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， 因此对于适应性免疫的产生和维持至关重要。衰老过程中胸腺退化， 称为退化，导致幼稚T细胞产生的急剧下降，并且是主要的贡献因素。 免疫衰老的因素。尽管这一主题对人类健康很重要，但分子和 在出生后胸腺中调节胸腺稳态和退化的细胞机制是 大部分未知。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 胸腺退化生理学是目前争论的主题。在目前的项目中，我们提供的证据表明， 一个只在胸腺上皮细胞中表达的单一基因Foxn 1转录因子的下调， 足以引起快速和过早的胸腺退化。转录因子Foxn 1是必需的， 足以测试胎儿胸腺上皮细胞（TEC）分化，且广泛（尽管不是普遍） 在出生后的TEC中表达;然而，出生后胸腺中的功能以前没有被证实。 使用Foxn 1的一个新等位基因Foxn 1 lacZ，我们发现出生后胸腺中Foxn 1表达的减少， 导致出生后胸腺变性表型，重现大部分或全部基质和胸腺细胞特异性 与老化有关的退化特征的缺陷。这种表型与Foxn 1基因的减少有关。 表达，并提供了功能证据，Foxn 1是需要在出生后TEC，以维持出生后 稳态胸腺对Foxn 1的需求对剂量非常敏感，Foxn 1的变化很小 水平对胸腺表型有很大影响，其中表达较高Foxn 1水平的TEC亚群最多 对下调敏感。基于这些和其他数据，我们提出不同的TEC子集需要 特定水平的Foxn 1用于它们的分化、增殖和维持，并且减少Foxn 1 随着年龄的增长，随着年龄的增长， 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卷的原因 方面的影响.因此，我们的数据表明，我们已经确定Foxn 1及其在TEC中的功能是一个关键因素， 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，以测试 假设Foxn 1基因表达下调是诱导胸腺细胞凋亡所必需和充分的， 对合目的1将定义Foxn 1在出生后胸腺中特定TEC群体中的表达， 测试其下调是否是触发胸腺退化的必要和充分条件。目标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。目标3将 测试Foxn 1维持TEC稳态和功能的分子机制。

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.