Foxn1 and Molecular Mechanism of Thymic Involution

摘要 胸腺是负责产生功能性T细胞的主要淋巴器官， 因此对产生和维持获得性免疫至关重要。在衰老过程中胸腺退化， 被称为退化，导致幼稚T细胞的产生急剧下降，这是一个主要的贡献 免疫衰老因子。尽管这一主题对人类健康很重要，但分子和 在出生后的胸腺中调节胸腺动态平衡和退化的细胞机制是 很大程度上是未知的。因此，胸腺退化的许多方面都缺乏了解和/或有争议。 例如，胸腺基质细胞、造血干细胞和 从生理学到胸腺退化是当前争论的话题。在当前的项目中，我们提供的证据表明 只在胸腺上皮细胞中表达的单个基因Foxn1转录因子的下调是 足以导致胸腺迅速和过早退化。转录因子Foxn1是必需的 足够分化胎儿胸腺上皮细胞(TEC)，而且广泛(尽管不是无处不在) 在出生后的TECs中表达；然而，出生后胸腺中的功能尚未被证实。 利用Foxn1的新等位基因Foxn1lacZ，我们发现Foxn1在出生后胸腺中的表达减少 导致出生后胸腺变性表型，该表型概括了大部分或全部间质和胸腺细胞特异性 与衰老相关的退化的缺陷。这种表型与Foxn1基因减少有关 表达，并提供了功能证据表明，在出生后的TEC中需要Foxn1来维持出生后的 稳定状态的胸腺。对Foxn1的需求对剂量非常敏感，Foxn1的变化很小 水平对胸腺表型有较大影响，且表达较高Foxn1水平的TEC亚群最多 对其下调监管很敏感。基于这些和其他数据，我们认为不同的TEC子集需要 Foxn1在分化、增殖和维持中的特异性水平，以及降低Foxn1的水平 随年龄增长的水平通过减少TEC的增殖和通过减少 生成特定TEC子集的能力。这些TEC特有的缺陷是导致大多数内陷的原因 效果。因此，我们的数据表明，我们已经确定Foxn1及其在TEC中的功能是关键 在出生后胸腺的复杂网络中的调节节点。我们提出了三个具体目标来测试 假设下调Foxn1基因表达是诱导胸腺的必要条件和充分条件 内卷曲。目的1确定Foxn1在出生后胸腺中特定的TEC群体中的表达，以及 测试它的下调是否是触发胸腺退化的必要条件和充分条件。AIM 2将测试 Foxn1介导的对TECs增殖的调节是否是退缩的关键组成部分。目标3将 检测Foxn1维持TEC动态平衡和功能的分子机制。

Abstract 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 Foxn1 is 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 idenified. 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 Foxn1 levels 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 Foxn1 gene 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.