Reasons for the increase and decrease in helper T-lymphocytes (CD4). Why are lymphocytes in the blood low, what does this mean? Cd3 cd8 increased cytotoxic lymphocytes

The main task of T-lymphocytes is to recognize foreign or altered self-antigens as part of a complex with MHC molecules. If foreign or altered molecules are present on the surface of its cells, the T-lymphocyte triggers their destruction.

Unlike B lymphocytes, T lymphocytes do not produce soluble forms of antigen recognition molecules. Moreover, most T lymphocytes are unable to recognize and bind soluble antigens.

In order for a T-lymphocyte to “pay attention to the antigen,” other cells must somehow “pass” the antigen through themselves and display it on their membrane in complex with MHC-I or MHC-II. This is the phenomenon of antigen presentation to the T lymphocyte. Recognition of such a complex by T-lymphocytes is double recognition, or MHC restriction of T-lymphocytes.

ANTIGEN RECOGNITION T-LYMPHOCYTE RECEPTOR

T cell antigen recognition receptors, TCRs, are composed of chains belonging to the immunoglobulin superfamily (see Figure 5-1). The TCR antigen recognition region protruding above the cell surface is a heterodimer, i.e. consists of two different polypeptide chains. There are two known TCR variants, designated αβTCR and γδTCR. These variants differ in the composition of the polypeptide chains of the antigen recognition region. Each T lymphocyte expresses only 1 variant of the receptor. αβT cells were discovered earlier and studied in more detail than γδT lymphocytes. In this regard, it is more convenient to describe the structure of the antigen recognition receptor of T lymphocytes using the example of αβTCR. The transmembrane-located TCR complex consists of 8 polypeptide

Rice. 6-1. Diagram of the T cell receptor and its associated molecules

chains (a heterodimer of the α- and β-chains of the TCR itself, two auxiliary ζ chains, as well as one heterodimer each of the ε/δ- and ε/ γ-chains of the CD3 molecule) (Fig. 6-1).

. Transmembrane chainsα and β TCR. These are 2 polypeptide chains of approximately the same size -α (molecular weight 40-60 kDa, acidic glycoprotein) andβ (molecular weight 40-50 kDa, neutral or basic glycoprotein). Each of these chains contains 2 glycosylated domains in the extracellular part of the receptor, a hydrophobic (positively charged due to lysine and arginine residues) transmembrane part and a short (5-12 amino acid residues) cytoplasmic region. The extracellular parts of both chains are connected by a single disulfide bond.

- V-region. The outer extracellular (distal) domains of both chains have a variable amino acid composition. They are homologous to the V region of immunoglobulin molecules and constitute the V region of the TCR. It is the V regions of the α and β chains that interact with the MHC-peptide complex.

-C-area. The proximal domains of both chains are homologous to the constant regions of immunoglobulins; these are the C regions of the TCR.

A short cytoplasmic region (both α- and β-chains) cannot independently ensure the conduction of a signal into the cell. For this purpose, 6 additional polypeptide chains are used: γ, δ, 2ε and 2ζ.

.CD3 complex. Chainsγ, δ, ε form heterodimers among themselvesγε and δε (together they are called the CD3 complex). This complex is required for expressionα- and β-chains, their stabilization and signal transmission into the cell. This complex consists of an extracellular, transmembrane (negatively charged and therefore electrostatically associated with transmembrane regionsα- and β-chains) and cytoplasmic parts. It is important not to confuse the chains of the CD3 complex withγ δ chains of the TCR dimer.

.ζ -Chains connected to each other by a disulfide bridge. Most of these chains are located in the cytoplasm. ζ-Chains carry out the signal into the cell.

.ITAM sequences. Cytoplasmic regions of polypeptide chainsγ, δ, ε and ζ contain 10 ITAM sequences (1 sequence in eachγ-, ε- and δ-chains and 3 - in each ζ-chain), interacting with Fyn, a cytosolic tyrosine kinase, the activation of which initiates the onset of biochemical reactions to conduct a signal (see Fig. 6-1).

Antigen binding involves ionic, hydrogen, van der Waals and hydrophobic forces; the conformation of the receptor changes significantly. Theoretically, each TCR is capable of binding about 10 5 different antigens, not only related in structure (cross-reacting), but also not homologous in structure. However, in reality, TCR polyspecificity is limited to the recognition of only a few structurally similar antigenic peptides. The structural basis of this phenomenon is the feature of simultaneous TCR recognition of the MHC-peptide complex.

Coreceptor molecules CD4 and CD8

In addition to the TCR itself, each mature T lymphocyte expresses one of the so-called coreceptor molecules - CD4 or CD8, which also interact with MHC molecules on APCs or target cells. Each of them has a cytoplasmic region associated

with the tyrosine kinase Lck, and probably contributes to the transmission of the signal into the cell during antigen recognition.

.CD4(β2 domain) of the MHC-II molecule (belongs to the immunoglobulin superfamily, see Fig. 5-1, b). CD4 has a molecular weight of 55 kDa and 4 domains in the extracellular part. When a T-lymphocyte is activated, one TCR molecule is “served” by 2 CD4 molecules: dimerization of CD4 molecules probably occurs.

.CD8 associated with the invariant part(α3-domain) of the MHC-I molecule (belongs to the immunoglobulin superfamily, see Fig. 5-1, a). CD8 - chain heterodimerα and β, connected by a disulfide bond. In some cases, a homodimer of two α chains is found, which can also interact with MHC-I. In the extracellular part, each of the chains has one immunoglobulin-like domain.

T cell receptor genes

Genes α-, β-, γ- and δ-chains (Fig. 6-2, also see Fig. 5-4) are homologous to immunoglobulin genes and undergo DNA recombination during the differentiation of T-lymphocytes, which theoretically ensures the generation of about 10 16 -10 18 variants of antigen-binding receptors (in reality, this variety is limited by the number of lymphocytes in the body to 10 9).

.α-chain genes have ~54 V-segments, 61 J-segments and 1 C-segment.

.β-chain genes contain ~65 V segments, 2 D segments, 13 J segments and 2 C segments.

.δ-chain genes. Between the V- and J-segments of the α-chain are the genes of the D-(3), J-(4) and C-(1) segments of the δ-chainγ δTCR. The V segments of the δ chain are interspersed among the V segments of the α chain.

.γ-chain genes γ δTCRs have 2 C segments, 3 J segments before the first C segment and 2 J segments before the second C segment, 15 V segments.

Gene rearrangement

.DNA recombination occurs when the V-, D- and J-segments combine and is catalyzed by the same recombinase complex as during the differentiation of B lymphocytes.

