Cells and Cellular Activities of the Immune System: Lymphocytes and Plasma Cells

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Cells and Cellular Activities of the Immune System

Lymphocytes and Plasma Cells

Lymphocytes and Plasma Cells

The adaptive immune system is comprised of the humoral and cellular systems. Each of the two arms of the adaptive immune system has fundamental mechanisms allowing the body to attack an invading pathogen. The immunologically specific cellular component of the immune system is organized around two classes of specialized cells, T lymphocytes and B lymphocytes. Lymphocytes recognize foreign antigens, directly destroy some cells, or produce antibodies as plasma cells. The total immune response involves the interaction of many different cell types and cell-mediated and antibody-mediated responses. Recent studies have shown that T cells are not just the latecomers in inflammation, but might also play a key role in the early phase of this response. T cell subsets, including NK cells, together with classic innate immune cells, contribute significantly to the development and establishment of acute and chronic inflammatory diseases.

Lymphoid and Nonlymphoid Surface Membrane Markers

Before 1979, human lymphocytes could be classified as T or B cells based on observation of these cells with electron microscopy (Fig. 4-1). T lymphocytes have a relatively smooth surface compared with the rough pattern of the B lymphocytes.

The introduction of monoclonal antibody (MAb) testing (see Chapter 2) led to the present identification of surface membrane markers on lymphocytes and other cells. In practical terms, surface markers are used to identify and enumerate various lymphocyte subsets, establish lymphocyte maturity, classify leukemias, and monitor patients on immunosuppressive therapy.

Cell surface molecules recognized by MAbs are called antigens, because antibodies can be produced against them, or markers, because they identify and discriminate between, or “mark,” different cell populations. Originally, surface markers were named according to the antibodies that reacted with them, but a uniform nomenclature system has now been adopted.

In this system, a surface marker that identifies a particular lineage or differentiation stage with a defined structure, and that can be identified with a group or cluster of MAbs, is called a member of a cluster of differentiation (CD; Fig. 4-2). Markers can be categorized as follows:

In addition to using CD classification for the identification and separation of lymphocytes, CD antigens are involved in various lymphocyte functions, usually the following:

Sites of Lymphocyte Development

In mammalian immunologic development, the precursors of lymphocytes arise from progenitor cells of the yolk sac and liver (Fig. 4-3). Later in fetal development, and throughout the life cycle, the bone marrow becomes the sole provider of undifferentiated progenitor cells, which can further develop into lymphoblasts. Continued cellular development and proliferation of lymphoid precursors occur as the cells travel to the primary and secondary lymphoid tissues.

Primary Lymphoid Tissue

In mammals, both the bone marrow (and/or fetal liver) and thymus are classified as primary or central lymphoid organs (Fig. 4-4).

Thymus

Early in embryonic development, the stroma and nonlymphoid epithelium of the thymus are derived from the third and fourth pharyngeal pouches. The characteristics of the thymus gland change with aging. Older persons are immunologically challenged because aging causes a reduction in the production of naïve T cells by the thymus. Intrinsic defects in mature T cell function, alterations in the life span of naïve T cells and in naïve or memory T cell ratios in the peripheral lymphoid tissues, occur as the result of the decline of the T cell response in older persons.

The thymus, located in the mediastinum, exercises control over the entire immune system. It is believed that the development of diversity occurs mainly in the thymus and bone marrow, although clonal expansion can occur anywhere in the peripheral lymphoid tissue.

Progenitor cells that migrate to the thymus proliferate and differentiate under the influence of the humoral factor, thymosin. These lymphocyte precursors with acquired surface membrane antigens are referred to as thymocytes.

The reticular structure of the thymus allows a significant number of lymphocytes to pass through it to become fully immunocompetent (able to function in the immune response), thymus-derived T cells. The thymus also regulates immune function by the secretion of multiple soluble hormones.

Many cells die in the thymus and apparently are phagocytized, a mechanism to eliminate lymphocyte clones reactive against self. It is estimated that approximately 97% of the cortical cells die in the thymus before becoming mature T cells. Viable cells migrate to the secondary tissues. The absence or abnormal development of the thymus results in a T lymphocyte deficiency.

Involution of the thymus is the first age-related change occurring in the human immune system. In postnatal life, the thymus is the primary organ that produces naïve T cells for the peripheral T cell pool but production of cells declines as early as 3 months of age. The thymus gradually loses up to 95% of its mass during the first 50 years of life (Fig. 4-5). The accompanying functional changes of decreased synthesis of thymic hormones and the loss of ability to differentiate immature lymphocytes are reflected in an increased number of immature lymphocytes within the thymus and as circulating peripheral blood T cells. Most changes in immune function, such as dysfunction of T and B lymphocytes, elevated levels of circulating immune complexes, increases in autoantibodies, and monoclonal gammopathies are correlated with involution of the thymus (see Chapter 27). Immune senescence may account for the increased susceptibility of older adults to infections, autoimmune disease, and neoplasms.

