Organization of the immune-lymphatic system

The lymphatic system includes primary and secondary lymphoid organs.

The primary lymphoid organs produce the cell components of the immune system. They are (1) the bone marrow and (2) the thymus. The main cell type is the lymphocyte originated from a lymphoid stem cell in bone marrow (Figure 10-1).

Figure 10-1 Lineage origin of the lymphoid progeny within the context of hematopoiesis

The secondary lymphoid organs are the sites where immune responses occur. They include (1) the lymph nodes, (2) the spleen, (3) the tonsils, and (4) aggregates of lymphocytes and antigen-presenting cells in the lung (bronchial-associated lymphoid tissue [BALT] and the mucosa of the digestive tract (gut-associated lymphoid tissue [GALT] including Peyer’s patches). The lymphatic system is widely distributed because pathogens can enter the body at any point.

The main function of the lymphoid organs, as components of the immune system, is to protect the body against invading pathogens or antigens (bacteria, viruses, and parasites). The basis for this defense mechanism, or immune response, is the ability to distinguish self from nonself.

The two key cell components of the immune system are lymphocytes and accessory cells (Figure 10-2). Lymphocytes include two major cell groups: (1) B cells, responding to cell-free and cell-bound antigens; and (2) T cells, subdivided into two categories: helper T cells and cytolytic or cytotoxic T cells. T cells respond to cell-bound antigens presented by specific molecules.

Figure 10-2 Major cells participating in immune reactions

After leaving the two primary organs (bone marrow and thymus), mature B and T cells circulate in the blood until they reach one of the various secondary lymphoid organs (lymph nodes, spleen, and tonsils).

B and T cells can leave the bloodstream through specialized venules called high endothelial venules, so called because they are lined by tall endothelial cells instead of the typical squamous endothelial cell type.

The accessory cells include two monocyte-derived cell types: macrophages and dendritic cells. An example of a dendritic cell is the Langerhans cell found in the epidermis of the skin. A third type, the follicular dendritic cell, is present in lymphatic nodules of the lymph nodes. Follicular dendritic cells differ from ordinary dendritic cells in that they do not derive from a bone marrow precursor.

Before we start our discussion of the origin, differentiation, and interaction of lymphocytes and accessory cells, we will define the characteristics of the immune system. Then, we will be able to integrate the structural aspects of each major lymphatic organ with the specific characteristics of the immune responses.

Properties of adaptive or acquired immunity

Both humoral and cell-mediated immunity developed against foreign pathogens have the following characteristics:

Major histocompatibility complex and human leukocyte antigens

The presentation of antigens to T cells is carried out by specialized proteins encoded by genes in the major histocompatibility locus and present on the surface of antigen-presenting cells. Antigen-presenting cells survey the body, find and internalize antigens by phagocytosis, break them down into antigenic peptide fragments, and bind them to major histocompatibility complex (MHC) molecules (Figure 10-5) so that the antigen peptide fragment–MHC complex can be exposed later on the surface of the cells. The MHC gene locus expresses gene products responsible for the rejection of grafted tissue between two genetically incompatible hosts.

Figure 10-5 Structure of the T cell receptor and class I and II major histocompatibility complex (MHC)

There are two types of mouse MHC gene products: class I MHC and class II MHC. The class I MHC molecule consists of two polypeptide chains: an α chain, consisting of three domains (α₁, α₂, and α₃) encoded by the MHC gene locus, and β₂-microglobulin, not encoded by the MHC gene locus. Antigens are housed in a cleft formed by the α₁ and α₂ domains. CD8, a coreceptor on the surface of cytolytic T cells, binds to the α₃ domain of class I MHC.

Class II MHC consists of two polypeptide chains, an α chain and a β chain. Both chains are encoded by the MHC gene locus. The α₁ and β₁ domains form an antigen-binding cleft. CD4, a coreceptor on the surface of helper T cells, binds to the β domain of class II MHC. CD4 and CD8 are cell surface identifiers, members of the cluster of differentiation or designation (abbreviated as CD). See Box 10-B.

All nucleated cells express class I MHC molecules. Class II MHC molecules are restricted mainly to antigen-presenting cells (macrophages, dendritic cells, and B cells), stromal epithelial cells of the thymus, and endothelial cells.

The MHC-equivalent molecules in the human are designated human leukocyte antigens (HLAs). HLA molecules are structurally and functionally homologous to mouse MHC molecules and the gene locus is present on human chromosome 5 (β₂-microglobulin is encoded by a gene on chromosome 15).

The class I MHC locus encodes three major proteins in the human: HLA-A, HLA-B, and HLA-C. The class II MHC locus encodes HLA-DR (R for antigenically related), HLA-DQ, and HLA-DP (Q and P preceding R in the alphabet).

