**1. The aging thymus**

Transcription factor TBX-1 is a mastermind in the formation of the third pharyngeal pouch involved in thymus organogenesis during embryonic development [1]. Patients with 22q11.2DS that impairs TBX-1 often present thymus hypoplasia. Similarly, Tbx-1null mice develop hypoplasia of the thymus [2, 3]. In both cases, defective thymus organogenesis leads to impaired thymocyte development [4]. However, as reported recently, the role of TBX-1 in thymus organogenesis is not straightforward. Ectopic forced expression of TBX-1 can inhibit transcription factor FoxN1, the mastermind of thymic epithelial identity thus indirectly impair thymus identity via sustained presence [5]. The thymus contains developing T cells (aka thymocytes) along with the non-lymphoid thymic stromal elements comprising the microenvironment that promotes thymocyte differentiation. Stromal elements include thymic epithelial cells (aka TECs), mesenchymal cells, endothelial cells as well as non-lymphoid hematopoietic cells (e.g., dendritic cells or macrophages). TECs constitute the main functional stromal cell type necessary to promote thymocyte differentiation [6, 7]. Soon after birth the thymus expands to increase the output of naive T cells, in order to colonize available niches in the periphery [8–10]. Cortical TECs (aka cTECs) are required for T lineage commitment, along with thymocyte expansion and differentiation, and positive selection. Medullary TECs (mTECs) are necessary for the induction of central tolerance and subsequent stages of thymocyte maturation before leaving the thymus. Of note, in order to maintain the well organized cortical and medullary compartments active (reverse) intercellular signaling is also required from developing thymocytes towards TECs

(aka cross-talk) [11, 12]. At a vaguely defined time point, the thymus begins to show involution, resulting in adipose degeneration of the organ; hence the process termed adipose involution. This senescent process is accompanied by the stepwise disorganization of thymic compartments, also shifting TEC subset ratios and reducing naive T cell production. Although the detailed mechanisms triggering these processes remain to be fully elucidated, they finally deteriorate thymus structure and function, severely impairing the output of fresh naive T cells. Decline of fresh naive T cells results in the inverse increase of memory T cell representation due to aging [13–15]. The observed bias in cTEC:mTEC ratio, and the fading of the most differentiated MHC class II-expressing TEC subsets leads to the development of a less complex medullary architecture along with the blurring of the cortico-medullary junction. This is followed by the focal disappearance of epithelial cells, gradually being replaced by adipose tissue in the perivascular spaces [16–20]. There is mounting evidence that adipose cells may have an thymic origin. Thymic adipose cells also produce an array of cytokines and signaling molecules that directly affect (impair) thymopoiesis [21–25]. As a result, although the appearance of thymic adipocytes may not trigger involution, their increasing presence with senescence can indirectly facilitate or perhaps even directly deteriorate thymus function. Thymus involution likely develops as a sum of failure of thymocyte progenitors and the inappropriate function of TEC compartments. It has been reported that the number of early T lineage precursors (aka ETPs) shows a gradual decline with senescence [26]. Reconstitution experiments of senescent thymi with bone marrow precursors from young donors cannot restore thymic compartments nor rescue impaired thymopoiesis. The opposite, however, reconstitution of young recipients using senescent bone marrow cells does not impair thymopoiesis [27, 28]. The genetic inactivation of cell cycle inhibitor p27 (aka Cdkn1b) also leads to the development of an enlarged thymus and enhances fresh naive T cell output along with normal stromal organization [29–33]. Recent thymic emigrants (aka RTEs) show a decrease upon enhanced expression of LIF, SCF, IL-6, and M-CSF [34, 35].

### **2. Characterization of thymic adipose tissue**

There are significant differences between adipose tissue subtypes. At least three subtypes are distinguished: white adipose tissue (WAT), brown adipose tissue (BAT) and the recently described beige adipose tissue. White adipose tissue stores energy, brown adipose tissue generates heat (via NST or non-shivering thermogenesis), while beige adipocytes act as intermediates. It has currently been described that thymic adipose involution yields beige adipose tissue based on its gene expression, miRNA, histology and metabolic profile [36]. In terms of gene expression and histology characteristic epithelial markers show down-regulation (FoxN1, EPCAM1, MHCII, Wnt4) (see **Figure 1**). Considering the miRNA profile beige-adipose tissue-associated miRNA species show supportive changes (miR27a, miR106b, miR155) (see **Figure 1**). While PPARgamma is the mastermind of all adipose tissue subtypes, TBX-1 has been acknowledged as a key and specific marker of beige adipose tissue development [17–20]. Beige adipocytes respond to adrenergic stimuli by thermogenesis via mitochondrial uncoupling of biochemical degradation and energy production [21]. Along with TBX-1 other beige-indicative markers have also been reported. These include mitochondrial uncoupling proteins (mostly UCP-1), EAR2 (also known as Nr2f6) and CD137 (also known as Tnfrsf9) [22]. The above-mentioned adipose and beige markers show up-regulation along (see **Figure 1**). The adult thymus expresses PPARgamma, TBX-1 and UCP-1 in the epithelial compartment, and latter two have been reported to initiate beige adipose

**39**

**Figure 1.**

*Thymic Senescence*

*DOI: http://dx.doi.org/10.5772/intechopen.87063*

TBX-1 showing bimodal expression [36].

