Lymphocytes and their products

T-lymphocytes,or T-cells,are the thymus-derived lymphocytes that are found mainly in the interfollicular and paracortical areas of lymph nodes as well as in the periarteriolar sheaths of the spleen. T-cells constitute 70% to 80% of circulating peripheral blood lymphocytes. T-cells recognize antigens in their environment through an antigen-specific T-cell receptor (TCR).

About 95% of T-cells have a TCR composed of two disulfide-linked polypeptide chains, and P, which are noncovalently bound to the CD3 molecular complex. Antigen binds the variable region of the TCR. Postrecognition steps leading to T-cell activation appear to be a function of the CD3 complex.

About 5% of T-cells have a TCR composed of two other distinct chains, y and 8. These cells do not have either CD4 or CD8 on their surface.

Antigenic diversity of the TCR develops through a process of gene rearrangements analogous to those found in immunoglobulin (Ig) synthesis.

T-cell subtypes. Cytotoxic T-cells are capable of antigen-directed killing. They are important in delayed hypersensitivity reactions, rejection of solid organ transplants, immunity to certain bacteria and viruses, and, possibly, tumor immunity. Monoclonal antibodies that react with cell-surface glycoproteins of these cells are designated CD8, and the T-cell subtype is referred to as CD8⁺.

Helper T-cells are regulatory cells that help B-cells and other T-cells to respond appropriately to antigen. Monoclonal antibody markers for these cells are designated CD4, and the T-cell subtype is referred to as CD4⁺.

Suppressor T-cells act to suppress antibody production by B-cells and, thus, partially control the T- and B-cell immune response. These cells are also designated CD8⁺.

T-cellproducts. A variety of substances known as lymphokines are produced and secreted by T-cells and play important roles in cell-mediated immunologic activities. Examples of lymphokines that regulate lymphocyte activities are following.

Interleukin-2 (IL-2) is a T-cell growth factor that is important for long-term proliferation of activated T-cells. Interleukin-3 (IL-3) stimulates differentiation of bone-marrow stem cells. Interleukin 4 (IL-4) acts as a growth factor for activated T-cells, B-cells, and mast cells. IL-4 up-regulates class II major histocompatibility complex (MHC) expression on B-cells. Interleukin 5 (IL-5) aids in differentiation of B-cells into antibody-producing plasma cells. Interleukin-6 (IL-6) aids in maturation of T-cells and B-cells, stimulates growth of hematopoietic precursor cells, and inhibits fibroblasts. Gamma interferon (INF-γ) is one of a family of interferons that are proteins with diverse functions. INF-γ has antiviral activity, activates macrophages, promotes B-cell differentiation, suppresses hematopoietic precursor cells, and induces the expression of class II MHC on many cell types.

B- lymphocytes,or B-cells,constitute up to 15% of circulating peripheral blood lymphocytes and typically are defined by the presence of endogenously produced immunoglobulins.

B-cells are derived from lymphoid progenitor cells, which are believed to differentiate in the fetal liver and spleen and in the adult bone marrow, although

the exact origin in humans is unknown. Once formed, B-cells come to reside in the blood, the germinal centers and superficial cortex of lymph nodes, the lymphoid follicles of the white pulp in the spleen, and the bone marrow.

B-cells normally pass through a maturation sequence characterized by gene rearrangements that produce distinct phenotypes and enormous diversity in the B-cell repertoire. Pre-B-cells contain cytoplasmic μ heavy chain but no surface immunoglobulin. Immature B-cells have surface immunoglobulin M (IgM). With further gene rearrangement, B-cells acquire surface IgD and then proceed to «switch» heavy-chain isotypes to express IgG, IgA, or IgE.

Competent B-cells recognize antigen through their surface immunoglobulin receptors. B-cell responses are expanded with the help of T-cells and macrophages. If antigen stimulation persists, B-cells differentiate into their final stage- the antibody-synthesizing plasma cell. If not, the activated B-cells return to a resting, or «memory», stage.