T cell receptor complex

In addition to MHC molecules, subsets of T cells have cell surface receptors that enable each of them to recognize a different antigen peptide–MHC combination. Antigen recognition involves an immunologic synapse mechanism consisting in the formation of stable antigen-presenting cell–T cell adhesiveness followed by an activating signaling cascade by T cells (see Box 10-C).

The receptor that recognizes specific antigenic peptides presented by class I and class II MHC molecules is the T cell receptor (TCR). TCR acts together with accessory cell surface molecules, called coreceptors, to stabilize the binding of antigen-presenting cells to T cells.

The TCR consists of two disulfide-linked transmembrane polypeptide chains: the α chain and the β chain (see Figure 10-5). A limited number of T cells have a TCR composed of γ and δ chains. Each α and β chain consists of a variable (Vα and Vβ) domain and a constant (Cα and Cβ) domain. When compared with the immunoglobulin molecule, the Vα and Vβ domains are structurally and functionally similar to the antigen-binding fragment (Fab) of immunoglobulins.

The TCR molecule is associated with two proteins, CD3 and ζ (not shown in Figure 10-5), forming the TCR complex. CD3 and ζ have a signaling role and are present in all T cells. CD3 contains the ITAM cytoplasmic domain previously mentioned as part of the B cell antigen receptor complex and involved in signaling functions.

CD4 and CD8 coreceptors

CD4 and CD8 are T cell surface proteins interacting selectively with class II MHC and class I MHC molecules, respectively. When the TCR recognizes an antigen bound to the cleft of MHC, CD4 or CD8 coreceptors cooperate in the activation of T cell function (see Figure 10-5).

CD4 and CD8 are members of the immunoglobulin (Ig) superfamily. In Chapter 1, Epithelium, we discussed the function and structure of cell adhesion molecules belonging to the Ig superfamily.

Members of the Ig superfamily have a variable number of extracellular Ig-like domains. The two terminal Ig-like domains of CD4 bind to the β₂ domain of the class II MHC (see Figure 10-5). The single Ig-like domain of CD8 binds to the α₃, domain of the class I MHC.

Thus, CD4⁺ helper T cells recognize antigens associated with class II MHC, and CD8⁺ cytolytic T cells (cytolytic thymus-derived lymphocytes [CTL]) respond to antigens presented by class I MHC (Figure 10-6).

Figure 10-6 General features of helper and cytolytic T cells

MHC molecules and adaptive immune responses

T cells are MHC-restricted. T cells are able to react against a foreign antigen fragment bound to their own (self-) MCH molecules and contribute to adaptive immune responses. T cells should not respond to self-antigen peptide fragments bound to self-MHC molecules. This lack of response is called self-tolerance. Developing T cells express unique TCRs generated by random rearrangement of a variety of gene segments. These randomly produced TCRs provide the diversity required to identify numerous foreign peptides.

During their maturation in the cortex of the thymus, T cells are selected to be self-MHC–restricted and self-tolerant. This selective process, known as positive selection, occurs only when self-MCH–restricted T cells are selected (see Figure 10-7). Negative selection takes place when T cells do not bind to any MHC or bind to the body’s tissue-specific antigens (self-molecules). We discuss later how a portfolio of self-antigens expressed in the thymus permits the elimination of autoreactive T cells by apoptosis. Only those T cells that can recognize foreign peptides and self-MHC survive, leave the thymus, and migrate into the secondary lymphoid organs.

Figure 10-7 Maturation of T cells involves changes in cell surface molecules

The cortex of the thymus contains branching and interconnected thymic cortical epithelial cells involved in the positive selection of T cells. The medulla of the thymus houses thymic medullary epithelial cells involved in the negative selection of potentially autoreactive T cells. Contact between MHC molecules on the thymic epithelial cell surfaces and TCRs of developing T cells is an important feature in positive selection. This is another example of the importance of the immunological synapse (see Box 10-C).

T cells developing in the thymus express specific cell surface molecules

Two major events take place in the thymus during T cell maturation: (1) a rearrangement of the gene-encoding protein components of the TCR and (2) the transient coexistence of TCR-associated coreceptors CD4 and CD8.

When precursor cells—derived from the bone marrow—enter the cortex of the thymus, they lack surface molecules typical of mature T cells. Because they still do not express CD4 and CD8, they are called “double-negative” T cells.

After interacting with thymic epithelial cells, double-negative T cells proliferate, differentiate, and express the first T cell–specific molecules: TCR and coreceptors CD4 and CD8.

TCR consists of two pairs of subunits: αβ chains or γδ chains (see Figure 10-3). Each chain can vary in sequence from one T cell to another. This variation is determined by the random combination of gene segments and has a bearing on which foreign antigen T cells can recognize.

