David C. Dale M.D.1
1Professor of Medicine, University of Washington Medical Center
The author has received grants for clinical research from and served as consultant for Amgen Inc.
Leukocytes, or white blood cells, protect the body against infections and participate in many types of immunologic and inflammatory responses. There are two main types of leukocytes: lymphocytes, which are responsible for antibody production and cell-mediated immunity, and phagocytes, which are responsible for the ingestion and killing of microorganisms. Neutrophils, monocytes, macrophages, and eosinophils are all phagocytes [see Figures 1a, 1b, 1c, and 1d]. Leukocytes interact with one another and modulate immune responses through the release of cytokines (interleukins and growth factors), enzymes, and vasoactive substances. This chapter covers the diagnosis of disorders of neutrophils, monocytes, and eosinophils and the treatment of neutropenia; the functions and disorders of lymphocytes are discussed elsewhere [see Section 6 Immunology and Allergy].
Figure 1a. Schematic of a Neutrophil
Shown is a schematic of a neutrophil.
Figure 1b. Electron Micrograph of Neutrophil
Shown is a electron micrograph of a neutrophil, corresponding to the schematic in Figure 1a.
Figure 1c. Electron Micrograph of a Monocyte
Shown is an electron micrograph of a monocyte.
Figure 1d. Electron Micrograph of an Eosinophil
Shown is an electron micrograph of an eosinophil.
The White Blood Cell Count
The total white blood cell (WBC) count and differential count are often the first studies performed in evaluating a patient with a suspected infection or with susceptibility to infections. Most laboratories measure the WBC count using automated cell-counting techniques.1 The normal WBC count ranges from 4,300 to 10,000/mm3, with a median of 7,000/mm3 [see Table 1]. A differential count gives the percentage for each type of leukocyte. The absolute count is determined by multiplying the total WBC count by this percentage (e.g., WBC × percent neutrophils = absolute neutrophil count). Because the blood level of each type of leukocyte is separately regulated, it is always better to use the absolute count rather than the percentage in assessing abnormalities.
Table 1 Normal Leukocyte Values in Peripheral Blood
Indications of the Presence of a Phagocytic Cell Disorder
Because the phagocytes, particularly neutrophils, represent the first line of defense against invading microorganisms, disorders in the number or function of these cells often result in an increased susceptibility to infection. A quantitative or qualitative disorder of phagocytic cells should be suspected when a patient has an increased number of bacterial or fungal infections, increasingly severe infections, or infections with unusual organisms.
Neutrophils are derived from the common stem cell, which also gives rise to erythrocytes, platelets, and other leukocytes. The proliferation and differentiation of the neutrophil precursors are governed by a family of regulatory cytokines. Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) are two important cytokines affecting neutrophil production and function. G-CSF selectively stimulates progenitor cells to differentiate into neutrophils and rapidly increases blood neutrophils in hematologically normal individuals [see Figure 2].2,3 GM-CSF stimulates progenitor cells to differentiate into neutrophils, eosinophils, monocytes, macro phages, and dendritic cells.4 The life cycle of the neutrophil consists of bone marrow, blood, and tissue phases. Neutrophil production in the bone marrow takes approximately 10 to 14 days, and the bone marrow produces approximately 1 × 109neutrophils/kg/day.5 Most of the body's neutrophils are found in the bone marrow. The mitotic compartment, which contains about 20% of the total neutrophil pool, consists of myeloblasts (the earliest morphologically recognizable precursors), promyelocytes, and myelocytes. The postmitotic pool or maturation compartment—the metamyelocytes, bands, and mature neutrophils—contains about 70% of the body's neutrophils. The marrow neutrophils and bands are sometimes called the storage compartment or marrow reserve. As neutrophils mature, they develop the capacity to enter the blood through increasing deformability and through changes in the adhesion proteins on their surface membranes. Entry into the blood involves interactions of the mature cells and the endothelial cells of the marrow sinusoids that are not yet well understood. Agents that stimulate release of neutrophils from the marrow (e.g., G-CSF, GM-CSF, corticosteroids, or endotoxin administration) can result in a doubling of the blood neutrophil count within 3 to 5 hours. The peripheral blood contains fewer than 10% of the body's neutrophils. In the blood, the neutrophils are divided approximately evenly between the circulating pool and the marginating pool; these pools are in dynamic equilibrium. Cells in the marginating pool can be swept rapidly (within minutes) into the circulation by endogenous or exogenous epinephrine or as a result of exercise or any cause of rapid increase in cardiac output. This response, called demargination, can double the blood neutrophil count very rapidly and is also quickly reversible. The blood half-life of the neutrophils is approximately 6 to 10 hours. Neutrophils leave the blood and enter the tissues by migrating between endothelial cells and penetrating the capillary basement membrane. It is now believed that neutrophils that do not leave the circulatory system die by apoptosis and are removed by mononuclear phagocytes in the spleen, liver, and other tissues.6
Figure 2. Neutrophil Maturation
The process of neutrophil maturation begins in the bone marrow (top). After about 12 days, approximately 10% of the mature neutrophils are released into the peripheral blood, where they have a half-life of approximately 6 to 10 hours. Eventually, the neutrophils migrate into the tissues by diapedesis. The percentage of neutrophils at each stage of development (bottom) ranges from about 2% at the myoblast stage to almost 25% at the mature neutrophil stage.
As neutrophil precursors mature, their nuclear chromatin becomes condensed and segmented. Mature cells have no nucleoli, few mitochondria, and very little endoplasmic reticulum. The cytoplasm is filled with granules and glycogen. The primary granules, which appear at the myeloblast and promyelocyte stages, contain myeloperoxidase (MPO), proteases, defensins, and other antibacterial substances.7,8Secondary granules, produced primarily during the myelocyte stage, predominate in mature cells. They contain collagenase, lactoferrin, lysozyme, vitamin B12-binding protein, and several other proteins. Small tertiary granules are also found in mature neutrophils. Neutrophils also may have cytoplasmic vesicles containing lactases, alkaline phosphatases, and components of nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase.
