Foxn1 and Molecular Mechanism of Thymic Involution

描述（由申请方提供）：胸腺是负责产生功能性T细胞的主要淋巴器官，因此对于产生和维持适应性免疫至关重要。衰老过程中的胸腺退化，称为退化，导致幼稚T细胞的产生急剧下降，并且是免疫衰老的主要促成因素。尽管这一主题对人类健康的重要性，但在出生后胸腺中介导胸腺稳态和退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知之甚少和/或有争议。例如，胸腺基质细胞、造血干细胞和生理学变化对胸腺退化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，一个单一的基因表达的下调，仅在胸腺上皮细胞，Foxn 1转录因子，足以诱导快速和过早的胸腺退化。转录因子Foxn 1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TEC中广泛表达（尽管不是普遍表达）;然而，出生后胸腺中的功能先前尚未被确定。使用一种新的等位基因的Foxn 1，Foxn 1 lacZ，我们表明，减少Foxn 1在出生后胸腺表达导致出生后胸腺变性表型，重演大部分或所有的基质和胸腺细胞特异性缺陷的特征与衰老相关的退化。这种表型与Foxn 1基因表达降低有关，并提供了Foxn 1在出生后TEC中维持出生后稳态胸腺所需的功能证据。对Foxn 1的需求是非常剂量敏感的，Foxn 1水平的微小变化对胸腺表型有很大的影响，表达较高Foxn 1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Foxn 1的分化，增殖和维护，并随着年龄的增长，减少Foxn 1水平直接有助于胸腺退化减少TEC增殖和减少能力，以产生特定的TEC亚群。这些TEC特定的缺陷是大多数内卷效应的原因。因此，我们的数据表明，我们已经确定Foxn 1和它的功能TECs作为一个关键的调节节点在复杂的网络出生后胸腺。我们提出了三个具体的目标来检验这一假设，即下调Foxn 1基因的表达是必要的和足够的诱导胸腺退化。目的1将定义Foxn 1在出生后胸腺特定TEC群体中的表达，并测试其下调是否是触发胸腺退化的必要和充分条件。目的2将测试Foxn 1介导的TEC增殖调节是否是退化的关键组成部分。目的3将测试Foxn 1维持TEC稳态和功能的分子机制。虽然衰老相关的胸腺退化或退化是衰老相关的免疫衰老的主要原因，但胸腺退化的许多方面仍知之甚少和/或存在争议。鉴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，并具有明确的翻译潜力，用于治疗疾病、实体器官和骨髓移植以及正常衰老引起的免疫缺陷。

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.