.After rearrangement of VJ in α-chain genes and VDJ in β-chain genes, as well as after the addition of non-coding N- and P-nucleotides to DNA

Rice. 6-2. Genes of α- and β-chains of human T-lymphocyte antigen recognition receptor

transcribed by RNA. Fusion with the C segment and removal of excess (unused) J segments occurs during splicing of the primary transcript.

.α-chain genes can be rearranged repeatedly while β-chain genes are already correctly rearranged and expressed. This is why there is some possibility that a single cell can carry more than one TCR variant.

.TCR genes are not subject to somatic hypermutagenesis.

CONDUCTING SIGNALS FROM ANTIGEN RECOGNITION RECEPTORS OF LYMPHOCYTES

TCR and BCR have a number of common patterns of registration and transmission of activation signals into the cell (see Fig. 5-11).

. Receptor clustering. To activate a lymphocyte, clustering of antigen recognition receptors and coreceptors is necessary, i.e. “cross-linking” of several receptors with one antigen.

. Tyrosine kinases. The processes of phosphorylation/dephosphorylation of proteins at the tyrosine residue under the action of tyrosine kinases and tyrosine phosphatases play a significant role in signal transmission.

leading to activation or inactivation of these proteins. These processes are easily reversible and “convenient” for fast and flexible cell reactions to external signals.

. Src kinases. Tyrosine-rich ITAM sequences of the cytoplasmic regions of immunoreceptors are phosphorylated by non-receptor (cytoplasmic) tyrosine kinases of the Src family (Fyn, Blk, Lyn in B lymphocytes, Lck and Fyn in T lymphocytes).

. ZAP-70 kinases(in T lymphocytes) or Syk(in B-lymphocytes), binding to phosphorylated ITAM sequences, adapter proteins are activated and begin to phosphorylate: LAT (Linker for Activation of T cells)(ZAP-70 kinase), SLP-76 (ZAP-70 kinase) or SLP-65 (Syk kinase).

. Adapter proteins recruit phosphoinositide 3-kinase(PI3K). This kinase in turn activates the serine/threonine protein kinase Akt, causing increased protein biosynthesis, which promotes accelerated cell growth.

. Phospholipase Cγ(see Fig. 4-8). Kinases of the Tec family (Btk - in B-lymphocytes, Itk - in T-lymphocytes) bind adapter proteins and activate phospholipase Cγ(PLCγ ).

PLCγ cleaves cell membrane phosphatidylinositol diphosphate (PIP 2) into inositol 1,4,5-trisphosphate (IP 3) and diacylglycerol

(DAG).

DAG remains in the membrane and activates protein kinase C (PKC), a serine/threonine kinase that activates the evolutionarily “ancient” transcription factor NFκB.

IP 3 binds to its receptor in the endoplasmic reticulum and releases calcium ions from the stores into the cytosol.

Free calcium activates calcium-binding proteins - calmodulin, which regulates the activity of a number of other proteins, and calcineurin, which dephosphorylates and thereby activates the nuclear factor of activated T-lymphocytes NFAT (Nuclear Factor of Activated T cells).

. Ras and other small G proteins in the inactive state are associated with GDP, but adapter proteins replace the latter with GTP, thereby transferring Ras to the active state.

Ras has its own GTPase activity and quickly cleaves off the third phosphate, thereby returning itself to an inactive state (self-inactivation).

In a state of short-term activation, Ras manages to activate the next cascade of kinases called MAPK (Mitogen Activated Protein Kinase), which ultimately activate the AP-1 transcription factor in the cell nucleus. In Fig. Figure 6-3 provides a schematic representation of the major TCR signaling pathways. The activation signal is turned on when the TCR binds to a ligand (MHC peptide complex) with the participation of a coreceptor (CD4 or CD8) and the co-stimulatory molecule CD28. This leads to activation of the kinases Fyn and Lck. ITAM regions in the cytoplasmic parts of CD3 polypeptide chains are marked in red. The role of Src kinases associated with the receptor in the phosphorylation of proteins: both receptor and signal ones is reflected. Noteworthy is the extremely wide range of effects of coreceptor-associated Lck kinase; the role of Fyn kinase is less certain (reflected in the discontinuous nature of the lines).

Rice. 6-3. Sources and direction of triggering activation signals during stimulation of T-lymphocytes. Designations: ZAP-70 (ζ -associated protein kinase, they say mass 70 kDa) - protein kinase p70 associated with the ζ-chain; PLCγ (Phospholipase Cγ ) - phospholipase C, isoform γ; PI3K (Phosphatidyl Inositol 3-kinase)- phosphatidylinositol 3-kinase; Lck, Fyn -tyrosine kinases; LAT, Grb, SLP, GADD, Vav - adapter proteins

The tyrosine kinase ZAP-70 plays a key role in mediating between receptor kinases and adapter molecules and enzymes. It activates (via phosphorylation) the adapter molecules SLP-76 and LAT, and the latter transmits an activation signal to other adapter proteins GADD, GRB and activates the y-isoform of phospholipase C (PLCy). Before this stage, signal transduction involves exclusively factors associated with the cell membrane. An important contribution to the activation of signaling pathways is made by the costimulatory molecule CD28, which realizes its action through the associated lipid kinase PI3K (Phosphatidyl Inositol 3-kinase). The main target of PI3K kinase is the cytoskeleton-associated factor Vav.

As a result of the formation of a signal and its transmission from the T-cell receptor to the nucleus, 3 transcription factors are formed - NFAT, AP-1 and NF-kB, which induce the expression of genes that control the process of T-lymphocyte activation (Fig. 6-4). The formation of NFAT is caused by a signaling pathway that does not depend on costimulation, which is activated due to the activation of phospholipase C and is realized with the participation of ions

Rice. 6-4. Scheme of signaling pathways during T cell activation. NFAT (Nuclear factor of activated T cells), AP-1 (Activation protein-1), NF-κB (Nuclear factor ofTo -gene of B cells)- transcription factors

Ca 2+. This pathway causes the activation of calcineurin, which, having phosphatase activity, dephosphorylates the cytosolic factor NFAT-P. Thanks to this, NFAT-P acquires the ability to migrate into the nucleus and bind to the promoters of activating genes. The AP-1 factor is formed as a heterodimer of the c-Fos and c-Jun proteins, the formation of which is induced due to the activation of the corresponding genes under the influence of factors formed as a result of the implementation of the three components of the MAP cascade. These pathways are activated by the short GTP-binding proteins Ras and Rac. A significant contribution to the implementation of the MAP cascade is made by signals dependent on costimulation through the CD28 molecule. The third transcription factor, NF-kB, is known as the main transcription factor of innate immune cells. It is activated by cleavage of the blocking IκB subunit by IKK kinase, which in T cells is activated by protein kinase C isoform ϴ-dependent signaling (PKC9). The main contribution to the activation of this signaling pathway is made by costimulatory signals from CD28. The formed transcription factors bind to the promoter regions of genes and induce their expression. Gene expression is particularly important for the initial stages of the T cell response to stimulation IL2 And IL2R, which determines the production of T-cell growth factor IL-2 and the expression of its high-affinity receptor on T-lymphocytes. As a result, IL-2 acts as an autocrine growth factor, causing the proliferative expansion of T-cell clones involved in the response to the antigen.

