Which type of leukocyte constitutes about 70 of the total circulating white blood cells?

Granulocytic Phagocytes

Granulocytic phagocytes are the cellular effectors of microbe killing, engulfing them and enzymatically lysing their cell membranes or walls. Two major types are polymorphonuclear leukocytes (neutrophils) and macrophages (the tissue version of circulating monocytes). Macrophages have surface receptors that recognize nonvertebrate carbohydrates, such as mannose, which form the cell wall of some microorganisms. Hence, they can identify and attack “invaders” rather than “self.”

Two other types of granulocytes, eosinophils and basophils, are less involved in the ingestion of organisms. Eosinophils mediate the destruction of certain parasitic helminths through release of toxic proteins. Normally only 3% of total granulocytes, this cell type can reach 20% during times of high parasite load. Basophils (rare in circulation) and their tissue counterparts, mast cells, have high affinity for IgE. On exposure to antigens, they release granules with histamine, prostaglandins, and leukotrienes, which affect the allergic-inflammatory response with increased vascular permeability, bronchospasm, and vasodilation.

Neutrophils constitute 90% of circulating granulocytes and spend only 6 to 8 hours of their average 4-day life in circulation (the remainder in tissues). Effective antibacterial activity depends on the ability of neutrophils to travel to sites of infection, a process known aschemotaxis. The locomotion of neutrophils along vascular endothelium is facilitated by adherence to cell surface proteins whose production is enhanced in the initial inflammatory response.

Half of all neutrophils that leave the bone marrow circulate in the plasma. The other half become marginated, adhering to endothelium, primarily in the lungs, liver, and spleen. During periods of stress or with endogenous or exogenous catecholamines or corticosteroids, these neutrophils demarginate and enter the circulation. As long as the patient is not neutropenic, demargination causes an increased peripheral neutrophil count composed of mature cells, whereas with bacterial infection, an increased proportion of immature (band) forms and is more typically seen.

Neutrophils (and macrophages in tissue) bind to and ingest bacteria through phagocytosis. This process is enhanced by proteins calledopsonins that bind to bacterial surfaces. C-reactive protein, one of the initial inflammatory response proteins, fulfills this function for certain bacteria, includingS. pneumoniae. IgG and complement protein C3b also opsonize bacteria, again illustrating the interdependence of the immune system. Actual killing takes place within granulocytes when cytoplasmic granules enzymatically produce potent oxidants. Granulocytes further control bacterial proliferation at the site of infection by elaborating lactoferrin, which locally binds free iron necessary for bacterial replication.

In addition to phagocytosis, macrophages (located in the spleen, alveoli, liver, and lymph nodes) modulate the immune response by presenting antigens to lymphocytes and releasing cytokines and complement components. Activation of macrophages to ingest bacteria depends on interaction with interferon-γ, a cytokine manufactured by T cells.2 Thus the once clear demarcation between cellular and humoral immunity is breaking down as more is understood about the interdependent immune system.

Low-Density Granulocytes in SLE

Low-density granulocytes are a specific subset of aberrant neutrophils that have recently been identified in peripheral blood mononuclear cell (PBMC) preparations from pediatric and adult SLE patients (Kaplan, 2011). Low-density granulocytes display phenotypic characteristics of immature neutrophils with nonsegmented nuclei and greater expression of myeloperoxidase, neutrophil elastase, and defensin-3 (Hacbarth and Kajdacsy-Balla, 1986; Bennett et al., 2003). Low-density granulocytes have a proinflammatory phenotype characterized by augmented secretion of tumor necrosis factor (TNF) and type I and II IFNs after stimulation, which could promote and increase tissue damage. They also have a markedly increased ability to kill endothelial cells upon cell–cell contact (Denny et al., 2010), and to form NETs compared with normal density SLE-derived neutrophils and control neutrophils (Villanueva et al., 2011) (Figure 14.1). Low-density granulocytes are suggested to account for the increased type I IFN production that leads to abnormal function of endothelial progenitor cells and/or circulating myeloid angiogenic cells in vitro and, possibly, in vivo in SLE. This aberrant subset of neutrophils has an increased propensity to undergo NETosis (Knight and Kaplan, 2012), and SLE patients with cutaneous involvement and vasculitis have a higher percentage of low-density granulocytes.

