Development

Monocytes, the circulating precursors of tissue macrophages, develop more rapidly in the bone marrow and remain longer in the circulation than do neutrophils (see Table 121-1). The first recognizable monocyte precursor is the monoblast, followed by the promonocyte, with cytoplasmic granules and an indented nucleus, and, finally, the fully developed monocyte. A mature monocyte is larger than a neutrophil and has cytoplasm filled with granules containing hydrolytic enzymes. The transition from monoblast to mature circulating monocyte requires about 6 days. Monocytes retain a limited capacity to divide, and they undergo considerable further differentiation after entering the tissues, where they may live for weeks to many months.

Among multiple subsets of blood monocytes, 2 major ones can be identified on the basis of surface antigens: CD14++ CD16–, originally termed “classical monocytes” because they constitute 90-95% of total monocytes, and the more mature CD14+ CD16+ “proinflammatory” monocytes, which produce more proinflammatory tumor necrosis factor-α (TNF-α) and less immunosuppressive IL-10 in response to microbial stimuli. Monocytes of either subset migrate into different tissues in response to localized inflammation or, apparently, randomly in the absence of inflammation. Once in the tissues, monocytes undergo transformation into tissue macrophages with morphologic and functional properties that are characteristic for the tissue in which they reside (see Table 122-1).

Organ-specific factors influence monocyte differentiation and endow each tissue macrophage with its characteristic features. Monocytes in the liver become Kupffer cells that bridge the sinusoids separating adjacent plates of hepatocytes. Those at the lung airway surface become large ellipsoid alveolar macrophages, and those in the bone become osteoclasts. All macrophages have at least 3 major functions in common: phagocytosis, presentation of antigens to lymphocytes, and enhancement or suppression of the immune response through release of a variety of potent hormone-like factors termed cytokines. At sites of inflammation, monocytes and macrophages can fuse to form multinucleated giant cells; these cells maintain the antimicrobial functions of macrophages.

Activation

The most important step in the maturation of tissue macrophages is the conversion from a resting to a more functionally active cell, a process driven primarily by certain cytokines and microbial products. Macrophage activation is a generic term, with the functional characteristics of an activated macrophage population varying with the cytokine or other stimulus (microbial, chemical) to which the population has been exposed. Classical activation refers to a response to infection that is driven by specifically activated T-helper 1 (Th1)–type lymphocytes and natural killer cells through their release of interferon-γ (IFN-γ). TNF-α secreted by activated macrophages amplifies their activation, as does bacterial cell wall protein or endotoxin. Alternative activation is driven by T-helper 2 (Th2)–type lymphocytes through release of interleukin (IL)-4 and IL-13, cytokines that regulate antibody responses, allergy, and resistance to parasites. Alternatively activated macrophages may have particular functional advantages, such as in wound healing and immunoregulation. In the traditional context of host defense, the term activated macrophage indicates that the “classically activated” cell has an enhanced capacity to kill microorganisms or tumor cells. These macrophages are larger, with more pseudopods and pronounced ruffling of the plasma membrane, and they exhibit accelerated activity of many functions (Table 122-2). Considering the variety of macrophage activities essential to the maintenance of homeostasis, it seems likely that so-called classically and alternatively activated macrophages are examples of a continuum of physiologic functions expressed by these long-lived cells in response to the specific task at hand.

Classical macrophage activation is accomplished during infection with intracellular pathogens (e.g., mycobacteria, Listeria) through crosstalk between Th1 lymphocytes and antigen-presenting macrophages mediated by the engagement of a series of ligands and receptors on the 2 cell types, including CD40 on macrophages and CD40 ligand on helper T cells, and through secretion of cytokines. Macrophages encountering microorganisms release IL-12, which stimulates T cells to release IFN-γ. These interactions constitute the basis of cell-mediated immunity. IFN-γ is an especially important macrophage-activating cytokine that is currently used as a therapeutic agent.

Functional Activities

Numerous functions are upregulated when the macrophage is activated in response to infection (see Table 122-2). Obviously important are the ingestion and killing of intracellular pathogens such as mycobacteria, Listeria, Leishmania, Toxoplasma, and some fungi; however, splenic and hepatic macrophages also clear extracellular pathogens such as pneumococci from the bloodstream. Killing of the ingested organisms depends heavily on products of the respiratory burst (e.g., hydrogen peroxide) and on nitric oxide, and release of these metabolites is enhanced in activated macrophages.

The activity of mononuclear phagocytes against cancers in humans is less well understood. This activity may not involve the phagocytic process. Rather, macrophages may kill tumor cells by means of secreted products, including lysosomal enzymes, nitric oxide, oxygen metabolites, cytolytic proteinases, and TNF-α. Proteolytic enzymes and cytocidal factors present on the surface membrane of monocytes may have a role in tumor rejection. In contrast, tumor-associated macrophages also appear to stimulate growth of certain tumors through secretion of growth and angiogenic factors.