DIFFERENTIATION OF T-LYMPHOCYTES

The basis for identifying the stages of T-lymphocyte development is the state of receptor V genes and TCR expression, as well as coreceptors and other membrane molecules. The differentiation scheme of T-lymphocytes (Fig. 6-5) is similar to the above scheme for the development of B-lymphocytes (see Fig. 5-13). Key characteristics of the phenotype and growth factors of developing T cells are presented. The accepted designations for the stages of T-cell development are determined by the expression of coreceptors: DN (from Double-Negative CD4CD8) - double negative, DP (from Double-Positive, CD4 + CD8 +) - double positive, SP (from Single-Positive, CD4 + CD8 - and CD4CD8 +) are single positive. The division of DNthymocytes into stages DN1, DN2, DN3 and DN4 is based on the nature

Rice. 6-5. Development of T lymphocytes

expression of CD44 and CD25 molecules. Other symbols: SCF (from Stem Cell Factor)- stem cell factor, lo (low; index mark) - low level of expression. Rearrangement stages: D-J - preliminary stage, connection of segments D and J (only in genes of the β- and δ-chains of TCR, see Fig. 6-2), V-DJ - final stage, connection of the germinal V gene with the combined DJ segment .

.Thymocytes differentiate from a common precursor cell, which, while still outside the thymus, expresses membrane markers such as CD7, CD2, CD34 and the cytoplasmic form of CD3.

.Precursor cells committed to differentiation into T lymphocytes migrate from the bone marrow to the subcapsular zone of the thymic cortex, where they slowly proliferate over approximately one week. New membrane molecules CD44 and CD25 appear on thymocytes.

.Then the cells move deeper into the thymic cortex, and the CD44 and CD25 molecules disappear from their membrane. At this stage, the rearrangement of β genes begins-, γ- and TCR δ chains. If genesγ- and δ-chains have time to be productive, i.e. without a frameshift, rearrange earlier than the β-chain genes, then the lymphocyte differentiates further asγ δT. Otherwise, the β-chain is expressed on the membrane in complex with pTα (an invariant surrogate chain that replaces the real α chain at this stage) and CD3. This serves

a signal to stop rearrangement of γ- and δ-chain genes. Cells begin to proliferate and express both CD4 and CD8 - double positive thymocytes. In this case, a mass of cells accumulates with a ready-made β-chain, but with not yet rearranged α-chain genes, which contributes to the diversity of αβ-heterodimers.

.At the next stage, the cells stop dividing and begin to rearrange Vα genes, several times over the course of 3-4 days. Rearrangement of the α-chain genes results in irreversible deletion of the δ-locus located between the segments of the α-chain genes.

.Expression of TCR occurs with each new variant of the α-chain and selection (selection) of thymocytes based on the strength of binding to the MHC-peptide complex on the membranes of thymic epithelial cells.

Positive selection: thymocytes that have not bound any of the available MHC-peptide complexes die. As a result of positive selection, about 90% of thymocytes die in the thymus.

Negative selection eliminates thymocyte clones that bind MHC-peptide complexes with too high an affinity. Negative selection eliminates from 10 to 70% of cells that have undergone positive selection.

Thymocytes that have bound any of the MHC-peptide complexes with the correct one, i.e. with average strength and affinity, they receive a signal for survival and continue differentiation.

.For a short time, both coreceptor molecules disappear from the thymocyte membrane, and then one of them is expressed: thymocytes that recognize the peptide in complex with MHC-I express the CD8 coreceptor, and with MHC-II, the CD4 coreceptor. Accordingly, two types of T-lymphocytes reach the periphery (in a ratio of about 2:1): CD8 + and CD4 +, whose functions in the upcoming immune responses are different.

-CD8+ T cells play the role of cytotoxic T-lymphocytes (CTLs) - they recognize and directly kill cells modified by the virus, tumor and other “altered” cells (Fig. 6-6).

-CD4+ T cells. The functional specialization of CD4 + T lymphocytes is more diverse. A significant part of CD4 + T-lymphocytes during the development of the immune response become T-helpers (helpers), interacting with B-lymphocytes, T-lymphocytes and other cells during

Rice. 6-6. The mechanism of action of a cytotoxic T-lymphocyte on a target cell. In the killer T cell, in response to an increase in Ca 2+ concentration, granules with perforin (purple ovals) and granzymes (yellow circles) merge with the cell membrane. The released perforin is incorporated into the membrane of the target cell with the subsequent formation of pores permeable to granzymes, water and ions. As a result, the target cell is lysed

direct contact or through soluble factors (cytokines). In certain cases, they can develop into CD4 + CTLs: in particular, such T lymphocytes are found in significant quantities in the skin of patients with Lyell's syndrome.

T helper subpopulations

Since the late 80s of the 20th century, it has been customary to distinguish 2 subpopulations of T-helpers (depending on what set of cytokines they produce) - Th1 and Th2. In recent years, the spectrum of CD4 + T cell subsets has continued to expand. Subpopulations such as Th17, T-regulators, Tr1, Th3, Tfh, etc. were discovered.

Major CD4+ T cell subsets:

. Th0 - CD4+ T lymphocytes in the early stages of the development of the immune response, they produce only IL-2 (a mitogen for all lymphocytes).

.Th1- differentiated subpopulation of CD4 + T-lymphocytes, specializing in the production of IFNγ, TNF β and IL-2. This subpopulation regulates many cellular immune responses, including delayed-type hypersensitivity (DTH) and CTL activation. In addition, Th1 stimulate the production of opsonizing IgG antibodies by B lymphocytes, which trigger the complement activation cascade. The development of excessive inflammation with subsequent tissue damage is directly related to the activity of the Th1 subpopulation.

.Th2- a differentiated subpopulation of CD4 + T lymphocytes specialized in the production of IL-4, IL-5, IL-6, IL-10 and IL-13. This subpopulation is involved in the activation of B lymphocytes and promotes their secretion of large quantities of antibodies of different classes, especially IgE. In addition, the Th2 subpopulation is involved in the activation of eosinophils and the development of allergic reactions.

