Phagocytes Ingest Target Cells
A key mechanism by which white blood cells destroy invading microorganisms is phagocytosis (Figure 54–4). Phagocytic leukocytes recognize and bind target cells using receptors that recognize either endogenous surface groups, such as bacterial lipopolysaccharides, or peptidoglycans. In most cases, however, infective pathogens are recognized indirectly, by the presence of antibodies or complement factors that have previously adhered to their surface (see Chapter 52). The process of tagging an invader with protective proteins to facilitate recognition by phagocytic leukocytes is called opsonization.
Phagocytosis. This figure depicts the destruction of an opsonized microorganism, shaded in ORANGE, by a neutrophil via phagocytosis. The multilobed nucleus of the neutrophil is shown in purple, secretory granules in green. The presence of an antibody or complement tag is indicated by a yellow triangle, with the corresponding cell surface receptor as a bright orange square. Cellular debris from the microorganism is represented as orange line segments. (A) The neutrophil binds an antigen molecule on the opsonized microbe via a receptor. (B) The neutrophil envelops the microbe. (C) Secretory granules fuse with the newly internalized phagosome, delivering their contents. (D) Granule-derived enzymes and cytotoxins destroy the microorganism. (E) The phagosome then fuses with the cell membrane, expelling any remaining debris.
Receptor binding triggers dramatic alterations in the shape of the phagocyte, which proceeds to envelop the target cell until it is encased within an internalized membrane vesicle called a phagosome (phagolysosome). The internalized cell is then destroyed using a combination of hydrolytic enzymes (eg, lysozyme, proteases), antimicrobial peptides (defensins), and reactive oxygen species. The enzymes and toxins responsible for the lysis of the enveloped cell and breakdown of its macromolecular components (Table 54–2) are stored in cytoplasmic vesicles that fuse with the phagosome. These vesicles are oftentimes referred to as granules, and the cells that harbor them as granulocytes, on the basis of their appearance when examined under a microscope. Eventually, the phagosome migrates to the plasma membrane of the white blood cell, where it fuses and expels the remaining debris.
TABLE 54–2Enzymes and Proteins of the Granules of Phagocytic Leukocytes ||Download (.pdf) TABLE 54–2 Enzymes and Proteins of the Granules of Phagocytic Leukocytes
|Enzyme or Protein ||Reaction Catalyzed or Function ||Comment |
|Myeloperoxidase (MPO) || where X- = Cl-, HOX = hypochlorous acid ||Responsible for the green color of pus Genetic deficiency can cause recurrent infections |
|NADPH oxidase || ||Key component of the respiratory burst Deficient in chronic granulomatous disease |
|Lysozyme ||Hydrolyzes link between N-acetylmuramic acid and N-acetyl-d-glucosamine found in certain bacterial cell walls ||Abundant in macrophages. Hydrolyzes bacterial peptidoglycans |
|Defensins ||Basic antibiotic peptides of 20-33 amino acids ||Apparently kill bacteria by causing membrane damage |
|Lactoferrin ||Iron-binding protein ||May inhibit growth of certain bacteria by binding iron and may be involved in regulation of proliferation of myeloid cells |
|Proteases ||Abundant in phagocytes; Breakdown protein components of infectious organisms; Generate fragments for antigen presentation |
The components of this debris, which include fragments of proteins, oligosaccharides, lipopolysaccharides, peptidoglycans, and polynucleotides, provide an important source of antigens for stimulating the production of new antibodies. Lymphocytes and other white blood cells absorb these materials via endocytosis (see Figure 40–21). The phagocyte often will absorb some of the debris from the phagosome and route it to the cell surface in association with a membrane protein called the major histocompatibility complex (MHC). The MHC serves as a scaffold for presenting potential antigens to surrounding lymphocytes in a form that stimulates the production of new antibodies.