Granulocytes

In normal circumstances, neutrophils, sometimes accompanied by eosinophils, congregate at the site of inflammation and are followed by macrophages. Formation of this inflammatory exudate is the result of activation of humoral factors and normal function of the vascular endothelium (seeChapter 8). Meanwhile, in the peripheral blood, granulocytosis evolves as a consequence of release of the marrow reserve and increased granulocytopoiesis, which is regulated by hematopoietic growth factors such as interleukin (IL)-3, granulocyte-macrophage colony-stimulating factor, and granulocyte colony-stimulating factor.5

Virtually all cytotoxic drugs used in the treatment of malignant diseases have a deleterious effect on the proliferation of normal hematopoietic progenitor cells. Therefore, after obliteration of the mitotic pool and depletion of the marrow pool reserve, neutropenia ensues. Likewise, therapeutic radiation can induce clinically important neutropenia, depending on the dose rate, total dose given, and irradiated area of the body. Total-body irradiation, as used to prepare for HSCT, is the most obvious illustration of the possible negative impact of irradiation. Thus, profound neutropenia is an unavoidable consequence of the treatment of malignancy and may persist for 3 or 4 weeks or even longer. Neutropenia or a treatment-related decrease in the granulocyte count is probably the most important primary risk factor for infection. Fever develops in nearly all cases of profound neutropenia (i.e., a granulocyte count <100/mm3 for more than 2–3 weeks), whereas only one-fifth of the febrile episodes in cancer patients occur when granulocyte counts are normal.6 Moreover, during iatrogenic neutropenia, the risk for infection and infection-related mortality increases proportionally with time.

Granulocytes that accumulate at the site of infection are of little use if they are unable to function normally. Antineoplastic drugs and irradiation interfere with these nonproliferating cells and their function, resulting in decreased chemotaxis, diminished phagocytic capacity, and defective intracellular killing by granulocytes. Glucocorticosteroids seem to enhance granulocytopoiesis and mobilize the marginal and the marrow pool reserve, but these putative positive effects on neutrophilic granulocytes are offset by numerous disadvantages. These drugs curb the accumulation of neutrophils at the site of inflammation by reducing their adherent capacity and diminishing their chemotactic activity. Furthermore, they decrease phagocytosis and intracellular killing of microorganisms. The lack of functioning neutrophils deprives the host of a primary defense mechanism against invading microorganisms, which are consequently able to readily establish themselves, initiate local infection, disseminate unhindered, and eventually lead to fulminant sepsis and death unless managed promptly and effectively.

Granulocyte Transfusion

The ability to collect and administer granulocytes as a therapeutic option has been around since the 1970s. However, the use of granulocytes fell out of favor due to equivocal results from it use, variable yields from collections, as well as the cumbersome collection process as more efficacious antimicrobials were emerging. However, more recently, there has been a renewed interest in granulocytes as a therapeutic option as apheresis technology has improved as well as improved granulocyte collection from donors through the addition of granulocyte colony stimulating factor, thus allowing increased mobilization of granulocytes from the donor [31].

Granulocytes should be considered as a therapeutic option when

The patient’s absolute neutrophil count (ANC) is less than 500/μL

There is a bacterial or fungal infection or suspected infection, for 24–48 h

The patient shows lack of improvement to antimicrobial treatment

There is a reasonable chance that the patient will have marrow recovery

Other indications for which granulocytes may show improvement include neonates with sepsis or patients with neutrophil dysfunction, such as patients with chronic granulomatous disease.