Maturation of T cells proceeds through a stage where both CD4 and CD8 coreceptors and low levels of TCR are expressed by the same cell. These cells are known as “double-positive” T cells. Double-positive T cells can or cannot recognize self-MHC. Those cells that can recognize self-MHC eventually mature and express one of the two coreceptor molecules (CD4 or CD8) and become “single-positive” T cells. Double-positive cells that cannot recognize self-MHC fail positive selection and are discarded.

T cell-mediated immunity

When T cells complete their development in the thymus, they enter the bloodstream and migrate to the peripheral lymphoid organs in search of an antigen on the surface of an antigen-presenting cell.

Helper T cells contain both the TCR and CD4 coreceptor. Helper T cells recognize class II MHC on antigen-presenting cells.

There are two distinct subtypes of helper T cells derived from the same CD4⁺ T cell precursor: Th1 and Th2 cells.

Th1 cells participate in the regulation of immune responses caused by intracellular pathogens (viruses causing infections, certain bacteria, or single-cell parasites) with the significant participation of macrophages. Th1 cells produce interferon-γ, which can suppress the activity of Th2 cells.

Immune responses controlled by TH2 cells are observed in patients with helminthic (Greek helmins, worm) intestinal parasites. Th2 cells produce interleukin-4 (IL-4) and interleukin-13 (IL-13), among other cytokines, and determine the production of immunoglobulin E by plasma cells to activate the responses of mast cells, basophils, and eosinophils. The activation of macrophage responses is minimal in Th2-driven immune responses.

Cytolytic (or killer) T cells display both the TCR and CD8 coreceptor. Cytolytic T cells recognize class I MHC on antigen-presenting cells. We will return to the clinical significance of helper and cytolytic T cells when we discuss their involvement in the pathology of human immunodeficiency virus-type 1 (HIV-1) infection, allergy, and cancer immunotherapy.

How do helper T cells help?

Helper T cells are activated when they recognize the antigen peptide–class II MHC complex (Figure 10-8).

Figure 10-8 Helper T cells

In the presence of cells with antigen peptide bound to class II MHC, helper T cells proliferate by mitosis and secrete cytokines, also called interleukins. These chemical signals, in turn, attract B cells, which also have receptor molecules of single specificity on their surface (immunoglobulin receptor). Unlike helper T cells, B cells can recognize free antigen peptides without MHC molecules.

When activated by interleukins produced by the proliferating helper T cells, B cells also divide and differentiate into plasma cells secreting immunoglobulins, a soluble form of their receptors. Secreted immunoglobulins diffuse freely, bind to antigen peptides to neutralize them, or trigger their destruction by enzymes or macrophages.

Plasma cells synthesize only one class of immunoglobulin (several thousand immunoglobulin molecules per second; lifetime of a plasma cell is from 10 to 20 days). Five classes of immunoglobulins are recognized in humans: IgG, IgA, IgM, IgE, and IgD (see Box 10-D). Abnormal plasma cells may accumulate in bones and bone marrow, causing bone destruction and affecting the production of normal blood cells. This pathologic condition is called multiple myeloma (see Box 10-E).

Box 10-D Immunoglobulins

• An immunoglobulin (Ig) molecule or antibody is composed of four polypeptide chains: two identical light chains and two identical heavy chains. One light chain is attached to one heavy chain by a disulfide bond. The two heavy chains are attached to each other by disulfide bonds.

• Heavy and light chains consists of amino terminal variable regions that participate in antigen recognition (Fab region) and carboxyl terminal constant regions. The constant region (Fc region) of the heavy chains mediate effector functions.

• Immunoglobulins can be membrane-bound or secreted.

• Types of immunogloblins: IgA forms dimers linked by a J chain and participate in mucosal immunity. IgD is a receptor for antigens of inmature B cells. IgE participates in mast cell and basophil activation (degranulation). IgG is the most abundant immunogloblin and the only one to cross the placental barrier. It participates in opsonization, a mechanism that enhances phagocytosis of pathogens. IgM molecules normally exist as pentamers.

Some T and B cells become memory cells, ready to eliminate the same antigen if it recurs in the future. The secondary immune response (re-encounter with the same antigen that triggered their production) is more rapid and of greater magnitude. Memory cells recirculate for many years and provide a surveillance system directed against foreign antigens.

How do cytolytic T cells kill?

Another function of helper T cells is to secrete cytokines to stimulate the proliferation of cytolytic T cells that recognize the antigen peptide–class I MHC complex on the surface of antigen-presenting cells.

The subset of cytolytic T cells initiates a target cell destruction process (Figure 10-9) by (1) attaching firmly to the antigen-presenting cell with the help of integrins and cell adhesion molecules (CAMs) on the cell surface of the target cell and (2) inducing cell membrane damage by the release of pore-forming proteins (called perforins). These pores facilitate the unregulated entry of the pro-apoptotic protease granzyme, water, and salts. The cytolytic T cell protects itself by a membrane protein, protectin, that inactivates perforin, blocking its insertion into the cytolytic T cell membrane.