The capacity to undergo diapedesis across the endothelial wall of blood vessels and to migrate to sites of microbial invasion is essential to monocyte function. Chemotactic factors for monocytes include complement products and chemokines derived from neutrophils, lymphocytes, and other cell types. Phagocytosis of the invading organisms can then occur, influenced by the presence of opsonins for the invader (antibody, complement, mannose-binding and surfactant proteins), the inherent surface properties of the microorganism, and the state of activation of the macrophage.

Monocytes migrating to intestinal mucosa are modified by stromal factors so that they lose innate receptors for microbial products such as endotoxin, and they do not effectively produce proinflammatory cytokines. They retain, however, the capacity to ingest and kill microbes. They have been modified to allow the absence of inflammation that exists in normal intestinal mucosa in spite of its constant exposure to huge numbers of microbes and their inflammatory by-products.

Macrophages play an essential role in the disposal of damaged and dying cells, helping resolve the immune response and heal wounds. Brain microglia demonstrate these functions particularly well. In conditions such as stroke, neurodegenerative disease, and tumor invasion, these cells can become activated, surround damaged and dead cells, and clear cellular debris. Macrophages lining the sinusoids of the spleen are especially important in ingesting aged or autoantibody-coated erythrocytes or platelets; splenectomy is used to manage autoimmune cytopenias. Macrophages in inflammatory sites can recognize changes in phosphatidylserine on the membrane of neutrophils undergoing apoptosis, and these can be removed before they become necrotic and spill their toxic contents into the tissue. Macrophages are phylogenetically primitive and can be identified early in fetal development, where they function to remove debris as one maturing embryonic tissue replaces another. They are also important in removing immune complexes, protein fragments, and inorganic particles such as elements of cigarette smoke that enter the alveoli.

Macrophages are integrally involved in the induction and expression of adaptive immune responses, including antibody formation and cell-mediated immunity. This involvement depends on their capacity to break down foreign material in phagocytic and pinocytic vesicles and then present individual antigens on their surface as peptides or polysaccharides bound to class II major histocompatibility complex (MHC) molecules. B lymphocytes and, especially, dendritic cells can also present antigens to T cells for the specific immune response. Expression of MHC class II molecules is increased in activated macrophages, and antigen presentation is more effective.

The heightened capacity of activated macrophages to synthesize and release various hydrolytic enzymes and potentially microbicidal materials (see Table 122-2) probably plays a part in their increased killing capacity, although not every macrophage product is secreted in increased amounts when the cell is activated. The macrophage is an extraordinarily active secretory cell. It has been shown to secrete over 100 distinct substances, including cytokines, growth factors, and sterol hormones, placing it in a class with the hepatocyte. Because of the profound effect of some of these secretory products on other cells and the large number and widespread distribution of macrophages, the mononuclear phagocyte system can be viewed as an important endocrine organ. IL-1 illustrates this point well. Microbes and microbial products, burns, ischemia-reperfusion, and other causes of inflammation or tissue damage stimulate the release of IL-1, mainly by monocytes and macrophages. In turn, IL-1 elicits fever, sleep, and release of IL-6, which induces production of acute phase proteins.

As traumatic damage and infection subside, the macrophage population shifts toward playing an essential role in tissue repair and healing through removal of apoptotic cells and secretion of IL-10, transforming growth factor-β, lipoxins, and omega-3 fatty acid–derived resolvins, protectins, and maresins (macrophage mediators in resolving inflammation).

Dendritic Cells (DCs)

DCs are bone marrow–derived immune cells that are specialized to capture, process, and present antigens to T cells in order to induce adaptive immunity or tolerance to self-antigens. They express retractable dendritic (branched) extensions and potent endocytic capacity but are a heterogeneous population from the standpoint of location, surface markers, level of antigen-presenting activity, and function. Designated subsets include “classical” splenic DCs; Langerhans cells in the epithelial surfaces of skin and mucosa; dermal or interstitial DCs in subepithelial skin and interstitia of solid organs; monocyte-derived DCs, which can leave the circulation and enter a site of pathogen invasion; and plasmacytoid DCs, believed to be a principal source of IFN-α and IFN-β in response to viral infection.

DCs migrating from the bloodstream enter skin, epithelial surfaces, and lymphoid organs where, as immature cells, they internalize self- and foreign-antigens. Microbial products, cytokines, or molecules exposed in damaged tissue (“danger signals” or “alarmins”) induce DC maturation, with upregulation of cytokine receptors and MHC class II and co-stimulatory molecules. Stimulated DCs in the periphery migrate to lymphoid organs where they continue to mature. They function there as the most potent cells that present antigens to T lymphocytes and induce their proliferation, activities that are central to the antigen-specific immune response. Macrophage IL-10 acts to suppress DC maturation during resolution of inflammation.

