The development of T cells in the thymus occurs under the direct influence and as a result of direct contacts of thymocytes with stromal epithelial cells, nurse cells, thymic macrophages, as well as under the influence of thymic hormones (α 1 -, β 1 -, β 4 -thymosin, thymopoietin, thymic humoral factor (m.m. 3220), thymostimulin (m.m. 12000). Under the influence of thymic hormones, the processes of proliferation and differentiation of thymocytes occur. In the thymus, T cells, during their development, acquire the ability to recognize antigen in the context of MHC molecules and tolerance to their own tissue antigens The morphogenesis of T-lymphocytes in the thymus is shown in the figure.
The earliest T cell to appear in the thymus is the prothymocyte, which is formed in the organ from a pre-T lymphocyte that migrated here from the bone marrow. Prothymocytes populate the cortical zone of the thymus. These cells are characterized by the presence in their cytoplasm of terminal nucleotidyl transferase (TdT) (DNA polymerase), which ensures the incorporation of additional nucleotides into DNA segments encoding variable regions of the T-cell receptor. Maturing cortical thymocytes first express the CD-1 marker, specific only for cortical thymocytes, then the constant marker of mature T cells CD2. Further, as they mature, thymocytes express a marker specific for inflammatory/helper cells – CD4 and a marker specific for cytotoxic cells – CD8. The cells then begin to express the T cell receptor (TCR) coupled to the T3 complex (CD3). After cells move from the thymic cortex to the medulla, some cells express only CD4 molecules, while others express only CD8 molecules. As a result, the entire population of thymocytes is divided into 2 phenotypes: cells expressing CD4 markers and cells expressing CD8 markers. Thus, two types of cells appear: one with the CD2 + , TKP + , CD3 + , CD4 + phenotype, which has inducer-helper properties, and the second, with the CD2 + , TKP + , CD3 + , CD8 + phenotype, which has cytotoxic properties . The question of the formation of a separate cell line in the thymus - T-suppressor cells, which have their own phenotypic markers, still remains open.
Major histocompatibility complex (MHC) antigens are expressed on T cells starting from the prothymocyte stage. In the process of the appearance of T cells with specific properties (inducer/helper cells and cytotoxic cells), thymocytes lose TdT and CD1 markers, which contain only immature T cells - thymocytes of the thymic cortex.
Markers that appear during the process of lymphocyte differentiation are called differentiation markers (CD) (cluster of differentiation) or differentiation antigens.
As T cells mature in the thymus, they acquire receptors for mitogens and the ability to respond to PHA and Con-A by blast transformation.
In the thymus, T lymphocytes differentiate, acquiring T-cell receptors (TCRs) and various co-receptors (surface markers). Play an important role in the acquired immune response. They ensure recognition and destruction of cells carrying foreign antigens, enhance the effect of monocytes, NK cells, and also take part in switching immunoglobulin isotypes (at the beginning of the immune response, B cells synthesize IgM, later switch to the production of IgG, IgE, IgA).
- 1 Types of T lymphocytes
- 1.1 T helper cells
- 1.2 Killer T cells
- 1.3 T-suppressors
- 2 Differentiation in the thymus
- 2.1 β-selection
- 2.2 Positive selection
- 2.3 Negative selection
- 3 Activation
- 4 Notes
Types of T lymphocytes
T-cell receptors (TCR) are the main surface protein complexes of T lymphocytes responsible for recognizing processed antigens bound to Major Histocompatibility Complex (MHC) molecules on the surface of antigen-presenting cells . The T-cell receptor is associated with another polypeptide membrane complex, CD3. The functions of the CD3 complex include transmission of signals into the cell, as well as stabilization of the T-cell receptor on the surface of the membrane. The T-cell receptor can associate with other surface proteins, TCR coreceptors. Depending on the coreceptor and the functions performed, two main types of T cells are distinguished.
T helper cells
T-helpers (from the English helper - assistant) - T-lymphocytes, the main function of which is to strengthen the adaptive immune response. They activate T-killers, B-lymphocytes, monocytes, NK cells through direct contact, as well as humorally, releasing cytokines. The main feature of T helper cells is the presence of the CD4 coreceptor molecule on the cell surface. Helper T cells recognize antigens when their T cell receptor interacts with an antigen bound to Major Histocompatibility Complex II (MHC-II) molecules.
Killer T cells
Helper T cells and killer T cells form a group effector T lymphocytes, directly responsible for the immune response. at the same time there is another group of cells, regulatory T lymphocytes, whose function is to regulate the activity of effector T lymphocytes. By modulating the strength and duration of the immune response through regulation of the activity of T-effector cells, regulatory T cells maintain tolerance to the body's own antigens and prevent the development of autoimmune diseases. There are several mechanisms of suppression: direct, with direct contact between cells, and distant, carried out at a distance - for example, through soluble cytokines.
T-suppressors
γδ T lymphocytes are a small population of cells with a modified T cell receptor. Unlike most other T cells, the receptor of which is formed by two α and β subunits, the T cell receptor of γδ lymphocytes is formed by γ and δ subunits. These subunits do not interact with peptide antigens presented by MHC complexes. It is assumed that γδ T lymphocytes are involved in the recognition of lipid antigens.
Differentiation in the thymus
All T cells originate from red bone marrow hematopoietic stem cells, which migrate to the thymus and differentiate into immature thymocytes. The thymus creates the microenvironment necessary for the development of a fully functional T cell repertoire that is MHC-restricted and self-tolerant.
