Thymic nurse cells (TNCs) are large epithelial cells found in the cortex of the thymus and also in cortico-medullary junction.They have their own nucleus and are known to internalize thymocytes through extensions of plasma membrane. The cell surfaces of TNCs and their cytoplasmic vacuoles express MHC Class I and MHC Class II antigens. The interaction of these antigens with the developing thymocytes determines whether the thymocytes undergo positive or negative selection.
In anatomy and zoology, the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the best-known of all cortices.
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. Histologically, each lobe of the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule. The cortex and medulla play different roles in the development of T cells. Cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin. Developing T cells are referred to as thymocytes and are of hematopoietic origin. Stromal cells include epithelial cells of the thymic cortex and medulla, and dendritic cells.
In cell biology, the nucleus is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many.
Thymic nurse cells (TNCs) are a sub-population of cortical thymic epithelial cells (cTECs).pH91, which is a TNC-specific monoclonal antibody, that can be used to identify TNCs. Thymic nurse cells express both MHC Class I and II antigens, and are found in the cortico-medullary junction in addition to the cortex of the thymus.
Epithelial reticular cells, or epithelioreticular cells(ERC), some called thymic epithelial cell (TEC), are a structure in both the cortex and medulla of the thymus. However, histologically, they are more easily identified in the medulla. These cells contain secretory granules which are thought to contain the thymic hormones.
Monoclonal antibodies are antibodies that are made by identical immune cells that are all clones of a unique parent cell. Monoclonal antibodies can have monovalent affinity, in that they bind to the same epitope. In contrast, polyclonal antibodies bind to multiple epitopes and are usually made by several different plasma cell lineages. Bispecific monoclonal antibodies can also be engineered, by increasing the therapeutic targets of one single monoclonal antibody to two epitopes.
The thymic nurse cells in the cortico-medullary junction express cytokeratin 5 (K5) and cytokeratin 8 (K8), whereas the ones in the cortex express only cytokeratin 8.Thymic nurse cells expressing only cytokeratin 5 have not been identified so far. Hendrix et. al. found in their study that one-fourth of the nurse cells isolated from mice were double-positives for K5 and K8, while the rest of them were positive only for K8.
Cytokeratins are keratin proteins found in the intracytoplasmic cytoskeleton of epithelial tissue. They are an important component of intermediate filaments, which help cells resist mechanical stress. Expression of these cytokeratins within epithelial cells is largely specific to particular organs or tissues. Thus they are used clinically to identify the cell of origin of various human tumors.
The extensions of plasma membrane from thymic nurse cells form a cage-like structure, which trap (Hendrix et. al., 2010) triple positive T cells, αβTCRlowCD4+CD8+ within the spaces formed by the interlocking of the membrane. Some of these T cells retain their mobility and undergo maturation to the developmental stage of αβTCRhighCD69+; they are then released from the TNC complex.The enclosed thymocytes have been found to remain intact and retain both metabolic and mitotic activities despite lacking any contact with the extracellular environment.
Although initially thought to be involved only in positive selection, thymic nurse cells have now been discovered to facilitate negative selection of thymocytes as well.Negative selection refers to the degradation of thymocytes, and has been found to occur through the help of lysosomes. Lysosomes are present near the nucleus in the cytoplasm of TNCs. If the internalized thymocytes are selected for negative selection, vacuoles containing the thymocytes move closer to the area with lysosomes and eventually fuse with the lysosomes. This leads to the degradation of the T cells within the vacuoles. Macrophages have also been found actively moving in and out of the vacuoles inside the TNCs during the times of high apoptotic activity suggesting their involvement in the elimination of negatively selected T lymphocytes.
A lysosome is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins, and its lumenal proteins. The lumen's pH (~4.5–5.0) is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism.
MHC restriction within TNCs
Whether the thymocytes undergo positive or negative selection is determined through MHC restriction, which refers to the interaction between the αβTCR (αβ T cell receptor) of the T cells and MHC antigens on the antigen-presenting cells.
