Chapter 8 The immune system and the oral cavity
Immunology is the branch of biology concerned with the body’s defence reactions. The word ‘immunity’ is derived from the Latin word immunis, meaning ‘free of burden’. In essence, the immune system exists to maintain the integrity of the body by excluding or removing the myriad of potentially burdensome or threatening microorganisms, which could invade from the environment. Internally derived threats, mutant cells with malignant potential, may also be attacked by the immune system.
During its development, the immune system must be educated specifically to avoid reacting against all normal components of the body (tolerance). Immunology can be considered ‘the science of self-non-self discrimination’.
The vital importance of the immune system is evident in the life-threatening infections suffered by patients with immune defects (immunodeficiency). In other situations, there may be too much immunity. A by-product of a successful immune response may be damage to normal ‘bystander’ cells, but this is normally limited by stringent immune regulatory mechanisms. Deficiencies of immunoregulation may be the root causes of hypersensitivity diseases such as autoimmunity and allergy.
These intrinsic defence mechanisms are present at birth prior to exposure to pathogens or other foreign macromolecules. They are not enhanced by such exposures and are not specific to a particular pathogen.
Intact skin is usually impenetrable to microorganisms. Membranous linings of the body tracts are protected by mucus, acid secretions and enzymes such as lysozyme, which breaks down bacterial cell wall proteoglycan. In the lower respiratory tract, the mucous membrane is covered by hair-like protrusions of the epithelial cell membrane called cilia. The movement of cilia can propel mucus-entrapped microorganisms from the tract (mucociliary escalator). Although most pathogens enter the body by binding to and penetrating mucous membranes, several defence mechanisms, including saliva, tears and mucous secretions, are involved in preventing this entry. Apart from acting to wash away potential invaders, these secretions also contain antibacterial or antiviral substances.
Defensins and cathelicidins are two major families of mammalian antimicrobial proteins. They contribute to host innate antimicrobial defences by disrupting the integrity of the bacterial cell membrane. Further, several members of defensins and cathelicidins have been shown recently to have chemotactic effects on host cells. Their capacity to mobilize various types of phagocytic leukocytes, immature dendritic cells and lymphocytes, together with their other effects, such as stimulating interleukin-8 production and mast cell degranulation, provides evidence for their participation in alerting, mobilizing and amplifying innate and adaptive antimicrobial immunity of the host (Table 8.1). In brief, upon microbial invasion, epithelial cells/keratinocytes and tissue macrophages are induced to produce β-defensins (especially HBD2 and 3) and cathelicidin/LI-37. The defensins and cathelicidin form gradients that, in tandem with other chemotactic mediators (e.g. chemokines), lead to extravasation of various types of leukocytes to the site of infection in order to overcome the invading pathogens (Table 8.2).
|Chemical||Antimicrobial function(s)||Major cell source(s)|
|Calprotectin||Divalent cation chelator, restricts microbe nutrition||Oral epithelial cells and neutrophils|
|Defensins (α and β types)||Membrane pore-forming peptides, cause osmotic lysis||Leukocytes and epithelial cells|
|Cathelicidins||Lysosomal antimicrobial polypeptides||Macrophages and neutrophils|
|Saliva||Ig, lysozyme, lactoferrin, peroxidases and GCF||Salivary acinar cells|
|Lysozyme||Muramidase activity, aggregates microbes and amphipathic sequences||Macrophages, epithelial cells and neutrophils|
|Peroxidase||Oxidizes bacterial enzymes in glycolytic pathways||Salivary acinar cells, neutrophils, eosinophils|
|His-, Cis- statins||Various effects||Salivary acinar cells|
|SLPI, PRP||Antiviral activities||Various cell types|
|GCF||Provides blood components||Various cell types|
|Mucins||Aggregates bacteria, various effects, homotypic and heterotypic complexes||Salivary acinar cells|
SLPI, secretory leukocyte protease inhibitor; PRP, proline-rich proteins; GCF, gingival crevicular fluid; Ig, immunoglobulin.
Phagocytosis is a process by which phagocytic cells ingest extracellular particulate material, including whole pathogenic microorganisms. If the mechanical defences are breached, the phagocytic cells become the next barrier. These include polymorphonuclear leukocytes (polymorphs) and macrophages. The former are short-lived circulating cells, which can invade the tissues, while the latter are the mature, tissue-resident stage of circulating monocytes.
