CHAPTER 73 MORPHOLOGY OF MONOCYTES AND MACROPHAGES
CHAPTER 73 MORPHOLOGY OF MONOCYTES AND MACROPHAGES
STEVEN D. DOUGLAS
Mononuclear Phagocyte System
Morphology of Monocyte Precursors
Morphology of Monocytes
Scanning Electron Microscopy
Transmission Electron Microscopy (TEM)
Histochemistry of Monocytes
Morphology of Macrophages
Light and Phase-Contrast Microscopy
Monocyte-Macrophage Surface Receptors
Receptors for Peptides and Small Molecules
Monocyte-Macrophage Surface Antigens
HLA Class II Receptors
CD14 and CD68 Receptors
The monocyte is a spherical cell with prominent surface ruffles and blebs when examined by scanning electron microscopy. When reconstructed from sections examined under transmission electron microscopy, the monocyte has a reniform nucleus containing a small nucleolus. The cytoplasm has many mitochondria, microtubules, and microfilaments. The Golgi apparatus is well developed and has neighboring centrioles. Numerous microvilli and microcytotic vesicles are evident at or near the cell surface. The cytoplasm contains scattered granules, akin to lysosomes. The granule contents share features with the primary granules of neutrophils, although, in contrast to the neutrophil, the monocyte granule is characterized by fluoride-inhibitable esterases. As the monocyte enters the tissue and differentiates into a macrophage, there is an increase in cell volume and number of cytoplasmic granules. The cell shape varies depending on the tissue type in which the macrophage resides (e.g., lung, liver, spleen, brain, etc.). A characteristic feature of macrophages is their prominent electron-dense membrane-bound lysosomes that can be seen fusing with phagosomes to form secondary lysosomes. The latter contain ingested cellular and noncellular material in stages of degradation. A broad range of surface receptors for many ligands, including the Fc portion of immunoglobulin, complement proteins, cytokines, chemokines, lipoproteins, and others are on the cell surface. Macrophages differ in appearance, biochemistry, and function based on the environment in which they mature from monocytes. These differences are exemplified by the diversity among dendritic cells of lymph nodes, histiocytes of connective tissue, osteoclasts of bone, Kupffer cells of liver, microglia of the central nervous system, and macrophages of the serosal surfaces, each fashioned to meet the local needs of the mononuclear phagocyte system.
Acronyms and abbreviations that appear in this chapter include: ADCC, antibody-dependent cellular cytotoxicity; CR1, complement receptor 1; CR3, complement receptor 3; FcR, Fc receptors; GM-CSF, granulocyte-monocyte colony stimulating factor; HIV-1, human immunodeficiency virus; HLA, human leukocyte antigens; IL-4, interleukin-4; IMP, intramembrane particles; LFA-1, lymphocyte function-associated antigen; LPS, lipopolysaccharide; MHC, major histocompatibility complex; PAS, periodic acid–Schiff; TEM, transmission electron microscopy.
MONONUCLEAR PHAGOCYTE SYSTEM
Modern study of mammalian phagocytes began with Metchnikoff in the nineteenth century. An understanding of the ontogeny, kinetics, and function of phagocytic cells in animals has led to the concept of the mononuclear phagocyte system.1,2 The system consists of marrow monoblasts and promonocytes, blood monocytes, and both free and fixed-tissue macrophages. Vascular endothelium, reticular cells, and dendritic cells of lymphoid germinal centers are not usually included in the mononuclear phagocyte system, although the now-obsolete term reticuloendothelial system3 denoted these cells as playing some complementary part with mononuclear phagocytes. Studies indicate that monocytes can differentiate into dendritic cells in vitro.4 Monocytes and macrophages comprise the functional system formerly thought to be the reticuloendothelial system. Tissue macrophages share many functional characteristics such as phagocytic and microbial killing capabilities and adherence to glass or plastic surfaces in vitro. Kinetic studies indicate that macrophages are transformed monocytes and that the monocyte is derived from the differentiation of the hematopoietic stem cell.