Figure 10-9 Cytolytic T cells

Cytolytic T cells can also destroy target cells by the Fas-Fas ligand mechanism seen during apoptosis (see Chapter 3, Cell Signaling). When the cytolytic T cell receptor recognizes an antigen on the surface of a target cell, Fas ligand is produced in the cytolytic T cell. The interaction of Fas ligand with the trimerized Fas receptor on the target cell surface (see Figure 10-9) triggers the apoptotic cascade by activation of procaspases into caspases that determine cell death.

Regulatory, suppressors, and effector T cells

B cells can differentiate into immunoglobulin-secreting plasma cells. Plasma cells are effector cells. T cells differentiate into regulatory, suppressor, and effector T cells.

Regulatory T cells include helper T cells, which cooperate with B cells to stimulate the proliferation and differentiation of B cells into immunoglobulin-secreting plasma cells and the cytolytic activation of killer T cells.

Suppressor T cells include Th1 and Th2 cells, two subsets of T cells. Suppressor T cells acting on helper T cells to moderate or inhibit their activities, also modulate the differentiation of B cells into plasma cells. Suppressor T cells produce different cytokines with distinct functions. Th1 cells produce interferon-γ, whereas Th2 cells produce IL-4 and IL-13. Interferon-γ, produced by Th1 cells, stimulates the differentiation of Th1 cells but suppresses the proliferation of Th2 cells. Furthermore, TH2-derived IL-4 suppresses the activation of Th1 cells.

Effector T cells include cytolytic ? cells and natural killer cells. Cytolytic T cells can lyse cells that bear antigens for which they are specific. Cell killing is caused by the release of perforin or Fas ligand as already discussed.

Natural killer cells destroy virus-infected cells and tumor cells, but this activity does not depend on antigen activation. Natural killer cells do not belong to the T or B cell types (they do not express TCR). They have CD56 receptors as well as inhibitory and activating receptors interacting respectively with class I MHC and activating ligand of normal cells. Target cells lacking class I MHC activate the destructive function of natural killer cells. The mechanism by which natural killer cells destroy target cells is described in Figure 10-10.

Figure 10-10 Natural killer cells

Clinical significance: Acquired immunodeficiency syndrome

The acquired immunodeficiency syndrome (AIDS) is caused by HIV-1 and is characterized by significant immunosuppression associated with opportunistic infections, malignancies, and degeneration of the central nervous system.

HIV infects macrophages, dendritic cells, and predominantly CD4-bearing helper T cells. HIV is a member of the lentivirus family of animal retroviruses and causes long-term latent cellular infection. HIV includes two types, designated HIV-1 and HIV-2. HIV-1 is the cause of AIDS. The genome of the infectious HIV consists of two strands of RNA enclosed within a core of viral proteins and surrounded by a lipid envelope derived from the infected cell. The lipid envelope contains viral proteins designated gp41 and gp120, encoded by the env viral sequence. The glycoprotein gp120 has binding affinity for CD4 and a coreceptor. HIV particles are present in blood, semen, and other body fluids. Transmission is by sexual contact or needle injection.

Figure 10-11 presents a summary of the cellular events associated with HIV infection. Box 10-F summarizes the steps of the HIV reproductive cycle. A relevant event of HIV infection is the destruction of CD4⁺ helper T cells responsible for the initiation of immune responses, leading to the elimination of HIV infection. Cytolytic T cells (that attach to virus-infected cells) and B cells (that give rise to antibody-producing plasma cells) represent an adaptive response to HIV infection. Antibodies to HIV antigens are detected within 6 to 9 weeks after infection.

Figure 10-11 Immune system and HIV infection

Clinical significance: Allergy

Allergy refers to immune responses characterized by the participation of IgE bound to a special receptor, designated FcRI. When an antigen or allergen binds to two adjacent IgE molecules, it induces aggregation of the IgE molecules and associated FcRI receptors. This event results in a signaling cascade that leads to the release of mediators and cytokines (Figure 10-12).

Figure 10-12 Allergy

Note that two subtypes of helper T cell, Th1 and TH2, trigger distinct responses when activated by specific antigens.

Complement system

The main function of the complement system is to enable the direct destruction of pathogens or target cells by phagocytes (macrophages and neutrophils) by a mechanism known as opsonization (Greek opsonein, to buy provisions) by producing proteolytic enzyme complexes (Figure 10-13).

Figure 10-13 Complement system

Complement provides a rapid and efficient mechanism for eliminating pathogens to prevent tissue injury or chronic infection. Host tissues have cell surface–anchored regulatory proteins, which can inhibit complement activation and prevent unintended damage.

