UC Davis

Systemic Salmonella enterica infections are caused by serovars Typhi and Paratyphi, causing typhoid, and other serovars, which cause invasive non-typhodial Salmonellosis (iNTS). These infections cause about 200,000 deaths per year and thus are a large public health burden. Currently, the vaccines that are approved for typhoid are not given to individuals who live in endemic areas due to low efficacy and there are no approved vaccines for iNTS. Thus, an effective vaccine is desperately needed to combat these infections. How the host naturally protects itself from pathogens is required to make an effective vaccine. CD4 T cells are required for protection against systemic Salmonella. CD4 T cell memory is composed of three subsets: central, effector and tissue resident memory. Central memory T cells (TCM) circulate throughout the secondary lymphoid organs (SLOs) and blood while effector memory T cells (TEM) circulate throughout the SLOs, blood and non-lymphoid tissues (NLTs). Both TCM and TEM circulate the body looking for cognate antigen to be reactivated and therefore, together are termed circulating memory. The third subset tissue resident memory T cells (TRM) do not circulate and resides in previously infected tissues and become the front line of defense for subsequent infections. Previously, CD4 TRM in the liver has been shown to be required for complete protection against systemic Salmonella. Thus, to make an effective vaccine, this memory subset needs to be induced. However, how to induce the formation of CD4 TRM in the liver during vaccination is currently unknown. Previously, transcription factors have been shown to be important in the formation of CD8 TRM in a variety tissues and allow CD8 TRM to adapt and survive in the tissue microenvironments. In this study we demonstrate a novel way transcription factors impact TRM formation. T-bet acts upstream of CD4 TRM differentiation in tissues and T-bethi CD4 T cells express higher levels of LFA-1 which allow these TRM precursors to compete for the competitive liver niche where they can access signals in the liver microenvironment which will induce the CD4 T cells to become TRM. These TRM differentiation signals have previously been interrogated in CD8 TRM formation in a variety of tissues with inflammation and pro-inflammatory cytokines driving CD8 TRM formation. In this study we demonstrated that liver inflammation increases CD4 TRM formation in the liver with pro-inflammatory cytokines, IL-2 and IL-1 required for optimal CD4 TRM formation. Additionally, adding liver inflammation enhances the protection provided by Salmonella specific subunit vaccination which is partially protective. Thus, this study has demonstrated there are two phases in CD4 TRM formation in the liver. In the secondary lymphoid organs, CD4 T cell priming impacts T-bet expression and T-bethi CD4 T cells have a higher chance of accessing the liver niche by increased LFA-1 expression. Once in the liver microenvironment, liver inflammation, specifically IL-2 and IL-1, stimulated by Salmonella infection induces CD4 T cells to become TRM. This understanding of CD4 TRM formation can further be applied to vaccine design for systemic Salmonella infection and aid in combating this large public health burden.