The blood monocyte is a medium to large motile cell that can marginate along vessel walls and has a propensity for adherence to surfaces. Monocytes respond to inflammation and chemotactic stimuli by active diapedesis across vessel walls into inflammatory foci, where they can mature into macrophages, with greater phagocytic capacity and increased content of hydrolytic enzymes. Free macrophages also are present in mammary glands, alveolar spaces, pleura, peritoneum, and synovia. The somewhat less motile fixed-tissue macrophages are found in different tissues and serous cavities (Table 73-1). The functions of mononuclear phagocytes include the following: phagocytosis and digestion of microorganisms, particulate material, or tissue debris; secretion of chemical mediators and regulators of the inflammatory response; interaction with antigen and lymphocytes in the generation of the immune response; cytotoxicity, such as killing of some tumor cells; and other functions specific for macrophages of particular tissues.
TABLE 73-1 DISTRIBUTION OF MONONUCLEAR PHAGOCYTE
The development of techniques to isolate monocytes from blood of adult subjects has led to the discovery that monocytes are heterogeneous with regard to cell volumes. Isolation of purified monocytes by adherence to glass substrates or to gelatin-coated flasks or by centrifugal elutriation reveals distinct populations of monocytes.1,2 In addition to the usual 12- to 15-µm diameter monocyte, a somewhat smaller cell has been identified that is less active than its larger, more mature counterpart. This cell is referred to as a small immature monocyte, yet its functional significance is not clear.
Monocytes continuously emigrate from the blood into peripheral tissue, with a half-life in the blood of about 1 day in mice.5 Non-dividing monocytes can be induced to differentiate into dendritic-like cells in vitro. However, this requires culture of the cells for 7 to 10 days with exogenous cytokines, typically interleukin-4 (IL-4) and granulocyte-monocyte colony stimulating factor (GM-CSF).6 In the presence of endothelial cells grown on an extracellular matrix, monocytes differentiate along two distinct pathways, toward dendritic cells or macrophages. Monocytes that migrate across endothelium in an abluminal to luminal direction differentiate into dendritic cells. In contrast, monocytes that remain in the subendothelial matrix differentiate into macrophages.
MORPHOLOGY OF MONOCYTE PRECURSORS
The monoblasts and promonocytes are the precursors of the monocytes, bearing finely dispersed nuclear chromatin and nucleoli when observed in the stained film of the blood or marrow. The monoblast is a very low prevalence marrow cell, indistinguishable by light microscopy from the myeloblast.
In animal studies, a small percentage of marrow cells are phagocytic, synthesize DNA, adhere to glass surfaces, and contain nonspecific esterases.7 These have been referred to as promonocytes and considered to be intermediate between monoblasts and the monocytes of the blood.7 Cytochemical studies identify the promonocyte in normal human marrow. These cells have deeply indented and irregularly shaped nuclei and bundled and scattered single filaments in the cytoplasm; these morphologic features distinguish the promonocyte from the progranulocyte.8,9 Peroxidase is present throughout the cell secretory apparatus in all cisternae of the rough-surfaced endoplasmic reticulum, the Golgi complex, associated vesicles, and all immature and mature granules. Cytochemical reaction products for acid phosphatase and arylsulfatase are also deposited throughout the secretory apparatus of the promonocyte.
MORPHOLOGY OF MONOCYTES
The morphology of the monocytes has been investigated by light and phase-contrast optics,10 scanning and transmission electron microscopy, and freeze-fracture and freeze-etch procedures.11
In the stained blood film, the monocyte has a diameter of 12 to 15 µm. Its nucleus occupies about half the area of the cell and is usually eccentrically placed. The nucleus is most often reniform but may be round or irregular. It contains a characteristic chromatin net with fine strands bridging small chromatin clumps. Chromatin aggregates are arranged along the internal aspect of the nuclear membrane. The nuclear chromatin pattern has been called “raked” because of its fine-stranded appearance. The cytoplasm is spread out, stains grayish-blue with Wright stain, and contains a variable number of fine, pink-purple granules, which at times are sufficiently numerous to give the entire cytoplasm a pink hue. Clear cytoplasmic vacuoles and a variable number of larger azurophilic granulations are often encountered in these cells.
