Tissue-Specific Immunity for Transplantable Endocrine Glands in the Context of HLA Expression

Abstract

Understanding the dynamic of the immune system, it is becoming clear that the characteristics of the tissue can be important as immune cells to determine the initiation and progression of an immune response. Among the various responses, tissue-specific immunity can be characterized by determining the Major Histocompatibility Complex (MHC). Human major histocompatibility antigens are known as the “human leukocyte antigen” (HLA) system. HLA contains more than 200 genes and has essential activities in immunology, diseases, and transplantation with gene regions of diverse functions. One of the significant roles in transplantation is donor and recipient selection. In allorecognition, once the recipient antigen-presenting cells (APC) recognize the donor tissue, this leads to activation and migration of the immune cells, which can promote rejection or tolerance. In solid organ transplantation, cultured tissue cells were presumed as passenger-leukocytes free, ensuring mainly prolonged graft survival. However, the current literature paves the way for understanding HLA peptides and allorecognition dynamics to prevent rejection or provide a definition for the donor-recipient match. Based on the given information, this chapter summarizes the HLA expression dynamics and allorecognition status from a transplantation perspective for endocrine glands, including the Adrenal glands, Pancreas, Parathyroid, and Thyroid glands.

1. Introduction

The transplant immune response points out the importance of the characteristic features of the related tissues, triggering capacity, and continuation of the immune response. In order to understand the specific immune response of a particular tissue, it is necessary to determine the immunogenicity of that tissue. Immunogenicity is the expression of how one or more molecule stimulates the immune system and/or its ability to elicit a response. The concept of tissue immunogenicity can be defined and categorized by the Major Histocompatibility (MHC) antigens.

MHC antigens are Human Leukocyte Antigens (HLA). Related genes are located on the short arm of chromosome six, close to the centromere part, which occupies a four-megabase pair that encodes more than 200 gene regions [1, 2]. Proteins found on the cell surface or in the cytoplasm are examined in three groups according to their distribution, structure, and function in tissues; Class I, II, and III molecules. Class I includes HLA-A, -B, -C, -E, -F, -G, -H antigens, and Class II contains HLA-DR, -DP, -DQ , -DO, -DM antigens. In addition, Class III encodes various components involved in the complement system [3, 4].

HLA class I molecules -A, -B, -C are classical HLAs expressed in many tissues and found in almost all nucleated cells. HLA Class I molecules -E,-F,-G are designated non-classical HLAs [5, 6]. These molecules’ primary purpose involves presenting peptide antigens to T cells [7]. When this situation is evaluated in terms of transplantation, the presented peptide is referred to as an allo-peptide. Moreover, the APCs of the recipient, presents the donor’s peptides to T cells after transplantation. APCs mainly present peptides of HLA Class I molecules in two ways; transporter antigen processing complex (TAP) dependent or TAP-independent. While foreign (or allo-) peptides are transported from the endoplasmic reticulum to the cell membrane. During this subcellular process, ATP is not always required, whether TAP-dependent or TAP-independent [8].

HLA Class II molecules; -DR, -DP, -DQ are known as classical antigens, and -DO, -DM is defined as non-classical antigens. These exogenous antigens are expressed in B and T lymphocytes, macrophages, dendritic, epithelial cells, and endothelium [1]. For antigen presentation, these molecules enable foreign antigens (or allo-peptides) that enter those cells by phagocytosis or endocytosis [9]. The classical antigens of the HLA Class II molecules have heteronanomeres structures in the endoplasmic reticulum. These molecules are responsible for introducing the endosomal peptides from APCs to the T-cell membrane. In transplantation, the exogenous proteins (allo-molecules) are taken up by APCs via endocytosis. These proteins are degraded by lysosomal proteases/hydrolases. Usually, endogenous peptides carried by Class II antigens in vesicles produced in the normal cell cycle process are degraded over time and then excreted from the cell membrane after lysosomal degradation. However, in the presence of the exogenous peptides, the endogenous peptide attaches to the Class II molecule. Then exogenous peptides must compete before this degradation and binds it for presentation [10, 11].

