Once recognition occurs, innate immunity is activated to increase production of proinflammatory signals that have three key effects: (1) to kill invaders and recruit other immune cells to the area, (2) to block the infection from causing disease beyond the local site of inflammation, and (3) to aid in repairing the damaged barrier. The cells that exert these functions can be categorized as antigen-presenting cells (APCs), granulocytes, and innate lymphocytes called NK cells.
Antigen-Presenting Cells: Macrophages & Monocytes
All nucleated cells express a protein called class I major histocompatibility complex (MHC), and cells present this protein on their surface in complex with peptides from cell cytosol for recognition by cytotoxic T cells (see Chapter 60). Some cells, for example, macrophages and dendritic cells, also express a different protein called class II MHC. This protein is presented on the cell surface in complex with peptides contained within endosomes, or vesicles, in the cell, and they can only be recognized by helper T cells. For this reason, the cells that are capable of presenting peptides by class II MHC proteins are referred to as “professional” APCs.
The most abundant professional APCs are myeloid cells called macrophages. They derive from precursors in the yolk sac and liver during fetal development, or from bone marrow in adults. In every tissue in the body, there are long-lived, resident macrophages that arrive before birth. These are usually the first cells to encounter foreign invaders or injured tissue, and some examples include the microglial cells in the brain, the alveolar macrophages in the lung, and the Kupffer cells in the liver. In addition to tissue resident macrophages, there are other cells, called monocytes, which are short-lived myeloid cells that patrol the body throughout life, reacting to inflammation by rapidly entering inflamed tissue and differentiating into macrophages or dendritic cells on demand.
Tissue-resident macrophages and monocyte-derived macrophages have three main functions: phagocytosis, antigen presentation, and cytokine production (Table 58–3 and Figure 58–2).
The functions of phagocytic antigen-presenting cells. A: (1) Microbes such as bacteria, viruses, or fungi are sensed by surface pattern recognition receptors (PRRs) and/or receptors for antibody (opsonins), facilitating phagocytosis by macrophages. The receptors are engulfed together with the microbe into the phagosome within the cell. Once there, the phagosome and lysosome are fused, exposing the microbe to degradative enzymes and free radicals. (2) The microbe is killed and its proteins are cleaved into short peptides, which are then complexed with class II major histocompatibility complex (MHC) proteins for presentation on the cell surface. (3) At the same time, killing of the microbe exposes more ligands for PRRs, which leads to transcription of inflammatory cytokine genes, activation of the inflammasome, and expression of co-stimulatory signals and cytokines that drive further inflammation. B: (1) Intracellular (cytosolic) microbes are degraded by the proteasome, releasing (1) antigens for loading onto class I MHC proteins and (2) ligands for PRRs that activate the macrophage to express co-stimulatory signals and inflammatory cytokines. C: Dendritic cells perform these functions in much the same fashion, and in addition, some dendritic cells can (4) complex endosomal antigens with both class I and class II MHC proteins. (For simplification, some aspects of the pathway are omitted.)
TABLE 58–3Important Features of Macrophages ||Download (.pdf) TABLE 58–3 Important Features of Macrophages
Ingestion and killing of microbes in phagolysosomes. Killing caused by reactive oxygen intermediates such as superoxides, reactive nitrogen intermediates such as nitric oxide, and lysosomal enzymes such as proteases, nucleases, and lysozyme.
Presentation of short peptide antigens in association with class II MHC proteins to helper T cells. Co-stimulatory signals are also required (see Chapter 60).
Synthesis and release of cytokines, such as IL-1, IL-6, IL-8, and TNF.
Phagocytosis. Macrophages, neutrophils, and dendritic cells ingest bacteria, viruses, and other foreign particles. They are activated to do this when their PRRs recognize foreign molecular patterns (see Table 58–2). Phagocytes also have two other important types of receptors: one type for C3b, part of the complement system that binds to microbes making them easier to ingest (see Chapter 63), and another type for immunoglobulins that similarly enhance the uptake of Ig-bound microbes. (Factors such complement and immunoglobulins that bind to microbes and enhance phagocytosis are called opsonins.)