The monocyte nucleus has a distinct chromatin pattern on a cloudy background when examined by phase-contrast microscope. The cytoplasm is clear gray. Mitochondria are extremely fine and on occasion form a small, juxtanuclear rosette surrounding the centrosome. The phase-dense cytoplasmic granules, varying in number, are generally at the limit of resolution of light microscopy and appear as fine intracytoplasmic dust. Monocytes contain several types of cytoplasmic vacuoles. Characteristic of the monocyte are its reniform nucleus with a juxtanuclear depression filled by a centrosome and its active undulating movement similar to that of other leukocytes. The locomotion of the monocyte has the same pattern of undulating cytoplasmic veils seen in macrophages. The monocyte generally assumes a triangular shape as it moves, with one point trailing behind and the other two points advancing before the cell. Blood monocytes undergo adherence and cytoplasmic spreading following attachment to glass surfaces.12 The extent of spreading is increased in the presence of antigen-antibody complexes, certain divalent metals, and proteolytic enzymes.12,13 The spread form of the monocyte reveals that the nucleus and granules are located centrally and the abundant hyaloplasm is in the periphery of the cell, terminating in a fringed border that displays undulating movement. The small monocyte may be difficult to distinguish from the large lymphocyte when examined by phase-contrast microscopy.
A striking feature on phase-contrast microscopy is the ruffled plasma membrane that forms prominent phase-dense folds at the cell surface and edges. Some cells have a dense thickening at the edge of the cytoplasm, with microextensions on the thickened edge.
SCANNING ELECTRON MICROSCOPY
The monocyte surface has very prominent ruffles and small surface blebs.14,15 Extensive ruffling on the monocyte plasma membrane is of functional significance. The monocyte is both motile and phagocytic, and these functions require physical contact with particles or cell surfaces. Reduction in the radius of curvature of the cell surface by formation of ruffles or microvilli may reduce repulsive forces when surface negative-charge groups on the cell approach and contact a negatively charged substratum or cell. Also, redundancy of the cell membrane may provide reserve membrane required for locomotion and for phagocytosis.
TRANSMISSION ELECTRON MICROSCOPY (TEM)
The nucleus of the monocyte contains one or two small nucleoli surrounded by nucleolar-associated chromatin (Fig. 73-1).16 The cytoplasm contains a relatively small quantity of endoplasmic reticulum and a variable quantity of ribosomes and polysomes. The mitochondria are numerous, small, and elongated. The Golgi complex is well developed and is situated about the centrosome within the nuclear indentation. Centrioles and filamentous centriolar satellites are often visualized in this region. Microtubules are numerous, and microfibrils are found in bundles surrounding the nucleus. In cultured macrophages collections of microfilaments are present underneath the plasma membrane near sites of cell attachment either to a substratum or to phagocytizable particles.17 The cell surface is characterized by numerous microvilli and vesicles of micropinocytosis. The cytoplasmic granules resemble the small granules found in the granulocytic series, measuring approximately 0.05 to 0.2 µm in diameter. They are dense and homogeneous and are surrounded by a limiting membrane. These granules, as with the lysosomal granules of other leukocytes, are packaged by the Golgi apparatus after their enzymatic content has been produced by the ribosomal complex of the cell.7,8,18 These cytoplasmic granules contain acid phosphatase and arylsulfatase and are therefore primary lysosomes. After endocytosis, lysosomes fuse with the phagosome, forming secondary lysosomes. Some monocyte granules stain positive for peroxidase, whereas others are peroxidase-negative.7,8
FIGURE 73-1 Transmission electron micrograph of a monocyte. The eccentric reniform nucleus has a thinly dispersed chromatin pattern. The Golgi complex (G) is in a juxtanuclear position. Small electron-dense granules can be seen evolving in the Golgi complex. A small amount of rough endoplasmic reticulum (er) and polyribosomes (r) are present, particularly about the cell periphery. Mitochondria (m) are concentrated in the region of the Golgi apparatus and are scattered in the cell periphery as well. Lysosomes (L) are small, electron-dense granules surrounded by a limiting membrane. The irregular ruffled cell margin is apparent with numerous microprojections. ×24,000.
In this technique a cell suspension is frozen, placed in a high-vacuum chamber, and struck with a blunt edge so as to produce a fracture that is propagated through the frozen specimen. The utility of the procedure comes from the remarkable finding that when the fracture encounters a cell, it tends to propagate along the interior of the plasma membrane and thus split the lipid bilayer in half. After fracture, the specimen is coated with platinum, which is electron-dense when viewed with TEM. All cell types examined thus far by the freeze-fracture technique reveal intramembrane particles (IMP) as the predominant feature of the topography of the interior of the bilayer. Studies of the erythrocyte have shown that at least some particles may contain intercalated membrane proteins, and this has been assumed to be the case with nucleated cells as well. The distribution of IMP is dramatically altered in a number of cell systems by physiologic stimuli, for example, hormonal stimulation.