MHC-restricted antigen recognition, or MHC restriction, refers to the fact that a given T cell can interact with both the self-major histocompatibility complex molecule and the foreign peptide that is bound to it, but will recognize and respond to the antigen, only when it is bound to a particular MHC molecule.
An antigen-presenting cell (APC) or accessory cell is a cell that displays antigen complexed with major histocompatibility complexes (MHCs) on their surfaces; this process is known as antigen presentation. T cells may recognize these complexes using their T cell receptors (TCRs). APCs process antigens and present them to T-cells.
This role of MHC restriction was observed in a study conducted by Martinez et. al in HY-TCR transgenic mice. Since HY is a male specific antigen, the developing thymocytes would be expected to undergo degradation in males but not in females. However, both males and females were found to contain TNCs.Furthermore, female mice TNCs were found to contain five times more thymocytes than male mice, and less than 4% of them were apoptotic compared to almost 50% in the male TNCs. Also, almost 90% of all thymocytes extracted from the female TNCs were found to be double positives (CD4 + CD8 +), whereas no such phenotype was present within the male thymic nurse cells. Thus, since not all thymocytes internalized by TNCs went through apoptotic pathways, this was used to conclude that thymic nurse cells are involved in MHC restriction process.
Negative selection has been proposed to occur when the αβTCR in developing T cells interact with MHC present on antigen-presenting cells like dendritic cells and macrophages with strong affinities, which then leads T cells down the apoptotic pathway inside the TNCs. Similarly, extremely weak affinities lead to the death of T lymphocytes through neglect. Only intermediate affinity interaction between the αβTCR of the T cells and MHC antigens in the TNCs results in positive selection.
The thymic cortical cells take up early thymocytes migrating from the bone marrow to the thymus and form the thymocyte-TNC complexes. The formation of finger-like projections has been found to facilitate this uptake; which also requires the participation of membrane and cytoskeleton proteins of TECs and thymocytes. Other players that mediate this process are ICAM-1, which is a cell adhesion molecule found on the surface of vacuoles and TNCs, and other extracellular glycoproteins like fibronectin, laminin and type IV collagen, which are produced by TNCs.
Similarly, the cytoplasmic vacuoles present in the cytoplasm near the membrane network also facilitate the uptake of thymocytes that have been negatively selected to undergo apoptosis.
The molecules like gal-3 (Galectin-3) and gal-1(Galectin-1), on the other hand, produce antagonistic effects. They inhibit thymocytes/TEC interaction and affect the movement of thymocytes in and out of TNC, in particular by increasing thymocyte release from TNCs.
Thymocytes within TNC
Incubation of TNC at 37°C in tissue culture releases thymocytes (TNC-T) present within it.Incubation of TNC at 4°C or room temperature inhibits release of TNC-T. Incubation of TNC at 37°C in presence of 0.1% sodium azide prevents the release of TNC-T from within even though the TNC-T are viable. This suggests that metabolic activity of epithelial thymocyte complex is essential for the release of TNC-T.
TNC-T are functionally mature than those external to TNC (ET).Unlike ET, mouse TNC-T cells proliferate following stimulation with alloantigen , mitogen and help B cell make antibody when tested in tissue culture experiments. Chicken TNC-T cells exhibit greater graft vs host reactivity than peripheral blood T cells or ET cells when TNCs from one strain of chicken were placed on egg choriallantoic membrane of another strain of chicken with different MHC antigen . Based on the observation that TNC harbor functionally mature population of TNC-T and the electronmicroscopic studies suggesting that TNCs are localized in close proximity of blood capillaries in both cortex and cortico-medullary region of thymus, Vakharia & Mitchison have hypothesized that TNC-T are potential thymus emigrant cells.
A T cell is a type of lymphocyte which developes in the thymus gland and plays a central role in the immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor on the cell surface. These immune cells originate as precursor cells, derived from bone marrow, and develop into several distinct types of T cells once they have migrated in to the thymus gland - for which these cells are named. T cell differentiation continues even after they have left the thymus.
A cytotoxic T cell is a T lymphocyte that kills cancer cells, cells that are infected, or cells that are damaged in other ways.