Epithelial cells in the gastrointestinal tract express Class II molecules and act as APCs with particular CD74 positivity. In this mechanism, a trimer is formed by the binding of CD74 to the heterodimer Class II structure (α and β) within the endoplasmic reticulum (this structure is also known as the invariant chain or Class II histocompatibility antigen gamma chain). “class II-associated invariant chain peptide” (CLIP) is prepared for presentation in a short fragment-linked state (CLIP-HLA-Class II structure) by splicing a portion of CD74 by various proteases in the endosomal/lysosomal system. With the cleavage of the exogenous peptide-CLIP, it binds to the Class II molecule and is presented from the cell membrane surface [9, 12, 13, 14, 15]. In this mechanism, the non-classical Class II molecule HLA-DM keeps Class II molecules stable within the endosome, until an exogenous peptide binds. In the presence of an exogenous peptide (if that peptide has sufficient affinity), it induces the CLIP to dissociate from the Class II molecule. After binding to the exogenous molecule, it rapidly presented through the cell membrane. Especially in B lymphocytes and dendritic cells, -DM must interact with -DO (non-classical Class II molecules). Afterward, the acidity level enhances proteolysis and promotes efficient peptide loading [12, 13, 14, 15, 16, 17].

Based on the given information about the antigen presentation process, transplantation of composite tissues/cells or endocrine glands antigenic determination and presentation process differs depending on the HLA load of the donor’s tissue. Therefore, this chapter assesses and summarizes the current literature on HLA expression status for transplantable endocrine glands. Throughout this chapter, the main focus is the tissue expression profile contained in databases between all organs and particular endocrine glands. Additionally, the HLA expression studies performed by many researchers and their relationship with transplantations are reviewed.

2. Adrenal gland transplantation

The primary role of the adrenal gland is defined by its functional layers; zona glomerulosa, zona reticular, and the zona fasciculata. Each layer has distinct features and regulates various roles such as the reabsorption of sodium and water, production of sex hormones, mammalian stress response, etc. [18, 19].

Adrenal insufficiency was described in 1855 by Thomas Addison [20]. Since the developmental strategies in transplantation gained therapeutical advancements involving replacement therapies with glucocorticoids, and stem cell developments, those features have been combined by clinicians and physicians.

Transcriptomic Analysis of the adrenal gland showed the differential distribution of specific genes. Other genes that share with other organs are also classified by databases including The Human Protein Atlas (HPA), and The Genotype-Tissue Expression (GTEx) platforms (Figure 1) [22, 23, 24, 25]. The adrenal gland has 24 specific and 220 elevated genes compared to other organs in humans. The development of the adrenal gland also contains 69 genes expressed in adult and fetal tissues [19]. Immunologically none of the genes are related to the HLA classes. The adrenal gland cells showed mononuclear infiltrations and high HLA expression in mRNA and protein levels for normal and diseased tissues [26]. In 1997, Marx et al. showed 17-hydroxylase expression in the zone fasciculata, and it started to express HLA Class II antigens in the early stages of life [27]. Afterward, in 2014 Leite et al. reported the relationship between the aggressiveness of pediatric adrenocortical tumors and low expression of HLA Class II molecules including -DRA, -DPA1, and -DPB1 [28]. A recent preprint from Altieri et al. determined immune clusters of the adult normal and adrenocortical adenomas in the human adrenal gland. They showed HLA class I (-A, -B, and -C) decreased in adenomas compared to the normal adrenal gland [29].

Figure 1.

Representative image of the particular shared genes and their relations between the adrenal gland and other organs. The image is reprinted from the human protein atlas database [21, 22, 23] and customized with BioRender.