Thymocyte differentiation is divided into different stages depending on the expression of various surface markers (antigens). At the earliest stage, thymocytes do not express the CD4 and CD8 coreceptors and are therefore classified as double negative (DN) (CD4-CD8-). At the next stage, thymocytes express both coreceptors and are called double positive (DP) (CD4+CD8+). Finally, at the final stage, there is a selection of cells that express only one of the coreceptors (Single Positive (SP)): either (CD4+) or (CD8+).
The early stage can be divided into several substages. So, at the DN1 substage (Double Negative 1), thymocytes have the following combination of markers: CD44+CD25-CD117+. Cells with this combination of markers are also called early lymphoid progenitors (ELP). Progressing in their differentiation, ELP cells actively divide and finally lose the ability to transform into other types of cells (for example, B lymphocytes or myeloid cells). Moving to the DN2 (Double Negative 2) substage, thymocytes express CD44+CD25+CD117+ and become early T-cell progenitors (ETPs). during the DN3 substage (Double Negative 3), ETP cells have a combination of CD44-CD25+ and enter the process β-selection.
β-selection
T-cell receptor genes consist of repeating segments belonging to three classes: V (variable), D (diversity), and J (joining). In the process of somatic recombination, gene segments, one from each class, are joined together (V(D)J recombination). The combined sequence of the V(D)J segments results in unique sequences for the variable domains of each receptor chain. The random nature of the formation of variable domain sequences makes it possible to generate T cells capable of recognizing a large number of various antigens, and, as a result, provide more effective protection against rapidly evolving pathogens. However, this same mechanism often leads to the formation of nonfunctional T-cell receptor subunits. The genes encoding the TCR-β subunit of the receptor are the first to undergo recombination in DN3 cells. To exclude the possibility of the formation of a non-functional peptide, the TCR-β subunit forms a complex with the invariable pre-TCR-α subunit, forming the so-called. pre-TCR receptor. Cells that are unable to form a functional pre-TCR receptor die by apoptosis. Thymocytes that have successfully passed β-selection move to the DN4 substage (CD44-CD25-) and undergo the process positive selection.
Positive selection
Cells expressing the pre-TCR receptor on their surface are still not immunocompetent, since they are not able to bind to molecules of the major histocompatibility complex (MHC). Recognition of MHC molecules by the TCR receptor requires the presence of CD4 and CD8 coreceptors on the surface of thymocytes. The formation of a complex between the pre-TCR receptor and the CD3 coreceptor leads to inhibition of rearrangements of the β subunit genes and at the same time causes activation of the expression of the CD4 and CD8 genes. Thus, thymocytes become double positive (DP) (CD4+CD8+). DP thymocytes actively migrate to the thymic cortex where they interact with cortical epithelial cells expressing both MHC complexes (MHC-I and MHC-II). Cells that are unable to interact with the MHC complexes of the cortical epithelium undergo apoptosis, while cells that successfully undergo such interaction begin to actively divide.
Negative selection
Thymocytes that have undergone positive selection begin to migrate to the corticomedullary border of the thymus. Once in the medulla, thymocytes interact with the body's own antigens presented on the MHC complexes of medullary thymic epithelial cells (mTECs). Thymocytes that actively interact with their own antigens undergo apoptosis. Negative selection prevents the emergence of self-activating T cells capable of causing autoimmune diseases, being an important element of the body’s immunological tolerance.
Activation
T-lymphocytes that have successfully passed positive and negative selection in the thymus and reached the periphery of the body, but have not had contact with the antigen, are called naive T cells(eng. Naive T cells). The main function of naïve T cells is to respond to pathogens previously unknown to the body's immune system. Once naïve T cells recognize an antigen, they become activated. Activated cells begin to actively divide, forming many clones. Some of these clones turn into effector T cells, which perform functions specific to of this type lymphocyte (for example, they secrete cytokines in the case of T-helper cells or lyse affected cells in the case of T-killer cells). The other half of the activated cells are transformed into Memory T cells. Memory cells remain in an inactive form after initial contact with an antigen until a second interaction with the same antigen occurs. Thus, memory T cells store information about previously active antigens and form a secondary immune response, which occurs in a shorter time than the primary one.
The interaction of the T-cell receptor and co-receptors (CD4, CD8) with the major histocompatibility complex is important for the successful activation of naive T cells, but in itself is not sufficient for differentiation into effector cells. For the subsequent proliferation of activated cells, the so-called interaction is necessary. costimulatory molecules. For T helper cells, these molecules are the CD28 receptor on the surface of the T cell and immunoglobulin B7 on the surface of the antigen-presenting cell.
Notes
- Murphy K., Travers P., Walport M. Janeway's Immunobiology. - New York: Garland Science, 2011. - 888 p. - ISBN 0-8153-4123-7.
- Alberts B., Johnson A., Lewis J., Raff M., Roberts K., Walter P. Molecular Biology of the Cell. - New York: Garland Science, 2002. - 1367 p. - ISBN 0-8153-3218-1.
- Holtmeier W., Kabelitz D. Gammadelta T cells link innate and adaptive immune responses // Chemical immunology and allergy. - 2005. - Vol. 86. - P. 151–83. - ISBN 978-3-8055-7862-2. - DOI:10.1159/000086659 - PMID 15976493.
- Schwarz B. A., Bhandoola A. Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis // Immunol. Rev.. - 2006. - Vol. 209. - P. 47–57. - DOI:10.1111/j.0105-2896.2006.00350.x - PMID 16448533.