The surface of the neutrophil is replete with deep folds and ruffles. On the neutrophil surface, there are numerous receptors, including receptors for immunoglobulins (e.g., FcγRII [CD32], FcγRIII [CD16]), complement (e.g., CR3 [CD11b18], CR1 [CD35]), chemokines, the colony-stimulating factors G-CSF and GM-CSF, Fas, tumor necrosis factor receptor (TNF-R), and the apoptosis-related receptors.9
The cytoskeleton of the neutrophil is composed of microtubules and microfilaments that are critical for phagocytic shape and movement, including migration through the vascular endothelium. The microfilaments, which consist primarily of actin polymers, are dispersed throughout the cytoplasm.10
The major function of neutrophils is to respond rapidly to microbial invasion to kill the invaders. This response has several distinct steps—adherence, migration, recognition, phagocytosis (or ingestion), degranulation, oxidative metabolism, and bacterial killing [see Figure 3]. Susceptibility to infection results from abnormalities in any one or a combination of these processes.
Figure 3. Neutrophil Response to Bacterial Invasion
The neutrophil response to bacterial invasion involves several stages. A bacterium infects a host cell and injures it (a). Bacterial products, antibodies, and complement cause the release of chemotactic factors, which activate a neutrophil in the adjacent blood vessel. The neutrophil adheres to the vessel wall and undergoes chemotaxis and diapedesis into tissue (b) to follow the chemoattractants to their sites of generation or expression. The neutrophil recognizes (c) and ingests (d) the bacterium-antibody-complement complex, forming a phagosome. The neutrophil then undergoes degranulation, a process in which granule membranes fuse with the plasma membrane (e). Degranulation releases various enzymes and enhances oxidative metabolism, the products of which are bactericidal (f). For example, hydrogen peroxide (H2O2), produced from superoxide (O2-), can interact with O2- in the presence of iron (Fe) to produce hydroxyl radicals (OH•) and singlet oxygen (1O2), both of which are highly toxic to bacteria. In addition, H2O2 and chloride (Cl-) combine in the presence of the myeloperoxidase (MPO) released in the phagosome to produce hypochlorite (OCl-), which is also bactericidal.
For neutrophils to move to an inflammatory site, they must first adhere to a capillary wall.11 Loose adherence is facilitated by L-selectins, such as sialyl-Lewisx (sLex), on the neutrophil and E-selectin and P-selectin on capillary endothelial cells [see Figure 4]. Bacterial invasion increases local selectin expression and neutrophil accumulation. Other neutrophil surface proteins, called β2 integrins, facilitate firmer adhesion to endothelial cells and interact with actin, myosin, and actin-binding proteins to initiate movement of neutrophils to the tissue.11The three proteins in this family have a common β subunit (CD18) and a different α subunit (CD11a, CD11b, or CD11c). There is generally increased expression of these proteins (e.g., CD 11b/C18) on neutrophils in response to inflammation. Concomitantly, there is increased expression of the intracellular adherence molecules (ICAMs) on the endothelial cells, with a net result of increased trafficking of neutrophils to the inflammatory focus.
Figure 4. Neutrophils in Peripheral Blood
Neutrophils in the peripheral blood exist in either the circulating or the marginating pool. The marginated neutrophils roll along a vessel wall, where their surface carbohydrates interact with selectins on the endothelial cells. After activation by chemotactic agents, the neutrophils change shape and change the affinity of their integrin molecules for endothelial cell intercellular adhesion molecules. The neutrophils then crawl and undergo diapedesis by interacting with platelet-endothelial cell adhesion molecules on the endothelial surface and by liberating hydrolases that permit passage of the neutrophils through the capillary basement membrane. (PECAM-1—platelet-endothelial cell adhesion molecule-1; sLex—sialyl-Lewisx carbohydrate; ICAM-1—intercellular adhesion molecule-1; VCAM-1—vascular cell adhesion molecule-1)
Chemotaxis, the directed movement of cells, occurs when neutrophils detect a chemoattractant at low concentrations and move up the concentration gradient toward its source, which is usually a site in the extravascular spaces.12 Well-characterized stimulators of neutrophil chemotaxis are the complement proteins C5a, leukotriene B4, interleukin-8 (IL-8), and a family of small peptides, the chemokines. The trafficking of neutrophils from the blood is unidirectional; they do not return from the tissues to the circulation.
Recognition and Phagocytosis
At the site of inflammation, neutrophils utilize their immunoglobulin and complement receptors to recognize bacteria and other particles coated or opsonized by immunoglobulins or complement. Inflammation stimulates neutrophils to express increased numbers of the high-affinity IgG receptor FcγRI (CD64).13 As the neutrophil internalizes a particle, a phagocytic vesicle, or phagosome, develops around it. This process stimulates degranulation and activates a burst of oxidative metabolism.
When the neutrophil is activated, the granule membranes come in contact with the plasma membranes surrounding the phagosome. The membranes fuse, which leads to the release of granule proteins into the phagosome and to the reorganization of the components of the critical NADPH oxidase system.14
Oxidative Metabolism and Bacterial Killing
Resting granulocytes are primarily anaerobic cells that rely on anaerobic glycolysis for adenosine triphosphate (ATP) production. Although chemotaxis, ingestion, and degranulation require some energy, they also proceed quite well anaerobically. However, bacterial killing generally is associated with a rapid increase (within seconds) in oxygen use. This respiratory burst occurs as a result of the activation of an NADPH oxidase.15 Before activation, the components of the oxidase are located separately in the plasma and granule membranes and in the cytosol. The membranes contain two components: gp91phox and p22phox. The cytosol includes a p47 protein and a p67 protein. When the neutrophil is activated, the cytosolic proteins first associate and then combine with the membrane components to produce the complete NADPH oxidase. NADPH oxidase can reduce oxygen by one electron to superoxide O2-; in the process, NADPH is converted to NADP+. The NADPH is then regenerated through the hexose monophosphate shunt. Dismutation of the superoxide in the presence of superoxide dismutase produces hydrogen peroxide (H2O2), which can then be converted to hydroxyl radical (OH•). H2O2, O2-, and OH• are highly toxic. In addition, within the phagocyte vacuole, hydrogen peroxide and chloride (Cl-) can combine in the presence of myeloperoxidase to produce hypochlorous acid (HOCl), which is bactericidal.15 These products of the respiratory burst can also be released from the activated neutrophil and subsequently damage the surrounding cells and tissues.