The complement system consists of about 20 plasma proteins, synthesized mainly in the liver, that “complement,” or enhance, a tissue response to pathogens. Several components of this system are proenzymes converted to active enzymes.

Activation of the complement cascade can be triggered by (1) antibodies bound to a pathogen (classic pathway); (2) binding of mannose-binding lectin to a bacterial carbohydrate moiety (lectin pathway); and (3) by spontaneous activation of C3, a proenzyme of the complement sequence (alternative pathway).

The critical molecule of the complement cascade is C1, a hexamer, called C1q, with binding affinity to the Fc region of an immunoglobulin. C1q is also associated with two molecules, C1r and C1s.

When the globular domains of C1q bind to the Fc regions of immunoglobulins already bound to the surface of a pathogen, C1r is activated and converts C1s into a serine protease. Activation of C1s marks the initiation of the complement activation cascade.

The second step is the cleavage of complement protein C4 by C1s. Two fragments are produced: (1) the small fragment C4a is discarded; and (2) the large fragment C4b binds to the pathogen surface.

The third step occurs when complement protein C2 is cleaved by C1s into C2a (discarded) and C2b. C2b binds to the already bound C4b, forming the complex C4b-2b, also called C3 convertase, on the surface of a pathogen.

The fourth step takes place when complement protein C3 is cleaved by C3 convertase into C3a (discarded) and C3b. C3b binds to C3 convertase. The C4b-2b-3b complex, now designated C5 convertase, cleaves complement protein C5 into C5a (discarded) and C5b. C5b binds to C5 convertase.

The last steps consist in the binding of the opsonized pathogen to complement receptors on the surface of the phagocyte. Additional complement proteins are C6, C7, C8, and C9. C9 binds to the protein complex and forms the membrane attack complex (MAC), a cytolytic pore that directly initiates the cell destruction process.

The complement system has the following specific characteristics important to remember:

Structure of a lymph node

A lymph node is surrounded by a capsule, and the parenchyma is divided into a cortex and a medulla (Figure 10-14). The capsule consists of dense irregular connective tissue surrounded by adipose tissue. The capsule at the convex surface of the lymph node is pierced by numerous afferent lymphatic vessels. Afferent lymphatic vessels have valves to prevent the reflux of lymph entering a lymph node.

Figure 10-14 Lymph node

The cortex has two zones: the outer cortex and the inner or deep cortex. The outer cortex contains B cell–rich lymphoid follicles. The deep cortex houses CD4⁺ helper T cells and venules lined by high endothelial cells. The inner cortex is a zone in which mainly CD4⁺ helper T cells interact with B cells to induce their proliferation and differentiation when exposed to a specific lymph-borne antigen.

A lymphoid follicle (Figure 10-15) consists of a mantle (facing the cortex) and a germinal center containing mainly proliferating B cells or lymphoblasts, resident follicular dendritic cells (FDCs), migrating dendritic cells (see Box 10-F), macrophages, and supporting reticular cells, which produce reticular fibers (type III collagen). A primary lymphoid follicle lacks a mantle and germinal center. A secondary lymphoid follicle has a mantle and a germinal center. The mantle and germinal center develop in response to antigen stimulation.

Figure 10-15 Lymphatic follicle

FDCs are branched (hence the name dendritic) cells forming a network within the lymphoid follicle. In contrast to migrating dendritic cells, which derive from bone marrow and interact with T cells, resident FDCs do not derive from a bone marrow cell precursor. FDCs are observed at the edge of the germinal centers and interact with mature B cells. FDCs trap antigens on their surface for recognition by B cells. The interaction of mature B cells with FDCs rescues the B cell from apoptosis. Only B cells with low-affinity surface immunoglobulin are induced to apoptosis. Macrophages in the lymphoid follicle phagocytose apoptotic B cells.

Lymphatic sinuses are spaces lined by endothelial cells. They are located under the capsule (subcapsular sinus) and along trabeculae of connective tissue derived from the capsule and entering the cortex (paratrabecular sinus). Highly phagocytic macrophages are distributed along the subcapsular and paratrabecular sinuses to remove particulate matter present in the percolating lymph. Lymph entering the paratrabecular sinus through the subcapsular sinus percolates to the medullary sinuses and exits through a single efferent lymphatic vessel. Lymph in the subcapsular sinus can bypass the paratrabecular and medullary sinuses and exit through the efferent lymphatic vessel.

High endothelial venules (HEVs) (see Figure 10-14), located in the inner cortex, are the sites of entry of most B and T cells into the lymph node (by the lymphocyte homing mechanism).

The medulla is surrounded by the cortex, except at the region of the hilum (see Figure 10-14). The hilum is a concave surface of the lymph node where efferent lymphatic vessels and a single vein leave and an artery enters the lymph node.

