**Sticking Up for the Immune System Integrity: Should the Thymus Be Preserved During Cardiac Surgery?**

Sara Ferrando-Martínez1,2,

M. Ángeles Muñoz-Fernández2 and Manuel Leal1

*1Laboratory of Immunovirology, Infectious Diseases Service, HU Virgen del Rocío, Institute of Biomedicine of Seville (IBiS), Seville 2Laboratory of Molecular Immuno-Biology, HGU Gregorio Marañón, Madrid Spain* 

## **1. Introduction**

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Thymus is the site of maturation of T lymphocytes (TL) from bone marrow-derived precursors and, therefore, a major organ in the immune system generation and maintenance. Thymic function starts as early as during embryonic development and its activity is maximal after birth. However, maintain the high mitotic rate that the thymus shows during fetal life and first months of life is not cost-effective after childhood. A progressive atrophy, enhanced by adolescence, will lead to the thymic output exhaustion later on life. Accordingly, it has been long accepted that TL repertory is fixed during childhood and thymus activity is no longer necessary in either adulthood or elderly. Thymus anatomical situation, in the anterior mediastinum above the heart, situates it in the surgical field of important open heart procedures and is therefore routinely removed (partially or totally) in patients of any age. Thymic ablation impact during children's cardiac surgery is being studied to determine its potential influence in peripheral TL dynamics, despite only partial results are still available. Conversely, since adult and elderly thymic function is usually disregarded, thymic ablation during adult's cardiac surgery is accepted as appropriate and harmless. Recent results, however, indicate that thymus could have an active role in elderly immune system maintenance, increasing the importance of maximal preservation during any-age cardiac surgery. Thus, the aim of this chapter is to analyze the importance of thymic function during the different stages of life and to review current evidences of the potential clinical impact of both, early childhood and adulthood thymectomy.

### **2. Overview of the human thymic gland**

#### **2.1 Embryonic development, anatomy and histology**

During embryonic development thymic epithelium and neighboring mesenchyme are derived from the cephalic region of the neural crest. Initially, thymus originates from the third pharyngeal pouch as two endodermic buds. Sprouts descend upon the superior mediastinum and will thereafter be fused as a V-shaped solid epithelial mass. Epithelium undergoes critical morphological changes and express the major histocompatibility class (MHC)-II complex at high levels as well as epidermal growth factor-like proteins that control several steps of tissue development and homeostasis, thus playing an important role in subsequent TL maturation processes. During the third gestation month thymus is colonized, via the bloodstream, by bone marrow-derived multipotential lymphoid stem cells (CFU-L), transforming the thymus in a lymphoepithelial organ. Under the inductive influence of modified thymic epithelial cells (TECs) lymphoid precursors mature in their passage through the thymus to be dumped back to peripheral blood as fully functional TL.

Developed thymus is a bilobed organ, surrounded by a connective-tissue capsule, located in the anterior mediastinum, just behind the sternum, above the heart and ahead the great vessels. Fibrous septa divide each thymic lobe into multiples smaller lobes. Thymocytes (lymphocytes undergoing maturation process in the thymus) locate heavily packed in the periphery of each lobe while the center is only sparsely populated. Therefore, a cortex and medulla region are usually identified despite there is no defined anatomical limitation. Cortex and medulla compose the true thymic epithelial space (TES), keratin-positive thymic epithelium that nourishes immature thymocytes, in which thymopoiesis actually occurs. In the human thymus a non-thymopoietic perivascular space (PVS), keratin-negative stroma, can also be found. In addition, non-lymphoid epithelial cells, bone marrow-derived macrophages and dendritic cells are spread over the thymus. Finally, Hassall's corpuscles, isolated tight whorls of epithelial cells located in the medullar region, are a thymic hallmark. Despite its function is still unclear, Interleukin (IL)-4 and IL-7 production could be its important contribution to thymocyte maturation and tuition. Vascular supply is rich in the thymus, and efferent lymphatic vessels drain into mediastinal lymph nodes. Young human thymus description and microphotography are showed in Figure 1A-B.