The three principal classes of phagocytic leukocytes are neutrophils, eosinophils, and macrophages. Neutrophils, which comprise roughly 60% of the white blood cells present in the circulation, phagocytize bacteria and small eukaryotic microorganisms such as fungi. The less numerous eosinophils, which make up 2% to 3% of the leukocytes in the blood, ingest larger eukaryotic microorganisms such as paramecia. Macrophages are derived from monocytes, which comprise about 5% of the leukocytes in the blood. Monocytes migrate from the bloodstream into tissues throughout the body where, upon receipt of a stimulus, they differentiate to form macrophages. While macrophages also can ingest invading microbes, the signature function of these large phagocytes is to remove human host cells that have been compromised by infection, malignant transformation, or programmed cell death, also known as apoptosis. These functionally compromised cells are recognized by the appearance of aberrant proteins and oligosaccharides on their surface. Precocious activation of macrophages is associated with the etiology of many degenerative diseases such as osteoporosis, atherosclerosis, arthritis, and cystic fibrosis, and can facilitate the metastasis of cancer cells.
Phagocytic Leukocytes Generate Reactive Oxygen Species During the Respiratory Burst
Reactive oxygen species (ROS) such as , H2O2, OH•, and HOCl (hypochlorous acid) form a major component of the chemical and enzymatic arsenal employed by phagocytes to destroy ingested cells. Production of the various reactive oxygen derivatives takes place shortly (15-60 seconds) after internalization of an encapsulated cell, using O2 and electrons derived from NADPH. The accompanying surge in oxygen consumption has been termed the respiratory burst. The production of large quantities of NADPH is facilitated by the heavy reliance of phagocytes, which contain relatively few mitochondria, on aerobic glycolysis to generate ATP. The consequent need to maintain robust supplies of glycolytic precursors and intermediates ensures the availability of the glucose 6-phosphate required to reduce NADP+ to NADPH via the pentose phosphate pathway (see Chapter 20).
The formation of microbicidal reactive oxygen derivatives during the respiratory burst starts with the synthesis of superoxide, which is catalyzed by the NADPH oxidase system. Catalysis proceeds via a two-step mechanism. The first step is the reduction of molecular oxygen to form superoxide (Table 54–2):
This is followed by the spontaneous dismutation of hydrogen peroxide from two molecules of superoxide:
The NADPH oxidase system is comprised of cytochrome b558, a plasma membrane-associated heterodimer containing polypeptides of 91 kDa and 22 kDa, and two cytoplasmic peptides of 47 kDa and 67 kDa. Upon activation, the cytoplasmic peptides are recruited to the plasma membrane where they associate with cytochrome b558 to form the active complex. The NADPH is generated by the pentose phosphate cycle, whose activity also increases markedly during phagocytosis. Any superoxide from the phagosomes that enters the cytosol is converted to H2O2 by superoxide dismutase, which catalyzes the same reaction as the spontaneous dismutation shown above. In turn, H2O2 is used by myeloperoxidase (see below) or disposed of by the action of glutathione peroxidase or catalase.
Myeloperoxidase Catalyzes the Production of Chlorinated Oxidants
The formation of hypohalous acids during the respiratory burst is catalyzed by the enzyme myeloperoxidase.
Present in large amounts in neutrophil granules, this enzyme uses H2O2 and to oxidize Cl− and other halides to produce hypohalous acids such as HOCl. HOCl, the active ingredient of household liquid bleach, is a powerful oxidant that is highly microbicidal. When applied to normal tissues, its potential for causing damage is diminished because it reacts with primary or secondary amines present in neutrophils and tissues to produce various nitrogen-chlorine derivatives. While also oxidants, these chloramines are less powerful than HOCl, and therefore can act as microbicidal agents (eg, in sterilizing wounds) without causing tissue damage.
Mutations Affecting the NADPH Oxidase System Cause Chronic Granulomatous Disease
Functionally deleterious mutations in the genes encoding any of the four polypeptides of the NADPH oxidase system can cause chronic granulomatous disease. The resulting decrease in the production of reactive oxygen derivatives undermine the ability of neutrophils and other phagocytic leukocytes to kill bacteria and other infectious microbes. Persons suffering from this relatively uncommon condition experience recurrent infections. They also form granulomas (chronic inflammatory lesions) in the skin, lungs, and lymph nodes to wall off invading pathogens. In some cases, relief can be provided by the administration of gamma interferon, which may increase transcription of the 91-kDa component of cytochrome b558.