Adrenal gland transplantation is mainly performed as autotransplantation and in literature, approximately 52 studies were reported between 1948 and 2019. Among them, only four studies were written in languages other than English including Russian [30], Portuguese [31], Italian [32], and German [33], and the full text are not available among the other two cases as well [34, 35]. However, numerous in vivo studies evaluate autotransplantation, allotransplantation, and xenotransplantation approaches. Most of them were performed on rats [36, 37, 38, 39, 40, 41, 42, 43], second on sheep [44, 45, 46, 47, 48, 49], and third on dogs [50] and monkeys [51]. Clinically, routine hormone replacement therapy provides challenges over transplantation, and limited studies reported concise cases of adrenal transplants [52, 53, 54]. In literature, provided adrenal gland transplantation studies often contain in vivo studies and limited clinical cases. Considering the details, human embryonic stem cells, somatic cells such as fibroblast, hair follicles, and adipose tissue-derived stem cells were tested for personalized treatment options. Additionally, decellularization from cadaveric donors and subsequent recellularization from animal donor studies were performed in vivo only. Partial tissue allotransplantation in humans from related-living-donor was tested clinically [55]. Microencapsulation strategy is another tool for allotransplantation still, there are no clinical studies for adrenal gland cells [56, 57]. Bornstein et al. recently achieved in vitro 3D models for bovine adrenal cells and provided viable organoid structures [58].

3. Pancreas transplantation

The pancreas is a unique organ that combines endocrine and exocrine functions, determining the variety of its pathology [59]. For a long time, it was believed that the ability of the pancreas to meet insulin requirements was minimal throughout life. In addition, evidence of plasticity of the pancreatic endocrine fraction has been reported, showing that the number of β-cells changes under certain physiological parameters. Besides, the endocrine function increases during changing metabolic demands such as obesity, or normal physiological growth. All basal cell types of the pancreas arise from a single pluripotent cell with a ductal phenotype [59, 60].

Transcriptomic Analysis of the pancreas provides a distribution of specific genes. The genes that share with the other organs are classified by databases involving HPA and GTEx platforms. The pancreas has 60 specific and 311 elevated genes compared to other organs in humans (Figure 2) [23, 24, 25, 61], none of those reported genes involve HLA molecules. As a transplantable organ, the pancreas shows strong HLA expression profiles for HLA Class I and II. HLA-B and HLA-DRβ1 expressions were observed at high levels [25] and histologically, positivity could not be observed in either exocrine or endocrine cells [61]. Considering the possibility that HLA protein expression levels differ among pancreas tissues. HLA negative correlation between RNA profiling and histology does not guarantee a higher survival rate after transplantation.

Figure 2.

Representative image of the particular shared genes and their relations between the pancreas and other organs. The image is reprinted from the human protein atlas database [21, 22, 23] and customized with BioRender.

The first total pancreas transplant in 1966 was performed by Kelly et al. [62]. After the first pancreas transplant; there was a gap due to poor transplant results, with the significant effects of poor organ preservation playing an important role [63]. The separation and transplantation of pancreatic islet cells became a reality many years later, after numerous experimental studies. In particular, over the past two decades, various efforts have made islet cell transplantation a viable therapy for many patients with type 1 diabetes (T1D) [63]. Current cell therapy gives patients with T1D the ability to have their insulin levels regulated by healthy endocrine functioning cells, rather than daily insulin injections. Clinical outcomes, including insulin independence, graft, and patient survival, have gradually improved and show results comparable to other organ transplants [64]. This is possibly due to the medical advancements in biotechnology [63, 65].

Between 2004 and 2013, studies on pancreas and islet transplantations continued throughout the world via the Collaborative Islet Transplant Registry (CITR). As suggested, transplantation should be performed according to the course of the disease and the clinical characteristics. Additionally, the type of diabetes and secondary etiologies should be considered [65]. The Leiden University Medical Center shared their 30 years of experience in 2015 and reported that they could observe an over 80% survival rate for 349 transplants [66]. Remarkably, factors such as the duration of cold ischemia of the tissue, the donor’s age, the organ’s procurement process, and the type of transplantation are essential in predicting the rejection risk [66]. Immunosuppressive treatments are offered to patients within two categories; induction and maintenance. In the induction part, the depletion of T cells is targeted, and in the maintenance part, a calcineurin inhibitor (CNI), and steroids are commonly used [65, 66]. There are two main drawbacks reported for islet transplantation compared to pancreas transplantation including the increase in the pancreas need and the risk of sensitization of the recipient [67]. Furthermore, Instant blood-mediated inflammatory reaction (IBMIR) is a well-known outcome regarding immune rejection and activation of complement after transplantation. Thus, it causes a severe loss of islet cells (approximately 25%) after vena cava infusion [68]. As is known, the Edmonton protocol provided promising outcomes by combining with immunosuppressive treatment [67, 69, 70]. Over the years, 15,000 procedures have already been performed so far [69, 71] with an outcome of one- to five-year survival rate with insulin independence reported [66, 69].