Secondary Lymphoid Organs

Secondary lymphoid organs provide a unique microenvironment for the initiation and development of immune responses. The secondary lymphoid tissues include lymph nodes, spleen, GALT, thoracic duct, bronchus-associated lymphoid tissue (BALT), skin-associated lymphoid tissue, and blood. Mature lymphocytes and accessory cells (e.g., antigen-presenting cells) are found throughout the body, although the relative percentages of T and B cells vary in different locations (Table 4-1).

Table 4-1

Approximate Percentage of Lymphocytes in Lymphoid Organs

Lymphoid Organ T Lymphocytes (%) B Lymphocytes (%)
Thymus 100 0
Blood 80 20
Lymph nodes 60 40
Spleen 45 55
Bone marrow 10 90

Adapted from Claman HN: The biology of the immune response, JAMA 268:2790–2796, 1992.

The highly sophisticated structure of secondary lymphoid organs allows migration and interactions between antigen-presenting cells, T and B lymphocytes, and follicular dendritic cells (FDCs) and other stromal cells. The cooperative activities of lymphoid cells within secondary organs dramatically increase the probability of interactions of rare B, T, and APCs that results in effective generation of humoral immune responses.

Tumor necrosis factor (TNF) and lymphotoxin are essential to the formation and maintenance of secondary organs. These cytokines are produced by B and T lymphocytes. Proliferation of the T and B lymphocytes in the secondary or peripheral lymphoid tissues (Fig. 4-6) is primarily dependent on antigenic stimulation.

The T lymphocytes or T cells populate the following:

The B lymphocytes or B cells multiply and populate the following:

Circulation of Lymphocytes

Mature T lymphocytes survive for several months or years, whereas the average life span of B lymphocytes is only a few days. Lymphocytes move freely between the blood and lymphoid tissues. This activity, termed lymphocyte recirculation, enables lymphocytes to come into contact with processed foreign antigens and disseminate antigen-sensitized memory cells throughout the lymphoid system. Clonal expansion may occur regionally, as in lymph nodes draining a contact allergic reaction, and then the whole body becomes susceptible to rechallenge because T cells recirculate, but generally are excluded from returning to the thymus. Research has shown that a pool of T cell clonal elements is developed by a combination of positive selection of clones able to recognize and react to foreign antigens, and negative selection (purging) of clones able to interact with self-antigens in a damaging way.

Recirculation of lymphocytes back to the blood is through the major lymphatic ducts. Lymphocytes enter the lymph node from the blood circulation via arterioles and capillaries to reach the specialized postcapillary venules. From the venule, the lymphocytes enter the node and remain in the node or pass through the node and return to the circulating blood. Lymphatic fluid, lymphocytes, and antigens from certain body sites enter the lymph node through the afferent lymphatic duct and exit the lymph node through the efferent lymphatic duct (see Fig. 4-7).

Virgin or Naïve Lymphocytes

Virgin or naïve lymphocytes are cells that have not encountered their specific antigen. These cells do express high-molecular-weight variants of leukocyte common antigen.

Memory cells are populations of long-lived T or B cells that have been stimulated by antigen. They can make a quick response to a previously encountered antigen. Memory B cells carry surface IgG as their antigen receptor; memory T cells express the CD45RO variant of the leukocyte common antigen and increased levels of cell-adhesion molecules (CAMs), chemical mediators involved in inflammatory processes throughout the body (Fig. 4-8).

Development of T Lymphocytes

Most lymphocytes (see Color Plate 8) found in the circulating blood are T cells derived from bone marrow progenitor cells that mature in the thymus gland (Table 4-2). These cells are responsible for cellular immune responses and are involved in the regulation of antibody reactions in conjunction with B lymphocytes.

Table 4-2

Lymphocyte Characteristics

Type Function(s) Phenotypic Marker Peripheral Blood (% of Total)
Helper T (Th) cells Stimulate B cell growth and differentiation (humoral immunity); macrophage activation by secreted cytokines (cell-mediated immunity) CD3+, CD4+, CD8− 50-60
Cytotoxic T (Tc) cells Lysis of virus-infected cells, tumor cells, and allografts (cell-mediated immunity); macrophage activation by secreted cytokines (cell-mediated immunity) CD3+, CD4−, CD8+ 20-25
Natural killer (NK) cells Lysis of virus-infected cells, (antibody-dependent cellular cytotoxicity) Fc receptor for IgG or cells CD16 ∼10
B cells Antibody production (humoral immunity) Fc receptors, MHC class II, CD19, CD21 10-15

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During cellular development, T lymphocyte function–associated antigens vary in expression. Some antigens appear early in cellular development and remain on mature T cells. Others appear at an early or intermediate stage of cellular maturation and are lost before maturity.