Profound changes in the distribution of IMP on mononuclear phagocytes occur following binding of antibody-coated erythrocytes.11 Since redistribution of IMP also occurs in some nonphagocyte Fc receptor-bearing cells11 and after exposure to aggregated IgG, this alteration in IMP presumably reflects interaction with the Fc receptor. Freeze-etch electron micrographs of the monocyte show nuclear pores traversing both lamellae of the nuclear membrane and contours of cytoplasmic lysosomes and mitochondria (Fig. 73-2).
FIGURE 73-2 Freeze-etch electron micrograph of a monocyte. Fracture plane displays the large nucleus (N), with multiple nuclear pores (np) and the two lamellae of the fractured nuclear membrane (nm) evident in some regions. Membrane and cleaved surfaces of mitochondria (m) and lysosomal granules (L) can also be identified in the cytoplasm.
HISTOCHEMISTRY OF MONOCYTES
Hydrolytic enzyme contents of monocytes, neutrophils, and lymphocytes are compared in Table 73-2. Monocytes also give a weak but positive periodic acid–Schiff (PAS) reaction (for polysaccharides) and Sudan black B reaction (for lipids).
TABLE 73-2 CYTOCHEMICAL REACTIONS OF LEUKOCYTE ENZYMES
Nonspecific esterase19,20 and 21 is frequently used as a marker for monocytes. Monocyte esterases are inhibited by sodium fluoride, whereas the esterases of the granulocytic series are not. The nonspecific esterase reaction is positive in promyelocytes and myelocytes, and therefore analysis of fluoride inhibition is necessary to distinguish marrow monocytes from early myelocytes. Monocyte granules, although heterogeneous in size (0.3 to 0.6 µm), are not separable into populations by routine electron microscopic criteria (except in the rat).22 Identification of monocyte granule populations has depended on subcellular localization of monocyte enzymes by electron microscopic cytochemistry.8 Human marrow promonocytes and blood monocytes contain granules that comprise two functionally distinct populations.8,9 One population contains the enzymes acid phosphatase, arylsulfatase, and, in the human (but not in the rabbit), also peroxidase; these granules are therefore modified primary lysosomes and are analogous to the azurophil granules of the neutrophil. The monocyte azurophil granule population is heterogeneous in cytochemical reactivity for peroxidase, acid phosphatase, and arylsulfatase.23,24 Moreover, primary granules that are morphologically identical with other vesicles may be identified as lysosomes cytochemically. The content of the other population of monocyte granules is unknown; however, they lack alkaline phosphatase23 and hence are not strictly analogous to the specific granules of neutrophils. The lysosomes have a digestive function, whereas the function of the second population is unknown.
About 10 percent of granules in normal human blood monocytes stain with reagents that identify complex acid carbohydrates, or “acid mucosubstances.”25 These substances are found in leukemic monocyte granules as well as in granules of normal neutrophils, and their function is unknown.
The classic studies of Lewis and Lewis in 1926,26 Maximow in 1932,27 and Ebert and Florey in 193928 have shown that monocytes transform into macrophages and multinucleated giant cells in vitro.
These studies have been reproduced utilizing purified populations of monocytes, and the alterations of ultrastructure during the transformation into macrophages, epithelioid cells, and giant cells have been described.16 As the monocyte differentiates into the macrophage, the cell enlarges in size and the lysosomal content is increased, along with the amount of hydrolytic enzymes within the lysosomes (e.g., phosphatases, esterases, b-glucuronidase, lysozyme, arylsulfatase). At the same time the size and number of mitochondria increase, with a concomitant increase in their energy metabolism. Production of lactate is also increased. The Golgi complex, which packages lysosomes, increases in size and vesicle complexity (Fig. 73-3,Fig. 73-4). There are several stimuli (e.g., phorbol myristate acetate) which induce formation of multinucleated giant cells from monocytes.29
FIGURE 73-3 Electron micrograph of monocytes in vitro for 2 days. Nucleus (N), endoplasmic reticulum (thin arrow), mitochondria (thick arrow), lysosomes (empty arrow).
FIGURE 73-4 Electron micrograph of monocyte-derived macrophage in vitro for 9 days. Nucleus (N), endoplasmic reticulum (thin arrow), Golgi zone (G), mitochondria (thick arrow), lysosomes (empty arrow). ×7,600.