4. Parathyroid transplantation

The production of parathyroid hormone by the parathyroid glands is essential for controlling blood calcium levels. The human body needs precise calcium levels since even tiny variations can lead to issues with muscles and nerves [72]. If any or all of the parathyroid glands are underactive, individuals may have hypoparathyroidism. These four little glands exist in the body and they are situated near the thyroid gland in the neck region. The parathyroid gland keeps a balanced level of phosphorus, magnesium, and calcium in the blood. These glands do not produce enough parathyroid hormone if they are underactive (PTH). This primarily lowers the blood calcium level. All four parathyroid glands being damaged or removed is the most frequent cause. That may unintentionally occur during thyroid surgery. These glands are absent in some persons from birth. Alternatively, for unknown reasons, the glands do not function as well [72, 73].

Transcriptomic Analysis of the parathyroid gland presents a distribution of particular genes. In addition, different genes that share with other organs are classified by databases involving HPA and GTEx platforms. The parathyroid gland has 26 specific and 204 elevated genes compared to other organs in humans (Figure 3) [23, 24, 25].

Figure 3.

Representative image of the particular shared genes and their relations between the parathyroid gland and other organs. The image is reprinted from the human protein atlas database [21, 23] and customized with BioRender.

Immunological properties of the parathyroid glands are evaluated in detail for HLA expression status for both tissue and isolated parathyroid cells [74, 75, 76]. In 1997, Tolloczko et al. compared the allotransplantation status of cultured parathyroid cells in the presence of IFNγ to decrease HLA Class I and II molecule expression [77]. With reduced HLA expression levels, parathyroid cells become more suitable for allotransplantation. However, based on their culture system, the survival rate after transplantation was reported to be 55% [78]. Flow cytometry outcome of these cells regarding HLA expression showed cells were negative for Class I molecules and positive for Class II [79]. However, an in vivo study showed that HLA Class I molecule expression in parathyroid cells causes more rapid rejection [80]. A recent study from Goncu et al. compared the HLA molecule expression with mRNA and protein levels via cultured cells [74, 75]. In their study, HLA-A molecule expression remained stable during 10 days of cultivation; also, HLA-B and -C expression could not detect at the protein level [75] despite the HPA histology staining data. HLA Class II molecules were also evaluated, and it was found that HLA-DP has higher mRNA expression levels, -DP, -DR, and -DQα1 protein expression levels showed a permanent expression among parathyroid tissues [74].

Parathyroid transplantation was first performed in 1911 by transplanting a tissue extract with thyroid tissue and the survival rate was reported to be up to 2 months [81]. The treatment of permanent hypoparathyroidism requires transplantation or hormone replacement therapy. From the first parathyroid transplantation, the 110 years to the present was demonstrated in various ways in literature, including survival rate, varying follow-up parameters, cell or tissue particle type of transplantation, and different transplant sites [82].

Autotransplantation is more frequently performed than allotransplantation [83]. There are 554 allotransplants reported in the literature from 1911 to 2021; among them, physicians evaluated several transplant sites including deltoid muscle [84, 85, 86, 87, 88, 89, 90], forearm muscle (brachialis) [78, 91, 92, 93, 94, 95, 96, 97, 98], shoulder junction muscles (pectoralis major) [99], rectus muscle (rectus abdominis) [100, 101], sternocleidomastoid muscle [102], and omentum [103]. The Warsaw team in Poland, who performed the largest series of parathyroid transplants in the literature, reported that the intraperitoneal space acts as a natural incubator for parathyroid cells [104]. Therefore, parathyroid allotransplantation in recent studies uses omentum for its protective features [103, 105].

5. Thyroid transplantation

The thyroid gland plays a decisive role in homeostasis and development. The hypothalamic-pituitary-thyroid axis regulates thyroid function, and functional response is coordinated by thyrotropin-releasing hormone (TRH), thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) [106].