Early Cellular Differentiation and Development

Differentiation of a lymphocyte begins in the thymus as a thymocyte. Early surface markers on thymocytes that are committed to becoming T cells include CD44 and CD25. As thymocytes develop, there is an orderly rearrangement of the genes coding for an antigen receptor.

Maturation is a complicated process that lasts for a period of 3 weeks. During this period, cells filter through the cortex to the medulla of the thymus. Thymic stromal cells include fibroblasts, macrophages, epithelial cells, and dendritic cells; all these cell types play a role in T cell development.

Double-Negative Thymocytes

Early thymocytes lacking CD4 and CD8 surface membrane markers are referred to as double-negative thymocytes. These cells proliferate in the outer cortex of the thymus under the influence of interleukin-7 (IL7). IL-7 is critical for this growth and differentiation.

Rearrangement of the genes that code for the antigen receptor, the T cell receptor (TCR), begin at this developmental stage. CD3 constitutes the main part of the T cell antigen receptor. The configuration of two of the eight chains of the receptor have variable regions that recognize specific antigens. These are coded for by selecting gene segments and deleting others.

Rearrangement of the beta (β) chain occurs first; the appearance of a functional β chain on the cell surface sends a signal to suppress any further β chain gene rearrangements. The combination of the β chain with the CD3 forms the pre– T-cell antigen receptor (TRC). Signaling by the β chain promotes the development of a CD4+ and CD8+ thymocyte.

Thymocytes that express gamma (γ ) and delta (δ) chains follow a different developmental pathway. Cells expressing gamma-delta (γδ) chains typically remain both CD4− and CD8−. These double-negative cells represent most of the population of T lymphocytes in the skin and intestinal and pulmonary epithelium.

Circulating CD3+ double-negative lymphocytes are phenotypically and functionally distinct from single-positive CD3+CD4+ and CD3+CD8+ lymphocytes and are thought to represent a distinct T cell lineage. The presence of low numbers of double-negative T cells in healthy individuals and the increase observed in association with lymphoproliferative disorders, graft-versus-host disease, and autoimmune diseases suggest a pathogenic or immunoregulatory role for this population of T lymphocytes.

Double-Positive Thymocytes

Cells with both CD4+ and CD8+, or double-positive, surface markers represent the second stage of thymocyte development. These thymocytes begin to demonstrate rearranged genes coding for the alpha (α) chain. When the CD3-αβ receptor complex (TCR) is expressed on the cell surface, a process known as positive selection permits only double-positive cells with functional TCR receptors to survive. T cells must recognize foreign antigen in association with class I or II major histocompatibility complex (MHC) molecules. Any thymocyte that is unable to recognize self-MHC dies without ever leaving the thymus gland. Functioning T lymphocytes must be able to recognize a foreign antigen along with MHC molecules. A second selection process, negative selection, takes place among the surviving double-positive T cells. Only 1% to 3% of double-positive thymocytes survive in the cortex.

Double-positive (DP) CD4CD8 Tαβ cells have been reported in normal individuals as well as in different pathologic conditions, including inflammatory diseases, viral infections and cancer, but their function remains to be elucidated. Double-negative cells may act like natural killer (NK) cells because they are capable of binding to many natural, unprocessed cell surface molecules. In addition, these cells are capable of recognizing antigens without being presented by MHC proteins. Consequently, NK cells may represent an important bridge between natural and adaptive immunity.

Later Cellular Differentiation and Development of T Lymphocytes

When mature T cells leave the thymus, their T cell receptors (TCRs) are CD4+ or CD8+. Survivors of selection exhibit only one type of marker, CD4+ or CD8+ and migrate to the medulla. These cells gain functional maturity with their entry into the peripheral blood circulation.

T cells develop into a variety of clones. Each lymphocyte displays a single type of structurally unique receptor. The repertoire of antigen receptors in the entire population of lymphocytes is extremely large and diverse. This increases the probability that an individual lymphocyte will encounter an antigen that binds to its receptor, thereby triggering activation and proliferation of the cell. This process, clonal selection, accounts for most of the basic properties of the adaptive immune system.

