MACROPHAGES AND DENDRITIC CELLS

Macrophages and dendritic cells are important accessory cells in immune defence. Dendritic cells (e.g. Langerhans cells of the skin) are the professional antigen-presenting cells (APC) of the immune system, taking up foreign material by endocytosis and macropinocytosis, and they are uniquely capable of efficiently activating naïve as well as mature T lymphocytes. Macrophages are able to process and present antigen to lymphocytes, but are less effective than dendritic cells. In addition macrophages play an important role in the effector arm of the immune response, clearing the infectious agent by phagocytosis. Thus, monocytes and macrophages ingest particulate material, including potential pathogens, and they kill organisms intracellularly, like neutrophils. The third major cell type involved in antigen presentation and T cell activation is the B lymphocyte. Follicular dendritic cells of lymph nodes, MALT and the spleen form a distinct group which, although not classic APCs, participate as accessory cells of the immune system by presenting non-processed antigen to B lymphocytes.
APCs endocytose antigen, digest it intracellularly, mostly to peptide fragments, and present the fragments on their surfaces, in conjunction with MHC class II molecules. Some endocytosed material is presented on Class I molecules, which are expressed by all nucleated cells. Class II molecules are normally found only on APC, although there is evidence that certain other cells can express class II molecules in pathological situations.
Recognition of foreign antigen is controlled by a variety of APC cell surface receptors. In addition to Fc and complement receptors which mediate uptake of opsonized material, pathogen-derived molecules are directly recognized via pattern recognition receptors (PRR) such as Toll-like receptors (TLR) and scavenger receptors.

Macrophages

The mononuclear phagocyte system consists of the blood monocytes, from which the other types are derived, and various tissue macrophages, some of which have tissue-specific names. Certain dendritic cells are sometimes included in the mononuclear phagocyte system; however, although they share a common lineage ancestor, they appear to form a discrete branch of the family tree. Most monocytes and macrophages express class II MHC molecules (e.g. HLA-DR).
Macrophages are very variable in size (generally 15-25 μm) and are found in many tissues of the body. They are migrant cells in all general connective tissues, the alveolar macrophages in the lung (Fig. 5.18; Chapter 62), Kupffer cells in liver sinusoids (p. 1222), in bone marrow (p. 77) and in all lymphoid tissues (p. 74). Macrophages often aggregate in subserous connective tissue of the pleura and peritoneum, where they are visible as milky spots near small lymphatic trunks. They cluster around the terminations of small (penicillar) arterioles in the spleen and, are distributed more diffusely, throughout the splenic cords (p. 1242).
Osteoclasts (up to 100 μm) in bone (p. 92) are closely related to macrophages. However they are syncytial cells derived from the fusion of up to 30 progenitor monocytes in bone tissue, where they differentiate further (p. 92). Microglia of the central nervous system (CNS) are thought to be monocytic in origin: they migrate into the CNS during its development (p. 55). They differ from macrophages in that normally they are quiescent cells in which MHC class II expression is downregulated, and they display little phagocytic activity.
 Alveolar macrophages (dust cells, arrow) with ingested carbon particles, in a section through pulmonary alveoli
    Figure 5.18 Alveolar macrophages (dust cells, arrow) with ingested carbon particles, in a section through pulmonary alveoli. (Photograph by Sarah-Jane Smith.)
Macrophages vary in structure depending on their location in the body. All have a moderately basophilic cytoplasm containing some rough and smooth endoplasmic reticulum, an active Golgi complex and a large, euchromatic and somewhat irregular nucleus. These features are consistent with an active metabolism: synthesis of lysosomal enzymes continues in mature cells. All macrophages have irregular surfaces studded with filopodia and they contain varying numbers of endocytic vesicles, larger vacuoles and lysosomes. Some macrophages are highly motile, whereas others tend to remain attached and sedentary, e.g. in hepatic and lymphoid sinuses. Within connective tissues, macrophages may fuse to form large syncytia (giant cells) around particles which are too large to be phagocytosed, or when stimulated by the presence of infectious organisms, e.g. tubercle bacilli.
When blood-borne monocytes enter the tissues through the endothelial walls of capillaries and venules, they can undergo a limited number of rounds of mitosis as tissue macrophages before they die and are replaced from the bone marrow, typically after several weeks. There is some evidence that alveolar macrophages of the lung are able to undergo many more mitoses than other macrophages.

