Foxn1 and Molecular Mechanism of Thymic Involution

描述（由申请人提供）：胸腺是负责产生功能性T细胞的主要淋巴器官，因此对产生和维持适应性免疫至关重要。衰老过程中的胸腺退行性变，称为老年性变，导致幼稚T细胞的产生急剧下降，是导致免疫衰老的主要因素。尽管这一主题对人类健康很重要，但在出生后胸腺中调节胸腺稳态和内化的分子和细胞机制在很大程度上是未知的。因此，胸腺退化的许多方面都没有得到很好的理解和/或有争议。例如，胸腺基质细胞、造血干细胞和生理变化对胸腺复旧的相对贡献是当前争论的主题。在目前的项目中，我们提供的证据表明，仅在胸腺上皮细胞中表达的单个基因Foxn1转录因子的下调足以诱导快速和过早的胸腺退化。转录因子foxn1是胎儿胸腺上皮细胞（TEC）分化所必需和充分的，并且在出生后的TEC中广泛（尽管不是普遍）表达；然而，出生后胸腺的功能尚未被确定。利用Foxn1的一个新的等位基因Foxn1lacZ，我们发现Foxn1在出生后胸腺中的表达减少会导致出生后胸腺变性表型，这种表型再现了大多数或所有的基质和胸腺细胞特异性缺陷，这些缺陷是衰老相关退化的特征。这种表型与Foxn1基因表达减少有关，并提供了Foxn1在出生后tec中维持出生后稳定胸腺所需的功能证据。对Foxn1的需求是非常剂量敏感的，Foxn1水平的微小变化对胸腺表型有很大的影响，表达较高Foxn1水平的TEC亚群对其下调最敏感。基于这些和其他数据，我们提出不同的TEC亚群需要特定水平的Foxn1来实现其分化、增殖和维持，并且Foxn1水平随着年龄的增长而降低，通过减少TEC增殖和降低产生特定TEC亚群的能力，直接促进胸腺退化。这些tec特异性缺陷是导致大多数退化效应的原因。因此，我们的数据表明，我们已经确定Foxn1及其在tec中的功能是出生后胸腺复杂网络中的关键调控节点。我们提出三个具体目标来验证foxn1基因表达下调是诱导胸腺退化的必要和充分条件的假设。目的1将确定Foxn1在出生后胸腺特定TEC人群中的表达，并测试其下调是否足以触发胸腺复归。目的2将测试foxn1介导的TECs增殖调控是否是退化的关键组成部分。目的3将测试Foxn1维持TEC稳态和功能的分子机制。虽然衰老相关的胸腺退行性变是衰老相关免疫衰老的主要原因，但胸腺退化的许多方面尚不清楚和/或有争议。确定这一过程的分子和细胞机制可能为合理的药物设计提供新的靶点，并且在治疗疾病、实体器官和骨髓移植以及正常衰老引起的免疫缺陷方面具有明确的转化潜力。

DESCRIPTION (provided by applicant): The thymus is the primary lymphoid organ responsible for the generation of functional T cells, and is therefore critical for generation and maintenance of adaptive immunity. Thymic degeneration during aging, termed involution, results in a dramatic drop in the production of naive T cells, and is a major contributing factor to immune senescence. Despite the importance of this subject for human health, the molecular and cellular mechanisms operating in the postnatal thymus that mediate thymic homeostasis and involution are largely unknown. As a result, many aspects of thymic involution are poorly understood and/or controversial. For example, the relative contributions of changes in thymic stromal cells, hematopoietic stem cells, and physiology to thymic involution are topics of current debate. In the current project, we provide evidence that the down regulation of a single gene expressed only in thymic epithelial cells, the Foxn1 transcription factor, is sufficient to induce rapid and premature thymic involution. The transcription factor Foxn1is necessary and sufficient for t fetal thymic epithelial cell (TEC) differentiation, and is widely (although not ubiquitously) expressed in postnatal TECs; however, a function in the postnatal thymus has not been previously identified. Using a novel allele of Foxn1, Foxn1lacZ, we show that reduction of Foxn1 expression in the postnatal thymus causes a postnatal thymic degeneration phenotype that recapitulates most or all stromal and thymocyte-specific defects characteristic of aging-associated involution. This phenotype is associated with decreased Foxn1 gene expression, and provides functional evidence that Foxn1 is required in postnatal TECs to maintain the postnatal steady-state thymus. The requirement for Foxn1 is extremely dosage-sensitive, with small changes in Foxn1levels having large effects on thymus phenotypes, and TEC subsets that express higher Foxn1 levels being most sensitive to its down-regulation. Based on these and other data, we propose that different TEC subsets require specific levels of Foxn1 for their differentiation, proliferation, and maintenance, and that decreasing Foxn1 levels with age directly contribute to thymic involution both by reducing TEC proliferation and by reducing the capacity to generate specific TEC subsets. These TEC-specific defects are then causative for most involution effects. Thus, our data indicate that we have identified Foxn1 and its function within TECs as a critical regulatory node in the complex network of the postnatal thymus. We propose three specific aims to test the hypothesis that down regulation of Foxn1gene expression is both necessary and sufficient to induce thymic involution. Aim 1 will define the expression of Foxn1 in specific TEC populations in the postnatal thymus, and test whether its down regulation is both necessary and sufficient to trigger thymic involution. Aim 2 will test whether Foxn1-mediated regulation of proliferation in TECs is a critical component of involution. Aim 3 will test the molecular mechanisms by which Foxn1 maintains TEC homeostasis and function. While aging-associated degeneration of the thymus, or involution, is a major cause of ageing-associated immunosenescence, many aspects of thymic involution are poorly understood and/or controversial. Identification of the molecular and cellular mechanisms underlying this process may provide novel targets for rational drug design, and has clear translational potential for treatment of immunodeficiencies caused by disease, solid organ and bone marrow transplants, and normal aging.