MORPHOLOGY OF MACROPHAGES
Macrophage characteristics are heralded by a significant increase in cell size, increase in the number of cytoplasmic granules, increase in the heterogeneity of the cell shape, and increase in the number of cytoplasmic clear vacuoles.
An effective monocyte response to infection is predicated upon the ability to migrate and accumulate at an infection site. Monocytes are capable of both random and directed movement. Random migration is nondirected movement that occurs in the absence of attracting substances. Directed movement, or chemotaxis, refers to monocyte migration that occurs in response to chemotactic factors or stimuli and that is mediated by different types of receptors on phagocyte cell surfaces.30 A number of different methods have been used to study macrophage movement both in vivo31 and in vitro.32
LIGHT AND PHASE-CONTRAST MICROSCOPY
In vitro culture of monocytes purified from adult human blood has provided an opportunity to observe the maturation of these cells into mature macrophages.
The macrophages of the pulmonary alveoli, peritoneal and pleural cavities, and inflammatory exudates are hypermature cells that have undergone in vivo stimulation and maturation. This results in enhanced bactericidal activity1,2 due to augmentation of the number of lysosomes and acid hydrolase content.
Macrophages display attributes of morphologic specialization specific to their location and function. The fixed macrophages of the spleen (littoral cells) are involved in the sequestration and destruction of effete or abnormal red cells and exhibit stages of erythrophago cytosis and intracytoplasmic aggregates of ferritin (see Chap. 5). The macrophages of the marrow, the “nurse cells” of the erythroblastic island, play a similar role in erythrophagocytosis and iron storage and transfer (see Chap. 4). Hepatic macrophages (Kupffer cells), found in liver sinusoids, also phagocytize red cells and other cellular elements and are important sites of iron storage. Macrophages of the pulmonary alveoli, the lamina propria of the gastrointestinal tract, and the peritoneal and pleural fluids reflect in their morphology a specific function of phagocytosis of microorganisms, cells, and cellular and noncellular debris, characteristic of the specific organ location.
On Wright-stained films, most macrophages are 25 to 50 µm in diameter. They have an eccentrically placed reniform or fusiform nucleus with one or two distinct nucleoli and finely dispersed, loosely stranded nuclear chromatin that tends to clump in the nuclear interior and along the internal aspect of the nuclear membrane. A juxtanuclear clear zone (Golgi complex) is well defined. The cytoplasm shows fine granules and multiple pink-purple, large azurophil granules. The cytoplasmic borders are irregularly serrated. Cytoplasmic vacuoles are present near the cell periphery, reflecting the active pinocytosis in these cells.
On phase-contrast microscopy, living macrophages are large cells with a propensity to adhere to and spread on glass surfaces, leaving the cell organelles concentrated within the central portion of the cell and clear veils of hyaloplasm spreading about the cell, with intense ruffling of the membrane borders. Vesicles and contractile vacuoles are seen about the cell periphery and in the cell interior. The juxtanuclear clear zone bearing the centrosome and the Golgi complex is particularly dynamic and displays an undulating motion.
Macrophages show a variable degree of differentiation, nuclear “maturity,” ribosomes, mitochondria, and lysosome content. In thin sections, the nucleus varies from horseshoe-shaped to fusiform. The heterochromatin is disposed in fine clumps in the interior of the nucleus and along the internal aspect of the nuclear membrane. Clear spaces between membrane-fixed chromatin aggregates mark the sites of nuclear pores that are relatively abundant in freeze-etch electron micrographs of macrophages as well as monocytes (see Fig. 73-2). Polyribosomes and scant smooth and rough endoplasmic reticulum are seen about the cell periphery. A well-developed Golgi complex is in a juxtanuclear location. It is often multicentric and contains a concentration of vesicles, some with dense inclusions that mark them as early lysosomes. A relatively constant feature of cells engaged in endocytosis is the large number of microvilli at the cell surface, forming the equivalent of a “brush border.” The degree of development of this surface adaptation is related to the phagocytic activity of the cell and its rate of pinocytosis.
The number and size of the mitochondria vary with the phagocytic and hence metabolic activity of the cell. Mitochondria tend to be grouped about the region of the Golgi complex, although several are usually seen dispersed about the cell periphery, presumably supplying energy for the active endocytic processes occurring there.
