization. This study is the first to provide insights in the generation of local vaccine-specific responses in the elderly population and highlights the importance of evaluating tissue immune responses in the context of infection and aging., Trial Registration: This study was approved by the Institutional Review Board and registered on ClinicalTrials.gov (identifier NCT03970304 , Registered 29 May 2019 - Retrospectively registered).

Background: Salmonella enterica serovar Typhi (S. Typhi) is a highly invasive bacterium that infects the human intestinal mucosa and causes ~ 11.9-20.6 million infections and ~ 130,000-223,000 deaths annually worldwide. Oral typhoid vaccine Ty21a confers a moderate level of long-lived protection (5-7 years) in the field. New and improved vaccines against enteric pathogens are needed but their development is hindered by a lack of the immunological correlates of protection especially at the site of infection. Tissue resident memory T (T RM ) cells provide immediate adaptive effector immune responsiveness at the infection site. However, the mechanism(s) by which S. Typhi induces T RM in the intestinal mucosa are unknown. Here, we focus on the induction of S. Typhi-specific CD4+T RM subsets by Ty21a in the human terminal ileum lamina propria and epithelial compartments., Methods: Terminal ileum biopsies were obtained from consenting volunteers undergoing routine colonoscopy who were either immunized orally with 4 doses of Ty21a or not. Isolated lamina propria mononuclear cells (LPMC) and intraepithelial lymphocytes (IEL) CD4+T RM immune responses were determined using either S. Typhi-infected or non-infected autologous EBV-B cell lines as stimulator cells. T-CMI was assessed by the production of 4 cytokines [interferon (IFN)γ, interleukin (IL)-2, IL-17A and tumor necrosis factor (TNF)α] in 36 volunteers (18 vaccinees and 18 controls volunteers)., Results: Although the frequencies of LPMC CD103+ CD4+T RM were significant decreased, both CD103+ and CD103- CD4+T RM subsets spontaneously produced significantly higher levels of cytokines (IFNγ and IL-17A) following Ty21a-immunization. Importantly, we observed significant increases in S. Typhi-specific LPMC CD103+ CD4+T RM (IFNγ and IL-17A) and CD103- CD4+T RM (IL-2 and IL-17A) responses following Ty21a-immunization. Further, differences in S. Typhi-specific responses between these two CD4+T RM subsets were observed following multifunctional analysis. In addition, we determined the effect of Ty21a-immunization on IEL and observed significant changes in the frequencies of IEL CD103+ (decrease) and CD103- CD4+T RM (increase) following immunization. Finally, we observed that IEL CD103- CD4+T RM , but not CD103+ CD4+T RM , produced increased cytokines (IFNγ, TNFα and IL-17A) to S. Typhi-specific stimulation following Ty21a-immunization., Conclusions: Oral Ty21a-immunization elicits distinct compartment specific immune responses in CD4+T RM (CD103+ and CD103-) subsets. This study provides novel insights in the generation of local vaccine-specific responses. Trial registration This study was approved by the Institutional Review Board and registered on ClinicalTrials.gov (identifier NCT03970304, Registered 29 May 2019-Retrospectively registered, http://www.ClinicalTrials.gov/NCT03970304).