Williams Hematology



Identification and Kinetic Studies of Monocytes and Macrophages
Phylogeny and Ontogeny of Macrophages
Tissue Distribution of Monocytes and Macrophages
The Development of Monocytes in the Marrow
Kinetics of Monocytes in Circulation and in Inflammatory Lesions
Differentiation of Monocytes into Macrophages
Multinucleated Giant Cells
Dendritic Cells and their Relationship to Monocytes and Macrophages
The Fate of Monocytes and Macrophages
Chapter References

Macrophages are ancient, mesoderm-derived host defense cells. During embryogenesis, they appear first in the yolk sac, then in the liver, and finally in the marrow—a sequence that recapitulates the phylogeny of blood-forming tissues in vertebrates. Large populations of tissue macrophages exist in the small intestine, liver (Kupffer cells), and lungs. Tissue macrophages can replicate sufficiently to sustain steady-state macrophage populations. Blood monocytes arise in the marrow from precursor cells (monoblasts) that are derived from the differentiation of multipotential progenitors. Blood monocytes rapidly enter into inflamed or infected tissues, where they can mature into macrophages and substantially augment resident macrophage populations. Monocytes can also mature into dendritic cells that efficiently present antigen to T cells.

Acronyms and abbreviations that appear in this chapter include: FIM, factor increasing monocytopoiesis; GM-CSF, granulocyte-monocyte colony stimulating factor; IL-1, interleukin-1; M-CSF, monocyte colony-stimulating factor; TNF, tumor necrosis factor.