Responses to and Production of Cytokines
The growth factors that affect neutrophil production, such as G-CSF and GM-CSF, also influence neutrophil function.16 These cytokines upregulate stimulus-dependent NADPH oxidase activity and can enhance bactericidal and fungicidal activities. Although neutrophils contain very few ribosomes, they can respond to bacterial stimuli by synthesizing and secreting proinflammatory cytokines such as IL-1, IL-6, and tumor necrosis factor-α (TNF-α); monocytes, however, produce much larger quantities of these substances.17
Disorders of Neutrophil Number
Neutrophilia, or granulocytosis, is usually defined as a neutrophil count greater than 10,000/mm3.
Neutrophilia most often occurs secondary to inflammation, stress, or corticosteroid therapy. Cigarette smoking commonly causes neutrophilia as a result of inflammation in the airways and lungs. Malignancies, hemolytic anemia, and lithium therapy are less common causes. Neutrophilia is also associated with splenectomy. Extreme neutrophilia (i.e., neutrophil counts of more than 30,000 to 50,000/mm3), often called a leukemoid reaction, occurs with severe infections, sepsis, hemorrhagic shock, and severe tissue injury of any cause. Neutrophilia is also seen in patients with leukocyte adhesion deficiency (LAD), a rare disease in which neutrophils accumulate in the blood because they lack either the integrin CD11b18 or the selectin sLex (CD15s) required to leave the circulation.18
Serious bacterial infections and chronic inflammation are usually associated with changes in both the number of circulating neutrophils and their morphology. Characteristic changes include increased numbers of young cells (bands), of cells with residual endoplasmic reticulum (Döhle bodies), and of cells with more prominent primary granules (toxic granulation). These changes are probably caused by the endogenous production of G-CSF or GM-CSF and are also seen with administration of these growth factors.
Primary neutrophilia (i.e., neutrophilia attributed to defects in proliferation and maturation of neutrophil precursors) occurs in patients with myeloproliferative disorders, such as chronic myeloid leukemia (CML) and polycythemia vera [see 5:V The Polycythemias]. Hereditary and idiopathic neutrophilias have been described; they are benign and quite rare. One such uncommon idiopathic condition is Sweet syndrome, which is an acute febrile illness with painful cutaneous plaques and associated neutrophilia of any cause.19 Neutrophilia is also associated with congenital abnormalities. For example, infants with Down syndrome can have transient leukemoid reactions that must be distinguished from congenital leukemia.20
When neutrophilia cannot be readily attributed to an infection or inflammatory condition or to glucocorticosteroid therapy, the possibility of a myeloproliferative disease should be considered. The presence of splenomegaly, metamyelocytes, and myelocytes in the blood, together with increased basophils or eosinophils and a low leukocyte alkaline phosphatase (LAP) score, suggests CML [see 12:XVII Chronic Myelogenous Leukemia and Other Myeloproliferative Disorders]. A high LAP score or the presence of toxic granulations usually suggests an underlying infection. When there is uncertainty, bone marrow aspiration and biopsy, chromosomal analysis, as well as marrow cultures for bacteria (e.g., Salmonella, Brucella, Mycobacterium, and fungi), are warranted. The results of these tests will enable the clinician to make a diagnosis of CML (or another myeloproliferative disorder), a granulomatous infection, inflammatory disease, or metastatic malignancy. If no such cause can be found in an otherwise healthy-appearing person, a diagnosis of idiopathic or familial neutrophilia may be considered, and repeated neutrophil counts can be performed at monthly intervals until the diagnosis is clarified.
Except for the myeloproliferative syndromes, treatment of neutrophilia is not indicated. Neutrophil levels will return to normal when the inflammatory process is resolved.
Neutropenia is generally defined as a neutrophil count of less than 1,800/mm3, which is two standard deviations below the normal mean. In some populations (e.g., Africans, African Americans, and Yemenite Jews), neutrophil counts as low as 1,000/mm3, or 1.0 × 109/L, are probably normal.21,22
In otherwise healthy persons, the risk of bacterial infections is relatively low if the neutrophil count is greater than 500 mm3, or 0.5 × 109/L—the level usually defined as severe neutropenia. When neutropenia develops after myelotoxic chemotherapy, the risk of infection is much greater, particularly in patients whose age and medical history (e.g., diabetes, heart failure, renal failure, previous chemotherapy, and HIV infection) also predispose them to infection.22,23 Patients with neutropenia are also at greater risk for serious infections if they have disrupted mucosal or cutaneous barriers or are taking corticosteroids. Patients with neutropenia are at risk for infection by those pathogenic organisms that normally colonize body surfaces, particularly the skin, the oropharynx, and the GI tract. Thus, infections from staphylococci occur in neutropenic patients after breaks in the skin. Infection by mixtures of aerobic and anaerobic organisms of the oropharynx frequently causes gingivitis, pharyngitis, and sinusitis with neutropenia. Gram-negative bacilli often invade the blood from the GI tract in these patients. Antibiotic therapy, particularly therapy involving broad-spectrum antibiotics and protracted treatments, leads to colonization by resistant bacteria and to fungal infections.24
Neutropenia may be a primary or secondary condition. Primary neutropenia is caused by abnormalities of neutrophil formation derived from hematopoietic stem cells in the bone marrow; disorders with this underlying pathogenesis include the myeloid malignancies and several congenital disorders [see Primary Forms of Neutropenia, below]. Secondary neutropenia may be caused by drug therapy, infections, and immunologic disorders, including autoimmune diseases. Secondary neutropenia is far more common than primary neutropenia. In all of these conditions, the risk of infection depends on the level of blood neutrophils and the capacity of the marrow to respond to an inflammatory stimulus and increase production of these cells. Usually if blood neutrophils are greater than 0.5 × 109/L, the risk of serious infection is relatively low.
