Aging of the Thymic Epithelial Progenitor Pool is Determined by the p63-FoxN1 Regulatory Axis

Burnley, Preston I.
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The immune system is composed of various effector cells and molecules that must work in concert in order to protect the body against infections, auto-reaction, and tumor occurrence. These responses can be divided into two categories – innate and adaptive immunity. The innate response is the host’s first line of defense towards a pathogen by providing a physical and chemical barrier against infection. Once activated, innate cells such as macrophages and dendritic cells can engulf the bacterium, degrade it, and secrete proteins to destroy the pathogen. Although this response occurs immediately after an encounter with a pathogen, the innate immunity is neither long-lasting nor specific. In contrast, the adaptive immune response is initiated when the innate immune response is unsuccessful in eliminating the infection, allowing for recognition and response tailored for a particular pathogen. The cells that make up the adaptive response all originate from a common lymphoid progenitor found in the bone marrow. From this precursor arise natural killer (NK) cells (part of the innate response) and T and B lymphocytes. The T lymphocytes originate from the bone marrow but undergo development in the thymus, hence the name T cells. B lymphocytes, on the other hand, originate and develop in the bone marrow. With the exception of NK cells, these adaptive immune cells require antigen presentation in order to become activated. Once the T and B cells have matured and become activated they can work together to clear the infection by secreting cytokines and antibodies. The most important aspect of the adaptive immune response is its ability to produce immunological memory. Memory T and B cells are able to ensure a rapid and effective response to a second encounter, providing long-last immunity. Unfortunately, this well-ordered process, specifically the development of T cells, becomes compromised during aging. This is due to the fact that thymic involution (or shrinking of the thymus) occurs at the onset of puberty and continues throughout the lifespan, which is primarily resulted from age-related defect in thymic epithelial cells (TECs). The thymus is crucial for the generation of T cells so any compromise to the organ results in changes in the T cells, which can possibly lead to immune insufficiency and autoimmunity [1]. Additionally, these conditions are exacerbated with age [2, 3]. This research project will focus on the molecular mechanism(s) responsible for thymic involution. To do so, we focused on TECS and two genes associated with the homeostatic maintenance of the thymic microenvironment, p63 and FoxN1. These genes regulate the proliferation and differentiation, respectively, of thymic epithelial cells (TECs), thereby maintaining a properly functioning thymus. For this study we will utilize our mouse model (FoxN1 conditional knockout, FC) extensively because it mimics an aged thymus. This model allows us to study the thymic microenvironment of a mouse with a defect in the FoxN1 gene.