- Sleckman B. P. Lymphocyte antigen receptor gene assembly: multiple layers of regulation // Immunol Res. - 2005. - Vol. 32. - P. 153-8.
| Blood | |
|---|---|
| Hematopoiesis |
Human bone marrow Erythropoiesis Leukopoiesis |
| Components |
Plasma Red blood cells Hematocrit Platelets Leukocytes: Granulocytes (Neutrophils Eosinophils Basophils) Agranulocytes (Lymphocytes (T-B-NK-) Monocyte) |
| Biochemistry |
Blood group Rh factor Buffer systems (Acidosis Alkalosis) Serum Glycemia (Hyper-Hypo-) |
| Diseases |
Anemia Leukemia Coagulopathy (Hemophilia Von Willebrand's disease) Hemoblastosis |
| see also |
Hematology Oncohematology |
t lymphocytes are higher, t lymphocytes are normal, t lymphocytes are increased, t lymphocytes are decreased
T-lymphocytes Information About
Lymphocytes are special cells in the body of a living being. They are responsible for protecting it from external irritants, infections, and viruses. But the concept of “lymphocytes” itself is quite broad and general. Within themselves, these cells will be divided into several more groups. In this article we will take a closer look at one of them - T-lymphocytes. Functions, types of cells, their normal indicators, deviations from the norm in human blood - all these topics will be discussed further.
Origin of cells
Where are T-lymphocyte cells formed? Although their main place of “residence” is the bloodstream (lymphocytes also live in other tissues), they are not formed there. The place of their “birth” is the bone red marrow. It is known as the hematopoietic tissue of the body. That is, in addition to lymphocytes, erythrocytes and white blood cells (neutrophils, leukocytes, monocytes) will also be formed here.
The structure of lymphocytes
The "anatomical" features are as follows:
- Large kernel of round or oval shape.
- There will be no granularity in the cytoplasm (the contents of the cell itself).
- If there is little cytoplasm in a cell, it is called narrow-plasma, if there is a lot - wide-plasma.
In terms of their structure, the lymphocytes that inhabit the blood will be slightly different from their counterparts that live in other tissues. And that's okay. Moreover, cells “living” in one place will also have some external differences among themselves.
Types of lymphocytes
In addition to the types of T lymphocytes, there are various groupings of these cells in general. Let's look at them.
The first classification is by size:
- Small ones.
- Big ones.
The second classification is based on the functions performed:
- B lymphocytes. They can recognize foreign particles and produce deadly antibodies against them. In other words, they are responsible for humoral immunity.
- T lymphocytes. The main function is responsibility for cellular immunity. They come into contact with foreign bodies and destroy them.
- NK cells. Natural killers that can recognize cancerous, defective cells and destroy them. Responsible for preserving cellular normal composition the whole body.
Types of T lymphocytes
This group of lymphocytes within itself will be divided into several more types:
- Killer T cells.
- T-helpers.
- T-suppressors.
- Memory T cells.
- Amplicators-lymphocytes.
Killer T-cells: what type?
These are the most famous representatives of the group of T-lymphocytes. Their main task is the destruction of inferior, defective cells of the body. Another name for the group is cytotoxic T-lymphocytes. In other words, they are responsible for eliminating cells (“cyto”) that have a toxic effect on the entire body.
The main function of killer T cells is immune surveillance. Cells aggressively act on foreign protein. It is this useful function that can be harmful when organs are transplanted to a person. T-killers strive to quickly destroy the “stranger,” not realizing that it is he who is able to save the body. Therefore, the patient takes medications for some time after organ transplantation that suppress the immune system. The drugs reduce the percentage of T-killer cells in the blood and disrupt their interaction. Thanks to this, the transplanted organ takes root, and the patient does not face complications or death.
The mechanism of action of this type of lymphocyte on a foreign element is very interesting. Phagocytes, for example, aggressively “attack” a “stranger” for its subsequent devouring and digestion. T-killers against their background are “noble killers”. They touch the object with their processes, then break contact and move away. Only after such a “kiss of death” does the foreign microorganism die. Why?
When touched, killer T cells leave a piece of their membrane on the surface of the body. It has properties that allow it to corrode the surface of the object of attack - up to the formation of through holes. Through these holes, potassium ions leave the microorganism, and water and sodium ions take their place. The cellular barrier is broken, there is no longer a boundary between the internal and external environment. The microorganism is inflated by the water entering it, the cytoplasmic proteins and organelles are destroyed. The remains of the “stranger” are then devoured by phagocytes.
Helpers
The main function of these T-lymphocyte cells is to help. Hence their name, which comes from English word, translated the same way.
But who or what do these T-lymphocytes come to the rescue of? They are designed to induce and stimulate an immune response. It is under the influence of T-helpers that the T-killers, with whom we have already become acquainted, will activate their work.
Helpers will begin to transmit data about the presence of a foreign protein in the body. And this is valuable information for B lymphocytes - they, in turn, begin to secrete certain protective antibodies against it.
T helper cells also stimulate the work of another type of “guard” cells - phagocytes. In particular, they interact closely with monocytes.
Suppressors
This term itself means "suppression". From here the function of T-suppressors becomes clear to us. Helpers in our body will activate protective, immune function, and these T-lymphocytes, on the contrary, will suppress it.
Don't think that this has any negative impact on the system. Suppressor T cells are responsible for regulating the immune response. After all, somewhere you need to react to a certain stimulus with restraint and moderation, and somewhere you need to accumulate all available forces against it.
Amplifiers
Let us now turn to the functions of T-lymphocytes of this group. After one or another aggressor penetrates the body, the content of lymphocytes immediately increases in the blood and tissues of a living creature. For example, in just a few hours their volume can double!