The medulla contains two major components:

Clinical significance: Lymphadenitis and lymphomas

Lymph nodes constitute a defense site against lymph-borne microorganisms (bacteria, viruses, parasites) entering the node through afferent lymphatic vessels. This defense mechanism depends on the close interaction of B cells in the follicle nodules with CD4⁺ T cells in the inner cortex.

In Chapter 12, Cardiovascular System, we discuss that the interstitial fluid, representing plasma filtrate, is transported into blind sacs corresponding to lymphatic capillaries. This interstitial fluid—entering the lymphatic capillaries as lymph—flows into collecting lymphatic vessels becoming afferents to regional lymph nodes (see Box 10-G). Lymph nodes are linked in series by the lymphatic vessels in such a way that the efferent lymphatic vessel of a lymph node becomes the afferent lymphatic vessel of a downstream lymph node in the chain.

Soluble and particulate antigens drained with the interstitial fluid, as well as antigen-bearing dendritic cells in the skin (Langerhans cells; see Chapter 11, Integumentary System), enter the lymphatic vessels and are transported to lymph nodes. Antigen-bearing dendritic cells enter the CD4⁺ helper T cell–rich inner cortex. Soluble and particulate antigens are detected in the percolating lymph by resident macrophages and dendritic cells strategically located along the subcapsular and paratrabecular sinuses.

When the immune reaction is acute in response to locally drained bacteria (for example, infections of the teeth or tonsils), local lymph nodes enlarge and become painful because of the distention of the capsule by cellular proliferation and edema. This condition is known as acute lymphadenitis.

Lymphomas are tumors of the lymphoid tissue in the form of tissue masses. The designation lymphocytic leukemia is used for lymphoid tumors involving the bone marrow. Most of the lymphomas are of B cell origin (80%); the remainder are of T cell origin. Lymphomas include Hodgkin’s lymphoma (Figure 10-16) and non-Hodgkin’s lymphomas. They are clinically characterized by nontender enlargement of localized or generalized lymph nodes (nodal disease). The Hodgkin-Reed-Sternberg cell, found in classic Hodgkin’s lymphoma, is a large multinucleated or multilobulated tumor cell of B cell origin surrounded by T cells, eosinophils, plasma cells, and macrophages (mixed celullarity). Another group in the lymphoma category includes the plasma cell tumors, consisting of plasma cells, the terminally differentiated B cells. Plasma cell tumors (multiple myeloma) originate in bone marrow and cause bone destruction with pain due to fractures.

Figure 10-16 Hodgkin’s lymphoma

Development of the thymus

A brief review of the development of the thymus facilitates an understanding of the structure and function of this lymphoid organ. The mesenchyme from the pharyngeal arch gives rise to the capsule, trabeculae, and vessels of the thymus (Figure 10-17). The thymic epithelial rudiment attracts bone marrow–derived thymocyte precursors, dendritic cells, and macrophages required for normal thymic function.

Figure 10-17 Development of the thymus

During fetal life, the thymus contains lymphocytes derived from the liver. T cell progenitors formed in the bone marrow during hematopoiesis enter the thymus as immature thymocytes and mature to become immunocompetent T cells (predominantly CD4⁺ or CD8⁺), which are then carried by the blood into lymph nodes, spleen, and other lymphatic tissues (Figure 10-18).

Figure 10-18 Foxn 1 and Aire genes: two critical players in thymus development

The thymus in humans is fully developed before birth. The production of T cells is significant before puberty. After puberty, the thymus begins to involute and the production of T cells in the adult decreases. The progenies of T cells become established, and immunity is maintained without the need to produce new T cells.

A significant difference from the lymph node and the spleen is that the stroma of the thymus consists of thymic epithelial cells organized in a dispersed network to allow for intimate contact with developing thymocytes, the T cell precursors arriving from bone marrow. In contrast to the thymus, the stroma of the lymph node and the spleen contains reticular cells and reticular fibers but not epithelial cells.

There are two important aspects during the development of the thymus with relevance to tolerance for self-antigens and autoimmune diseases:

Thymic cortical epithelial cells are involved in the clonal selection of T cells. Medullary epithelial cells are involved in the clonal deletion of potentially au to reactive T cells.

Clinical significance: DiGeorge syndrome

DiGeorge syndrome is an inherited immunodeficiency disease in which thymic epithelial cells fail to develop, and the thymus and parathyroid glands are rudimentary or absent. The cause is a deletion of genes from chromosome 22. Patients have congenital heart defects, hypoparathyroidism, cleft palate, behavioral and psychiatric problems, and increased susceptibility to infections.