Antigen receptors for common pathogens need to be reinvented by every generation of cells. Because the binding sites of antigen receptors arise from random genetic mechanisms, the receptor repertoire contains binding sites that can react not only with infectious microorganisms, but also with innocuous environmental antigens and self antigens.

Helper T Lymphocytes

Helper T lymphocytes, or T-helper (Th) cells, can be assigned to one of several subsets, including the following:

These divisions are not absolute, with considerable overlap or redundancy in function among the different subsets. This classification is based on the in vitro blends of cytokines that they produce. Th1 and Th2 cells can promote the development of cytotoxic cells and are believed to develop from Th0 cells. Th1 cells interact most effectively with mononuclear phagocytes; Th2 cells release cytokines that are required for B cell differentiation (Fig. 4-9).

Characterized by high interferon-gamma (IFN-γ) production, Th1 responses promote the elimination of intracellular pathogens (Fig. 4-10, A). Characterized by IL-4 and IL-5, Th2 responses promote a different type of effector response that involves immunoglobulin E (IgE) production and eosinophils capable of eliminating larger extracellular pathogens, such as helminths (see Fig. 4-10, B). In situations of repeated pathogen exposure or persistent infection, the polarization of T cell responses serves to focus the antigen-specific response on a specific effector pathway.

The following factors can influence the terminal differentiation of lymphocytes:

A hierarchy is apparent among these factors and is determined by how they influence T cell differentiation. Certain cytokines acting directly on T cells during primary activation appear to be the most proximal or direct mediators of CD4+ T cell differentiation. The presence of IL-12 during primary T cell activation leads to strong development of Th1 responses, and IL-4 promotes Th2 development. Activation through the TCR is a requirement for initiating terminal differentiation, but the signals from the TCR appear to be phenotype-neutral.

Certain T cells carry out delayed hypersensitivity reactions. These T cells react with antigen MHC class II on APCs and create their effects mainly through cytokine production. These cells generally are of the CD4+ phenotype.

T cells can also be differentiated into two populations depending on whether they use an αβ (TCR2) or γδ (TCR1) antigen receptor. The TCR consists of a heterodimer and a number of associated polypeptides that form the CD3 complex. The dimer recognizes processed antigen associated with an MHC molecule. The CD3 complex is required for receptor expression and is involved in signal transduction. TCR1 cells constitute less than 5% of total lymphocytes but appear in greater proportions in some sites (e.g., skin, vagina). TCR1 cells appear to recognize different antigens than TCR2 cells, including carbohydrate and intact protein antigens. In addition, some TCR1 cells do not require antigen to be processed or presented by MHC molecules.

T Regulatory Lymphocytes

Treg cells are immunoregulatory Th cells that control autoimmunity in the peripheral blood through dominant tolerance. Types of Treg cells include the following:

Natural Treg cells, characterized by constitutive expression of CD25, are developed primarily in the thymus from positively selected thymocytes with a relatively high avidity for self antigens. Natural Treg cells represent approximately 5% to 10% of the total CD4+ T cell population. The signal to develop into Treg cells is thought to come from interactions between the TCR and MHC class II self-peptide complex expressed on the thymic stroma. In humans, natural Treg cells express CD4 and CD25.

Other types of Treg cells that can develop in the periphery are Tr1 and Th3 cells. Tr1 cells are CD4+ and are functionally induced by IL-10. These Treg cells, in turn, secrete IL-10 and regulate the immune system. Th3 progenitor cells are also CD4+. In vitro CD4+ cells have been shown to secrete transforming growth factor β (TGF-β). CD8+ Treg cells are less well characterized and are reportedly capable of suppressing CD4+ cells in vitro.

Cytotoxic T Lymphocytes

Cytotoxic T lymphocytes, or T cytotoxic (Tc) cells, are effector cells found in the peripheral blood that are capable of directly destroying virally infected target cells. Most Tc cells are CD8+ and recognize antigen on the target cell surface associated with MHC class I molecules (e.g., human leukocyte antigen [HLA] types A, B, and C) or MHC class I alone. This process is demonstrated by the immune response to virus-infected cells or tumor cells (Fig. 4-11).

In a primary viral infection, naïve CD8+ T cells are primed in secondary lymph nodes and consequently proliferate and differentiate into effector CD8+ T cells to eliminate virus-infected cells. After clearance of the virus, most effector CD8+ T cells contract because of apoptosis but a small number of these CD8+ T cells form a memory T cell pool.

Studies have demonstrated that human CD8+ T cells undergo a change in the expression of costimulatory molecules (e.g., CD27, CD28, and CD45RA) on their surface, according to their differentiation and maturation. Cytolytic effector molecules, perforin, and granzyme A-B, are considered to be markers for effector CD8+ T cells because they are the actual functional molecules for killing target cells.