What is the reason for this rapid growth armies of protective cells? Maybe the fact is that somewhere in the body they are “hidden” in reserve for the time being?
This is true. Some mass of mature, full-fledged lymphocytes lives in the thymus and spleen. Only up to a certain point these cells are not “defined” with their purpose and function. They will be called amplifiers. If necessary, these cells turn into one or another type of T-lymphocyte.
Memory cells
Experience, as you know, is the main weapon. Therefore, having coped with any threat, our T-lymphocytes remember it. In turn, the body produces special cells that will store this information until a new “battle” with this foreign element. These elements will be memory T cells.
A secondary aggressor (of the type that the immune system has already resisted) enters the body. The memory T cell recognizes it. Then this particle begins to actively multiply in order to give the foreign organism a secondary worthy immune response.
Normal indicators of T-lymphocytes in human blood
It is impossible to provide any specific figure in this category - normal values will vary depending on the age of the person. This is due to the peculiarities of the development of his immune system. With age, the volume of the thymus gland will decrease. Therefore, if in childhood lymphocytes predominate in the blood, then with adulthood they transfer the leading position to neutrophils.
The level of T-lymphocytes in the blood helps determine a general clinical blood test. The normal indicators here are:
- (50.4±3.14)*0.6-2.5 thousand.
- 50-70%.
- The “helper/suppressor” ratio is 1.5-2.
What do increased and decreased indicators indicate?
An increased level of T-lymphocytes in the blood may indicate the following:
- Chronic or acute lymphocytic leukemia.
- Hyperactive immunity.
- Sézary's syndrome.
On the contrary, a reduced content of T-elements indicates the following pathologies and diseases:
- Chronic infections- purulent processes, HIV, tuberculosis.
- Reduced production of lymphocytes.
- Genetic diseases, causing immunodeficiency.
- Tumors of lymphoid tissue.
- Renal and heart failure observed at the last stage.
- T-cell lymphoma.
- The patient is taking medications that destroy lymphocytes.
- Consequence of radiation therapy.
We got acquainted with T-lymphocytes - the protective cells of our body. Each type performs its own special function.
A well-functioning immune system of a healthy person is able to cope with most external and internal threats. Lymphocytes are blood cells that are the first to fight for the cleanliness of the body. Viruses, bacteria, fungus are the daily concern of the immune system. Moreover lymphocyte functions are not limited to detecting external enemies.
Any damaged or defective cells of one's own tissues must also be detected and destroyed.
Functions of lymphocytes in human blood
The main performers in the work of immunity in humans are colorless blood cells - leukocytes. Each variety fulfills its function, most important of which are allocated specifically to lymphocytes. Their number relative to other leukocytes in the blood sometimes exceeds 30% . Functions of lymphocytes are quite diverse and accompany the entire immune process from beginning to end.
In essence, lymphocytes detect any fragments that do not correspond to the body genetically, give a signal to start a battle with foreign objects, control its entire course, actively participate in the destruction of “enemies” and end the battle after victory. As conscientious guards, they remember each violator by sight, which gives the body the opportunity to act faster and more efficiently at the next meeting. This is how living beings manifest a property called immunity.
The most important lymphocyte functions:
- Detection of viruses, bacteria, etc. harmful microorganisms, as well as any cells of one’s own body that have abnormalities (old, damaged, infected, mutated).
- A message to the immune system about the “invasion” and the type of antigen.
- Direct destruction of pathogenic microbes, production of antibodies.
- Management of the entire process using special “signal substances”.
- Winding down the active phase of the “battle” and managing the cleanup after the battle.
- Preservation of memory of each defeated microorganism for subsequent rapid recognition.
The production of such immune soldiers occurs in the red bone marrow; they have different structures and properties. It is most convenient to distinguish immune lymphocytes by their functions in defense mechanisms:
- B lymphocytes recognize harmful inclusions and synthesize antibodies;
- T-lymphocytes activate and inhibit immune processes, directly destroy antigens;
- NK lymphocytes perform a function control over the tissues of the native organism, are capable of killing mutated, old, degenerated cells.
Based on their size and structure, they are distinguished between large granular (NK) and small (T, B) lymphocytes. Each type of lymphocyte has its own characteristics and important functions, which are worth considering in more detail.
B lymphocytes
TO distinctive features This means that for normal functioning the body requires not just young lymphocytes in large quantities, but hardened, mature soldiers.
Maturation and education of T cells take place in the intestines, appendix, and tonsils. In these "training camps" young bodies are trained to perform three important functions:
- “Naive lymphocytes” are young, not activated blood cells that have no experience of encountering foreign substances, and therefore do not have strict specificity. They are able to show a limited reaction to several antigens. Activated after meeting an antigen, they are sent to the spleen or bone marrow for re-maturation and rapid cloning of their own kind. After ripening, plasma cells very quickly grow from them, producing antibodies exclusively to this type of pathogen.
- Mature plasma cells, strictly speaking, are no longer lymphocytes, but factories for the production of specific soluble antibodies. They live only a few days, eliminating themselves as soon as the threat that caused the defensive reaction disappears. Some of them will later be “preserved” and will again become small lymphocytes with memory of the antigen.
- Activated B-lymphocytes, with the assistance of T-lymphocytes, can become repositories of the memory of a defeated foreign agent; they live for decades, perform a function transmitting information to their “descendants”, providing long-term immunity, accelerating the body’s response to meeting the same type of aggressive influence.