The thyroid gland affects the metabolism of trace elements, and the level of trace elements also affects the metabolism and normal function of the thyroid gland. Changes in trace mineral levels will affect endocrine and other body systems causing thyroid dysfunction including hyperthyroidism, hypothyroidism, autoimmune thyroid diseases (Graves and Hashimoto’s), thyroid cancer, and other systemic diseases [106]. Trace elements are essential for human survival and many physiological processes, including those of the thyroid gland, where concentrations of many trace elements are higher than in other tissues [106]. Thyroid disease is a common endocrine disorder and the incidence is increasing. As a result, thyroid disease is attracting attention and some regional challenges remain strong such as the Black Sea region and Turkiye being an endemic goiter region [107, 108]. Although several research studies report inconclusive outcomes about the relationship between trace minerals and thyroid diseases.

Transcriptomic Analysis of the thyroid gland presents a distribution of certain genes. Different genes that share with other organs are classified by databases involving HPA and GTEx platforms. The thyroid gland has ten specific and 171 elevated genes compared to other organs in humans (Figure 4) [22, 23, 24, 25]. However, differential expression profiles in HLA molecules in thyroid glands show various changes by cancer progression. In 2007, one study reported that the frequency of Class II antigen expression in papillary thyroid cancer (PTC) samples, particularly the expression of HLA-DR/-DQ antigen, was found positive at 46.8 and 53.2%, respectively (n = 77). Clinicopathologically -DR and -DQ expressions were also present without nodal metastasis [109]. Notably, most differentiated thyroid tumors derived from thyroid epithelial cells are slow-growing cancers. Thyroid tumorigenesis is a complex process regulated by the activation of oncogenes, inactivation of tumor suppressors, and alterations in programmed cell death [110].

Figure 4.

Representative image of the particular shared genes and their relations between the thyroid gland and other organs. The image is reprinted from the human protein atlas database [21, 22, 23] and customized with BioRender.

In 2017, Selieger et al. defined the role of the HLA Class II presentation pathway in tumors [111]. HLA expression (particularly Class II antigens) influences the tumor antigen (TA)-specific immune responses. Several studies and databases provided concordant information to the literature about the frequency of HLA molecules for tumor types [21, 76, 109, 111]. It is noteworthy that methodology and lab-to-lab variations such as used antibodies, the patients’ population’s characteristics, and the disease’s molecular pathogenesis may reflect those differences [111].

The first record of thyroid transplantation in the literature belongs to the study of Albert Kocher in 1923 [112]. In patients with congenital thyroid insufficiency, administration of thyroid extract was used as a treatment of hypothyroidism, as a result, a certain level of success was achieved. A decrease in hypothyroidism symptoms was reported and 14% of the patients in this cohort (n = 204) showed otherwise [112]. Between 1946 and 1991 there were limited publications about thyroid transplantation studies written in languages other than English including Russian [113], French [114], and Japanese [115], and some of the studies did not have full text either. The first study that may be considered a starting point for thyroid transplantation was reported in 1978 by Perloff et al. The different sites were evaluated, and it was stated that the intrasplenic site was found to be the most unsuccessful transplantation area. Also, the thyroid was accepted as a “relatively privileged” tissue depending on the thyroid transplants that were considered successful. The originality of this study is that it is the first evaluation of thyroid and histocompatibility in terms of survival [116].

Approximately eight in vivo studies exist between 1981 and 2022 [117, 118, 119, 120, 121, 122, 123]. In these studies on thyroid transplantation; auto- and allotransplantation of irradiated thyroid tissue in rats with syngenic rats have been evaluated. At the 60-day follow-up, it was reported that thyroid tissue was functioning and irradiated thyroid auto-grafts would not show any pathological transformation [122]. Later, in 1988 Braun et al. reported an encapsulated transplantation of thyroid tissue in vivo, and the graft was placed beneath the kidney capsule. As an outcome thyroid encapsulated graft remained functional for 12 weeks [123]. Another study demonstrated a cultivation method for thyroid grafts in a hyperbaric oxygen condition and evaluated the survival, cellular infiltration, and HLA Class I molecule expressions. As a result, indicated cultivation systems provided immune cell depletion and decreased antigenicity of the thyroid graft [117]. At the beginning of the 2000s, Lee et al. provided donor-specific tolerance by administration of donor bone marrow cells via the portal venous route. Long-term follow-up was achieved and a combination of myeloablation therapy during 100-day survival was required for transplant success. That in vivo study has a significant role in stem cell and thyroid transplantation [119]. Furthermore, a different study reported the greater omentum feasibility as a transplant site for thyroid tissue via in vivo auto-transplantation model [120]. A more recent study from Wiseman et al. described a cell-pouch system for the thyroid tissue and determined the efficiency with viability and thyroid functionality after the subcutaneous transplantation model [121]. Intriguingly, the biomedical approach resulted in the fresh thyroid grafts showing a higher survival rate and functionality when compared to cryopreserved thyroid transplants [121].