A fully functional thymus contains 10% of immature precursors and 15% of mature thymocytes waiting to reach the bloodstream as naive (antigen-inexperienced) TL. Remaining 75% of the thymocytes are in an intermediate maturation step (CD4+CD8+ double positive – DP – thymocytes) and undergoing selection processes. Approximately 99% of these immature thymocytes will not fulfilled the strict criteria to be safely poured to peripheral blood (extended information about the maturation process is provided in the next section) and endure programmed cell death in the cortical region, which is full of dying individual thymocytes. Phagocyted cells can also be found inside macrophages or cortical epithelial cells. Thus, the thymus has a dramatically high rate of both, mitosis and cell death. This situation of extensive proliferation and mass death cannot be cost-effectively maintained and thymus undergoes a chronically age-related atrophy as evolutionary energy-saving method. Atrophy starts from the first year of life (Steinmann et al., 1985) and is enhanced by hormonal changes during puberty (Chiodi, 1940). During the atrophy process the PVS (adipocytes, peripheral blood lymphocytes and stroma) increases, intensely diminishing the amount of TES. Loss of TES - it can be as low as 10% in elderly thymus together with architecture damage that breaks up the cortex/medulla structure leads to a less efficient thymopoiesis. Immature DP thymocytes percentages diminish and *in situ* TCR rearrangement is impaired (Sempowski et al., 2000). In addition, lymphoid component within the remaining TES also decreases in around 3%/year during the first 35 to 45 years of life and 1%/year thereafter (reviewed in Lynch et al., 2009). As a consequence, elderly thymuses are mostly adipocyte-filled PVS with isolated lymphoepithelial islets. Figure 1 C-D show a schematic representation and microphotography of an atrophied thymus.

undergoes critical morphological changes and express the major histocompatibility class (MHC)-II complex at high levels as well as epidermal growth factor-like proteins that control several steps of tissue development and homeostasis, thus playing an important role in subsequent TL maturation processes. During the third gestation month thymus is colonized, via the bloodstream, by bone marrow-derived multipotential lymphoid stem cells (CFU-L), transforming the thymus in a lymphoepithelial organ. Under the inductive influence of modified thymic epithelial cells (TECs) lymphoid precursors mature in their passage through the thymus to be dumped back to peripheral blood as fully functional TL. Developed thymus is a bilobed organ, surrounded by a connective-tissue capsule, located in the anterior mediastinum, just behind the sternum, above the heart and ahead the great vessels. Fibrous septa divide each thymic lobe into multiples smaller lobes. Thymocytes (lymphocytes undergoing maturation process in the thymus) locate heavily packed in the periphery of each lobe while the center is only sparsely populated. Therefore, a cortex and medulla region are usually identified despite there is no defined anatomical limitation. Cortex and medulla compose the true thymic epithelial space (TES), keratin-positive thymic epithelium that nourishes immature thymocytes, in which thymopoiesis actually occurs. In the human thymus a non-thymopoietic perivascular space (PVS), keratin-negative stroma, can also be found. In addition, non-lymphoid epithelial cells, bone marrow-derived macrophages and dendritic cells are spread over the thymus. Finally, Hassall's corpuscles, isolated tight whorls of epithelial cells located in the medullar region, are a thymic hallmark. Despite its function is still unclear, Interleukin (IL)-4 and IL-7 production could be its important contribution to thymocyte maturation and tuition. Vascular supply is rich in the thymus, and efferent lymphatic vessels drain into mediastinal lymph nodes. Young human

thymus description and microphotography are showed in Figure 1A-B.

A fully functional thymus contains 10% of immature precursors and 15% of mature thymocytes waiting to reach the bloodstream as naive (antigen-inexperienced) TL. Remaining 75% of the thymocytes are in an intermediate maturation step (CD4+CD8+ double positive – DP – thymocytes) and undergoing selection processes. Approximately 99% of these immature thymocytes will not fulfilled the strict criteria to be safely poured to peripheral blood (extended information about the maturation process is provided in the next section) and endure programmed cell death in the cortical region, which is full of dying individual thymocytes. Phagocyted cells can also be found inside macrophages or cortical epithelial cells. Thus, the thymus has a dramatically high rate of both, mitosis and cell death. This situation of extensive proliferation and mass death cannot be cost-effectively maintained and thymus undergoes a chronically age-related atrophy as evolutionary energy-saving method. Atrophy starts from the first year of life (Steinmann et al., 1985) and is enhanced by hormonal changes during puberty (Chiodi, 1940). During the atrophy process the PVS (adipocytes, peripheral blood lymphocytes and stroma) increases, intensely diminishing the amount of TES. Loss of TES - it can be as low as 10% in elderly thymus together with architecture damage that breaks up the cortex/medulla structure leads to a less efficient thymopoiesis. Immature DP thymocytes percentages diminish and *in situ* TCR rearrangement is impaired (Sempowski et al., 2000). In addition, lymphoid component within the remaining TES also decreases in around 3%/year during the first 35 to 45 years of life and 1%/year thereafter (reviewed in Lynch et al., 2009). As a consequence, elderly thymuses are mostly adipocyte-filled PVS with isolated lymphoepithelial islets. Figure 1 C-

D show a schematic representation and microphotography of an atrophied thymus.

Fig. 1. Schematic representation and microphotography of young (A-B) and atrophied (C-D) thymus. C = Cortex; HC = Hassall's corpuscles; M = Medulla; PVS = Perivascular space; TES = Thymic epithelial space; V = blood vessels;