B cells are very specific. Each of them is activated only when encountering a certain type of threat (a strain of a virus, a type of bacteria or protozoa, a protein, a chemical). The lymphocyte will not react to pathogens of a different nature. Thus, the main function of B lymphocytes is to provide humoral immunity and produce antibodies.
T lymphocytes
Young T-bodies are also produced by the bone marrow. This type of red blood cells undergoes the most stringent step-by-step selection, which rejects more than 90% of young cells. “Nurturing” and selection occur in thymus gland(thymus).
Note!The thymus is an organ that enters the phase of greatest development between 10 and 15 years, when its mass can reach 40 g. After 20 years, it begins to decrease. In old people, the thymus weighs the same as in babies, no more than 13 g. The working tissues of the gland after 50 years are replaced by fatty and connective tissue. Accordingly, the number of T cells decreases and the body’s defenses weaken.
As a result of the selection occurring in the thymus gland, T-lymphocytes are eliminated that are not capable of binding any foreign agent, as well as those that have detected a reaction to proteins of the native organism. The remaining ripened bodies are considered suitable and are scattered throughout the body. A huge number of T cells (about 70% of all lymphocytes) circulate in the bloodstream; their concentration is high in the lymph nodes and spleen.
Three types of mature T lymphocytes leave the thymus:
- T-helpers. They help perform functions B lymphocytes, other immune agents. They guide their actions during direct contact or give orders by releasing cytokines (signal substances).
- Killer T cells. Cytotoxic lymphocytes that directly destroy defective, infected, tumor, and any modified cells. Killer T cells are also responsible for the rejection of foreign tissue upon implantation.
- T-suppressors. Execute important function supervision of the activity of B lymphocytes. Slow down or stop the immune response, if necessary. Their immediate responsibility is to prevent autoimmune reactions, when protective bodies mistake their cells for hostile ones and begin to attack them.
T-lymphocytes have the main properties: regulate the speed of the protective reaction, its duration, serve as an obligatory participant in certain transformations and provide cellular immunity.
NK lymphocytes
Unlike small forms, NK cells (null lymphocytes) are larger and contain granules consisting of substances that destroy the membrane of the infected cell or destroy it entirely. The principle of defeating hostile inclusions is similar to the corresponding mechanism in T-killers, but is more powerful and does not have pronounced specificity.
NK lymphocytes do not undergo the maturation procedure in lymphatic system, are able to react to any antigens and kill such formations that T-lymphocytes are powerless against. For such unique qualities they are called “natural killers.” NK lymphocytes are the main killers of cancer cells. Increasing their number and increasing activity is one of the promising directions for the development of oncology.
Interesting! Lymphocytes carry large molecules that transmit genetic information throughout the body. Important Feature These blood cells are not limited only to protection, but extend to the regulation of restoration, growth, and differentiation of tissues.
When necessary, null lymphocytes can function as B or T cells, thus serving as universal soldiers of the immune system.
In the complex mechanism of immune processes, lymphocytes play a leading, regulatory role. Moreover, they carry out their work both through contact and at a distance, developing special chemical substances. Recognizing these command signals, all links of the immune chain are coordinated into the process and ensure the purity and durability of the human body.
Article for the “bio/mol/text” competition: Immune system cells travel through the lymph and bloodstream in search of an antigen that they can recognize and initiate a protective immune response. But a significant portion of T-lymphocytes are not found in the blood or lymph nodes, but in organs that are not related to the immune system. This article explains what tissue-resident T cells do, how they get there, and the medical benefits of studying them.
Note!
This work took first place in the “Best Article on Immunology” category of the “bio/mol/text” competition - 2015.
The sponsor of the nomination “Best article on the mechanisms of aging and longevity” is the Science for Life Extension Foundation. The audience award was sponsored by Helicon.
Competition sponsors: Biotechnology Research Laboratory 3D Bioprinting Solutions and Scientific Graphics, Animation and Modeling Studio Visual Science.
An adequate protective reaction when infected with a pathogenic virus is to destroy the infected cells, preventing the infection from spreading throughout the body and killing more cells. A cell infected with a virus can notice the virus in itself and begin autophagy or apoptosis - or receive instructions for programmed cell death from a T-killer cell.
Classical human immunology is based on the study immune cells blood simply due to the fact that a blood test can be taken from any patient and examined for normal and pathological conditions. It is on blood cells that the classification of T-lymphocytes was built: division into T-killers and T-helpers, which check the antigen specificity of T-killers, give them a “license to kill” and are able to control the entire course of the immune response through soluble signaling molecules - cytokines. As well as the later isolation from the T-helper branch of a group of regulatory T cells that suppress excess adaptive immunity.
But as yogurt commercials remind us, a significant portion of the immune system's cells are concentrated around the lining of the digestive tract and in other tissues. While there are about 6–15 billion lymphocytes in 5–6 liters of blood in an adult, the number of T cells found in the epidermis and skin is estimated at 20 billion, and another 4 billion in the liver of an adult man. Is studying blood cells enough to full description functions of T cells if there are more T cells in peripheral organs than in the bloodstream? And are classical subpopulations sufficient to describe all the types of T cells found in the human body?
Life cycle of a T lymphocyte
Each T cell, after assembling a T-cell receptor, is tested for the functionality of the randomly assembled receptor (positive selection) and the lack of specificity for the body's own antigens (negative selection), that is, for the absence of an obvious autoimmune threat. The stages of selection occur in the thymus gland, thymus; in this case, more than 90% of precursor cells die, failing to correctly assemble the receptor or undergo selective selection. Surviving T cells proliferate and exit the thymus into the bloodstream - these are naïve T lymphocytes that have not encountered the antigen. The naive T cell circulates through the blood and periodically enters the lymph nodes, where in the T-cell zone it contacts specialized antigen-presenting cells.