After ingestion, the phagosome containing the microbe fuses with a lysosome. The microbe is killed within this phagolysosome by proteases as well as by reactive oxygen and reactive nitrogen radicals (generated by NADPH oxidase and nitric oxide synthase, respectively), which can directly attack the microbe or can be converted to other toxic species such as peroxide and hypochlorite. This reaction is called the oxidative burst, and it is a critical innate immune mechanism for killing many microorganisms.
Genetic defects in NADPH oxidase cause chronic granulomatous disease (CGD), a condition in which phagocytes are unable to generate an oxidative burst. This causes severe infections as the macrophages and neutrophils, unable to kill the microbes they have ingested, resort to forming large granulomas (see Chapter 68). Note that the ingestion and killing of microbes is further enhanced by adaptive immunity: antibodies, especially IgG, can act as opsonins (see Chapter 61), and phagocytes’ killing mechanisms are enhanced by cytokines, especially gamma interferon (IFN-γ), from activated T cells (i.e., T-cell–mediated immunity; see Chapter 61).
Antigen presentation. After foreign material is ingested and degraded, fragments of antigen are presented on the macrophage cell surface in conjunction with class II MHC proteins (for interaction with helper T cells). The fragments presented on MHC as antigens are short peptides. (See Table 58–3 and Chapters 60, 61, and 62 for more details about T-cell interactions with class I MHC and class II MHC proteins.) Degradation of the foreign protein stops when the fragment associates with the MHC protein in the cytoplasm. The peptide–MHC complex is then transported to the cell surface to be presented to T cells. Professional APCs also provide surface “co-stimulatory” signals for the T cells, providing a “red flag” that the peptide came from a foreign source, and these signals are enhanced during macrophage activation (see below and Chapter 60).
Cytokine production. In addition to co-stimulatory signals, which are increased in an inflammatory context, macrophages produce several cytokines that further enhance inflammation. The most important of these are interleukin (IL)-1, IL-6, and tumor necrosis factor-α (TNF-α). These are important mediators of inflammation. In addition, macrophages produce IL-8, a “chemokine” that attracts neutrophils and T cells to the site of infection. (Chemokines are cytokines that attract leukocytes to where they need to go.)
The macrophage’s phagocytic ability, antigen presentation, and cytokine production are greatly enhanced when a process called macrophage activation occurs. Macrophages are activated initially by substances such as bacterial LPS (endotoxin), bacterial peptidoglycan, or bacterial DNA. These substances are PAMPs that interact with TLRs and other macrophage PRRs, as described above, and signal the cell to increase its expression of co-stimulatory molecules and its production of cytokines, including TNF-α.
Macrophages are also activated by cytokines produced by other cells. For example, the cytokine IFN-γ, produced by T cells and NK cells, increases the synthesis of class II MHC proteins, which enhances antigen presentation. IFN-γ also increases the microbicidal activity of macrophages by inducing the synthesis of NADPH oxidase, which produces reactive oxygen species (ROS).
Antigen-Presenting Cells: Dendritic Cells
Dendritic cells are another “professional” APC (i.e., they express class II MHC proteins and present antigen to helper T cells). They are particularly important because they are the main inducers of the primary adaptive immune response, thus serving as a bridge between innate and adaptive immunity. They are called “dendritic” because their long, narrow branches make them very efficient at making contact with foreign material (déndron is Greek for “tree”).
Dendritic cells are primarily located in barrier tissues, including the skin and the mucosa of the gastrointestinal, respiratory, and genitourinary tracts. As mentioned earlier, some dendritic cells are also derived from monocytes that are recruited into infected tissue by inflammatory signals. As with macrophages, dendritic cells engulf foreign material, process it into peptide fragments, or antigens, and present the antigens on MHC proteins to interact with cytotoxic T cells (through class I MHC proteins) and with helper T cells (through class II MHC proteins).
But two very important features of dendritic cells distinguish them from macrophages. First is their ability to collect antigens and then migrate from these barrier locations, through the draining lymphatic vessels, and into local lymph nodes. To do this, the dendritic cell uses the C-C chemokine receptor 7, or CCR7, a receptor on its cell surface, to sense and migrate toward chemokines that are produced by stromal cells (called fibroblastic reticular cells) in the lymphoid tissue.