In aggregate, drug reactions are probably the most common cause of neutropenia in adults [see Table 2].25,26 Many cancer chemotherapy drugs, some of which are also used as cytotoxic immunosuppressive agents (e.g., cyclophosphamide, methotrexate, and azathioprine), predictably cause dose-dependent neutropenia. The use of these agents requires careful attention to medical history, dosages, treatment schedules, and serial neutrophil counts to avoid serious and life-threatening toxicity. Other drugs cause neutropenia idiosyncratically. Many of these reactions probably occur because drugs can act as immunogens or as haptens, causing immunologic injury to neutrophils and their precursors. Other mechanisms of drug-induced neutropenia may involve direct toxicity of marrow cells in susceptible persons. Most patients recover from drug-induced neutropenia; the time for recovery can vary from 2 days to 2 weeks or more.
Table 2 Drugs Associated with Neutropenia
Viral infections often cause mild neutropenia, especially in children. Such infections include measles and other viral exanthems, infectious mononucleosis, hepatitis, and HIV infection. The mechanisms are diverse. For example, in HIV infection, possible mechanisms include infections of tha hematopoietic precursor cells and the marrow stromal cells, which lead to decreased production; induction of autoantibodies, which leads to accelerated turnover of mature neutrophils; and accelerated apoptosis of mature cells.27 HIV-associated neutropenia generally develops late in AIDS and is often compounded by the use of antiviral agents (e.g., zidovudine, ganciclovir), antibiotics (e.g., sulfonamides), or the presence of hematologic malignancies (e.g., lymphoma, Kaposi sarcoma).28 With other viral infections, the neutropenia is usually mild and without serious consequences. In rare instances, infectious mononucleosis causes severe hypoplasia, which has more severe consequences.29 Neutropenia and anemia are common features of human parvovirus B19 infection.30
With severe bacterial infections, neutropenia occurs as a consequence of endotoxemia, which results in rapid neutrophil mobilization and turnover, especially in patients with a marrow reserve that is impaired because of previous chemotherapy, other drugs, or alcohol. In this setting, neutrkpenia generally portends a grave prognosis. Neutropenia occurs in parasitic infections associated with splenomegaly, such as kala-azar and acute malaria, presumably as a result of splenic trapping of the cells.
Autoimmune and idiopathic neutropenia
Autoimmune neutropenia occurs as an isolated phenomenon or secondary to other autoimmune disorders.31 For example, in patients with Evans syndrome, autoimmune neutropenia may be associated with immune thrombocytopenia and hemolytic anemia. The bone marrow cellularity in patients with autoimmune neutropenia is either normal or increased, with a relative decrease in the number of cells in the late stages of the neutrophil formation. The diagnosis of autoimmune neutropenia requires specific antineutrophil antibody tests.32 The specificity of these tests probably varies considerably, and they are performed by a limited number of laboratories. It is often difficult to distinguish autoimmune neutropenia from cases otherwise categorized as idiopathic neutropenia. Neutropenia with antineutrophil antibodies also occurs in systemic lupus erythematosus,33 Sjögren syndrome,34,35 rheumatoid arthritis,36 and Felty syndrome (i.e., rheumatoid arthritis, splenomegaly, and neutropenia).37
Patients with rheumatoid arthritis and neutropenia may have clonal expansion of large granular lymphocytes (usually CD2+, CD3+, CD8+, and CD57+ cells), which impair neutrophil production by excessive Fas ligand or interferon-gamma production.38 Recent studies indicate that this same mechanism may be involved in cases diagnosed as idiopathic neutropenia.39 The marrow typically shows increased lymphocytes with reduced neutrophils in the later stages of development. In most patients, the lymphocytosis is clonal and may evolve very gradually into a lymphoid malignancy.40
In sarcoidosis, cirrhosis, and congestive splenomegaly of diverse causes, neutropenia and thrombocytopenia often occur concomitantly, presumably because of splenic sequestration. In most instances, the neutropenia is mild and without recognizable consequences.
Other secondary causes of neutropenia
Neonates can have severe neutropenia because of transplacental transfer of maternal IgG antibodies to the FcγRIII (CD16) isotype (previously called NA-1 or NA-2) that is inherited from the infant's father.41 This abnormality is transient, usually lasting less than 3 months. Transient severe neutropenia also can occur in an infant as a result of transplacental transfer of other antibodies (e.g., transfer of IgG) from a mother with autoimmune neutropenia. Pure white cell aplasia is a rare acquired condition characterized by a complete absence of myeloid precursors.42 Pure white cell aplasia may be associated with a thymoma; if so, the aplasia may respond on removal of the thymoma. The short-term consequences of all these conditions depend primarily upon the level of blood neutrophils and the proliferative response of the marrow when inflammation or infections occur. The causes of other forms of neutropenia in children and adults are often difficult to establish and usually require referral to an expert hematologist.
Primary forms of neutropenia
There are a number of congenital and inherited causes of neutropenia [see Table 3].
Table 3 Intrinsic Disorders of Neutrophils That Cause Neutropenia
Neutropenia is easily diagnosed by performing a white blood cell count and differential count. Patients with acute, severe neutropenia are often febrile and are frequently referred to as having acute febrile neutropenia. In this circumstance, attention is immediately focused on determining whether the patient has an infection, as well as focused on instituting empirical antibiotic therapy. Hematologic studies (e.g., bone marrow examination) are generally not necessary, because the cause of neutropenia is recognized from the patient's history, and it will resolve if the inciting cause has been eliminated.