After meeting the antigen in the lymph node, the T cell acquires the ability to divide again - it becomes the precursor of memory T cells (T SCM, stem cell memory T cells). Among the clone of its descendants, central memory cells (T CM), short-lived effector cells that carry out the immune reaction (SLEC or T EMRA cells), and effector memory precursor cells T EM appear, which in turn, when dividing, produce T EMRA. All these cells leave the lymph node and travel through the blood. Effector cells can then leave the bloodstream to carry out an immune response in the peripheral tissue of the organ where the pathogen resides. What then - another journey through the blood and lymph nodes?
Figure 1. Emigration of an effector T cell into tissue during viral infection. Inflammatory signals from infected epithelial cells with the participation of resident cells are transmitted to the vascular endothelium; endothelial cells attract effector T cells with chemokines CXCL9, CXCL10. Rolling: When moving along a postcapillary venule in tissue, the effector cell slows down, forming temporary contacts between E-selectins and P-selectins on endothelial cells. Arrest: The effector cell adheres tightly to the endothelium when LFA-1 and other alpha integrins interact with ICAM-1/VCAM-1/MAdCAM-1 (on the endothelium). Transmigration: The effector T cell binds endothelial JAM-1 with molecules PECAM, CD99, LFA-1 and penetrates through endothelial cells into the submucosa. Drawing from.
The process of leukocyte transmigration.
The cells of the stroma, that is, the base of the lymph node, secrete signaling substances in order to call the T cell to the lymph node - chemokines. Lymph node chemokines are recognized by homing receptors CCR7 and CD62L. But effector cells lack both of these receptors. Because of this for a long time It was a mystery how effector cells could get from peripheral tissue back to the secondary lymphoid organs - the spleen and lymph nodes.
At the same time, data began to accumulate on differences in membrane marker repertoires and transcriptional profiles between memory T cells in the blood (T EM) and memory T cells in other organs, which did not fit into the concept of constant migration of T cells between tissues and tissues. blood. It was decided to isolate a new subpopulation: resident memory cells that inhabit a specific organ and do not recycle - T RM cells.
Origin of tissue resident T cells
Where do tissue resident cells first appear? These are the descendants of effector cells that have lost their ability to recycle. Some tissues peripheral to the immune system, for example, the mucosa of the small intestine, abdomen, - allow effector T lymphocytes to penetrate freely; others - very limited, a large flow of effector T cells into these tissues is observed only during an inflammatory reaction. The second type of tissue includes those separated by a barrier from the immune system, for example, the brain and spinal cord, as well as many others: peripheral ganglia, mucous membranes of the genital organs, lungs, epidermis, eyes. The difference between the two tissue types is the expression of additional homing molecules for effector T cells, such as the epithelial infiltration adhesion molecule MadCAM-1.
Figure 3. “To home or not to home?” - complex choice of effector cell. To home is the process of homing, or migration of T cells, for example, to the place most familiar to naive cells - the lymph node. The alternative is not to travel through the body and become a resident tissue cell.
Resident T cells in aging human tissues
A map of the relationships between the presence of individual subpopulations of T cells in different human organs, oddly enough, was compiled only in 2014. Donna Farber's team from medical center Columbia University of New York compared the phenotypes of T cells isolated from the blood and tissues of organ donors of all age groups from 3 to 73 years, for a total of 56 donors. Analysis of T cell subpopulations using flow cytometry confirmed many data obtained using methods with lower resolution and lower statistics, and some features of the description of the immune system transferred from mouse immunology to humans, for example, a decrease in the content of naïve T lymphocytes with aging in all organs .
The decrease in the number of naïve T cells with age is associated with rapid aging of the thymus gland, in which future T cells undergo the stages of T-cell receptor assembly, testing the functionality of the receptor, and selection for a lack of autoimmune potential. It is important not only to reduce the absolute number of naïve T cells, but also to reduce the diversity of the T-cell receptor repertoire, and therefore the ability to form an adaptive immune response to a previously unfamiliar infection. For naive T-killers, a progressive decline in numbers in the blood and lymph nodes was confirmed, although for naive T-helpers there was a negative correlation of numbers with age in this study was significant only in secondary lymphoid organs, but not in the blood.
Isolation of memory T lymphocytes, memory effector cells and short-lived effector cells from the mucous membranes of the lungs, small and large intestines, inguinal and mesenteric lymph nodes of organ donors made it possible for the first time to assess the dynamics of these populations in human tissues during aging. The proportion of central memory cells expectedly increases over the course of life, in accordance with the increase in the number of infections that the body has encountered and entered the memory library of the immune system. The percentage of terminally differentiated effector killer T cells (T EMRA) also increases, but only in the lymph nodes and spleen; in non-lymphoid tissues, the number of T EMRA decreases. Effector memory cells T EM rapidly fill the niche for T cells in the child’s tissues, quickly displacing naïve T cells by about 12 years of age. Short-lived terminally differentiated killer T cells are most often found in the blood, spleen and mucous membranes of the lungs at any age, but among T helper cells this subpopulation is represented by a vanishingly small number of cells. Similarly, there are few central memory cells among killer T cells; they are predominantly located in the mucous membranes of two barrier tissues: the lungs and intestines.