PHAGOCYTOSIS

The uptake of particulate material and microorganisms is carried out by macrophages in many tissues and organs. When present in general connective tissue, they ingest and kill invading microorganisms and remove debris produced as a consequence of tissue damage. They engulf apoptotic cells in all situations. In the lung, alveolar macrophages constantly patrol the respiratory surfaces, to which they migrate from pulmonary connective tissue. They engulf inhaled particles including bacteria, surfactant and debris and many enter the sputum (hence their alternative names, dust cells or, in cardiac disease, heart failure cells, which are full of extravasated erythrocytes). They perform similar scavenger functions in the pleural and peritoneal cavities. In lymph nodes, macrophages line the walls of sinuses and remove particulate matter from lymph as it percolates through them. In the spleen and liver, macrophages are involved in particle removal and in the detection and destruction of aged or damaged erythrocytes. They begin the degradation of haemoglobin for recycling iron and amino acids.
Macrophages bear surface receptors for the Fc portions of antibodies and for the C3 component of complement. Phagocytic activity is greatly increased when the target has been coated in antibody (opsonized) or complement, or both. Once phagocytosis has occurred, the vacuole bearing the ingested particle fuses with endosomal vesicles which contain a wide range of lysosomal enzymes, including many hydrolases, and oxidative systems capable of rapid bacteriocidal action. These activities are much enhanced when macrophages are stimulated (activated macrophages) by cytokines, e.g. interferon (IFN)-γ, which are secreted by other cells of the immune system, especially T lymphocytes.
Close antibody-mediated binding may initiate the release of lysosomal enzymes onto the surfaces of the cellular targets to which the macrophages bind. This process of antibody-dependent cell-mediated cytotoxicity (ADCC), is also used by other cells, including NK cells, neutrophils and eosinophils, particularly if the targets are too large to be phagocytosed (e.g. nematode worm parasites).
SECRETORY ACTIVITIES
Activated macrophages can synthesize and secrete various bioactive substances, e.g. IL-1, which stimulate the proliferation and maturation of other lymphocytes, greatly amplifying the reaction of the immune system to foreign antigens. They also synthesize tumour necrosis factor (TNF)-α, which is able to kill small numbers of neoplastic cells. TNF-α depresses the anabolic activities of many cells in the body, and may be a major factor mediating cachexia (wasting) which typically accompanies more advanced cancers. Other macrophage products include plasminogen activator, which promotes clot removal; various lysosomal enzymes; several complement and clotting factors; and lysozyme (an antibacterial protein). In pathogenesis, these substances may be released inappropriately and damage healthy tissues, e.g. in rheumatoid arthritis and various other inflammatory conditions.
Dendritic cells (Fig. 5.19)
 