Initial evaluation of patients with chronic neutropenia should include a careful family history and review of the incidence and severity of infections, including oral ulcers, gingivitis, cellulitis, and more serious problems. A complete blood count will reveal whether the neutropenia is isolated or associated with other hematologic abnormalities. Medications should be discontinued if they can be implicated as causes of the neutropenia. A bone marrow biopsy and aspirate are indicated if there is any question of a primary disease affecting the marrow (e.g., metastatic carcinoma, tuberculosis) or if myelodysplasia or a hematologic malignancy is suspected. Serologic testing for infectious mononucleosis, hepatitis, and HIV is often warranted, as is measurement of antinuclear antibodies and rheumatoid factor titers. Broader immunologic assessments (i.e., lymphocyte subtypes and immunoglobulin levels) are warranted if the history suggests a susceptibility to infections by viruses, parasites, or bacteria; and they are also useful to detect clonal proliferation of lymphocytes and to diagnose the large granular lymphocyte syndrome. Neutrophil mobilization with corticosteroids and demargination tests with epinephrine are rarely helpful.
Treatment of Neutropenia
Evidence-based guidelines for management and prevention of acute febrile neutropenia associated with cancer chemotherapy have been developed by the Infectious Diseases Society of America (http://www.idsociety.org) and the American Society of Clinical Oncology (http://www.asco.org). Other guidelines are also available (http://www.guideline.gov) [see Table 4]. In general, acute management of severe, idiosyncratic, drug-induced neutropenia should be similarly managed.43,44
Table 4 Guidelines for Management and Prevention of Febrile Neutropenia45
Treatments for chronic neutropenia vary with the severity of neutropenia and the pattern of susceptibility to infection. Mild or moderate neutropenia (i.e., counts above 0.5 × 109/L, determined by serial counts over several weeks) rarely requires treatment. The neutropenia in Felty syndrome often responds to splenectomy and weekly doses of methotrexate.45,46 With few other exceptions, long-term use of corticosteroids, γ-globulin injections, androgens, and splenectomy is not indicated for management of chronic neutropenia. With suspected infections, short-term, broad-spectrum antibiotic therapy is indicated, usually initiated after culture of blood and other body fluids for bacteria. Long-term antibiotic therapy is of unproven benefit in preventing infections, and it carries the risk of colonization by antibiotic-resistant organisms. G-CSF, usually in doses of 1 to 5 mg/kg/day, is of proven benefit for the treatment of congenital, idiopathic, and cyclic neutropenia and hastens the recovery of marrow from neutropenia after cancer chemotherapy.47 G-CSF and GM-CSF have been widely used to treat other forms of chronic neutropenia, including the neutropenia associated with HIV infection.
Disorders of Neutrophil Function
In patients who have recurrent, severe, or unusual infections but who have a normal number of neutrophils, the presence of a neutrophil function disorder must be considered. Neutrophil function disorders are caused by defects in neutrophil adherence, chemotaxis, degranulation, or oxidative metabolism [see Table 5].
Table 5 Selected Disorders of Neutrophil Function
The evaluation of patients with confirmed, recurrent, or unusual infections is first to review the family history and then to examine the patient [see Figure 5]. A complete blood count and examination of the granulocytes in a blood smear can show neutrophilia or neutropenia, specific granule deficiency, or giant granules such as those that occur in Chédiak-Higashi syndrome. Evaluation of immunoglobulin levels (IgG, IgM, IgA, and IgE) and complement levels (C3 and CH50) are also potentially helpful, especially if there is a pattern of infection by encapsulated bacteria or unusual organisms. After these considerations, neutrophil function should be evaluated with the nitroblue tetrazolium (NBT) test, superoxide production assays, and chemotactic assays. The NBT test and superoxide assays can determine whether a patient has chronic granulomatous disease (CGD), severe glucose-6-phosphate dehydrogenase (G6PD) deficiency, or a glutathione-pathway disorder14; chemotactic assays can be used to confirm the diagnosis of Chédiak-Higashi syndrome and acquired chemotactic defects.12Leukocyte adhesion deficiency types I and II are diagnosed by flow cytometry.11 If the results of all of these tests are normal, ingestion assays using the patient's serum and cells and staining for MPO may be helpful. In this manner, all of the known neutrophil function abnormalities can be diagnosed, often with the aid of specialty consultations and a research laboratory.48
Figure 5. Evaluation of Recurrent Infections
Steps in the evaluation of a patient with recurrent infections for a phagocytic cell disorder.
Monocytes and Macrophage Physiology
Monocytes and macrophages play critical roles in homeostasis and in host defense mechanisms. Monocytes and macro phages perform tissue maintenance functions, such as clearance of particles—including bacteria—from the blood and removal of old red blood cells. They process antigens by interacting with T cells and B cells and are essential for containment of mycobacte-rial, parasitic, fungal, and viral infections.
Monocytes develop from hematopoietic progenitor cells in the bone marrow. Once the progenitor cells are committed to a monocyte lineage, they develop morphologically into mono blasts, then promonocytes, and then monocytes. Monocytes are present in the bone marrow and blood. They are the precursors for the tissue mononuclear phagocyte system (including alveolar, peritoneal, and splenic macrophages), Kupffer cells, osteoclasts, dendritic cells, and Langerhans cells. In addition to having phagocytic capabilities, monocytes and macrophages play a central role in the immune response through the generation of numerous cytokines, including growth factors for white blood cells.
With the exception of the alveolar macrophages, which are uniquely dependent on aerobic metabolism for energy production, monocytes and macrophages are facultative anaerobes. Phagocytosis by monocytes and macrophages is associated with an oxidative burst and stimulation of the hexose monophosphate shunt. Adhesion, chemotaxis, and activation are similar for monocytes and neutrophils, although macrophages are better than neutrophils at phagocytosis and perform chemotaxis less rapidly and efficiently. Macrophages are also capable of oxygen-independent bactericidal activity that may depend on lytic activity. Stimulated macrophages are capable of producing nitric oxide. Macrophages are capable of secreting many cytokines, growth factors, and acute-phase reactants.