Macrophages are found in areas of blood filtration where they are most likely to encounter foreign particles, e.g. liver sinusoids, kidney mesangium, alveoli, lymph nodes and spleen. Phagocytes attach to microorganisms by non-specific cell membrane ‘threat’ receptors, after which pseudopodia extend around the particle and internalize it into a phagosome. Lysosomal vesicles containing proteolytic enzymes fuse with the phagosome, and oxygen and nitrogen radicals are generated, which kill the microbe. The phagocytes have several ways of dealing with the phagocytosed material. For example, macrophages reduce molecular oxygen to form microbicidal-reactive oxygen intermediates that are secreted into the phagosome.
Unlike adaptive immunity, innate immunity does not recognize every possible antigen. The cells involved in innate immune responses such as phagocytes (neutrophils, monocytes, macrophages) and cells that release inflammatory mediators (basophils, mast cells and eosinophils) are designed to recognize only a few highly conserved structures present in many different microorganisms. These cells recognize microbial structures called pathogen-associated molecular patterns (PAMPs) in order to activate the innate immune response. PAMPs are molecular components common to a variety of microorganisms but not found as a part of eukaryotic cells and include:
This promotes the attachment of microbes to phagocytes and their subsequent engulfment and destruction. Most defence cells (macrophages, dendritic cells, endothelial cells, mucosal epithelial cells, lymphocytes) have on their surface a variety of receptors called pattern-recognition receptors (PRRs) capable of binding specifically to conserved portions of PAMPs so there is an immediate response against invading microbes. These receptors enable phagocytes to attach to microbes so they can be engulfed and destroyed by lysosomes. There are two functionally different classes of PRRs:
Signalling PRRs bind a number of microbial molecules such as flagellin, pilin, glycolipids, zymosan from fungi and viral double-stranded RNA. A major class of signalling PRRs is Toll-like receptors (TLRs), so named because of their similarity to the protein coded by the Toll gene identified in Drosophila melanogaster.
Binding of PAMPs to signalling PRRs promotes the synthesis and secretion of regulatory molecules such as cytokines that are crucial to initiating innate immunity. Various types of TLRs bind different PAMPs and initiate different types of innate immune responses (Fig. 8.2). PAMPs can also be recognized by a series of soluble PRRs in the blood that function as opsonins and initiate the complement pathway.
Natural killer (NK) cells are non-phagocytic lymphocytes that account for up to 15% of blood lymphocytes and have a special role in the killing of virus-infected and malignant cells (Fig. 8.3). These cells have two kinds of receptors with opposing action: antigen receptors able to recognize specific molecules on target cells, through which activation signals are transmitted, and receptors that recognize self major histocompatibility complex I (MHC I) antigens (see below) through which inactivation signals are transmitted. Activation of NK cells can only occur when there is no inactivation signal, so virus-infected and tumour cells with downregulated MHC I antigens are susceptible to NK cytotoxicity, but normal MHC I-positive cells are protected. The killing mechanism is activated by cytokines released by virus-infected cells, tissue cells, lymphocytes and NK cells themselves. The NK cells are also important in the adaptive immune response, being the effector cells for killing antibody-coated microorganisms.
Acute-phase proteins are serum proteins produced by the liver in response to tissue-damaging infections and other inflammatory stimuli such as cytokines (e.g. interleukins-1 and -6). Although the physiological role of the acute-phase proteins is not fully understood, it has been recognized to enhance the efficiency of innate immunity. Positive acute-phase proteins increase in plasma concentration in the acute-phase response to inhibit or kill microbes through opsonization, coagulation, antiprotease activity and/or complement activation. Negative acute-phase proteins including human serum albumin and transferrin are reduced in concentration in the acute-phase response and act to limit inflammation. Together acute-phase proteins provide immediate defence and enable the body to recognize and react to foreign substances prior to more extensive activation of the immune response. The concentration of the following positive acute-phase proteins in body fluids increases rapidly during tissue injury or infection:
Interferon, produced by virus-infected cells, comprises a group of cytokines that mediate innate immunity and includes those that protect against viral infection and those that initiate inflammatory reactions that protect against bacterial pathogens.
The complement system is very much involved in the inflammatory response and is one of the key effector mechanisms of the immune system. It consists of at least 30 components – enzymes, regulators and membrane receptors – which interact in an ordered and tightly regulated manner to bring about phagocytosis or lysis of target cells.
Complement components are normally present in body fluids as inactive precursors. The alternative pathway of complement activation can be stimulated directly by microorganisms and is important in the early stages of the infection before the production of antibody. It is part of the innate immune system. The classical pathway requires antibody, which may take weeks to develop. Both pathways can lead to the lytic or membrane attack pathway. During the course of complement activation, numerous split products of complement components, with important biological effects, are produced.