Naïve and central memory CD8+ T cells express the membrane marker, CCR7, for homing to secondary lymph nodes, but effector memory and effector CD8+ T cells express the chemokine receptors for inflammatory cytokines, which enable the cells to migrate toward infected and inflamed sites. A unique subset of the effector CD8+ T cell population expresses CXCR1. These CXCR1 CD8+ T cells possess chemotactic activity toward the CDCR1 ligand IL-8, a potent inflammatory cytokine produced in inflamed tissues and in tissues infected with some viruses, such as human cytomegalovirus (HCMV) or influenza A. This suggests that these CXCR1+ effector CD8+ T cells immediately migrate to inflamed and infected sites to exert their effector function in the initial stage of an immune response. It is possible that effector CD8+ T cell subsets are functionally distinct populations of T lymphocytes.

In addition to destruction of virally infected, MHC class I–bearing targets, Tc cells are major effectors in allograft organ rejection. Tc cells express CD4 or CD8, depending on the MHC antigen restriction that governs their antigen recognition (i.e., class I or II antigens; Fig. 4-12).

Suppressor T lymphocytes, or T suppressor (Ts) cells, are functionally defined T cells that downregulate the actions of other T and B cells. Ts cells have no unique markers. Although antigen-specific suppression was described in 1970, and many investigators believe that Ts cells are critical in various phases of immunoregulation, peripheral tolerance, and autoimmunity, their mode of action is unclear. Many Ts cells are CD8+ and may operate through secretion of free TCRs.

Antigen Processing and Antigen Presentation to T Cells

Antigen-presenting cells (APCs) are a group of functionally defined cells capable of taking up antigens and presenting them to lymphocytes in a form that they can recognize. APCs take up antigens (e.g., dendritic cells, macrophages, B cells, even tissue cells) in various ways. Some are collected in the periphery and transported to the secondary lymphoid tissues; other APCs normally reside in lymphoid tissues and intercept antigen as it arrives. B cells recognize antigen in a native form.

There are two major pathways of antigen processing for the APC and target cell, endogenous and exogenous. The endogenous pathway processes proteins that have been internalized, processed into fragments, and reexpressed at the cell surface membrane in association with MHC molecules. In this pathway, proteins in the cytoplasm are cleaved into peptide fragments about 20 amino acids in length. These fragments are then transported into the lumen of the endoplasmic reticulum by the transporter associated with the antigen-processing complex, where the fragments encounter newly formed, heavy-chain molecules of MHC class I and their associated beta2-microglobulin (β2m) light chains. The heavy chain, light chain, and peptide form a trimeric complex, which is then transported to and expressed on the cell surface.

T cells that express the CD8+ cell surface marker recognize antigens presented by MHC class I molecules. CD8+ functions as a coreceptor in this process, binding to an invariant region of the MHC class I molecule. Pathogen clearance requires that CD8+ effector cells produce inflammatory cytokines and develop cytolytic activity against infected target cells, after which a small number of memory cells survive that rapidly regain effector function in the event of rechallenge. During this process, a relatively homogeneous pool of naïve CD8+ T cells differentiates into heterogeneous pools of effector and memory CD8+ T cells.

In the exogenous pathway, soluble proteins are taken up from the extracellular environment, generally by specialized or so-called professional APCs. The antigens are then processed in a series of intracellular acidic vesicles called endosomes. During this process, the endosomes intersect with vesicles that are transporting MHC class II molecules to the cell surface. CD4+ T cells recognize antigens that are presented by MHC class II molecules. As with CD8, the CD4 molecule functions as a coreceptor, increasing the strength of the interaction between the T cell and APC.

For both systems of antigen presentation, recognition of the antigen by the T cells is described as being MHC-restricted, a process whereby T cells recognize only antigen presented by self MHC molecules.

Antigen Recognition by T Cells

T cells are clonally restricted, so that each T cell expresses a receptor that can interact with a given peptide. Each lymphocyte makes only one type of antigen receptor and can recognize only a very limited number of antigens. Because receptors differ on each clone of cells, the entire lymphocyte population has an enormous number of different, specific antigen receptors.

The TCR of most T lymphocytes is composed of an alpha and beta polypeptide chain, with constant regions located close to the cell surface and the part that binds to the antigenic peptide of appropriate fit located away from the cell surface. The difference in structure of the distal regions of the alpha and beta chains allows the development of different clones of T cells. The TCR reacts with antigen in the context of MHC class I or II molecules on an APC (Fig. 4-13).

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Figure 4-13

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