The most constant and characteristic ultrastructural features of the macrophages are the electron-dense membrane-bound lysosomes that can often be seen fusing with phagosomes to form secondary lysosomes. Within the secondary lysosomes, ingested cellular, bacterial, and noncellular material can be seen in various stages of degradation, often recognizable as degenerating mitochondria or nuclear material. These secondary lysosomes also contain partially degraded material from the late stages of the endocytic process, often appearing as multilamellar lipid bodies.
Microtubules and microfilaments are prominent in macrophages, and actin- and myosin-like proteins have been isolated from monocytes and partially characterized.
Resting macrophages have irregular cell borders and pseudopodia pushed out in all directions. Their cytoplasm has rough endoplasmic reticulum and Golgi complex in the perinuclear area. Lipid globules, primary lysosomes, and mitochondria are characteristically prominent. Activated monocytes/macrophages are motile cells that extend a leading pseudopod as they move forward.33
MONOCYTE-MACROPHAGE SURFACE RECEPTORS
Monocyte-macrophage cells have surface receptors that have been characterized by their binding to specific monoclonal antibodies. These receptors (Table 73-3) are markers for origin, growth, differentiation,34 activation, recognition, migration, and function of the monocyte-macrophage.
TABLE 73-3 SURFACE RECEPTORS OF MONOCYTES AND MACROPHAGES
RECEPTORS FOR PEPTIDES AND SMALL MOLECULES
Fc RECEPTORS (FcR)
Fc receptors for IgG are expressed on the surface of mononuclear cells, macrophages, granulocytes, and platelets.35,36 There are three distinct classes of FcR: FcRI, FcRII, and FcRIII. These receptors have broad ranges of expression on different cells. The first, FcRI (CD64), is a receptor found on monocytes, macrophages, and activated neutrophils. This receptor binds monomeric IgG through the Fc portion of the molecule. This immunoglobulin receptor has increased expression on activated monocytes and macrophages. CD64 allows for receptor-mediated endocytosis of IgG-antigen complexes for presentation to T cells, can trigger release of cytokines and reactive oxygen intermediates, and can play a role in granulocyte-mediated antibody-dependent cytotoxicity. The second IgG receptor, FcRII (CD32), is a widely distributed receptor present on many cell types, including monocytes, platelets, neutrophils, B cells, some T cells, and some capillary endothelium. This receptor can bind complexed IgG rather than monomeric IgG. This Fc receptor regulates B cell function when coengaged with the B cell receptor for antigen, namely surface Ig. It also can induce mediator release from myeloid cells and phagocytosis of Ig-coated particles in vitro. Finally, this Fc receptor also can target antigen into presenting pathways. The third IgG receptor, FcRIII (CD16) is expressed by neutrophils, natural killer cells, and tissue macrophages.37 This receptor can bind Ig in immune complexes and Ig bound to cell surface membranes and is the main Fc receptor responsible for antibody-dependent cellular cytotoxicity (ADCC). All three FcR specifically bind the human IgG subclasses IgGl and IgG3 (see Chap. 83). The interaction of the FcR on macrophages with immune complexes results in cell “activation,” with an increase in phagocytosis, superoxide production, and prostaglandin and leukotriene release.
Activation of the complement system results in the liberation of numerous ligands that bind to specific receptors on mononuclear phagocytes (Chap. 6). Four receptors have been identified that bind fragments of the complement component C3.38 Complement receptor 1 (CR1, or CD35) binds dimeric C3bi and is found on both monocytes and macrophages. Complement receptor 3 (CR3, or CD11b) binds the complement fragment C3b. CR3 is a heterodimeric glycoprotein that is composed of two noncovalently linked polypeptides. The a chain of the polypeptide has an Mr of 185,000, and the b subunit an Mr of 95,000. This receptor, along with the leukocyte antigens LFA-1 (lymphocyte function-associated antigen, CD11a) and alpha-X integrin chain (CD11c), compose a family of heterodimers that share a common b subunit (CD18).39 This family is designated the leukocyte integrin (b2) subfamily.40 These heterodimers are involved in cell-to-cell interactions, in the binding of opsonized particles and plasma proteins, and in attachment to various substrates. They may also modulate intercellular adhesion.