In broad strokes, the distribution map of human T-lymphocytes can be outlined as follows: naïve T-cells travel through the blood and periodically enter secondary lymphoid organs; killer T-EMRA cells are found in the blood, spleen and lungs. Apparently, central memory cells are characterized by a more individual distribution across tissues than other subpopulations: in any case, patterns of dynamics during aging of different tissues could not be identified. Effector memory cells, including the T RM subpopulation, dominate among T cells of mucosal barrier tissues. In general, with aging T cell immunity non-lymphoid tissues greater age-related dynamics of T-cell types. The stability of tissue cells is easier to explain if we understand which of the effector cells TEM remain in the tissue, become resident TRM, and what events their life consists of after they stop traveling throughout the body.
Figure 4. Circulation pathways of T lymphocytes of different subpopulations. Tnaive - naive T cells, together with the T CM subpopulation, move through the blood and enter the T-cell zone of various lymph nodes; they are found in tissue capillaries, but do not enter the tissue (red trajectory). Effector T-cells (blue color) move through the lymph and bloodstream; when they enter a lymph node, they do not enter the T-cell zones (center of the lymph node) - trajectory lilac color. Tissue resident T cells (shown in green in the skin and different colors in mucous membranes) move only within the tissue - green trajectory. Drawing from , with modifications.
How to distinguish resident tissue cells from admixtures of blood cells?
Resident T cells can be correctly, but inconveniently, determined each time by the ability of an individual cell to migrate to lymph nodes, so a list must be compiled characteristic features, by which one can determine membership in this subpopulation. Resident T lymphocytes in the body's natural barrier tissues (for example, in the lungs and small intestinal mucosa) are somewhat similar to classical effector blood cells: they express the activated cell marker CD69, and the expression is stable throughout life during adulthood and aging and is characteristic of all non-lymphoid tissues . But in addition, CD69 colocalizes with the marker CD103, which designates a group of adhesion molecules - integrins, which promote the attachment of resident T cells to the epithelium and fibroblasts in the submucosa of the selected organ. For effector T cells in secondary lymphoid organs, the expression of CD103 integrins is completely uncharacteristic: T EM cells constantly maintain a motile phenotype.
The map compiled by Donna Farber's team has a major flaw: it is unclear how cleanly T-lymphocytes can be isolated from an organ, and what proportion of the analyzed cells are actually blood T-lymphocytes from capillaries inside the organ.
The issue of contamination with blood cells is especially acute for the lungs; it is no coincidence that the subpopulation composition of T cells in the lungs is unexpectedly similar to T cells in the blood and lymph nodes. The issue of blood cell contamination was elegantly solved for mouse T-lymphocytes: experimental mice were infected with a virus lymphocytic choriomeningitis after transplantation of a transgenic clone of P14 T cells specific to this virus. As a result, during infection, the majority of circulating cells were represented by the virus-specific P14 clone, and its presence in tissues could be monitored by immunofluorescence with a P14-specific antibody. Before the mice were killed, they were injected with an antibody to the killer T-cell marker anti-CD8, which quickly spread through the bloodstream and bound to all killer T-cells in the blood (but not in the tissues). By microscopy of organ sections, it was easy to distinguish resident killer T RMs from cells that had just recently emerged from the blood into the organ and were labeled with anti-CD8 antibody. The number of resident cells calculated by this method was 70 times higher than the numbers determined by flow cytometry; a difference of less than twofold was observed only for resident cells of the lymph nodes and spleen: it turns out that standard methods for isolating lymphocytes from organs are poorly suited for the analysis of killer resident cells and significantly underestimate the population size.
The work of resident T cells: do not confuse tourism with emigration
In a normal situation, mouse resident tissue cells hardly move within non-lymphoid tissue and are quite firmly attached by adhesion molecules to the stroma of the organ. When resident macrophages of the same tissue initiate an inflammatory response by secreting cytokines, T RMs become more motile and patrol the nearby epithelium in search of infected cells.
If the inflammatory reaction intensifies, then the cells understand this as a signal of reinforcement: T CM and T EM cells newly arriving from the blood are involved in the work of patrol T RM cells. These blood cells are much more mobile and move better in the epithelium: does this mean that it is in the blood that killer T-cells among TEMs are ready to act, and CD8+ TRMs perform helper and regulatory functions in the tissue?
On the one hand, T-helpers are more tissue-specific in the spectrum of T-cell receptors, that is, there is very little overlap between the repertoires of T-cell receptors of cells taken from different tissues, while cells of the same T-killer clone are found in different tissues among T EM . The range of functions and repertoire of antigen specificity of T RM remains to be studied, but T RM killers definitely have the ability to destroy infected tissue cells. Moreover, the affinity of virus-specific T-cell receptors (TCRs) of resident killer cells is higher than that of virus-specific central memory cells in a mouse model of polyomavirus infection in brain tissue.
However, the size of the T-cell population depends not only on the specificity of T-cell receptors for infections that previously occurred in a given organ, but also on the homeostatic proliferation of T-cells - the proliferation of more successful cells to fill the organ's capacity according to the number of T-lymphocytes. By using the markers CD28 and CD127 on the cell surface, it is possible to distinguish cells recently and long ago activated through the T-cell receptor from those that received only a homeostatic signal to proliferation from the growth factor IL-7. With tissue aging, homeostatic cell proliferation begins to prevail over the proliferation of TCR-activated cells.