.Th17- a subpopulation of CD4 + T lymphocytes specialized in the production of IL-17. These cells provide antifungal and antimicrobial protection of epithelial and mucosal barriers, and also play a key role in the pathology of autoimmune diseases.

.T-regulators- CD4 + T lymphocytes that suppress the activity of other cells of the immune system through the secretion of immunosuppressive cytokines - IL-10 (inhibitor of macrophage and Th1 cell activity) and TGFβ - inhibitor of lymphocyte proliferation. The inhibitory effect can also be achieved through direct intercellular interaction, since the inducers of apoptosis of activated and “spent” lymphocytes - FasL (Fas ligand) - are expressed on the membrane of some T-regulators. There are several populations of CD4 + regulatory T lymphocytes: natural (Treg), maturing in the thymus (CD4 + CD25 +, expressing the transcription factor Foxp3), and induced - localized predominantly in the mucous membranes of the digestive tract and switching to the formation of TGFβ (Th3) or IL-10 (Tr1). Normal functioning of T-regulators is necessary to maintain homeostasis of the immune system and prevent the development of autoimmune diseases.

.Additional helper populations. Recently, there has been a description of ever new populations of CD4 + T lymphocytes, classi-

classified according to the type of cytokine they predominantly produce. So, as it turned out, one of the most important populations is Tfh (from the English. follicular helper- follicular helper). This population of CD4 + T lymphocytes is predominantly located in lymphoid follicles and exerts a helper function for B lymphocytes through the production of IL-21, causing their maturation and terminal differentiation into plasma cells. In addition to IL-21, Tfh can also produce IL-6 and IL-10, which are necessary for the differentiation of B lymphocytes. Dysfunction of this population leads to the development of autoimmune diseases or immunodeficiencies. Another “newly emerging” population is Th9 - IL-9 producers. Apparently, these are Th2, which switched to the secretion of IL-9, which is capable of causing the proliferation of T helper cells in the absence of antigenic stimulation, as well as enhancing the secretion of IgM, IgG and IgE by B lymphocytes.

The main subpopulations of T helper cells are presented in Fig. 6-7. The figure summarizes current ideas about adaptive subpopulations of CD4 + T cells, i.e. subpopulations forming-

Rice. 6-7. Adaptive CD4+ T cell subsets (cytokines, differentiation factors, chemokine receptors)

occurs during an immune response, and not during the natural development of cells. For all types of T helper cells, inducer cytokines are indicated (on the arrows leading to the circles symbolizing cells), transcription factors (inside the circles), chemokine receptors that direct migration (near the lines extending from the “cell surface”), and the cytokines produced ( in the rectangles to which the arrows extending from the circles are directed).

The expansion of the family of adaptive subpopulations of CD4 + T cells required a solution to the question of the nature of the cells with which these subpopulations interact (to whom they provide “help” in accordance with their helper function). These ideas are reflected in Fig. 6-8. It also provides an updated look at the functions of these subpopulations (participation in protection against certain groups of pathogens), as well as the pathological consequences of an unbalanced increase in the activity of these cells.

Rice. 6-8. Adaptive T cell subsets (partner cells, physiological and pathological effects)

γ δT lymphocytes

The vast majority (99%) of T lymphocytes undergoing lymphopoiesis in the thymus are αβT cells; less than 1% are γδT cells. The latter mostly differentiate outside the thymus, primarily in the mucous membranes of the digestive tract. In the skin, lungs, digestive and reproductive tracts, they are the dominant subpopulation of intraepithelial lymphocytes. Among all T lymphocytes in the body, γδT cells make up from 10 to 50%. In embryogenesis, γδT cells appear earlier than αβT cells.

.γδ T cells do not express CD4. The CD8 molecule is expressed on some γδT cells, but not as an ap-heterodimer, as on CD8 + apT cells, but as a homodimer of two a-chains.

.Antigen recognition properties:γδTCRs are more reminiscent of immunoglobulins than αβTCRs, i.e. capable of binding native antigens independently of classical MHC molecules - for γδT cells, preliminary processing of the antigen by APC is not necessary or not necessary at all.

.Diversityγδ TCR less than αβTCR or immunoglobulins, although in general γδT cells are able to recognize a wide range of antigens (mainly phospholipid antigens of mycobacteria, carbohydrates, heat shock proteins).

.Functionsγδ T cells have not yet been fully studied, although the prevailing opinion is that they serve as one of the connecting components between innate and acquired immunity. γδT cells are one of the first barriers to pathogens. In addition, these cells, secreting cytokines, play an important immunoregulatory role and are able to differentiate into CTLs.

NKT lymphocytes

Natural killer T cells (NKT cells) represent a special subpopulation of lymphocytes that occupy an intermediate position between innate and adaptive immune cells. These cells have features of both NK and T lymphocytes. NKT cells express αβTCR and the NK cell-specific receptor NK1.1, which belongs to the C-type lectin glycoprotein superfamily. However, the TCR receptor of NKT cells has significant differences from the TCR receptor of ordinary cells. In mice, most NKT cells express an invariant a-chain V domain, consisting of

segments Vα14-Jα18, sometimes referred to as Jα281. In humans, the V domain of the α chain consists of Vα24-JαQ segments. In mice, the α chain of the invariant TCR is predominantly complexed with Vβ8.2, and in humans, with Vβ11. Due to the structural features of the TCR chains, NKT cells are called invariant - iTCR. The development of NKT cells depends on the CD1d molecule, which is similar to MHC-I molecules. Unlike classical MHC-I molecules that present peptides to T cells, CD1d presents only glycolipids to T cells. Although the liver is thought to be the site of NKT cell development, there is strong evidence for a role for the thymus in their development. NKT cells play an important role in regulating immunity. In mice and humans with various autoimmune processes, the functional activity of NKT cells is severely impaired. There is no complete picture of the significance of such disorders in the pathogenesis of autoimmune processes. In some autoimmune processes, NKT cells can play a suppressor role.

In addition to controlling autoimmune and allergic reactions, NKT cells participate in immune surveillance, causing tumor rejection when their functional activity increases. Their role in antimicrobial protection is great, especially in the early stages of the development of the infectious process. NKT cells are involved in various inflammatory infectious processes, especially in viral liver lesions. In general, NKT cells are a multifunctional population of lymphocytes that still carry many scientific mysteries.

In Fig. 6-9 summarizes data on the differentiation of T lymphocytes into functional subpopulations. Several levels of bifurcation are presented: γ δТ/ αβТ, then for αβТ cells - NKT/ other T lymphocytes, for the latter - CD4 + /CD8 +, for CD4 + T cells - Th/Treg, for CD8 + T lymphocytes - CD8αβ/CD8αα. Differentiation transcription factors responsible for all developmental lines are also shown.