Complement factor C3 is the central component of both the classical and alternative pathways (Fig. 8.4). Products of C3 activation, C3b and inactivated C3b (iC3b) bind to microorganisms and are recognized by complement receptors (CRs) on phagocytes. If any C3b molecules bind to a normal host cell surface, they can then bind the next component in the sequence, factor B. Factor D (the only complement factor present in body fluids as an active enzyme) splits off a small fragment, Ba, leaving an active C3 convertase, C3bBb, on the cell surface. However, the normal host cell is able actively to dissociate and inactivate C3bBb. This is achieved by the concerted action of regulatory proteins decay-accelerating factor (DAF), membrane cofactor protein (MCP), β1H globulin (factor H), CR1 and factor I.
Fig. 8.4 Alternative pathway of complement activation. B, factor B; CR, complement receptor; D, factor D; DAF, decay-accelerating factor; H, β1H-globulin; I, C3 inactivator; MCP, membrane cofactor protein; P, properdin.
Activator surfaces are those that inhibit the regulatory proteins, allowing C3bBb to remain intact. For example, bacterial endotoxins and LPSs inhibit factor H. The enzyme C3bBb converts C3 into C3a and C3b. The latter is incorporated, along with properdin (factor P), to form PC3bBbC3b. This is a stable enzyme whose substrates are C3 and C5. It amplifies C3b production and activates the membrane attack pathway.
Classical pathway of complement activation (Fig. 8.5) is mainly initiated by complexes of antigen with antibody. Antibodies of the immunoglobulin (Ig) IgG1, IgG2, IgG3 and IgM classes, but not IgG4, IgA, IgD or IgE, can activate the classical pathway.
The first component of the classical pathway, C1, is actually a complex of C1q, C1r and C1s. This complex can bind very weakly to monomeric IgG, but when IgG complexes with antigen in such a way that adjacent IgG molecules are close together, C1q binds firmly between the two molecules. The C1 complex can bind strongly to a single molecule of pentameric IgM, but only after the conformation of the latter has been altered by binding to antigen.
Activated C1 reacts with fluid-phase C4 and C2, splitting off small peptides C4a and C2a. The resulting C4b2b is deposited on a surface and performs a similar job to C3bBb of the alternative pathway: it can convert C3 into C3a and C3b, and the latter can either opsonize particles for phagocytosis or bind to C4b2b. Cell-bound C4b2b3b is more stable than C4b2b, being somewhat protected from the regulatory proteins DAF and C4-binding protein. Like PC3bBbC3b, it activates the membrane attack pathway.
The peptides Bb and C2b, bound into their respective alternative (PC3bBbC3b) and classical (C4b2b3b) pathway enzymatic complexes, initiate membrane attack (Fig. 8.6) by splitting a small peptide, C5a, from C5 to form C5b. This molecule binds C6 and C7. Cell-bound C5b67 acts as a template for the binding of one molecule of C8 and up to 18 molecules of C9. Normal cells in the body are largely protected from bystander lysis by homologous restriction factor (HRF), which intercepts C8 and C9 before they can be properly assembled into the membrane attack complex (MAC). The MAC, with a molecular weight of 1–2 × 106, forms transmembrane channels, which permit osmotic influx so that the target cell swells up and bursts.
Probably the most important function of the complement system is to opsonize antigen–antibody (immune) complexes, microorganisms and cell debris for phagocytosis (Fig. 8.7). This is achieved by deposition of C3b and iC3b on the particle. Phagocytes bind to the particle via CR1, CR3 and CR4. Also, CR1 is found on erythrocytes, which can bind immune complexes coated with C3b and transport them to the spleen or liver for digestion by macrophages.
The peptides C3a, C4a and C5a are anaphylatoxins that cause mast cell degranulation and smooth-muscle contraction. They increase vascular permeability, which permits cells and fluids to enter the tissues from the circulation. They are regulated by anaphylatoxin inactivator, which splits off the C-terminal arginine so that binding to cellular receptors can no longer occur.
The local inflammatory response is usually accompanied with a systemic response known as the acute-phase response. The manifestation of this response includes the induction of fever and increased production of leukocytes, and the production of soluble factors, including acute-phase proteins in the liver. Injured or infected tissues become inflamed in order to direct components of the immune system to where they are needed. The blood supply to the tissues is increased, capillaries become more permeable to soluble mediators and leukocytes, and leukocytes migrate towards the site of infection as a result of the production of chemotactic factors.
