Mechanisms of CD4 T Cell Antigen Recognition and Effector Cell Differentiation and Function

Abstract

The ability for CD4 T cells to efficiently search for and subsequently respond to microbial pathogens is essential for protective immunity, but mechanisms controlling these responses are not completely understood. In this thesis I study the regulation of CD4 T cell responses at two different stages during an immune response. First, I analyze one of the most basic mechanisms by which T cells search for and become activated by an antigenic stimulus during the initial events in an adaptive immune response. Using human memory CD4 T cells in vitro I have identified a novel role for actin-rich invadapodia-like protrusions (ILPs) in overcoming the energy barrier required for the T cell receptor (TCR) to send signals into T cells when interacting with peptide-loaded MHC II. My studies show that ILPs, which are used during migration, are also essential for surveying the surface of other cells during cellular communication. Secondly, I explore the costimulatory requirements and function of T follicular regulatory \((T_{FR})\) cells, a newly identified subset of regulatory T \((T_{REG})\) cells. Using mouse models, I have discovered that the costimulatory receptor PD-1 inhibits the differentiation and function of \(T_{FR}\) cells in vivo. My work also has revealed that \(T_{FR}\) cells can circulate within the blood and that blood TFR cells can potently inhibit B cell mediated antibody production in vivo. Taken together, the studies presented here not only provide insights into the very initial events leading to adaptive immunity, but also demonstrate how adaptive immunity is controlled during the effector phase of an immune reaction.