Rice. 6-9. Natural subpopulations of T-lymphocytes and their differentiation factors

The total number of T-lymphocytes in the blood of adults is normal - 58-76%, absolute number - 1.1-1.7-10"/l.

Mature T-lymphocytes are “responsible” for cellular immune reactions and carry out immunological surveillance of antigenic homeostasis in the body. They are formed in the bone marrow and undergo differentiation in the thymus gland, where they are divided into effector (killer T-lymphocytes, delayed-type hypersensitivity T-lymphocytes) and regulatory (helper T-lymphocytes, suppressor T-lymphocytes). In accordance with this, T-lymphocytes perform two important functions in the body: effector and regulatory. The effector function of T lymphocytes is specific cytotoxicity towards foreign cells. The regulatory function (T-helper - T-suppressor system) is to control the intensity of the development of a specific reaction of the immune system to foreign antigens. A decrease in the absolute number of T-lymphocytes in the blood indicates a lack of cellular immunity, an increase indicates immune hyperactivity and the presence of immunoproliferative diseases.

The development of any inflammatory process is accompanied almost throughout its entire duration by a decrease in the content of T-lymphocytes. This is observed in inflammation of a wide variety of etiologies: various infections, nonspecific inflammatory processes, destruction of damaged tissues and cells after surgery, trauma, burns, heart attack, destruction of malignant tumor cells, trophic destruction, etc. The decrease in the number of T-lymphocytes is determined by the intensity of the inflammatory process, but this pattern is not always observed. T-lymphocytes react most quickly of all immunocompetent cells to the onset of the inflammatory process. This reaction manifests itself even before the development of the clinical picture of the disease. An increase in the number of T-lymphocytes during the inflammatory process is a favorable sign, and a high level of T-lymphocytes with pronounced clinical manifestations of such a process, on the contrary, is an unfavorable sign, indicating a sluggish course of the inflammatory process with a tendency to become chronic. The complete completion of the inflammatory process is accompanied by normalization of the number of T-lymphocytes. An increase in the relative number of T-lymphocytes is not of great clinical significance. However, an increase in the absolute number of T-lymphocytes in the blood is very important for the diagnosis of leukemia. Diseases and conditions leading to changes in the number of T-lymphocytes in the blood are presented in table. 7.19.

Table 7.19. Diseases and conditions leading to changes in the amount

T lymphocytes (CD3) in the blood

Continuation of Table 7.19

Helper T lymphocytes (CD4) in the blood

The number of helper T-lymphocytes in the blood of adults is normal - 36-55%, absolute

Quantity - 0.4-1.110"/l-

T-lymphocytes are helpers (inducers) of the immune response, cells that regulate the strength of the body’s immune response to a foreign antigen, control the constancy of the body’s internal environment (antigenic homeostasis) and cause increased production of antibodies. An increase in the number of helper T-lymphocytes indicates immune hyperactivity, while a decrease indicates immunological deficiency.

The ratio of T-helpers and T-suppressors in the peripheral blood is of key importance in assessing the state of the immune system, since the intensity of the immune response depends on this. Normally, cytotoxic cells and antibodies should be produced as much as they are necessary to remove a particular antigen. Insufficient activity of T-suppressors leads to the predominance of the influence of T-helpers, which contributes to a stronger immune response (pronounced antibody production and/or prolonged activation of T-effectors). Excessive activity of T-suppressors, on the contrary, leads to rapid suppression and abortive course of the immune response and even phenomena of immunological tolerance (an immunological response to the antigen does not develop). With a strong immune response, the development of autoimmune and allergic processes is possible. The high functional activity of T-suppressors in such a response does not allow the development of an adequate immune response, and therefore the clinical picture of immunodeficiency is dominated by infections and a predisposition to malignant growth. The CD4/CD8 index of 1.5-2.5 corresponds to a normergic state, more than 2.5 - hyperactivity, less than 1.0 - immunodeficiency. In severe cases of the inflammatory process, the CD4/CD8 ratio may be less than 1. This ratio is of fundamental importance when assessing the immune system in patients with AIDS. In this disease, the human immunodeficiency virus selectively infects and destroys CO4 lymphocytes, as a result of which the CD4/CD8 ratio decreases before values ​​significantly less than 1.

An increase in the CD4/CD8 ratio (up to 3) is often observed in the acute phase of various inflammatory diseases due to an increase in the level of T helper cells and a decrease in T suppressor cells. In the middle of an inflammatory disease, there is a slow decrease in T-helper cells and an increase in T-suppressor cells. When the inflammatory process subsides, these indicators and their ratio are normalized. An increase in the CD4/CD8 ratio is characteristic of almost all autoimmune diseases: hemolytic anemia, immune thrombocytopenia, Hashimoto's thyroiditis, pernicious anemia, Goodpasture's syndrome, systemic lupus erythematosus, rheumatoid arthritis. An increase in the CD4/CD8 ratio due to a decrease in the level of CD8 in the listed diseases is usually detected at the height of an exacerbation with high activity of the process. A decrease in the CD4/CD8 ratio due to an increase in CD8 levels is characteristic of a number of tumors, in particular Kaposi's sarcoma. Diseases and conditions leading to changes in the number of CD4 in the blood are presented in table. 7.20.

Table 7.20. Diseases and conditions leading to changes in the number of CD4 in the blood

Continuation of the table. 7.20

Lymphocytes are cells of the leukocyte unit of the blood that perform a number of important functions. A decrease or increase in the level of these cells may indicate the development of a pathological process in the body.

The process of formation and function of lymphocytes

Lymphocytes are produced in the bone marrow, then migrate to the thymus gland (thymus), where, under the influence of hormones and epithelial cells, they undergo changes and differentiate into subgroups with different functions. The human body also has secondary lymphoid organs, these include the lymph nodes and spleen. The spleen is also the site of lymphocyte death.

There are T and B lymphocytes. 10-15% of all lymphocytes in the lymph nodes are transformed into B lymphocytes. Thanks to these cells, the human body acquires lifelong immunity to past diseases - upon first contact with a foreign agent (virus, bacteria, chemical compound), B-lymphocytes produce antibodies to it, remember the pathogenic element and, upon repeated interaction, mobilize immunity to destroy it. Also, due to the presence of B-lymphocytes in the blood plasma, the effect of vaccination is achieved.

In the thymus, about 80% of lymphocytes are converted into T lymphocytes (CD3 is a common cell marker). T-lymphocyte receptors detect and bind antigens. T cells, in turn, are divided into three subtypes: killer T cells, helper T cells, and suppressor T cells. Each type of T-lymphocyte is directly involved in eliminating a foreign agent.