'Antigen processing or cytosolic' pathway is an immunological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes. It is considered to be a stage of antigen presentation pathways. This process involves two distinct pathways for processing of antigens from an organism's own (self) proteins or intracellular pathogens, or from phagocytosed pathogens ; subsequent presentation of these antigens on class I or class II major histocompatibility complex (MHC) molecules is dependent on which pathway is used. Both MHC class I and II are required to bind antigen before they are stably expressed on a cell surface. MHC I antigen presentation typically involves the endogenous pathway of antigen processing, and MHC II antigen presentation involves the exogenous pathway of antigen processing. Cross-presentation involves parts of the exogenous and the endogenous pathways but ultimately involves the latter portion of the endogenous pathway.
The T-cell receptor (TCR) is a molecule found on the surface of T cells, or T lymphocytes, that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is degenerate: that is, many TCRs recognize the same antigen peptide and many antigen peptides are recognized by the same TCR.
Cross-presentation is the ability of certain antigen-presenting cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Cross-priming, the result of this process, describes the stimulation of naive cytotoxic CD8+ T cells into activated cytotoxic CD8+ T cells. This process is necessary for immunity against most tumors and viruses that do not readily infect antigen-presenting cells, but rather intracellular tumors and viruses that infect peripheral tissue cells. Cross presentation is also required for the induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.
Central tolerance, also known as negative selection, is the process of eliminating any developing T or B lymphocytes that are reactive to self. Through elimination of autoreactive lymphocytes, tolerance ensures that the immune system does not attack self peptides. Lymphocyte maturation occurs in primary lymphoid organs such as the bone marrow and the thymus. In mammals, B cells mature in the bone marrow and T cells mature in the thymus.
Thymocytes are hematopoietic progenitor cells present in the thymus. Thymopoiesis is the process in the thymus by which thymocytes differentiate into mature T lymphocytes. The primary function of thymocytes is the generation of T lymphocytes. The thymus provides an inductive environment, which allows for the development and selection of physiologically useful T cells. The processes of beta-selection, positive selection, and negative selection shape the population of thymocytes into a peripheral pool of T cells that are able to respond to foreign pathogens and are immunologically tolerant towards self antigens.
Antigen presentation describes a vital immune process which is essential for T cell immune response triggering. Because T cells recognise only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, now bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as presentation, where it can be recognized by a T cell receptor. If there has been an infection with viruses or bacteria, the cell will present an endogenous or exogenous peptide fragment derived from the antigen bound to MHC molecules. There are two types of MHC molecules which differ in the of the antigens: MHC class I molecules (MHC-I) bind peptides from the cell cytosol, while peptides generated in the endocytic vesicles after internalisation are bound to MHC class II (MHC-II). Cellular membranes separate these two cellular environments - intracellular and extracellular. Each T cell can finally recognise only ten to hundreds copies of a unique sequence of a single peptide among thousands of other peptides presented on the very same cell because MHC molecule in one cell can bind quite a large range of peptides.
In immunology, the CD3 T cell co-receptor helps to activate both the cytotoxic T cell and also T helper cells. It consists of a protein complex and is composed of four distinct chains. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains. These chains associate with the T-cell receptor (TCR) and the ζ-chain (zeta-chain) to generate an activation signal in T lymphocytes. The TCR, ζ-chain, and CD3 molecules together constitute the TCR complex.
Minor histocompatibility antigen are receptors on the cellular surface of donated organs that are known to give an immunological response in some organ transplants. They cause problems of rejection less frequently than those of the major histocompatibility complex (MHC). Minor histocompatibility antigens (MiHAs) are diverse, short segments of proteins and are referred to as peptides. These peptides are normally around 9-12 amino acids in length and are bound to both the major histocompatibility complex (MHC) class I and class II proteins. Peptide sequences can differ among individuals and these differences arise from SNPs in the coding region of genes, gene deletions, frameshift mutations, or insertions. About a third of the characterized MiHAs come from the Y chromosome. The proteins are composed of a single immunogenic HLA allele. Prior to becoming a short peptide sequence, the proteins expressed by these polymorphic or diverse genes need to be digested in the proteasome into shorter peptides. These endogenous or self peptides are then transported into the endoplasmic reticulum with a peptide transporter pump called TAP where they encounter and bind to the MHC class I molecule. This contrasts with MHC class II molecules's antigens which are peptides derived from phagocytosis/endocytosis and molecular degradation of non-self entities' proteins, usually by antigen-presenting cells. MiHA antigens are either ubiquitously expressed in most tissue like skin and intestines or restrictively expressed in the immune cells.