When thymic epithelial cells fail to organize the thymus, bone marrow–derived T cell precursors cannot differentiate. Thymic epithelial cells express MHC class I and class II molecules on their surface, and these molecules are required for the clonal selection of T cells. Their absence in DiGeorge syndrome affects the production of functional T cells. The development of B cells is not affected in DiGeorge syndrome.

The nude (athymic) mouse—a strain of mice lacking the expression of the transcription factor Foxn1 necessary for the differentiation of thymic epithelial cells and epidermal cells involved in the normal development of the thymus and hair follicles—is the equivalent of DiGeorge syndrome. This syndrome and the nude mouse demonstrate the role of the thymus in cell-mediated immunity.

Structure of the thymus

The thymus consists of two lobes subdivided into incomplete lobules, each separated into an outer cortex and a central medulla (Figure 10-19). A connective tissue capsule with small arterioles surrounds the lobules. The capsule projects septa or trabeculae into the organ. Blood vessels (trabecular arterioles and venules) within the trabeculae gain access to the thymic epithelial stroma (Figure 10-20).

Figure 10-19 Thymus

Figure 10-20 Thymus

The cortex contains thymic epithelial cells forming a three-dimensional network supported by collagen fibers. Thymic epithelial cells, linked to each other by desmosomes, surround capillaries. A dual basal lamina is present in the space between epithelial cells and capillaries. One basal lamina is produced by the cortical thymic epithelial cells. The other basal lamina is of endothelial cell origin. Macrophages may also be present in proximity (Figure 10-21).

Figure 10-21 The blood-thymus barrier

Thymic cortical epithelial cells, basal laminae, and endothelial cells form the functional blood-thymus barrier (see Figure 10-21). Macrophages adjacent to the capillaries ensure that antigens escaping from blood vessels into the thymus do not react with developing T cells in the cortex, thus preventing the risk of an autoimmune reaction.

Most T cell development takes place in the cortex. In the outer area of the cortex adjacent to the capsule, double-negative thymocytes proliferate and begin the process of gene rearrangement leading to the expression of the pre-TCR along with coreceptors CD4 and CD8 (see Figures 10-7 and 10-20).

Deep in the cortex, maturing T cells are double-positive (CD4⁺ and CD8⁺) and become receptive to peptide-MHC complexes. The process of positive selection of T cells now starts in the presence of thymic cortical epithelial cells expressing both MHC class I and class II molecules on their surface. MHC class II molecules are required for the development of CD4⁺ T cells; MHC class I molecules are necessary for the development of CD8⁺ T cells.

T cells that recognize self-MHC molecules but not self-antigens are allowed to mature by positive selection. T cells unable to recognize MHC molecules are not selected and are eliminated by programmed cell death, or apoptosis (see Chapter 3, Cell Signaling).

T cells that recognize both self-MHC and self-antigens—produced by thymic medullary epithelial cells under the regulation of the aire gene—are eliminated by negative selection (clonal deletion), a task carried out by dendritic cells and macrophages.

About 95% of the developing T cells die within the cortex of the thymus without ever maturing. Double-positive T cells undergo apoptosis within three days in the absence of a surviving signal; positive signals enable the progression to single-positive. Within 1 week, single-positive cells will be eliminated by apoptosis unless they receive a positive signal for survival and export to the periphery.

The medulla of one lobule is continuous with the medulla of an adjacent lobule. The medulla displays few almost mature T cells (single-positive) migrating from the cortex. Maturation of T cells is completed in the medulla, and functional T cells enter postcapillary venules in the corticomedullary junction to exit the thymus toward the peripheral lymphoid organs (see Figure 10-20).

Thymic epithelial cells populate the medulla, many of them forming Hassall’s corpuscles. Hassall’s corpuscles are sites where thymic epithelial cells accumulate and form onion-like layers (see Figure 10-20). Hassall’s corpuscles produce cytokine thymic stromal lymphopoietin, which stimulates thymic dendritic cells to complete the maturation of single-positive T cells to optimize negative selection and ensure tolerance.

Note that the blood-thymus barrier is not present in the medulla and that Hassall’s corpuscles can be seen only in the medulla.

Vascularization of the spleen

The spleen is covered by a capsule consisting of dense, irregular connective tissue with elastic and smooth muscle fibers (it varies with the species).

Trabeculae derived from the capsule carry blood vessels (trabecular arteries and veins) and nerves to and from the splenic red pulp (Figure 10-22). A brief review of vascularization of the spleen, which is similar to that of many organs with a significant blood supply such as the kidneys and lungs, provides a useful background for understanding the function and structure of this organ.

The splenic artery enters the hilum, giving rise to trabecular arteries, which are distributed to the splenic pulp along the connective tissue trabeculae. As an artery leaves the trabecula, it becomes invested by a sheath of T cells forming a peri-arteriolar lymphoid sheath (PALS) and penetrates a lymphatic nodule (the white pulp). The blood vessel is designated the central artery (also called the follicular arteriole, because of the nodular or follicular arrangement of the white pulp).