Monocytes and macrophages are recognized outside the marrow as smaller (spread diameter of 10 to 18 µm) and larger (20 to 80 µm) cells that are mononuclear and phagocytic (see Chap. 73). Among the histochemical markers characteristic of mammalian monocytes and macrophages, “lipase” (a nonspecific esterase usually detected by its hydrolysis of a-naphthyl butyrate) and myeloperoxidase (detected by the peroxidation of diaminobenzidine) have been the most useful. Human monocytes and macrophages both express lipase activity, but only the monocytes and immature macrophages contain granules that react with peroxidase substrates. Marrow macrophages and monocytes are morphologically and histochemically similar to their extramyeloid counterparts. Myeloid lineage-specific genes that encode transcription factors regulate macrophage development. A transcription factor PU.1 encoded by an ETS family gene appears to be central in macrophage development. Transcription factor gene expression is probably induced by exogenous cytokine stimulation, especially by M-CSF, interacting with GM-CSF, and IL-3.43 Promonocytes, monocyte precursors in the marrow, are weakly phagocytic mononuclear cells 10 to 20 µm in diameter that contain cytoplasmic filaments visible under electron microscopy and a small number of peroxidase-positive cytoplasmic granules.1,2 Monoclonal antibodies and lectins variably specific for monocytes and macrophages have been developed.3
Monocytes or macrophages can be isolated from body fluids, labeled with lipophilic dyes or radioactive compounds, then reinfused and their fate followed by repeated sampling of blood or tissues. Alternatively, genetic markers can be employed to follow the fate of infused monocytes, macrophages, or marrow cells. Concerns have been raised about the effects of in vitro handling on the fate of reinfused cells. The kinetics of monocytes and macrophages after marrow transplantation also may be altered from normal by the effects of radiation and conditioning drugs on the recipient.4
Experimental animals treated with a brief infusion of 3H-thymidine incorporate the radioactive nucleotide into cells undergoing DNA replication. The labeled cells and their descendants can be detected by overlaying tissue sections, imprints, or thin films with photographic emulsions where the beta particles emitted by tritium cause black “grains” to develop. Cells that have divided more than once after incorporating 3H-thymidine are less radioactive, since each division splits the labeled DNA equally between the daughter cells. The films or tissue sections can be conventionally stained to allow the classification of the labeled cells according to their morphologic and staining characteristics. When a nondividing population arises only by maturation of a dividing precursor cell population, most of the precursors incorporate tritiated thymidine abundantly but the mature descendants incorporate comparatively little. As the precursors mature, the number of labeled precursors decreases while their labeled descendants increase. Quantitative analysis can yield kinetic models of traffic between various cell populations and their rates of proliferation. Since macrophages are labeled both directly (dividing macrophages) and indirectly (macrophages arising from dividing earlier marrow precursors), the interpretation of the experimental data can be complex and has led to controversy.4,5
Dual in vivo labeling of macrophage populations is largely avoided by using parabiotic animals,4 whose blood circulations are joined by a permanent cutaneous connection. The skin tunnel between the two animals can be clamped to temporarily separate their circulations while only one animal is infused with tritiated thymidine. When labeling is complete, cross-circulation is allowed to resume. In this case, the macrophages of the recipient animal that was not injected with tritiated thymidine are labeled only if they develop from labeled donor-derived circulating cells.
Large mononuclear phagocytic cells of mesodermal origin (macrophages) are the principal host defense cells in invertebrates6 (e.g., mollusks, crustaceans, or insects), where they are usually referred to as amebocytes or hemocytes. The premyeloid phylogenetic origin of macrophages may be mirrored during embryonic development. Primitive (weakly phagocytic) macrophages with an ameboid shape that react with the monocyte-macrophage lineage-specific monoclonal antibody F4/80 are found in the developing yolk sac when blood vessels and blood cells first appear.7,8,9 Promonocytes and monocyte-like cells appear subsequently. Before hematopoiesis shifts from the yolk sac to the liver, macrophages become more phagocytic, develop lysosomal structures, display lipase activity, and divide rapidly as indicated by incorporation of tritiated thymidine into DNA. At the same time, macrophages identified morphologically and by staining with Griffonia simplicifolia isolectin B4 are already present in the developing liver, brain, and lungs and persist there throughout embryonic development.8,9 It is not clear whether these tissue macrophages are of yolk sac origin or arise independently. Normal tissue macrophage populations can undergo prominent expansion in response to postnatal influences. For example, rabbit alveolar macrophages exposed to ambient microbes, their products, and various particulates proliferate rapidly during the first 2 weeks of life.10
In the adult, the major macrophage populations are found in the lamina propria of the small intestine, in the liver (Kupffer cells), the lungs (alveolar and interstitial macrophages), the spleen, the lymph nodes, the bone marrow, the serosal cavities (peritoneal and pleural), the kidney and endocrine glands, and in the brain (microglia).11,46 The heart and the muscles are relatively macrophage-poor. Additional cells thought to be closely related to macrophages functionally, antigenically, and developmentally are found in the skin (Langerhans or dendritic cells) and in the bone (osteoclasts). The precise lineage relationship of the latter two cells to monocytes is complex.47,48,51 Dendritic cells arise from both myeloid and lymphoid progenitors,51,53 and osteoclasts may develop from myeloid progenitors at an early stage.48 In the absence of inflammation, monocytes are found principally in the marrow and blood. Monocytes migrate from blood into inflammatory lesions, where they differentiate into typical macrophages.12,13,14,15 Macrophages, whether resident or inflammatory, assume different morphologic and functional features depending on their location in organs and tissues. The determinants of this tissue-specific differentiation are not known.
Blood monocytes arise from progenitor cells in the marrow,15,16 since they do not incorporate tritiated thymidine into their DNA, do not undergo mitosis while in blood, and carry the genotype of the donor after marrow transplantation. Labeled monocytes do not appear in blood for 13 to 24 h after intravenous injection of tritiated thymidine,17 indicating that blood monocytes arise from precursors that divided at least 13 h previously. Since blood monocytes have myeloperoxidase-containing granules, monocyte precursors in the marrow were sought among dividing cells that synthesized myeloperoxidase and resembled monocytes morphologically. The immediate monocyte precursors, promonocytes, were identified by intense thymidine labeling, peroxidase staining of rough endoplasmic reticulum and Golgi, and the presence of cytoplasmic filaments and cleft nuclei.1,2,18 Although similar to myelocytes under light microscopy, they could be distinguished from the latter under electron microscopy: Promonocyte cell membranes displayed many fingerlike projections, cytoplasmic filaments, and contained many fewer and smaller granules than did the myelocytes. The putative precursors of promonocytes, termed monoblasts, were recognized in macrophage-forming colonies as smaller dividing cells that contained large nuclei, scant cytoplasm, and few peroxidase-positive granules.19 Monoblasts probably develop from multipotential granulocyte-monocyte progenitors.20 A model of monocyte development has been proposed in which each monoblast gives rise to two promonocytes, each of which then divides into two monocytes.21
Human monocytes appear in the circulation 13 to 26 h after the last round of promonocyte DNA synthesis, followed by mitotic division. They leave the circulation at random times with a half-life that has been estimated at 8 to 70 h.17,22,23 The shorter half-lives were obtained in experiments in which monocytes were removed from blood, labeled, and reinfused, manipulations that may have shortened the half-life of labeled cells. The longer half-life was seen after labeling monocytes in vivo.17 The calculated basal monocyte output is approximately 9.4 × 108 cells per day for the average adult. In rabbits there is a large pool of monocytes transiently trapped in the lung vasculature, but it is not known whether human monocytes are similarly marginated. Within a few hours of the onset of infection or inflammation, monocytes migrating from the bloodstream are found in the lesions, although they are initially much less numerous than neutrophils. In model lesions, monocytes begin to predominate over neutrophils after 12 h.24 Endothelial transmigration is mediated by platelet/endothelial cell adhesion molecule-1 (PECAM-1) and other surface molecules.49 In rats, hematogenous infection with Salmonella enteritidis elicits transient monocytopenia followed by prolonged monocytosis.25 The monocytosis is a combined effect of the release of immature monocytes into the circulation, shortened monocyte generation time, and an expanded monocyte precursor pool. In this model of infection, the half-life of monocytes in blood is shortened to 50 percent of normal, probably due to more rapid efflux into tissues.
The interleukins IL-3 and IL-6 and the colony-stimulating factors GM-CSF and M-CSF, all cytokines (see Chap. 22), and a less extensively characterized protein named factor increasing monocytopoiesis (FIM) induce monocytosis in experimental animals,5,26,27,28 and 29 but the role of these factors in the physiologic regulation of monocyte and macrophage production and kinetics is not yet fully understood. Release of cytokines and hematopoietic growth factors by macrophages engaged in host defense contributes to the increase in monocyte/macrophage production during infections (Fig. 75-1). Pharmacologic doses of glucocorticoids induce monocytopenia and diminish monocyte recruitment into test skin lesions.30