Monocytes and macrophages present antigen to T cells in association with major histocompatibility complex (MHC) class II molecules. This association occurs in the lysosomes of a mononuclear cell before the MHC class II molecules are expressed on the cell surface. Monocytes and macrophages are involved in antibody-dependent and antibody-independent cell-mediated cytotoxicity. The cytotoxicity involves oxidative metabolism, the production of nitric oxide and cytokines, and the secretion of cytotoxic mediators.
Macrophages play a key role in metabolizing high-molecular-weight proteins, glycoproteins, and other material and are intimately involved in the destruction of senescent and killed cells. They also are required for angiogenesis and wound healing and are able to induce neovascularization and endothelial cell proliferation. Given these diverse products and functions, macro phages are involved in many metabolic, infectious, inflammatory, and degenerative diseases.
Increases in blood monocytes (usually less than two times the normal level or less than 1.0 × 109/L) are a common feature of chronic inflammatory diseases and malignancies. Higher counts should raise concern about a hematologic malignancy (e.g., monocytic or myelomonocytic leukemia) [see 12:XVI Acute Leukemia].
Disorders of Monocytes and Macrophages
Histiocytic syndromes are a group of malignant and nonmalignant disorders in which the macrophages and dendritic (Langerhans) cells are the principal cells of abnormality.49 The malignant disorders include acute monocytic leukemia, monocytic sarcoma, and histiocytic sarcoma. The nonmalignant disorders include the Langerhans cell histiocytosis (LCH) syndromes and the hemophagocytic syndromes, such as sinus histiocytosis with massive lymphadenopathy, hemophagocytic lymphohistiocytosis (HL), and infection-associated hemophagocytic syndrome (IAHS).
Langerhans Cell Histiocytosis Syndromes
The LCH syndromes include solitary eosinophilic granuloma, multifocal eosinophilic granuloma, Hand-Schüller-Christian disease, and Letterer-Siwe disease.49,50 These disorders predominantly affect children 1 to 15 years of age but also occur in young adults. The LCH syndromes represent a continuum of disease that has been divided on the basis of histologic studies, age at diagnosis, extent of disease, and organ involvement. The signs and symptoms of the LCH syndromes depend on the specific organs involved.51 The bones, skin, teeth, gingival tissue, ears, endocrine organs, lungs, liver, spleen, lymph nodes, and bone marrow can all be involved and become dysfunctional as a result of cellular infiltration.52 For example, diabetes insipidus is caused by histocyte infiltration of the pituitary gland,53 and Erdheim-Chester disease is a multisystem disease characterized by histiocyte infiltration of many tissues.54 LCH with solitary and multifocal eosinophilic granuloma is found predominantly in older children and young adults; more infiltrative disease is common in younger patients.55On presentation, patients with solitary lesions may have an inability to bear weight, or they may have tender swelling caused by tissue infiltrates that overlie a sharply marginated bony lesion. Diagnosis is usually made by demonstration of dendritic cells, eosinophils, and giant cells present in a biopsy specimen; electron microscopy and immunostaining may be helpful for further classification.
Treatment of local LCH is sometimes unnecessary; when it is necessary, surgery or local radiation therapy can be curative.50,51,52,53,54,55LCH syndromes respond to chemotherapeutic agents, including vinblastine, methotrexate, 6-mercaptopurine, etoposide, or 2-chlorodeoxyadenosine (cladribine). There is a long-term risk of secondary or treatment-related malignancies in these patients.
Sinus Histiocytosis with Massive Lymphadenopathy
Sinus histiocytosis with massive lymphadenopathy, or Rosai-Dorfman disease, is characterized by chronic, painless, massive lymphadenopathy that usually involves the cervical nodes and less frequently involves the axillary, hilar, peritracheal, or inguinal nodes.56It occurs in both adults and children. Extra-nodal disease in the respiratory tract, bones, orbits, skin, liver, and kidneys is present in almost 30% of patients. The disease is usually benign, but significant morbidity and even death may result if massive tissue invasion of the liver, kidneys, lungs, and other critical structures occurs. Patients are usually of African descent, and the incidence of this disease is highest in Africa and the West Indies.
The affected lymph nodes show marked sinusoidal dilatation and follicular hyperplasia with proliferation of foamy histiocytes and multinucleated giant cells in the sinuses. The etiology of this disorder is unknown and may be related to abnormal immune regulation. Attempts at treatment should be reserved for special circumstances that are potentially life threatening. Surgery, irradiation, corticosteroids, vinblastine, and cyclophosphamide have all been administered with varying degrees of success.
HL is a rapidly fatal disorder, occurring as a familial or acquired condition; it is characterized by fever, pancytopenia, hepatic dysfunction, and activated macrophages, which overproduce inflammatory cytokines.57 Family studies suggest that a portion of cases are attributable to mutations in the perforin gene.58,59 The disease is usually diagnosed in young children; however, secondary forms of HL account for numerous cases in adults and occur in association with bacterial, fungal, and parasitic infections and exposure to various drugs. Treatment is difficult. Chemotherapy may be helpful. If the disease is associated with infection, treatment with appropriate antimicrobials may resolve the disorder.
LYSOSOMAL STORAGE DISEASES
Monocytes and macrophages play a role in tissue remodeling and the removal of senescent cellular debris, and lysosomes are the organelles that perform these functions; therefore, enzymatic abnormalities that involve lysosomal constituents result in disorders of storage that are related to macrophage function. These disorders, usually diagnosed in early childhood, include the mucopolysaccharidoses, the glycoproteinoses, the sphingolipidoses, and the neutral lipid storage diseases [see Table 6]. Enzymatic defects have been described for most of these disorders, and diagnosis depends on demonstrating the enzymatic abnormality in macrophages or histiocytes. Most of these defects result from point mutations or genetic rearrangements at a single locus of the gene that codes for a single lysosomal hydrolase.