Killer T cells destroy and break down cells infected by bacteria and viruses, and cancer cells. Killer T cells are the main element of antiviral immunity. The function of T-helper cells is to enhance the adaptive immune response; such T-cells secrete special substances that activate the T-killer response.

Killer T cells and helper T cells are effector T lymphocytes whose function is to provide an immune response. There are also suppressor T cells - regulatory T lymphocytes that regulate the activity of effector T cells. By controlling the intensity of the immune response, regulatory T lymphocytes prevent the destruction of healthy cells in the body and prevent the occurrence of autoimmune processes.

Normal lymphocyte counts

Normal values ​​of lymphocytes are different for each age - this is due to the peculiarities of the development of the immune system.

With age, the volume of the thymus gland, in which the bulk of lymphocytes mature, decreases. Until the age of 6, lymphocytes predominate in the blood; as a person grows older, neutrophils become dominant.

• newborn children - 12-36% of the total number of leukocytes;
• 1 month of life - 40-76%;
• at 6 months - 42-74%;
• at 12 months - 38-72%;
• up to 6 years - 26-60%;
• up to 12 years - 24-54%;
• 13-15 years old - 22-50%;
• adult - 19-37%.

To determine the number of lymphocytes, a general (clinical) blood test is performed. With the help of such a study, it is possible to determine the total number of lymphocytes in the blood (this indicator is usually expressed as a percentage). To obtain absolute values, the calculation must take into account the total content of leukocytes.

A detailed determination of the concentration of lymphocytes is carried out during an immunological study. The immunogram reflects the indicators of B and T lymphocytes. The normal rate of T-lymphocytes is 50-70%, (50.4±3.14)*0.6-2.5 thousand. The normal rate of B-lymphocytes is 6-20%, 0.1-0.9 thousand. Ratio between T-helpers and T-suppressors is normally 1.5-2.0.

Increase and decrease in T-lymphocyte levels

An increase in T-lymphocytes in the immunogram indicates hyperactivity of the immune system and the presence of immunoproliferative disorders. A decrease in the level of T-lymphocytes indicates a lack of cellular immunity.

In any inflammatory process, the level of T-lymphocytes is reduced. The degree of reduction in the concentration of T cells is influenced by the intensity of inflammation, but this pattern is not observed in all cases. If T-lymphocytes are increased in the dynamics of the inflammatory process, this is a favorable sign. However, an increased level of T cells against the background of severe clinical symptoms, on the contrary, is an unfavorable sign that indicates the transition of the disease to a chronic form. After complete elimination of inflammation, the level of T-lymphocytes reaches normal values.

The cause of an increase in the level of T-lymphocytes may be disorders such as:

• lymphocytic leukemia (acute, chronic);
• Sézary syndrome;
• hyperactivity of the immune system.

T-lymphocytes can be reduced in the following pathologies:

• chronic infectious diseases (HIV, tuberculosis, purulent processes);
• decreased production of lymphocytes;
• genetic disorders causing immunodeficiency;
• tumors of lymphoid tissue (lymphosarcoma, lymphogranulomatosis);
• kidney and heart failure of the last stage;
• destruction of lymphocytes under the influence of certain medications (corticosteroids, cytostatics) or radiation therapy;
• T-cell lymphoma.

The level of T-lymphocytes must be assessed in conjunction with other blood elements, taking into account the patient’s symptoms and complaints. Therefore, only a qualified specialist should interpret the results of a blood test.

Cells expressing the CD8 antigen are represented by two main subpopulations - cytotoxic T cells and T lymphocytes with suppressor activity.

Over time, it became known that CD8 is expressed not only by these subpopulations of lymphocytes, but also by individual clones of other cells: macrophages, natural killer cells (NK), mast cells, dendritic cells (DC); the main ligands of CD8 are the a2 and a3 domains of class I antigens major histocompatibility complex (MHC).

It follows that CD8 is a nonspecific marker cytotoxic lymphocytes (CTL) and T-suppressor lymphocytes, but is considered as one of the main phenotypic characteristics of these cells.

That is why, when objectively assessing the level of T-lymphocyte suppression, it is necessary to study the suppressor activity of cells expressing CD8 using a method developed for this purpose, since only the determination of CD8 does not give grounds to talk about either the cytotoxic or suppressor activity of T-lymphocytes that have common marker CD8.

General understanding of CD8+CD28 T-lymphocytes

A general idea of ​​suppressor T-lymphocytes began to form already in the 70s of the last century, and by the mid-80s it became known that these cells are represented by various clones, differing in the conditions of their occurrence, kinetics of formation, features of action, diversity of properties, secreted mediators and etc.

Nevertheless, even then B.D. Brondz formulated that they have common features, which consist in the ability to block the differentiation and activity of other lymphoid cells and this fundamentally distinguishes them from CTLs. The differences between T-suppressors and other T-lymphocytes should also include their instability, high sensitivity to various influences, short life span, etc.

Thanks to the research of many famous immunologists of that time, some surface markers of these cells were identified (with the help of the methodological capabilities of that period), their differences from other cells were established, and some stages and mechanisms of activation of T-suppressors were identified.

As a result, it was concluded that T-suppressors and their various clones are regulatory cells that control the relationship between cellular and humoral immunity and largely determine the intensity of the response to tumors, transplants, and viruses. It should be added that this idea has not undergone fundamental changes to the present day.

Over time, the study of CD8+ T lymphocytes made it possible to establish that CD8+ T lymphocytes with suppressor activity are characterized by the absence of expression of CD28 molecules, so their phenotype was defined as CD8+CD28-.

When studying these cells in various systems (a mixed culture of lymphocytes was especially often used), it was shown that they have many inhibitory effects: inhibition of the proliferation of CD4 + T lymphocytes stimulated by allogeneic cells, inhibition of receptors associated primarily with cell activation (IL-2 receptors and transferrin), suppression of the expression of costimulatory molecules by antigen-presenting cells, which prevents their optimal interaction with CD4+ T lymphocytes, inability to maintain the secretion of cytokines, etc.

It was confirmed that when carrying out their inhibitory effects, CD8+CD28 T-lymphocytes recognize the complex MHC class I antigens - peptides with the participation of the TCR of these cells. It has also been established that CD8+CD28 T-lymphocytes are a heterogeneous subpopulation.

In the general characteristics of cells of this type, it is important that they are characterized by a decrease in proliferation in response to stimuli, inhibit cytotoxicity, have a high level of expression of CD11b, CD29, CD57, CD94 with a low level of CD25 compared to CD8+CD28+T lymphocytes; CD8+CD28 T-lymphocytes of peripheral blood have significantly reduced phosphorylation of the TCR-zeta chain and a high level of the cyclin-dependent kinase inhibitor p16.