Once in the lymph node, the dendritic cell presents antigen complexed with MHC proteins to “naïve” T cells in the T-cell zone. Thus, dendritic cells, and not macrophages, are responsible for priming naïve T cells to become activated during the initiation of an immune response. Macrophages only interact with already-activated T cells in the peripheral inflamed tissue. How naïve T cells undergo priming by dendritic cells is discussed in greater detail in Chapter 60.
The second special feature of dendritic cells is that some of them can present endosomal antigens on class I MHC. As described earlier, all nucleated cells express cytosolic peptides on class I MHC. Usually, these peptides are innocuous “self” antigens and do not elicit an immune response, but if a cell is infected by a virus, those viral peptides will be presented in complex with class I MHC for recognition by cytotoxic T cells.
A particular subset of dendritic cells is able to phagocytize viral particles into their endosomes and present them on both class I MHC and class II MHC, bypassing the step in which the cell becomes infected with the virus. This process is called cross-presentation, and it allows dendritic cells to prime naïve cytotoxic T cells to recognize tissue-tropic viruses, such as hepatitis B virus, without the dendritic cell itself actually being infected (Figure 58–3). The process by which certain dendritic cells are capable of cross-presentation is not fully known.
Key features of myeloid-derived innate immune cells. The figure lists some of the distinguishing characteristics and functions of myeloid cells, as well as their interactions with adaptive immunity (T cells and antibody). Note that only macrophages and dendritic cells are “professional” antigen-presenting cells (APCs) and that dendritic cells are primarily responsible for initial activation of the T-cell response. See text for abbreviations.
Neutrophils are the most abundant immune cell in the blood. They are phagocytes that belong to the family of myeloid white blood cells, and in addition, they are in a subgroup called granulocytes, named for their cytoplasmic granules visible with Wright stain. Neutrophils are a very important component of our innate host defenses, and severe bacterial and fungal infections occur if they are too few in number (neutropenia) or are deficient in function (as in some of the immune disorders discussed in Chapter 68).
Neutrophil granules stain a pale pink (neutral) color with Wright stain, in contrast to eosinophils and basophils, whose granules stain red and blue, respectively. (The differences in the staining color of the various types of granulocytes are due to differences in the charge of their various granules’ contents.) The pink granules are lysosomes, which contain a variety of degradative enzymes that are important in the microbicidal action of these cells. The process of phagocytosis and killing by neutrophils is described in detail in Chapter 8.
Like macrophages, neutrophils have surface receptors for IgG, making it easier for them to phagocytize opsonized microbes. Note that neutrophils do not display class II MHC proteins on their surface and therefore do not present antigen to helper T cells. This is in contrast to macrophages, which are both phagocytes and APCs, as discussed earlier.
Neutrophils can be thought of as a “two-edged” sword. The positive edge of the sword is their powerful microbicidal activity, but the negative edge is the tissue damage caused by the release of degradative enzymes. For example, the neutrophil gelatinase-associated lipocalin (NGAL, also known as lipocalin-2) is a protease that is also a urine biomarker of acute kidney injury, which can occur during acute poststreptococcal glomerulonephritis. In this disease, immune complexes composed of antibody, streptococcal antigens, and complement attach to the glomerular membrane. Neutrophils that are attracted into the glomeruli and activated by the immune complexes, release their enzymes causing kidney damage.
Eosinophils are white blood cells with cytoplasmic granules that appear red when stained with Wright stain. The red color is caused by the negatively charged eosin dye binding to the positively charged major basic protein in the granules. The eosinophil count is elevated in two medically important types of diseases: parasitic diseases, especially those caused by tissue-invading nematodes and trematodes (see Chapters 55 and 56, respectively) and hypersensitivity diseases, such as asthma and serum sickness (see Chapter 65). Diseases caused by protozoa are typically not characterized by eosinophilia.
Eosinophil major basic protein can damage respiratory epithelium and contributes to the pathogenesis of asthma. Interestingly, the protective function of eosinophils has not been clearly established. It seems likely that they defend against the migratory larvae of parasites, such as Strongyloides and Trichinella. These parasites become coated with IgE, and eosinophils, which have receptors for IgE, can then attach to the surface of larvae and discharge the contents of their eosinophilic granules, damaging the cuticle of the larvae. The granules of the eosinophils also contain leukotrienes and peroxidases, which can damage tissue and cause inflammation.