Table 6 Lysosomal Storage Diseases
The two types of therapy for lysosomal storage diseases that are currently available are cellular transplantation and enzyme therapy.60Gaucher disease was formerly treated with bone marrow transplantation, but it is currently treated with alglucerase, an α-mannosyl-terminated glucocerebrosidase. Bone marrow transplantation for the other lysosomal storage diseases is investigational and has yielded mixed results.
Eosinophils can enhance or suppress acute inflammatory reactions and mediate responses to helminthic infection, allergy, and certain tumors.61 Like neutrophils, eosinophils are capable of phagocytosis, but eosinophils are primarily secretory cells. Most of the functions they perform require the release of granule contents or reactive oxygen species. The eosinophils respond to unique chemotactic agents and growth factors that permit their accumulation at sites of inflammation.
The granules of eosinophils contain strongly basic proteins and stain intensely with acid dyes. They have a striking and unique appearance on electron microscopy [see Figures 1a, 1b, 1c, and 1d]. The granules consist of an electron-dense core surrounded by a relatively radiolucent matrix; eosinophil peroxidase is active in the matrix. The dense core has a crystalloid structure and contains eosinophil cationic proteins (ECPs), major basic proteins (MBPs), and eosinophil-derived neurotoxins. MBPs and ECPs are capable of inflicting considerable damage to parasites such as schistosomula by binding to and disrupting their cell membranes. In addition, MBPs enhance the adherence of eosinophils and neutrophils to schistosomula.62
Eosinophils respond to a variety of chemotactic factors that enable them to enter tissues and carry out their functions. Some chemokines and chemotactic factors, such as C5a, N-formylmethionyl-containing peptides, and leukotriene B4, stimulate both eosinophils and neutrophils. Several chemotactic stimuli, however, are highly specific for eosinophils. Among these eosinophil-specific stimuli are platelet-activating factor (PAF), eosinophil chemotactic factor of anaphylaxis, and a variety of parasite-derived factors. Responses to PAF, one of the most potent activators of normal eosinophils, include chemotaxis, adherence, enhanced binding of IgE, production of superoxide, release of granule proteins, and synthesis of prostanoids.
Both the production and activation of eosinophils are affected by GM-CSF, IL-5, and IL-3. IL-5 appears to be critical for eosinophil production and deployment.63 Exposure to low doses of IL-5 also specifically primes eosinophils for later actions by other stimulants. Once activated, the eosinophils have enhanced generation of reactive oxygen species, enhanced glucose utilization and transport, increased oxygen consumption, a reduced cell surface charge, and activation of acid phosphatases in specific granules.
Eosinophils enhance the immune response to helminths. They perform this function by binding to the surface of both larval and adult forms, by damaging target cells through oxygen-dependent mechanisms that are similar to those of neutrophils, and by damaging cell surfaces by releasing granule proteins such as MBP and ECP. Although the release of these proteins similarly damages normal tissues and tumor cells, these interactions between eosinophils and host cells are less well understood. Eosinophils also produce cytokines that enhance the inflammatory response.64 The presence of eosinophilia in patients with Hodgkin disease appears to be a function of the production of IL-5 by Reed-Sternberg cells. Eosinophils contribute to the fibrosis of the nodular sclerosis type of Hodgkin disease by producing transforming growth factor-β1.
Disorders of Eosinophil Number
Evaluation of the patient with eosinophilia (eosinophil count > 700/mm3) is difficult because the causes of this disorder are multiple and diverse.65 Common causes of secondary eosinophilia include allergic disorders, infections caused by parasites and other organisms, dermatologic diseases, pulmonary diseases, collagen vascular disease, neoplasms, and immunodeficiency diseases. There are also myriad uncommon causes, such as eosin ophilic gastroenteritis, inflammatory bowel disease, chronic active hepatitis, pancreatitis, and hypopituitarism.
The term hypereosinophilic syndrome (HES) is often used for patients with chronic eosinophilia of unknown cause.66 The criteria used to diagnose HES are an unexplained eosinophil count of greater than 1,500/mm3 for longer than 6 months and signs or symptoms of infiltration of eosinophils into tissues. Recent evidence points to a mutation in chromosome 4 that results in linkage of the Rhe gene and the PDGFRα gene.67
The clinical features of HES are rash, fever, cough, dyspnea, diarrhea, and peripheral neuropathy. Patients may have chronic congestive heart failure, valvular abnormalities, and distinctive, fibrous, biventricular endocardial thickening with mural thrombi.66 The blood smear of a patient with HES usually reveals normal mature eosinophils of typical morphology; however, the presence of hypogranulation and cytoplasmic vacuoles has been reported. The total leukocyte count is typically 10,000 to 30,000/mm3, 30% to 70% of which are eosinophils. The bone marrow is generally hypercellular, with eosinophils constituting 25% to 75% of the marrow elements.
HES can usually be distinguished from malignant disorders associated with eosinophilia, such as acute or chronic eosin o philic leukemias.68Allergic reactions must also be excluded; the exclusion of such a reaction is usually based on the history, physical examination, and review of current medications. Because many drugs may generate an allergic reaction accompanied by eosinophilia, all nonessential medication should be discontinued before the patient is evaluated.
Parasitic infections, most commonly with such tissue-invasive helminths as filariae and Strongyloides, Trichinella, Schisto soma, andToxocara species, frequently present with eosin ophilia. To eliminate parasitosis as the cause of eosinophilia, multiple stool samples and a small bowel aspirate are recommended, particularly in patients who are at particular risk for infection (e.g., those who frequently travel, those who are exposed to animals, and those who have immunodeficiencies). If these test results are negative, serologic assays, radiologic tests, and peripheral blood and bone marrow smears should be performed to exclude the presence of connective tissue diseases, occult lymphoproliferative syndromes and solid tumors, and hematologic malignancies, respectively. In patients with possible cardiac involvement, an echocardiogram should be performed.