FIGURE 75-1 Autoregulation of mononuclear phagocyte production in response to host defense stimuli. Macrophages exposed to microbial or immune stimuli release hematopoietic growth factors GM-CSF and M-CSF that increase the proliferation of monocyte/macrophage precursors. Additionally, by releasing interleukin-1 (IL-1) and tumor necrosis factor (TNF), activated macrophages induce GM-CSF and M-CSF production by endothelial cells and fibroblasts. Prostaglandin E produced by macrophages may act as a negative regulator of monocyte production.

Monocytes in cell culture12 and in tissues31 spontaneously transform into macrophages, and it is well established that monocytes migrating into inflamed tissues give rise to most of the reactive macrophage population13,16,32 Nevertheless, macrophages are capable of cell division, and resident (noninflammatory) macrophage populations may be largely self-sustaining.4 Serial analysis of gene expression during cytokine-induced maturation of human blood monocytes to macrophages has found a high frequency of expressed genes involved in lipid metabolism.50 In human marrow transplant recipients, alveolar and liver macrophages (Kupffer cells) are eventually replaced by donor-derived cells,33,34 occurring in the case of alveolar macrophages over a period of about 100 days. It is not certain whether the influx of donor-derived cells results from tissue inflammation or from damage to resident macrophages caused by radiation or cytotoxic therapy or reflects the natural dynamics of macrophage populations. The former possibilities are supported by studies on parabiotic mice and rats, wherein macrophage replacement from the cross-circulating monocytes is not seen unless inflammation is induced.4,35,36 In human liver transplant recipients, liver macrophages (Kupffer cells) were replaced by recipient-derived cells over a period of several months.37 It is likely that the migration of macrophage precursors into donor tissue was stimulated by the inflammation associated with low-grade transplant rejection.
Multinucleated giant cells are phagocytic and microbicidal cells found in areas of chronic tissue inflammation. They arise from macrophages either by cell fusion or perhaps by a process in which nuclear division occurs without cell division. In vitro, transformation of macrophages to giant cells occurs by fusion after about 1 to 2 weeks of culture12 and is stimulated by interferon-g or IL-3 or macrophage adherence to surfaces.38
Dendritic cells are found in peripheral tissues where they take up proteins and particulates, process them, then migrate into lymph nodes where they present antigenic fragments to T lymphocytes.39,40 Their ability to present antigen very efficiently and their characteristic morphology have been used as the defining characteristics of this cell type. In vivo, dendritic cells are especially important during primary immunization. “Veiled cells” are dendritic cells that migrate in lymph vessels to lymph nodes. Studies in animals pulsed with 3H-thymidine show that these cells arose from precursors that last divided a few days before. This time course can be reproduced in an in vitro model, where maturation of monocytes into dendritic cells takes place within 48 h after exposure to particles or microorganisms followed by a signal from endothelial cells during transmigration of monocytes from the luminal surface to the subendothelial matrix.41 In the absence of the second signal, monocytes develop into macrophages. Slower differentiation of marrow precursors or blood monocytes to dendritic cells occurs under the influence of mixtures of cytokines, typically including GM-CSF with IL-4 or TNF-a.42,43,44,45,52,53 Using different mixtures of cytokines not including GM-CSF, dendritic cells can also be generated in vitro from lymphoid cells.47,51 Dendritic cells of lymphoid origin are abundant in the thymus and may participate in lymphocyte selection.51,53
The fate of monocytes under noninflammatory conditions is not known with certainty. Some may develop into macrophages, while others may be destroyed in as yet unknown disposal sites. Alveolar macrophages leave the body in swallowed mucus from the airways, and other macrophages may migrate to local lymph nodes.42 However, the ultimate destination of the majority of senescent macrophages is not known. The lymph nodes appear to be the principal final destination of dendritic cells.39,40 and 41

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Copyright © 2001 McGraw-Hill
Ernest Beutler, Marshall A. Lichtman, Barry S. Coller, Thomas J. Kipps, and Uri Seligsohn
Williams Hematology



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