Immunoglobulins are globular proteins produced by B-cells or plasma cells in response to exposure to a particular antigen. The immunoglobulins react specifically to that antigen. Five isotypes of immunoglobulins — IgG, IgA, IgM, IgD, and IgE are identified by structural differences. The basic structure and functional properties of immunoglobulins are the same for all isotypes.

The basic structural unit of immunoglobulins consists of two identical heavy polypeptide chains and two identical light polypeptide chains covalently linked by disulfide bonds. Each heavy and light chain of the immunoglobulin molecule is divided into two regions.

The variable regions of immunoglobulins are highly heterogeneous and are the sites responsible for binding antigen. These regions are contained within the Fab fragments that are produced following pepsin degradation of the molecule.

The constant regions of immunoglobulins control other functions, such as the binding of complement and cytotropic reactions. The constant regions of the heavy chains are found in the Fc-fragment that is produced following papain degradation of the molecule individual regions into continuous sequence.

Null cellsare a population of cytotoxic lymphocytes that are large, possess cytoplasmic granules, and are negative for TCRs and surface immunoglobulin. Their name reflects this lack of traditional T- or B-cell markers.

Natural killer-cells(NK-cells) represent the majority of null cells. NK-cells possess cytolytic activity against a variety of targets in the absence of purposeful immunization. Their cytotoxic activity does not seem to be restricted by the MHC. NK-cells require cell—cell contact for killing, which is enhanced by interferon and IL-2.

Killer-cells(K-cells) are a related population of null cells, which possess Fc-receptors and mediate killing through antibody-dependent mechanisms.

Mononuclear phagocytesare widely distributed in the body; they are given different names depending on their location. In connective tissue, they are called histiocytes; in blood, monocytes; in bone marrow, macrophages; in the liver, Kupffer cells; and in the lung, alveolar macrophages. Mononuclear phagocytes in all of these sites show common properties that make them critical components of inflammatory and immune reactions.

Mast cells and basophilsare granulocytes whose electron-dense cytoplasmic granules contain many of the chemical mediators of inflammation, including histamine, heparin. Secretion of these products is mediated through type I hypersensitivity mechanisms.

The complement systemis a plasma-based system of proteins that play a major role in host defense (both specific and nonspecific), in the inflammatory response, and in the mediation of tissue injury. Activated complement components regulate a variety of biologic activities, including chemotaxis, opsonization, phagocytosis, and cytolysis. They also promote smooth muscle contraction and vascular permeability.

Complement is activated sequentially in a cascading manner such that a protein is activated only by the protein that directly preceded it in the sequence. Two pathways are possible.

1. Classic pathway. This pathway requires all nine major complement compo-nents. Activation occurs by direct binding of CI to an antibody (most often IgG or IgM) in the form of an antigen-antibody complex. Activated CI cleaves C4 and C2 to form the bimo-lecular complex C4b.

2. C3 then splits under the influence of C3 convertase to form C3a and C3b. C3a is a small fragment that, along with other low-molecular-weight peptides generated later in the cascade, causes the release of vasoactive amines and lysosomal.

Central organs of immune system producing immunecompetent cells are bone marrow and thymus.The bone marrow contains progenitor cells for the other lymphoid organs. The progenitor cells produced in the bone marrow circulate to the thymus or peripheral organs of immune system, where they develop into more mature lymphoid cells. Populations of bone marrow cells that may have recirculated back to the bone marrow can respond to antigens and are called B-lymphocytes.

The thymus produces and differentiates small lymphocytes (T-lymphocytes).

Main peripheral organs of immune system are spleen, lymphatic nodes and gastrointestinal associated lymphoid tissue (GALT) and bronchus associated lymphoid tissue (BALT). The main immunocompetent cells are T-lymphocytes, B-lymphocytes, macrophages.