*Key molecular events of thymic senescence.*

**3. Natural resistance to senescence**

tissue development. Thymic adipose tissue may also bee classified based on cellular analysis from an adipocyte perspective [23–26]. Thymus tissue appears to be unique expressing TBX-1 during embryonic development and also during senescence embedded in different contexts. It is appreciable that TBX-1 plays a role in thymus organogenesis (immune peak) and thymic adipose involution (metabolic peak). This suggests an intersection of immunity and metabolism, and a dual role of

A medical condition termed persistent thymus has been known for long [37]. In the affected population the thymus is rescued from involution. These individuals, however, have severe defects in their hormonal system, affecting the level of sex steroids. It is the lack of androgen-effect that prevents thymus involution on one hand, but hampers the endocrine system on the other hand. Recently another medical condition termed FPLD3 (familial partial lipodystrophy type 3) has also been associated with the lack of thymus involution [38]. FPLD3 also derails the hormonal system by affecting PPARgamma activity. As for all adipose tissue subtypes, PPARgamma plays a crucial role during thymus adipose involution as well [39]. It has been suggested by others previously based on direct fate-mapping experiments that with senescence thymic adipose tissue develops from the thymic stromal or epithelial compartment [22]. In further support, epithelial to adipose trans-differentiation has been reported to occur as indicated by the presence by EpCAM-1/PPARgamma doublepositive cells at a given time point during thymus senescence. Such cells express cell

#### **Figure 1.**

*Thymus*

(aka cross-talk) [11, 12]. At a vaguely defined time point, the thymus begins to show involution, resulting in adipose degeneration of the organ; hence the process termed adipose involution. This senescent process is accompanied by the stepwise disorganization of thymic compartments, also shifting TEC subset ratios and reducing naive T cell production. Although the detailed mechanisms triggering these processes remain to be fully elucidated, they finally deteriorate thymus structure and function, severely impairing the output of fresh naive T cells. Decline of fresh naive T cells results in the inverse increase of memory T cell representation due to aging [13–15]. The observed bias in cTEC:mTEC ratio, and the fading of the most differentiated MHC class II-expressing TEC subsets leads to the development of a less complex medullary architecture along with the blurring of the cortico-medullary junction. This is followed by the focal disappearance of epithelial cells, gradually being replaced by adipose tissue in the perivascular spaces [16–20]. There is mounting evidence that adipose cells may have an thymic origin. Thymic adipose cells also produce an array of cytokines and signaling molecules that directly affect (impair) thymopoiesis [21–25]. As a result, although the appearance of thymic adipocytes may not trigger involution, their increasing presence with senescence can indirectly facilitate or perhaps even directly deteriorate thymus function. Thymus involution likely develops as a sum of failure of thymocyte progenitors and the inappropriate function of TEC compartments. It has been reported that the number of early T lineage precursors (aka ETPs) shows a gradual decline with senescence [26]. Reconstitution experiments of senescent thymi with bone marrow precursors from young donors cannot restore thymic compartments nor rescue impaired thymopoiesis. The opposite, however, reconstitution of young recipients using senescent bone marrow cells does not impair thymopoiesis [27, 28]. The genetic inactivation of cell cycle inhibitor p27 (aka Cdkn1b) also leads to the development of an enlarged thymus and enhances fresh naive T cell output along with normal stromal organization [29–33]. Recent thymic emigrants (aka RTEs) show a decrease upon enhanced

expression of LIF, SCF, IL-6, and M-CSF [34, 35].

**2. Characterization of thymic adipose tissue**

There are significant differences between adipose tissue subtypes. At least three subtypes are distinguished: white adipose tissue (WAT), brown adipose tissue (BAT) and the recently described beige adipose tissue. White adipose tissue stores energy, brown adipose tissue generates heat (via NST or non-shivering thermogenesis), while beige adipocytes act as intermediates. It has currently been described that thymic adipose involution yields beige adipose tissue based on its gene expression, miRNA, histology and metabolic profile [36]. In terms of gene expression and histology characteristic epithelial markers show down-regulation (FoxN1, EPCAM1, MHCII, Wnt4) (see **Figure 1**). Considering the miRNA profile beige-adipose tissue-associated miRNA species show supportive changes (miR27a, miR106b, miR155) (see **Figure 1**). While PPARgamma is the mastermind of all adipose tissue subtypes, TBX-1 has been acknowledged as a key and specific marker of beige adipose tissue development [17–20]. Beige adipocytes respond to adrenergic stimuli by thermogenesis via mitochondrial uncoupling of biochemical degradation and energy production [21]. Along with TBX-1 other beige-indicative markers have also been reported. These include mitochondrial uncoupling proteins (mostly UCP-1), EAR2 (also known as Nr2f6) and CD137 (also known as Tnfrsf9) [22]. The above-mentioned adipose and beige markers show up-regulation along (see **Figure 1**). The adult thymus expresses PPARgamma, TBX-1 and UCP-1 in the epithelial compartment, and latter two have been reported to initiate beige adipose

**38**

*Key molecular events of thymic senescence.*

tissue development. Thymic adipose tissue may also bee classified based on cellular analysis from an adipocyte perspective [23–26]. Thymus tissue appears to be unique expressing TBX-1 during embryonic development and also during senescence embedded in different contexts. It is appreciable that TBX-1 plays a role in thymus organogenesis (immune peak) and thymic adipose involution (metabolic peak). This suggests an intersection of immunity and metabolism, and a dual role of TBX-1 showing bimodal expression [36].