Clonal deletion is the removal through apoptosis of B cells and T cells that have expressed receptors for self before developing into fully immunocompetent lymphocytes. This prevents recognition and destruction of self host cells, making it a type of negative selection or central tolerance. Central tolerance prevents B and T lymphocytes from reacting to self. Thus, clonal deletion can help protect individuals against autoimmunity. Clonal deletion is thought to be the most common type of negative selection. It is one method of immune tolerance.
Short Course Immune Induction Therapy or SCIIT, is a therapeutic strategy employing rapid, specific, short term-modulation of the immune system using a therapeutic agent to induce T-cell non-responsiveness, also known as operational tolerance. As an alternative strategy to immunosuppression and antigen-specific tolerance inducing therapies, the primary goal of SCIIT is to re-establish or induce peripheral immune tolerance in the context of autoimmune disease and transplant rejection through the use of biological agents. In recent years, SCIIT has received increasing attention in clinical and research settings as an alternative to immunosuppressive drugs currently used in the clinic, drugs which put the patients at risk of developing infection, cancer, and cardiovascular disease.
Seong Hoe Park is a Korean immunologist and pathologist and a distinguished professor of pathology at the Seoul National University College of Medicine. He served as the chair of the Department of Pathology (2000–2004), the chair of the Graduate Program of Immunology (2002–2006), the President of Center for Animal Resource Development (2004–2006) at Seoul National University. He was the president of the Korean Association of Immunologists (2000–2001). Throughout his career as a T cell immunologist, Park established the theory of T cell-T cell interaction in human thymus, in which T cells expressing MHC class II drive previously unrecognized types of T cells and provide another significant developmental mechanism of T cells.
Medullary thymic epithelial cells (mTECs) represent a unique stromal cell population of the thymus which plays an essential role in the establishment of central tolerance. Therefore, mTECs rank among cells relevant for the development of functional mammal immune system.
Antigen transfer in the thymus is the transmission of self-antigens between thymic Antigen presenting cells (APCs) which contributes to the establishment of T cell central tolerance.
Cortical thymic epithelial cells (cTECs) form unique stromal cell population of the thymus which critically contribute to the development of T cells.
Thymic epithelial cells (TECs) are specialized cells with high degree of anatomic, phenotypic and functional heterogeneity that are located in the thymic epithelium within the thymic stroma. Thymus, as a primary lymphoid organ, mediates T cell development and maturation. Thymic microenvironment is established by TEC network filled with thymocytes in different developing stages. TECs and thymocytes are the most important components in the thymus, that are necessary for production functionally competent T lymphocytes and self tolerance. Dysfunction of TECs causes several immunodeficiencies and autoimmune diseases.
Promiscuous gene expression (PGE), formerly referred to as ectopic expression, is a process specific to the thymus that plays a pivotal role in the establishment of central tolerance. This phenomenon enables generation of self-antigens, so called tissue-restricted antigens (TRAs), which are in the body expressed only by one or few specific tissues. These antigens are represented for example by insulin from the pancreas or defensins from the gastrointestinal tract. Antigen-presenting cells (APCs) of the thymus, namely medullary thymic epithelial cells (mTECs), dendritic cells (DCs) and B cells are capable to present peptides derived from TRAs to developing T cells and hereby test, whether their T cell receptors (TCRs) engage self entities and therefore their occurrence in the body can potentially lead to the development of autoimmune disease. In that case, thymic APCs either induce apoptosis in these autoreactive T cells or they deviate them to become T regulatory cells, which suppress self-reactive T cells in the body that escaped negative selection in the thymus. Thus, PGE is crucial for tissues protection against autoimmunity.