The production of monoclonal antibodies that strictly interact with CD8+CD28 T-lymphocytes made it possible to confirm that they represent an independent cell clone, different from cytotoxic lymphocytes; the use of these antibodies terminated the inhibitory effects of CD8+CD28-T lymphocytes both in vivo and in vitro, but did not affect the functions of CTLs.

Using monoclonal antibodies, one of the ganglioside epitopes, CD75s, was identified in these cells, which was not detected on other cells, which served as the basis for expanding the phenotypic characteristics, which were defined as CD8+CD28-CD75s+.

From the standpoint of understanding the general biological significance of CD8+CD28 T-lymphocytes, their ability to interact with epithelial cells of the mucous membrane is important. CD8+CD28-T cells with this ability express CD101 and CD103, interact with epithelial cells through the p180 protein, and perform regulatory functions.

The authors rightly conclude that in the mucous membrane there are CD8+CD28-CD101+CD103+T lymphocytes that exercise local immunological control. From these data it follows that the regulatory influences of CD8 + CD28 T lymphocytes are not limited to Th1 lymphocytes and have a wider spectrum of action.

The interaction of CD8+CD28-T lymphocytes with epithelial cells is presented in Fig. 58.

Rice. 58. Interaction between suppressor T-lymphocytes and epithelial cells:
CD101 is a glycoprotein involved in co-stimulation; CD103 - mucosal lymphocyte antigen

These general ideas about CD8+CD28 T lymphocytes have recently been supplemented with new data that reveal both their previously unknown inhibitory effects and some pathways and mechanisms for their implementation. Such data were obtained both in experimental studies and in the study of suppressor T-lymphocytes in the peripheral blood of healthy individuals, as well as in some pathologies.

Of significant interest is the information that the above-mentioned fact of the heterogeneity of this cell clone is receiving new light. When studying CD8+CD28 T-lymphocytes of human peripheral blood, three types were identified, united by the ability to inhibit the antigen-specific response of T-lymphocytes.

The first type is characterized by the ability to damage the expression of costimulatory molecules on DCs, an effect that requires direct intercellular interaction. The second has a pronounced ability to inhibit the secretion of cytokines such as IFNy and IL-6, which occurs without mandatory intercellular contact. The third mediates its effects by secreting IL-10.

Important findings have been obtained by studying both CD28-expressing and non-expressing T lymphocytes in the peripheral blood of people of different age groups. Firstly, it has been shown that with age the number of CD8+CD28 T-lymphocytes decreases; secondly, stimulation phytohemagglutinin(FGA) increases the ratio of these cell clones in all age groups and enhances the proliferation of not only CD8+CD28+-, but CD8+CD28-T cells with a higher level of proliferation of the latter in elderly individuals.

Finally, it was found that treatment of lymphocytes with PHA leads to apoptosis of all CD8+ T lymphocytes and the number of dead cells was the same in both clones - evidence that the ability to apoptosis was not dependent on age. To this should be added the data that an increase in the number of CD8+CD28 T-lymphocytes in elderly individuals without identified pathology can explain the decrease in proliferation, accompanied by an increase in the activity of cyclin-dependent protein kinase p16.

It is quite reasonable to believe that these new data, with further study of age-related changes in T-suppressor lymphocytes, may be significant for elucidating their role in age-related features of the regulation of immunological homeostasis.

To date, the main stages of the inhibitory effect of CD8+CD28 T-lymphocytes have become known, the activation of which can occur under the influence of allogeneic, xenogeneic, as well as heterogeneous antigen-presenting cells loaded with antigens. The main participants in the implementation of the inhibitory effect of suppressor T-lymphocytes are: CD8 + CD28 + T-lymphocytes, antigen-presenting cells and CD4 + T-helpers.

In this case, antigen-presenting cells act as a kind of bridge between T-suppressors and CD4+ T-lymphocytes. The general mechanism of the inhibitory effect of suppressor T-lymphocytes can be represented as follows: activation of human CD8+CD28-T-lymphocytes as a result of their recognition of the complex antigens of the major histocompatibility complex - peptide (the process occurs with the participation of TCR suppressor cells) on antigen-presenting cells, which deprives them of the ability to express costimulatory molecules and therefore, after recognition of the MHC class II antigens - peptide complex by T-helper lymphocytes, they do not receive the necessary costimulatory signal, become energetic and incapable of activation and proliferation. This process is shown schematically in Fig. 59.

Rice. 59. Stages of the inhibitory effect of CD8+CD28~T-lymphocytes on CD4+T-lymphocytes (helpers): APC - antigen-presenting cell

To understand the mechanism of action of suppressor T lymphocytes, it is also important that they do not require either proliferation or protein synthesis to carry out their inhibitory effect. When inhibitory signals from T-suppressor lymphocytes are implemented in antigen-presenting cells, the activity of NF-kappaB is inhibited, which plays a major role in their inability to send co-stimulatory signals to Th lymphocytes.

As already noted, significant attention is paid to the question of whether direct contact with antigen-presenting cells is necessary for CD8+CD28-T lymphocytes to exert an inhibitory effect. Currently, most authors are inclined to believe that such intercellular contact is necessary.

Many facts that allow us to expand our understanding of the regulatory capabilities of CD8+CD28 T-lymphocytes were obtained by studying them in conditions of tissue transplantation, as well as autoimmune pathology. According to the data obtained, the presence of CD8+CD28 T-lymphocytes largely determines the fate of the graft.

A comparative study of these peripheral blood cells of healthy individuals and people with a transplanted heart showed a significantly more pronounced activation in patients than in healthy individuals with parallel active expression of CD38, leukocyte antigen DR, and a higher content of perforin-positive cells.

It was noted that the expression of CD27 was more pronounced on the cells of CD8+CD28- patients who did not have transplant rejection, compared to patients who showed signs of rejection. In connection with these data, another aspect of the significance of T-suppressor cells is identified: there is a clone of regulatory cells CD8+CD28-CD27+ that play a role in graft protection.

It was also found that these cells isolated from transplants do not exhibit cytotoxic activity against donor cells and have a higher level of KIR94 expression - facts indicating that transplantation causes phenotypic changes in T-suppressor lymphocytes.

Considering that T-suppressor lymphocytes are present in the blood after transplantation, as well as their ability to suppress the expression of costimulatory molecules (CD80, CD86) on the donor's antigen-presenting cells, it is advisable to conduct appropriate monitoring during transplantation.

As already indicated, the study of CD8+CD28 T-lymphocytes in autoimmune pathology also provides information about the properties of these cells. In particular, it was found that in patients with systemic lupus erythematosus in the active phase, CD8+CD28 T-lymphocytes did not have inhibitory activity, which was combined with an imbalance between the inhibitory effects of IL-6 and the stimulating effects of IL-12.