MONOCYTE-MACROPHAGE SURFACE ANTIGENS
HLA CLASS II RECEPTORS
Monocytes and macrophages serve an important function as antigen-presenting cells: They bear the class II glycoproteins of the major histocompatibility gene complex, HLA-DR, -DP, and -DQ. There is a wide variation in expression of MHC class II antigens on macrophages from different tissues. While spleen macrophages contain a high percentage of HLA-DR-positive cells (50 percent), peritoneal macrophages have relatively few (10 to 20 percent)41; the percentage of Ia-positive alveolar macrophages is only about 5 percent.42 Lymphokines, primarily interferon-g, can induce macrophages to express higher levels of MHC class II antigens,43 while prostaglandin E, alpha-fetoprotein, and glucocorticoids44 downregulate the HLA-DR antigen expression on macrophages.
CD11 defines a family of three accessory adhesion surface glycoproteins: CD11a, CD11b, and CD11c. These proteins are distinct a subunits for three heterodimeric surface glycoproteins, each sharing a common b subunit, designated CD18. The a subunits have different isoelectric points, molecular weights, and cell distribution45 (see Chap. 14). Whereas CD11a is expressed on all leukocytes, CD11b or CD11c is expressed predominantly on monocytes and macrophages, a minor subset of B lymphocytes, and most polymorphonuclear leukocytes. CD11b is expressed on more than 95 percent of fresh human monocytes and macrophages but declines rapidly on cells maintained in vitro. Antibodies specific for CD11b, such as OKM1 or Mo1, may block this complement receptor’s ability to bind to CD3bi.46 Accordingly, these antibodies strongly inhibit complement receptor-mediated rosetting of erythrocyte-IgM antibody-complement complexes.
CD14 AND CD68 RECEPTORS
The CD14 molecule is one of the most characteristic surface antigens of the monocyte lineage. It is a polypeptide of 356 amino acids that is anchored to the plasma membrane by a phosphoinositol linkage.47 It is expressed strongly on the surface of monocytes and weakly on the surface of granulocytes and most tissue macrophages. It also can be detected on some nonmyeloid cells (e.g., hepatocytes and some epithelial cells). CD14 functions as a receptor for endotoxin (LPS). LPS binds to a serum protein, LPS-binding protein, that facilitates the binding of LPS to CD14. When LPS binds to CD14 expressed by monocytes or neutrophils, the cells become activated and release cytokines such as tumor necrosis factor and upregulate cell surface molecules, including adhesion molecules. In vitro, soluble CD14 binds to LPS and the complex stimulates cells that do not express CD14 to secrete cytokines and coregulate adhesion molecules.48
The CD68 antigen is a specific marker of monocytes and macrophages. Antibodies against the antigen label macrophages and other members of the mononuclear phagocyte lineage in routinely processed tissue sections and have been used to stain a range of lymphoid, histiocytic, and myelomonocytic proliferation.49
T lymphocytes express several surface receptors, with the surface antigen CD4 expressed exclusively in T-helper lymphocytes (see Chap. 95). CD4 and its corresponding mRNA have been demonstrated on monocytes, macrophages, and monocyte-like cell line, U-937.50 Although CD4 is present at low concentrations in blood monocytes, the percentage of cells that display this plasma membrane determinant ranges from fewer than 5 percent to 90 percent. Several monoclonal antibodies have been described that react with different epitopes of the CD4 antigen.51 The CD4 molecule is involved in the induction of T-lymphocyte helper functions (T4) and T-proliferative responses to antigen stimulation; however, its role in the function of monocyte-macrophages has not yet been determined. An important aspect of the monocyte-macrophage phenotype is the presence of CD4 molecules on the surface of monocytes that can act as the receptors for the human immunodeficiency virus (HIV-1). HIV-1 utilizes the CD4 receptors as an entry pathway for the infection of monocyte-macrophages.52
Chemokines mediate their activities by binding to target cell surface chemokine receptors that belong to a large family of G protein-coupled, seven transmembrane domain receptors. Human monocytes/macrophages express several chemokine receptors (Table 73-3). The chemokine receptor, CCR5, has been implicated in HIV infection of monocytes/macrophages.53,54,55,56 and 57 CCR5 is a major coreceptor on monocytes/macrophages for M-tropic HIV infection. A 32-nucleotide deletion within the CCR5 gene has a highly protective role against acquisition of HIV.58,59
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Ernest Beutler, Marshall A. Lichtman, Barry S. Coller, Thomas J. Kipps, and Uri Seligsohn