The central artery leaves the white pulp to become the penicillar artery. Penicillar arteries end as macrophage-sheathed capillaries.

Terminal capillaries either drain directly into splenic sinusoids (closed circulation) or terminate as open-ended vessels within the red pulp (open circulation). Splenic sinusoids are drained by pulp veins, to trabecular veins, to splenic veins.

White pulp

This component of the spleen is essentially nodular lymphoid tissue that contains a central artery or arteriole.

The white pulp includes (see Figure 10-19) (1) the central artery or arteriole surrounded by a sheath of T cells (PALS) and (2) the lymphatic nodules, consisting of B cells. Antigen-presenting cells and macrophages are also present in the white pulp.

There is a marginal sinus zone between the red and white pulps that receives radial arterioles from the central artery or arteriole (Figures 10-23 and 10-24). This marginal sinus zone drains into small sinusoids located on the outer portion of the marginal zone. At the marginal zone, blood contacts the splenic parenchyma, which contains phagocytic macrophages and antigen-presenting cells. T and B cells enter the spleen and become segregated in their specific splenic location.

Figure 10-23 Vascularization of the spleen

Figure 10-24 White pulp

Red pulp

The red pulp contains an interconnected network of splenic sinusoids lined by elongated endothelial cells separated by narrow slits. Splenic cords, also known as the cords of Billroth, separate splenic sinusoids (see Figure 10-23; Figure 10-25).

Figure 10-25 Red pulp

The splenic cords contain plasma cells, macrophages, and blood cells, all supported by a stroma of reticular cells and fibers. Cytoplasmic processes of macrophages lie adjacent to the sinusoids and may project into the lumen of the sinusoids through the interendothelial cell slits to sample particulate material.

Splenic sinusoids are discontinuous vascular spaces lined by rib-shaped endothelial cells oriented in parallel along the long axis of the sinusoids (see Figure 10-25). Junctional complexes can be found at the tapering ends of the endothelial cells.

Each splenic sinusoid is covered by a discontinuous basal lamina oriented like barrel hoops around the endothelial cells (see Figure 10-25). Adjacent hoops are cross-linked by strands of basal lamina material. In addition, a network of loose reticular fibers also encircles the splenic sinusoids. Consequently, blood cells have an unobstructed access to the sinusoids through the narrow slits between the fusiform endothelial cells and the loose basal lamina–reticular fiber network.

Two types of blood circulations have been described in the red pulp (see Figure 10-23): (1) a closed circulation, in which arterial vessels connect directly to splenic sinusoids; and (2) an open circulation, characterized by blood vessels opening directly into the red pulp spaces, with the blood flowing through these spaces and then entering through the interendothelial cell slits of the splenic sinusoids.

Clinical significance: Sickle cell anemia

Sickle cell anemia is discussed briefly in Chapter 6, Blood and Hematopoiesis, within the context of the structure of the red blood cell. Here, we focus on the fate of irreversibly sickled red blood cells when they travel through the narrow passages of the red pulp. We also consider the function of macrophages associated with the splenic sinuses in the disposal of destroyed sickle cells.

When the oxygen tension decreases, sickle cells show preferential adhesion to postcapillary venules followed by trapping of irreversibly sickled cells and retrograde obstruction of the blood vessel (Figure 10-26).

Figure 10-26 Sickle cell anemia and spleen

An increased destruction of sickle cells leads to anemia and to an increase in the formation of bilirubin from the released hemoglobin (chronic hyperbilirubinemia). The occlusion of splenic sinuses by sickle cells is associated with splenomegaly (enlargement of the spleen), disrupted bacterial clearance function of the spleen in cases of bacteremia, and painful crises in the affected region. Similar vascular occlusions can also occur in the kidneys, liver, bones, and retinas.

Asplenia (lack of development of the spleen) is a clear demonstration of the function of the spleen in bacteremia. Adults who already have antibodies to microorganisms are less prone to bacteremia. Children who have not developed antibodies are more vulnerable. To a certain extent, the Kupffer cells of the liver sinusoids complement the role of the white pulp in the detection and removal of bacteria circulating in blood.

The spleen can be removed surgically (splenectomy) in cases of traumatic rupture, as part of the treatment of autoimmune diseases, or because of a malignant tumor of the spleen (lymphomas).

Clinical significance: Adoptive cellular immunotherapy

Strategies are being developed to enhance the immune response against tumor cells expressing tumor-related antigens. One strategy, called adoptive cellular immunotherapy, consists of the transfer of activated immune cells with antitumoral activity into a tumor-bearing host.

Two procedures have been used (Figure 10-27):

Figure 10-27 Adoptive cellular immunotherapy

Concept mapping

Immune-Lymphatic System