Title

Investigating the adjuvant effects of lipopolysaccharide on T cells in mice

Date of Completion

January 2009

Keywords

Health Sciences, Immunology

Degree

Ph.D.

Abstract

Lipopolysaccharide (LPS) is an essential component of the cell wall of gram-negative bacteria and stimulates the mammalian immune system through Toll-like receptor 4, resulting in the activation of MyD88- and TRIF-dependent signaling pathways which induce inflammatory cytokines. When injected shortly after a T cell antigen, LPS enhances T cell clonal expansion, long-term survival, differentiation into IFN-γ-producing Th1 cells, and migration to extra-lymphoid tissues, and therefore has desirable properties of a vaccine adjuvant. The present studies were undertaken to explore the contribution of signaling pathways and cell types in LPS adjuvanticity, as well as to explore effects on T cell subsets other than Th1. LPS enhanced murine T cell clonal expansion and long-term survival through the MyD88 pathway on non-T cells, while Th1 differentiation and accumulation into the liver and lung were driven by TRIF. Since CD11c+ dendritic cells (DCs) are important antigen presenting cells, their contribution was determined by conditional depletion. CD11c+ cells were required for optimal Th1 differentiation, revealing their preferential involvement in the TRIF-dependent pathway. This data supports a model in which LPS stimulates macrophages in vivo to release T cell survival factors through MyD88 and DC activation factors through TRIF. Interactions between activated DCs and T cells then result in Th1 differentiation and migration to extra-lymphoid tissues such as liver and lung. Intraperitoneal immunization with antigen and LPS also potently drove T cell priming in small intestinal lamina propria (LP). In contrast to spleen, the LP contained high frequencies of IL-17-producing Th17 cells, nearly equaling the number of Th1 cells. Immunizing IL-23p19-deficient mice resulted in reduced frequencies of T cells producing IL-17, and reduced overall numbers of T cells in LP. Thus, IL-23 appears to be an intestinal niche-specific T cell survival factor that drives local Th17 differentiation. This project exemplified how individual signaling pathways and cell types cooperate to induce optimal T cell priming following LPS-based immunization, and that local factors such as IL-23 influence T cell differentiation. ^