However, another function of eosinophils may be to reduce inflammation. The granules of eosinophils contain histaminase, an enzyme that degrades histamine, which is an important mediator of immediate hypersensitivity (allergic) reactions. Eosinophils can phagocytize bacteria but they do so weakly, and they do not present antigen with class II MHC. Therefore, they are not sufficient to protect against pyogenic bacterial infections in neutropenic patients. The growth and differentiation of eosinophils are stimulated by the cytokine interleukin-5 (IL-5), and eotaxin is a chemokine (see below) that attracts eosinophils from the blood into tissues.
Granulocytes: Basophils & Mast Cells
Basophils are white blood cells with cytoplasmic granules that appear blue when stained with Wright stain. The blue color is caused by the positively charged methylene blue dye binding to several negatively charged molecules in the granules. Basophils circulate in the bloodstream, whereas mast cells, which are similar to basophils in many ways, are fixed in tissue, especially under the skin and in the mucosa of the respiratory and gastrointestinal tracts.
Basophils and mast cells have receptors on the cell surface for the Fc portion of the heavy chain of IgE. When adjacent IgE molecules are cross-linked by antigen, the cells release preformed inflammatory mediators from their granules. Some examples of these mediators are histamine, proteolytic enzymes, and proteoglycans such as heparin. They also release newly generated eicosanoids, such as prostaglandins and leukotrienes. These cause inflammation and, when produced in large amounts, cause a wide range of immediate hypersensitivity reactions: the mildest form is urticaria (hives), while the most severe form is systemic anaphylaxis.
Basophils and mast cells also release cytokines and chemokines that recruit and activate other cells during bacterial and viral infection. For example, the surface of mast cells contains TLRs that recognize bacteria and viruses. The mast cells respond by releasing cytokines and enzymes from their granules that mediate inflammation and attract neutrophils and dendritic cells to the site of infection.
NK cells play two important roles in immunity: (1) they kill virus-infected cells and tumor cells and (2) they produce IFN-γ that activates macrophages to kill bacteria that they have ingested (see Chapter 60). NK cells are called “natural” because, unlike adaptive cells, they do not recognize their target cells by detecting antigens presented by class I or class II MHC proteins, they are not enhanced by exposure, they have no memory, and they are relatively nonspecific for any one virus or tumor.
Rather, NK cells target cells to be killed by detecting other features of cell dysfunction, for example, the lack of class I MHC proteins on the cell surface. This detection process is effective because many cells lose their ability to synthesize class I MHC proteins after they have been infected by a virus.
NK cells can also detect cancer cells by recognizing a protein called MICA that is found on the surface of many cancer cells but not normal cells. Interaction of MICA with a receptor on NK cells triggers the production of cytotoxins. Table 58–4 summarizes some of the key features of NK cells.
TABLE 58–4Important Features of Natural Killer (NK) Cells ||Download (.pdf) TABLE 58–4 Important Features of Natural Killer (NK) Cells
I. Nature of NK Cells
Large granular lymphocytes
Lack T-cell receptor, CD3 proteins, and surface IgM and IgD
Thymus not required for development
Normal numbers in severe combined immunodeficiency disease (SCID) patients
Activity not enhanced by prior exposure
Have no memory
II. Function of NK Cells
Recognize virus-infected cells by detecting lack of class I MHC proteins on the surface of the infected cells
Kill virus-infected cells and cancer cells using perforin and granzyme
Killing is nonspecific and is not dependent on foreign antigen presentation by class I or II MHC proteins
Produce gamma interferon that activates macrophages to kill ingested bacteria
NK cells kill virus-infected cells and tumor cells by secreting cytotoxins (perforins and granzymes) that induce apoptosis. They can do this without antibody, but antibody (IgG) enhances their effectiveness, a process called antibody-dependent cellular cytotoxicity (ADCC) (see Chapter 61). IL-12 produced by macrophages and interferons alpha and beta produced by virus-infected cells are potent activators of NK cells. Approximately 5% to 10% of peripheral lymphocytes are NK cells. Humans who lack functional NK cells are predisposed to severe infections with herpesviruses and human papillomavirus, as well as various cancers.