The ability of T-suppressor lymphocytes to suppress the proliferation of antigen-specific CD4+T lymphocytes is associated with the appearance of remission in patients with autoimmune pathology. In in vitro systems, it was possible to characterize the precursors of CD8+CD28-T lymphocytes and show that a key role in their generation is played by monocytes, which secrete IL-10 after stimulation with GM-CSF (in these cases, direct cell-to-cell contact does not play a significant role).

The progenitors have the CD8+CD45RA-CD27-CCR-IL10Ra- phenotype. It has also been shown that T-suppressor lymphocytes suppress the activity of antigen-specific cytotoxic lymphocytes, reducing the expression of class I antigens of the major histocompatibility complex.

The presented data leave no doubt that CD8+CD28 T lymphocytes play an important role in maintaining immunological homeostasis together with CD4+ regulatory T lymphocytes.

There is sufficient evidence to believe that one of the main functions of these cells is the regulation of a specific T-cell response. Activation of suppressor T lymphocytes through specific recognition under physiological conditions protects Th lymphocytes from excessive activation, and therefore from excessive immunological response under certain conditions, in particular when the number of reactive T helper cells increases.

Available data indicate that the inhibitory effects of T-suppressor lymphocytes are one of the important participants in the induction of peripheral tolerance, and dysregulation of the control that these cells exercise over autoreactive clones of other T-lymphocytes may be the cause of the development of autoimmune pathology.

Along with this, one cannot but agree that understanding the physiological significance of the role of CD8 + CD28 T-lymphocytes requires further study, which will provide new data on the mechanisms of their action in tumor and autoimmune pathology.

A general idea of ​​CD8+CD28 T-lymphocytes is given in Fig. 60.

Rice. 60. Phenotypic and functional features of CD8+CD28-T lymphocytes

Regulatory functions of suppressor T lymphocytes

The results of studying suppressor CD8+CD28 T-lymphocytes leave no doubt that they perform important regulatory functions that are quite clearly defined under normal conditions.

It is also quite reasonable to assert that both suppressor T-lymphocytes (CD8+CD28-) and regulatory CD4+CD25+T-lymphocytes (Th3/Trl) are joint participants in the regulation of immunological homeostasis at all stages of its formation.

Along with this, it is also obvious that if the role of suppressor T-lymphocytes under normal conditions is quite clear, then the role of these cells during malignant growth remains to be clarified in the future. In this regard, the question is of particular interest: under what conditions do CD8+CD28-T-suppressors acquire the ability to have cytotoxic effects - a fact that was observed only when they were cultured with tumor cells.

Detailing of this issue (in all cases is it possible for the ability to become cytotoxic, to what extent is this related to the biological characteristics of tumor cells, what is the proportion of these cells in the implementation of cytotoxicity and what is the mechanism of influence of tumor cells on CD8+CD28-T lymphocytes) is undoubtedly , will provide new insight into the role of T-suppressor lymphocytes in the tumor process.

No less important is further study of the features of the interaction of T-suppressor lymphocytes with endothelial cells during tumor growth. The interest in clarifying this issue is understandable due to the fact that the interaction of CD8+CD28 T-lymphocytes with endothelial cells leads to pronounced manifestations of the activity of the latter, which can affect the process of malignant transformation.

Summarizing the presented materials, we can draw the following conclusions:

First

T-suppressor lymphocytes - CD8+CD28- are a separate clone of T-lymphocytes expressing CD8, have pronounced inhibitory effects on CD4+T-lymphocytes, and are able to interact with endothelial cells; play an important role in maintaining immunological homeostasis under normal and pathological conditions.

Second

The inhibitory effects of suppressor T-lymphocytes are due to intercellular interactions, in which, along with CD8+CD28-T-lymphocytes, dendritic cells and CD4+T-lymphocytes take part.

Third

In most cases, with various oncological diseases, the number of CD8 + CD28 T-lymphocytes in the blood increases, which is often combined with a poor prognosis; lymphocytes infiltrating the tumor also contain significant numbers of these cells.

Fourth

When T-suppressor lymphocytes are co-cultivated with autologous tumor cells, CD8+CD28-T lymphocytes appear that can have a cytotoxic effect.

Fifth

Determination of the number of CD8+CD28 T-lymphocytes can be used to monitor the influence

For activation disorder T lymphocytes characterized by the presence of a normal or increased number of T cells in the blood. These cells retain a normal phenotype, but the transmission of the signal from the receptors into the cell is impaired. Therefore, they do not proliferate or produce cytokines when stimulated by mitogens, antigens, or other signals from the TCR.

According to clinical manifestations, such defects are similar to other types of deficiency and in some cases are indistinguishable from severe combined immunodeficiency.

CD8 lymphopenia due to mutation of the zeta-associated protein 70 gene

In patients with a disorder T cell activation Severe, recurrent and often fatal infections develop during infancy. Most cases have been identified among Mennonites. The number of B lymphocytes in the blood is normal or increased; the concentration of immunoglobulins in serum is variable. Expression of CD3 and CD4 surface antigens on T lymphocytes is preserved, but CD8 cells are almost completely absent.

They do not respond to mitogens or allogeneic cells in vitro and do not produce cytotoxic T lymphocytes. NK cell activity is maintained. The thymus of one of the patients had a normal structure, and there were cells with both surface markers - CD4 and CD8. However, CD8 cells were absent. This condition is caused by mutations in the gene encoding zeta-associated protein 70 (ZAP-70), a tyrosine kinase that does not belong to the Src family and plays an important role in signal transmission to T lymphocytes.

ZAP-70 gene located on the long arm of chromosome 2 (section ql2). The normal number of T-lymphocytes with both markers (CD4 and CD8) is explained by the possibility of using another tyrosine kinase, Syk, for positive selection. The level of Syk in thymocytes is 4 times higher than its content in peripheral T-lymphocytes, which apparently determines the absence of a normal reaction of blood CD4 cells.

p56-Isk deficiency. A 2-month-old boy suffering from bacterial, viral and fungal infections was found to have lymphopenia and hypogammaglobulinemia. B and NK cells were present in the blood, but the number of CD4 T lymphocytes was low. Responses to mitogens were inconsistent. TCR stimulation did not result in CD69 expression. However, when stimulated with phorbol myristate acetate and the calcium ionophore CD69 (which is an activation marker), it appeared on T lymphocytes, indicating a defect in the proximal sections of the signal pathway into the cells.

Molecular research revealed alternative splicing of the transcript, resulting in the absence of a kinase domain in p56-lck.