title: Mucosal and peripheral blood responses to vaccination with the licensed live oral typhoid vaccine Ty21a
Creators: name: Thompson, Ameeka
Publisher: name: University of Liverpool Repository
Year: 2012

description: The mucosal membranes represent the most common portal of entry for human pathogens and are protected by a specialised mucosal immune system which is distinct from the systemic immune system. Salmonella enterica serovar Typhi (S. Typhi) penetrates the body through the intestinal mucosa and causes both mucosal and systemic illness. Mucosal vaccination against S. Typhi with the licensed live oral typhoid vaccine Ty21a should elicit protective cellular responses both systemically and at the mucosal surface. However, nothing is known about the cell-mediated immune responses that are induced locally in the mucosa of the small intestine. It is also unclear whether Ty21a can elicit cellular mucosal immune responses at other mucosal surfaces, a property that would make it a candidate vaccine vector. This study aimed to answer these questions and to determine whether there is a correlation between the immune response elicited at different mucosal sites or between the intestinal mucosa and blood. Healthy adults were either vaccinated with Ty21a (n=10) or allocated to the unvaccinated control group (n=7). Blood was collected from both groups on day 0 (pre-vaccination sample) and day 18 (post-vaccination sample) by venesection. Cells were separated by differential centrifugation and analysed by flow cytometry. Mucosal samples (duodenum ± sigmoid colon) were collected by endoscopy and mucosal pinch-biopsy on day 18. Cells were separated by collagenase digestion and mechanical disruption and analysed by flow cytometry. Two functional T cell assays, an intracellular cytokine staining assay (interferon-γ, tumour necrosis factor-α and interleukin-2) and a lymphoproliferation assay, were used to assess antigen-specific cellular immune responses. The results demonstrated that in blood, there was no statistically significant difference in the mean percentage of antigen-specific CD4+ T cells in the intracellular cytokine staining assay between the control and vaccinated groups at day 0 but there was at day 18. This was due to an unexpected fall in the mean percentage of antigen-specific CD4+ T cells in the control group between day 0 and day 18. In the blood lymphoproliferation assay, there was no statistically significant difference between the control and vaccinated groups at day 0 or day 18. There was, however, a statistically significant increase in the mean percentage of antigen-specific cytokine-producing CD4+ T cells in the vaccinated group compared to the control group in duodenal tissue, but not in the colon at day 18. In the vaccinated group, there was not a strong correlation between duodenum and colon or between duodenum and blood in terms of the percentage of antigen-specific cytokine-producing CD4+ T cells. This is the first study to report antigen-specific mucosal T cell responses to an oral vaccine, and we were able to demonstrate a significant increase in intracellular cytokine responses to Ty21a antigens in T cells from the mucosal surface. The results from this study support the theory that local immune responses against pathogens such as S. Typhi which gain access to the body through mucosal membranes are important. In addition, they suggest that the cellular immune response measured in peripheral blood may not correlate with that measured at the mucosal surface, emphasising the potential value of obtaining mucosal tissue for direct study. This has implications for the evaluation of future vaccine candidates which often relies on systemic immune responses to assess vaccine immunogenicity. This study will now be extended beyond this interim analysis by recruiting more individuals to confirm the findings.