The defence mechanisms in adaptive immunity can specifically recognize and selectively eliminate pathogens and foreign macromolecules. In contrast to innate immunity, adaptive immune responses are reactions to specific antigenic challenge and display four cardinal features: specificity, diversity, immunological memory and discrimination of self and non-self.
Adaptive immune responses are specific for distinct antigens. This unique specificity exists because B and T lymphocytes express membrane receptors that specifically recognize different antigens. Importantly, adaptive immunity is not dependent on innate immunity. Through delicately modulated interactions, the two types of defence mechanisms work synergistically to produce more effective immunity.
All the cells of the immune system (Fig. 8.8) are derived from self-regenerating haematopoietic stem cells present in bone marrow and foetal liver. These differentiate along either the myeloid or the lymphoid pathway. Myeloid precursor cells give rise to mast cells, erythrocytes, platelets, dendritic cells, polymorphs (eosinophils, basophils, neutrophils) and mononuclear phagocytes (monocytes in the blood, macrophages in the tissues). Lymphoid precursor differentiation gives rise to T (thymus-dependent) lymphocytes, B (bone marrow-derived) lymphocytes and NK lymphocytes.
During post-natal life, B cell genesis takes place in the bone marrow. Each newly formed B cell expresses a unique B cell receptor (BCR) on its membrane for antigen-binding. Although T lymphocytes also arise in the bone marrow, they migrate to the thymus to mature. During its maturation, the T lymphocyte expresses a specific antigen-binding molecule known as the T cell receptor (TCR) on its membrane.
The B lymphocytes are responsible for secreting Ig antibodies and can also function as highly efficient antigen-presenting cells (APCs) for T lymphocytes. The latter are divided into two major subsets: T-helper cells, which usually bear the ‘cluster of differentiation’ marker CD4, and T-cytotoxic cells, which usually carry CD8. The T-helper cells are required for activating the effector function of B cells, other T cells, NK cells and macrophages. They do this by transmitting signals via cell-to-cell contact interactions and/or via soluble hormone-like factors called lymphokines. The T-cytotoxic cells kill target cells such as virus-infected host cells. Another functional property of some T lymphocytes is to downregulate immune responses. These T-suppressor cells are usually CD8-positive. Dendritic cells and monocytes/macrophages play key roles in the immune system as APCs.
The primary sites of lymphocyte production are the bone marrow and thymus. Immature lymphocytes produced from stem cells in the bone marrow may continue their development within the bone marrow (B lymphocytes, NK cells) or migrate to the thymus and develop into T lymphocytes. ‘Education’ within the primary lymphoid organs ensures that emerging lymphocytes can discriminate self from non-self. They migrate through the blood and lymphatic systems to the secondary lymphoid organs – spleen, lymph nodes and mucosa-associated lymphoid tissue (MALT) of the alimentary, respiratory and urogenital tracts. Here, lymphocytes encounter foreign antigens and become activated effector cells of the immune response.
The spleen acts as a filter for blood and is the major site for clearance of opsonized particles. It is an important site for production of antibodies against intravenous antigens. The lymph nodes form a network of strategically placed filters, which drain fluids from the tissues and concentrate foreign antigen on to APCs and subsequently to lymphocytes. Spleen and lymph nodes are encapsulated organs, whereas MALT is non-encapsulated dispersed aggregates of lymphoid cells positioned to protect the main passages by which microorganisms gain entry into the body. Gut-associated lymphoid tissue (GALT) includes Peyer’s patches of the lower ileum, accumulations of lymphoid tissue in the lamina propria of the intestinal wall and the tonsils.
The T and B lymphocytes are responsible for specificity in the immune response. They have cell surface receptors whose purpose is to recognize foreign antigens. Each receptor usually binds only to a single antigen, though there may be a degree of cross-reactivity with other antigens of very similar structure. Since all antigen receptors on a given lymphocyte are identical, each B or T cell can usually recognize only one antigen. A single cell, on encountering its specific antigen, must proliferate to form a clone of identical cells able to deal with the offending antigen (clonal selection).
In humans, products of the highly polymorphic MHC genetic loci on chromosome 6 are known as histocompatibility locus antigens (HLAs). Their function is to bind APC-processed short antigenic peptides and present them on the APC surface to T cells. HLA phenotype is responsible for tissue transplant rejection when the recipient and donor are not HLA-matched.