Clinical trials have used DCs from cancer patients in an attempt to control their cancer. The patient’s DCs are amplified and matured from blood monocytes or marrow progenitor cells by cytokines, exposed to antigens from the patient’s tumor, then injected into the patient as a “vaccine” against the cancer.

Abnormalities of Monocyte-Macrophage or Dendritic Cell Function

Mononuclear phagocytes, as well as neutrophils, from patients with chronic granulomatous disease (CGD) exhibit a profound defect of phagocytic killing (Chapter 124). The inability of affected macrophages to kill ingested organisms leads to abscess formation and characteristic granulomas at sites of macrophage accumulation beneath the skin and in the liver, lungs, spleen, and lymph nodes. IFN-γ is currently used for preventing infection in patients with CGD and for treating the decreased bone resorption of congenital osteopetrosis, which is caused by decreased function of osteoclasts. Genetic deficiency of the CD11/CD18 complex of membrane adherence glycoproteins (leukocyte adhesion defect-1), which includes a receptor for opsonic complement component 3, results in impaired phagocytosis by monocytes (Chapter 124).

The monocyte-macrophage system is prominently involved in lipid storage diseases called sphingolipidoses (Chapter 80.3). In these conditions, the expression in macrophages of a systemic enzymatic defect permits the accumulation of cell debris that is normally cleared. Resistance to infection can be impaired, at least partly because of impairment in macrophage function. Gaucher disease is the prototype for these disorders. In this condition, the enzyme glucocerebrosidase functions abnormally, thus allowing accumulation of glucocerebroside from cell membranes in Gaucher cells throughout the body. In all locations, the Gaucher cell is an altered macrophage. These patients can be treated with infusions of the normal enzyme modified to expose mannose residues, which bind to mannose receptors on macrophages.

The cytokine IL-12 is a powerful inducer of IFN-γ production by T cells and natural killer cells. Individuals with inherited deficiency in macrophage receptors for IFN-γ or lymphocyte receptors for IL-12, or in IL-12 itself, suffer a severe, profound, and selective susceptibility to infection by nontuberculous mycobacteria such as Mycobacterium avium complex or bacillus Calmette-Guérin (BCG) (Chapter 120). About half of these patients have had disseminated Salmonella infection. These abnormalities are now grouped as defects in the IFN-γ–IL-12 axis.

Monocyte-macrophage function has been shown to be partially abnormal in various clinical conditions. Cultured mononuclear phagocytes of newborns are more readily infected than adult cells by human immunodeficiency virus (HIV)-1 and measles virus. Macrophages from newborns release less granulocyte colony–stimulating factor (G-CSF) and IL-6 in culture, and this deficiency is accentuated in cells from preterm infants. This finding supports the observations that levels of G-CSF are significantly decreased in blood from newborns, and that the marrow granulocyte storage pool is diminished in infants, particularly preterm infants. Mononuclear cells from newborns produce less IFN-γ and IL-12 than do adult cells, and macrophages cultured from cord blood are not activated normally by IFN-γ. This combination of deficiencies would be expected to blunt the newborn’s response to infection by viruses, fungi, and certain bacteria such as Listeria.

There are two disorders in which macrophage activation is pathologically excessive. Familial and acquired hemophagocytic lymphohistiocytosis is characterized by uncontrolled activation of T cells and macrophages, with resultant fever, hepatosplenomegaly, lymphadenopathy, pancytopenia, marked elevation of serum proinflammatory cytokines, and macrophage hemophagocytosis (Chapter 501). Up to 5% of children with systemic onset juvenile rheumatoid arthritis develop an acute severe complication termed macrophage activation syndrome, with persistent fever (rather than typical febrile spikes), hepatosplenomegaly, pancytopenia, macrophage hemophagocytosis, and coagulopathy, which can progress to disseminated intravascular coagulation and death if not recognized (Chapter 149). Two genetic autoinflammatory diseases result from dysregulation of the mononuclear phagocyte–produced proinflammatory cytokine IL-1. In neonatal onset multisystem inflammatory disorder (NOMID) monocytes overproduce IL-1. In deficiency of the IL-1-receptor antagonist (DIRA), normal activity levels of IL-1 go unopposed. In both conditions patients present in the 1st few days or weeks of life with pustular or urticarial rash, bony overgrowth, elevated sedimentation rate, and other evidence of systemic inflammation.

The term histiocyte was originally used to describe cells thought to be macrophages in fixed tissue preparations. Histiocytosis X represents a malignancy-like overgrowth of Langerhans-type dendritic cells (Chapter 501). Thus, the term Langerhans cell histiocytosis better describes this disorder, because histiocyte is a histologic term and not cell specific.