Therapy is directed toward lowering the eosinophil count and correcting specific symptoms. If symptoms involving the lungs or the heart are present, prednisone at a dosage of 1 mg/kg/day should be given for 2 weeks, followed by 1 mg/kg every other day for 3 months or longer. If this treatment fails or if an alternative is necessary to avoid steroid side effects, hydroxyurea at a dosage of 0.5 to 1.5 g/day should be given to lower the WBC count to less than 10,000/mm3 and the eosinophil count to less than 5,000/mm3. Study findings suggest that treatment with imatinib mesylate is effective.69 Alternative agents include interferon alfa, cyclosporine, and etoposide.
Basophil and Mast Cell Physiology
Basophils and mast cells are important in immediate hypersensitivity reactions, asthma, urticaria, allergic rhinitis, and anaphylaxis.70 They are derived from a common hematopoietic progenitor cell in the bone marrow and are stimulated by soluble mediators, primarily IgE, to release granule contents and arachidonic acid metabolites from their plasma membranes.
The cytoplasmic granules of both basophils and mast cells contain sulfated glycosaminoglycans; in normal basophils, the sulfated glycosaminoglycans are predominantly heparin. The sulfated glycosaminoglycans are the granule contents that are primarily responsible for the intense staining of the basophil. Most, if not all, of the circulating histamine in the body is synthesized by the basophil and stored in its granules. Degranulation causes the release of histamine, which mediates many immediate hypersensitivity effects and which, because it is a potent eosinophil chemoattractant, draws eosinophils to the site of degranulation. Other substances that are released on basophil degranulation include additional eosinophil chemotactic factors and a variety of arachidonic acid metabolites, the most important of which is leukotriene C4. In addition, the cell membranes of basophils contain high-affinity IgE receptors, the number of which tends to be increased in allergic persons.
Disorders of Basophil Number
Basophilia (basophil count > 150/mm3) is seen in myeloproliferative disorders, such as CML, polycythemia vera, and mye loid metaplasia; after splenectomy; in some hemolytic anemias; and in Hodgkin disease. The basophil count can also be increased in patients with ulcerative colitis or varicella infection. Although basophils and mast cells are involved in immediate hypersensitivity reactions and basophils are often seen in areas of contact dermatitis, basophilia is not seen in patients with these disorders.
Lymphocytes (e.g., B cells and T cells) are also derived from hematopoietic stem cells. These cells develop and mature in the bone marrow, thymus, spleen, and lymph nodes and in other specialized lymphoid tissues [see Section 6 Immunology and Allergy].
Disorders of Lymphocytes
Lymphocytosis in adults is defined as an absolute lymphocyte count greater than 4,000/mm3. In children with the disease, lymphocyte counts are higher than in adults and may be as high as 20,000/mm3 in the first year of life. The blood film of any patient with lymphocytosis should be carefully examined to determine the morphology and diversity of the lymphocytes (e.g., reactive lymphocytes, large granular lymphocytes, blasts, or smudge cells).
Lymphocytosis can be either primary or secondary. Primary lymphocytosis, often called lymphoproliferative disease, is caused by dysregulation in the production of lymphocytes. The primary lymphocytoses include the leukemias (e.g., chronic lymphocytic leukemia, acute lymphocytic leukemia, hairy-cell leukemia, or large granular lymphocyte leukemia), the lymphomas, and monoclonal B cell lymphocytosis [see Table 7].
Table 7 Causes of Lymphocytosis
The reactive, or secondary, lymphocytoses are conditions that involve absolute increases in lymphocytes caused by physiologic or pathophysiologic responses to infection, inflammation, toxins, cytokines, or unknown agents. The most common causes of reactive lymphocytosis are viral infections: Epstein-Barr virus, cytomegalovirus, herpes simplex virus, varicella-zoster virus, rubella, human T cell lymphotropic virus type I (HTLV-I), HIV, adenovirus, or one of the hepatitis viruses is frequently responsible for the disease. Other pathogens that produce lymphocytosis are Toxoplasma gondii and, in children, Bordetella pertussis (which causes the lymphocyte count to rise to as high as 70,000/mm3). Lymphocytosis is also associated with stress and consequent release of epinephrine, such as that seen in patients who have had cardiovascular collapse, septic shock, sickle cell crisis, status epilepticus, trauma, major surgery, drug reactions, or hypersensitivity. Persistent lymphocytosis may be seen in patients with autoimmune disorders, sarcoidosis, hyposplenism, or cancer and in those who are long-term cigarette smokers.
Lymphocytopenia is defined as a total lymphocyte count less than 1,000/mm3. Because in adults 80% of lymphocytes are T cells, most cases of lymphocytopenia are caused by a reduction in the T cell count. The mechanisms of lymphocytopenia are often unknown, and the causes are usually differentiated as inherited or acquired.
Inherited lymphocytopenias are usually caused by congenital immunodeficiency diseases. These diseases include severe combined immunodeficiency (e.g., adenosine deaminase deficiency, purine-nucleoside phosphorylase deficiency, and reticular dysgenesis), ataxia-telangiectasia, Wiskott-Aldrich syndrome, and cartilage-hair hypoplasia [see Table 8]. In addition, some persons have idiopathic CD4+ T cell lymphocytopenia.
Table 8 Causes of Lymphocytopenia
Acquired lymphocytopenia can be seen in patients with viral infections, such as HIV infection, hepatitis, influenza, and respiratory syncytial virus infection; in patients with certain bacterial infections, such as typhoid fever, pneumonia, sepsis, and tuberculosis; and in patients with aplastic anemia, autoimmune diseases, Hodgkin disease, sarcoidosis, renal failure, protein-losing enteropathies, and chylous ascites. Zinc deficiency and long-term alcohol ingestion are also associated with lymphocytopenia. Finally, immunosuppressive agents, such as antithymocyte globulin, corticosteroids, chemotherapeutic agents, and radiation, also produce lymphocytopenia.
Figure 1 Tom Moore. Electron micrographs courtesy of Dr. E. Chi, University of Washington School of Medicine, Seattle.
Figure 3 Tom Moore.
Editors: Dale, David C.; Federman, Daniel D.