The correction of thyroid deficiency remains a controversial clinical area despite the fact that more than 120 years having passed since the thyroid hormone was first used for therapeutic purposes [124]. Unfortunately, the cultivation of thyroid progenitor cells is a difficult task. Many attempts have been made to turn embryonic stem cells into functional thyroid cells, and here, limited success has been achieved mainly for mice [125]. Thyroid cell transplantation remains obscure for patients with congenital hypothyroidism and thyroid cancer. Besides, rejection occurs in the transplanted graft in patients with Hashimoto’s thyroiditis or those with Graves’ disease after the radioactive iodine or thyroidectomy process [125, 126].

6. Summary

Considering the antigen presentation and triggering an immune response after transplantation via HLA molecules, a common feature has been reported that 50% of endocrine tissues consist of non-immunogenic tissue [78, 94]. However, databases and existing studies report different expression outcomes for endocrine glands. When tissue and single-cell evaluations are mainly considered, two techniques come to the fore; histological evaluation and determination of RNA expression profile. Figure 5 demonstrates HLA expression outcome in endocrine tissues, including the adrenal gland, pancreas, parathyroid, and thyroid for classical and non-classical HLA molecules. Among these transplantable endocrine glands, expression levels according to the organs are adrenal gland>thyroid gland>pancreas>parathyroid. Notably, the HLA-DRβ1 molecule expression is even higher than the other organs and other classes. Considering the expression of whole HLA Class I molecules expressions (both classical and non-classical molecules) were shown a decreasing pattern. On the contrary, this pattern lost its linearity and revealed the existence of different tissue-specific profiles when Class II molecules were evaluated. Despite differences between expression profiles, the retrieval process causes a limitation in that the expression levels are observed in diseased tissue samples.

Figure 5.

HLA class I and II molecules expression in the particular endocrine organs involving the adrenal gland, pancreas, parathyroid, and thyroid gland, respectively. Expression levels refer to the number of transcripts per million (nTPM). RNA sequence expression data are retrieved from genotype-tissue expression portal (GTEx) [25]. Graphs were prepared in Microsoft excel and customized with BioRender.

Difficulties in obtaining healthy tissues hinder researchers from studying with large cohorts. Hence, comparing the normal and diseased states of endocrine tissues becomes a challenge to render. Despite all this, studies were carried out with tissues from diseased individuals who already contribute to transplantation. As an example; the donor tissues used in the literature for the transplantation of parathyroid tissues belong to parathyroid hyperplasia tissues obtained from individuals who had chronic kidney failure. Another similar example; the pancreas transplantation, the endocrine cells rather than exocrine cells are preferred. Endocrine cell content is isolated and then transplanted. Although, deceased donors are preferred in the transplantation of thyroid and adrenal glands. The typical target in the transplantations of these endocrine tissues is the elimination of organ deprivation. Thus, the goal is to maintain metabolic responses more regularly instead of hormone replacement therapy.

This chapter outlines the topics mentioned above that may provide a solid ground to understand the HLA and endocrine gland interactions as well as some facilitating features for transplantation and crucial immunogenicity characteristics. Further, these comparisons elucidate that tissue-specific immunity has distinctive roles in transplantable endocrine glands.

Acknowledgments

This chapter is dedicated to all patients undergoing treatment for endocrine-related diseases who agreed to participate in the studies from many researchers highlighted in this section. This chapter was supported by the Research Fund Unit of Bezmialem Vakif University.

Conflict of interest

The authors declare no conflict of interest.