NKT cells, a large type of liver resident cell that is also found in other tissues, often function independently of T-cell receptors. They can be activated by NK cell receptors through recognition not of individual antigens, but of general molecular patterns of danger and tissue stress. When activated, CD8 + NKT cells release cytotoxic granules and lyse suspicious tissue cells, for example, single tumor cells and virus-infected cells that express and display MHC-like stress molecules on the outer membrane. With aging, the tendency of T RM to activate without the T cell receptor through NK cell receptors or cytokine signals can lead to erroneous lysis of tissue cells, insufficient control of chronically infected or degenerating areas of the epithelium.
Pathological manifestations associated with the work of resident T cells include organ-specific autoimmune syndromes and chronic inflammation in fabric. Examples of chronic inflammation maintained by resident T lymphocytes - contact dermatitis and psoriasis, and the mechanism is the release of inflammatory factors IL-17 by resident T-killer cells and IL-22 by resident T-helper cells of the dermis. CD8+ effector killer T cells located in the brain are similar in their set of membrane marker molecules to T RM of the skin, intestines and lungs and are able to push the development of intermittent multiple sclerosis with periodic releases of inflammatory cytokines; It is unclear, however, whether there is a normal T RM population in the brain or whether these are T lymphocytes remaining in the tissue after a neurotropic viral infection.
The functions of resident memory cells normally, in the absence of infection or chronic inflammation, may include cross-talk (mutual regulation primarily through the secretion of cytokines and costimulatory molecules) with non-classical, poorly understood lymphoid cells such as mucosal-associated gamma/delta T cells, carriers Alternative option T cell receptor assemblies; or lymphoid cells innate immunity(innate lymphoid cells, ILC), which share with T and B lymphocytes share features of the epigenetic landscape, but do not have T/B or NK cell receptors.
T RM cells come into contact with antigen-presenting cells of tissues - these are dendritic cells of the skin and resident tissue macrophages. Resident myeloid cells in different tissues are differentiated and slightly similar to each other. For example, marginal zone macrophages of the spleen, liver macrophages, and microglia (brain macrophages) will differ greatly in both morphology and range of functions. In addition to detecting antigens in tissue, resident macrophages are involved in regulating the processes of aging and tissue self-renewal, in particular, they secrete growth factors and cytokines that stimulate the division of tissue stem cells. In adipose tissue, for example, macrophages stimulate the differentiation of new fat cells, but when they enter an activated M1 state, they trigger inflammation and, instead of differentiating, cause existing fat cells to enlarge and swell. Concomitant changes in adipose tissue metabolism lead to the accumulation of fat mass and in recent years have been associated with the mechanisms of development of obesity and type II diabetes. In the skin, cytokines released by macrophages and resident gamma/delta T cells stimulate stem cell division during the regeneration of the epidermis and hair follicle stem cells. It can be assumed that helper T RM cells, when patrolling the epithelium and forming contacts with tissue macrophages, can modulate the spectrum and volume of growth factors secreted by the latter for stem cells, inflammatory cytokines and epithelial remodeling factors, and thereby participate in tissue renewal.
Figure 5. Proposed functions of tissue resident T lymphocytes. Some functions can be performed in interaction with resident macrophages (see explanations in the text).
What can the study of Trm give to medicine?
Understanding how resident T cells work is absolutely necessary to fight infections that do not enter the bloodstream directly, but enter the body through barrier tissues - that is, for the vast majority of infections. Rational design of vaccines for protection against this group of infections can be aimed specifically at enhancing the first stage of protection with the help of resident cells: a situation in which optimally activated antigen-specific cells eliminate the pathogen in the barrier tissue is much more beneficial than triggering acute inflammation to call T lymphocytes from the blood, since there is less tissue damage.
The T-cell receptor repertoire of cells associated with mucosal barrier tissues is considered to be partially degenerate and public, that is, identical for many individuals in a population. However, biases in the isolation of T cells from organs, data bias resulting from the selection of only certain Caucasian donors into cohorts, and the overall small amount of accumulated sequencing data do not provide confidence in the public availability of T cell receptor repertoires of T RM cells. While this would be convenient, vaccine design could be reduced to finding and modifying the most affinity and immunogenic peptides from a pathogen to interact with one of the publicly available TCR variants in the pathogen's barrier tissue.
Of course, understanding what T-cell receptors T RM cells carry on their surface is not enough to effectively manipulate immune responses in tissue. It is necessary to study in detail the factors influencing the colonization of tissues by certain T-cell clones and to understand the mechanisms of activation of local tissue immunity and induction of T RM tolerance. How the niches of T-lymphocytes are populated in the mucous membranes of a child before meeting with a large number pathogens and, accordingly, until the formation of a significant pool of effector memory T cells - the precursors of resident cells and central memory cells? Why and how, instead of the classical activation of lymphocytes, ignoring and tolerance reaction to microbes of non-pathogenic mucosal flora is formed? These questions are on the agenda in the study of resident cells of the immune system.
Determining the patterns of T-lymphocyte homing into specific tissues may provide an advantage in cellular immunotherapy of tumor diseases. Theoretically, killer T cells with the desired specificity for a tumor antigen are activated in vitro, must kill the patient's tumor cells. In practice, such immunotherapy is complicated by the fact that tumor cells are able to suppress immune responses and render T-killer cells approaching the tumor into an inactive state of anergy. Often, anergic T-lymphocytes, primarily T RM of a given tissue, accumulate in and around the mass of a growing tumor. Of the many active tumor-specific T cells injected into a patient, few will reach the target, and even these may be virtually useless in the immunosuppressive tumor microenvironment.
Deciphering the mechanisms that drive specific T cell clones to specific tissues could allow laboratory-engineered T cells to be targeted more effectively to tumors and usher in the era of accessible, personalized immunotherapy.
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