Dendritic cells in the skin and lymphoid tissues
    Figure 5.19 Dendritic cells in the skin and lymphoid tissues. In the skin, these are known as Langerhans cells. They migrate (as veiled cells) with processed antigen to the paracortex of draining lymph nodes, where, as interdigitating dendritic cells, they make contact with and present antigen to T cells. Follicular dendritic cells in germinal centres of lymph nodes expose B cells to antigen. (By permission from Roitt I, Brostoff J, Male D 2001 Immunology, 6th edn. London: Mosby.)
There are two distinct groups of dendritic cells, myeloid and lymphoid. The groups are morphologically similar but have different developmental origins and functions. They are derived from haemopoietic stem cells, and a major subset are of myeloid lineage, closely related to monocytes and responsive to bacterial antigens. The other group may share lineage with the lymphocytic line and respond primarily to viral antigens. The myeloid dendritic cells are professional antigen-presenting cells (APC), which are able to process and present antigen to T lymphocytes, including naïve T cells. They are present as immature dendritic cells in the epidermis of the skin and other stratified squamous epithelia, e.g. the oral mucosa (Langerhans cells), and in the dermis and most other tissues (interstitial dendritic cells), where they are concerned with immune surveillance. Immature dendritic cells have an antigen-capturing function. They express pattern recognition receptors (e.g. Toll-like receptors) on their surface. Binding of bacterial molecules (e.g. carbohydrate or DNA) to these receptors stimulates the dendritic cells to migrate via the lymphatics to nearby secondary lymphoid tissues where they mature and acquire an antigen-presenting function. Mature dendritic cells are known as veiled cells when in the afferent lymphatics and the subcapsular sinuses of lymph nodes (p. 75), and as interdigitating dendritic cells once they are within the lymphoid tissue proper. Their function within the secondary lymphoid tissue is to present their processed antigen to T lymphocytes, and thus to initiate and stimulate the immune response.
LANGERHANS CELLS
Langhans cells are one of the most well-studied type of immature dendritic cell. They are present throughout the epidermis, but are most clearly identifiable in the stratum spinosum (p. 157). They have an irregular nucleus and a clear cytoplasm, and contain characteristic elongated membranous vesicles (Birbeck granules). Langerhans cells endocytose and process antigens, undergoing a process of maturation from antigen-capturing to antigen-presenting cells which express high levels of MHC class I and II molecules, co-stimulatory proteins and adhesion molecules. They migrate to lymph nodes to activate T lymphocytes.

INTERDIGITATING DENDRITIC CELLS

Immature dendritic cells are found all over the body and function in antigen-processing and immune surveillance. Mature dendritic cells are present in T cell-rich areas of secondary lymphoid tissue (paracortical areas of lymph nodes, interfollicular areas of MALT, periarteriolar sheaths of splenic white pulp), where they are frequently referred to as interdigitating dendritic cells. Within the secondary lymphoid tissues, they are involved in the presentation of antigen to T lymphocytes in the context of either MHC class I (CD8 T cells) or MHC class II (CD4 T cells). Binding is accompanied by co-stimulatory protein recognition by T cell surface molecules, and by adhesive interactions between the two cell types. Appropriate T cells are thus activated to proliferate and are primed for carrying out their immunological functions. Only T cells which possess receptors corresponding to the specific antigen presented to them in combination with MHC class I or II molecules can be triggered in this way. These processes are known as class I and class II MHC restriction, respectively. Naïve T cells can only respond to antigen presented by dendritic cells. Once primed, T cells can be stimulated by any APC, including macrophages. Mature dendritic cells not only present antigen to activate T lymphocytes, but also secrete cytokines which direct the nature of the T cell response (e.g. Th1 versus Th2).
FOLLICULAR DENDRITIC CELLS (Fig. 5.20)
Follicular dendritic cells, FDCs, are a non-migratory population of cells found in the follicles of secondary lymphoid tissues, where they attract and interact with B cells. Unlike other dendritic cells, FDCs are not haemopoietic in origin, but are probably derived from the stromal cells of lymphoid tissues. They are unable to endocytose and process antigen, and they lack MHC class II molecules. However, Fc receptors and complement receptors CD21 and CD35 on FDCs allow the cells to bind immune complexes (iccosomes) to their surface for subsequent presentation, as unprocessed antigen, to germinal centre B cells. Complex interactions between B cells, CD4 helper T cells and FDCs in the germinal centres are important in the selection of high affinity B cells and their maturation to either plasma cells or memory B lymphocytes.
  Follicular dendritic cells in a germinal centre of the palatine tonsil
    Figure 5.20 Follicular dendritic cells in a germinal centre of the palatine tonsil (immunoperoxidase labelled). (By kind permission from Dr Marta Perry, UMDS, London.)