There are T- and B-zones in the peripheral organs of the immune system. Thus, in the spleen, periarterial zone of the follicle is T-zone, marginal zone is inhibited by B-lymphocytes. There are T-, B-lymphocytes and macrophages in the red pulp of the spleen. In the lymphatic nodes, paracortical zone and peripheral zone of the follicle is T-zone, cortical layer, light centers of the follicles are B-zone. There are T-, B-lymphocytes and macrophages in the medullar substance. Gastrointestinal associated lymphoid tissue (GALT) and bronchus associated lymphoid tissue (B ALT) have different immunocompetent cells, i.e. T-lymphocytes, B-lymphocytes, macrophages without any zones.

For identification of immunocompetent cells we use monoclonal antibodies to different immune cells (Cluster of differentiation).

Histocompatibility antigens.In humans, the major histocompatibility genes are located on the short arm of chromosome 6. These genes code for the histocompatibility antigens termed human leukocyte antigens (HLAs). HLAs are products of genes of the MHC, which is a highly polymorphous set of membrane-associated glycoproteins that are critical for the recognition of self during cell—cell immunologic reactions. Two broad categories of MHC antigens are described.

Class I MHC antigens are composed of two noncovalently linked polypeptide chains. The smaller chain is a superficial protein termed P₂-microglobulin.

The heavy chain is a trans-membrane protein that bears the antigenic determinants for the alleles of the three major, class I loci, which are designated HLA-A, HLA-B, and HLA-C. The alleles of this antigen system are codominant.

Class II MHC antigens are composed of two noncovalently linked glycoproteins; however, both are transmembrane proteins. Class II antigens are encoded by the HLA-D gene, which is divided into three major loci designated HLA-DP, HLA-DQ, and HLA-DR.

Class II antigens are located mostly in macrophages, B-cells, activated T-cells, endothelial cells and dendritic cells.

THYMUS

Thymus is the organ regulating the whole immune system. At immunogenesis disturbances we usually see the following processes and pathology.

1. Accidental thymus transformation(involution), that is reduction in the size and mass due to thymocyte migration to the peripheral immune organs and blood as well as due to their partial decomposition and absorption by macrophages (this is called apoptoses).

According to T. Ivanovskaya (1976), accidental involution consists of 5 stages.

Stage 1 — «holey clearing» — accumulation of lymphocytes around the macrophages. It occurs in the cortex.

Stage 2 — transition of the lymphocytes from the cortex to the medullar substance. The boundary between the layers is either poorly seen or not seen at all.

Stage 3 — «layer inversion)), when the cortex layer looks light, and medullar layer looks dark as a result of transition of lymphocytes from the cortex to the medullar substance.

Stage 4 — Reduction in the lymphocyte amount in the both layers, reticular stroma growth.

Stage 5 — collapse of the lobe of the thymus and sclerosis and lobe atrophy.

Accidental transformation more often occurs in the newborn suffering from stress factors. The more powerful is the stimulus, the more pronounced is the degree of involution. Accidental involution occurs in infections, intoxications, in the children born from sick mothers. The process is reversible. Elimination of pathological agent results in thymus normalization.

2. Thymus hyperplasia(thymolymphatic state, thymomegaly) The weight and the size of thymus are considerably increased. Microscopic examination reveals a large number of immature lobules (zones are not distinct). The density of the thymocytes is high. If this condition is accompanied by hypoplasia of adrenal and sexual glands as well as narrow aorta and arteries, this pathological process is called «thymo-lymphatic state».

Sudden death syndrome (crib death) may occur in thymomegaly, it results from insufficiency of

T-lymphocytes of the cortex and medullar substance of the adrenal glands.

3. Thymus hypoplasiais characterized by absence of lobule division into cortical and medullar substance, poor development of reticuloepithelial component, responsible for hormonal function, as well as lymphocyte component. As a rule thymus hypoplasia is typical for congenital immune deficiency.