18 Innate Immunity
Innate immunity depends on recognition of conserved molecular patterns found in many microorganisms.
Several families of pattern recognition receptors are responsible for triggering innate immune responses.
Toll-like receptors and other pattern recognition receptors with leucine-rich repeat domains play a key role in innate immune recognition.
Antimicrobial peptides are important effectors of innate immunity.
Phagocytic cells and several types of innate-like lymphocytes are key cell types in mediating innate immunity.
Innate immune responses have a strong impact on the development of adaptive immunity.
Some defects in the innate immune system are associated with a predisposition to infection or to autoimmune disease.
It has become common practice in immunology to divide the mechanisms involved in host defense into adaptive and innate components; this provides a useful framework for classifying the numerous cells, receptors, and effector molecules that combine to make up the vertebrate immune system (Table 18-1). A specific immune response, such as the production of antibodies or T cells against a particular pathogen, is referred to as adaptive immunity because it represents an adaptation that occurs during the lifetime of an individual as a result of exposure to that pathogen. Adaptive immune responses involve the clonal expansion of T and B lymphocytes bearing a large repertoire of somatically generated receptors that can be selected to recognize virtually any pathogen. The adaptive immune system of any given individual is profoundly molded by the immunologic challenges encountered by that individual during the course of a lifetime. A hallmark of adaptive immune responses is that they are highly specific for the triggering agent, and they provide the basis for immunologic memory. This property of memory endows the adaptive immune response with its “anticipatory” property, which provides increased resistance against future infection with the same pathogen and also allows vaccination against future infectious threats.
|Property||Innate Immune System||Adaptive Immune System|
|Receptors||Relatively few (several hundred?)||Many (potentially 1014 or more)|
|Fixed in genome||Encoded in gene segments|
|Gene rearrangement not required||Gene rearrangement required|
|All cells of a class identical||All cells of a class distinct|
|Targets||Conserved molecular patterns||Details of molecular structure|
|Glycans and peptidoglycans||Carbohydrates|
|Self–non–self-discrimination||Perfect: selected over evolutionary time||Imperfect: selected in individual somatic cells|
|Action time||Immediate or rapid (seconds to hours)||Delayed (days to weeks)|
|Response||Microbicidal effector molecules||Clonal expansion or anergy of specific T and B lymphocytes|
|Antimicrobial peptides||Cytokines (IL-2, IL-4, IFN-γ, others)|
|Superoxide||Specific antibody production|
|Nitric oxide||Specific cytolytic T cell generation|
|Cytokines (IL-1, IL-6, others)|
|Chemokines (IL-8, others)|
IFN, interferon; IL, interleukin.
Modified from Medzhitov R, Janeway CA Jr: Innate immune recognition, Annu Rev Immunol 20:197, 2002.
Adaptive immunity is essential for the survival of all mammals and most other vertebrates, but a wide variety of other mechanisms that do not involve antigen-specific lymphocyte responses are also involved in successful immune protection. These diverse mechanisms are collectively known as innate immunity because they are not dependent on prior exposure to specific pathogens for their amplification. Such responses are controlled by the products of germline genes that are inherited and similarly expressed by all normal individuals. Innate immune mechanisms involve both constitutive and inducible components and use a wide variety of recognition and effector mechanisms. It has become clear in recent years that innate immune responses have a profound influence on the generation and outcome of adaptive immune responses. This ability of the innate immune system to instruct the responses of the adaptive immune system suggests many ways in which innate immunity can influence the development of both long-term specific immunity and autoimmune disease.
Evolutionary Origins of Innate Immunity
In spite of its obvious importance for most vertebrate organisms, the adaptive immune system is a relatively recent evolutionary development (Figure 18-1). In the great majority of present-day vertebrate species, the adaptive immune system is based on the ability to generate large families of variable lymphocyte receptors with immunoglobulin-like structures. This ability has been conserved owing to the acquisition of a specialized recombination system that mediates the assembly of gene segments in the T cell and B cell receptor families, which most likely occurred through invasion of the genome of a primitive vertebrate by a transposable element or virus carrying this machinery.1 This critical step in the evolution of the immune system can be traced back to the emergence of the ancestors of present-day jawed fish, which represent the most primitive extant species known to have adaptive immune systems based on the generation of large families of specific immunoglobulin-type receptors.2 Recently, other systems of variable lymphocyte receptors that are unrelated to immunoglobulins but also provide the basis for an adaptive immune response have been discovered in primitive jawless fish such as lampreys and hagfish.3 This finding shows that at least two different strategies for the creation of an adaptive immune system emerged at the dawn of vertebrate evolution about 500 million years ago, and it emphasizes the importance of adaptive immunity for survival and further evolution of the vertebrate lineages.
Figure 18-1 Ancient evolutionary origin of the innate immune system. Studies of the immune systems of a wide range of vertebrates and invertebrates have revealed that even the most primitive invertebrates possess many components of innate immunity (e.g., pattern recognition receptors of the lectin and Toll-like families, antimicrobial peptides, complement proteins). The innate immune system is thus extremely ancient, having arisen early in the evolution of multicellular life. In contrast, the adaptive immune system is a much more recent development that did not appear until emergence of the ancestors of present-day sharks and rays, approximately 400 million years ago. The first species to acquire an adaptive immune system based on immunoglobulin-type receptors must have arisen after the appearance of the direct ancestors of present-day jawless fish (lampreys and hagfish), which are the most highly evolved living species that lack the ability to generate large families of variable immunoglobulin-type lymphocyte receptors (arrow). ACP, alternative complement pathway; CCP, classic complement pathway; Ig, immunoglobulin; LCP, lectin-activated complement pathway; LRR, leucine-rich repeat domain; MAC, membrane attack complex; MHC, major histocompatibility complex; PRR, pattern recognition receptor.
(Adapted from Sunyer JO, Zarkadis IK, Lambris JD: Complement diversity: a mechanism for generating immune diversity? Immunol Today 19:519, 1998.)
Given this key role of adaptive immunity in the evolution and survival of vertebrates, it is surprising that all invertebrate animals, and possibly some of the lowest vertebrate species as well, completely lack the ability to generate lymphocyte populations bearing large families of clonally diverse antigen receptors.4,5 In these animals, protection against pathogen invasion depends entirely on innate immunity, elements of which appear to exist in all animals and plants and must have evolved with the earliest multicellular forms of life. In many cases, components of the innate immune system are significantly conserved in structure and function in animals from the lowliest invertebrates to the most complex vertebrates.4 This preservation of innate immune mechanisms, with their functions largely intact, over such vast evolutionary distances is a clear indication of their importance, even in animals that have developed sophisticated adaptive immune responses.
Pathogen Recognition by the Innate Immune System
Some mechanisms of innate immunity are constitutive, meaning that they are continuously expressed and are not significantly modulated by the presence or absence of infection. Examples include the barrier functions provided by epithelial surfaces continuously exposed to microbial flora, such as those of the skin and intestinal and genital tracts. In contrast, the inducible mechanisms of innate immunity involve increased production of mediators and upregulation of effector functions that eliminate microorganisms. Induction occurs as a result of exposure to a wide variety of microbes and represents a less specific form of immune recognition than that associated with the specific antibodies and T cells that mediate adaptive immunity. The basic principle underlying this form of response is a process known as pattern recognition. This recognition strategy is based on the detection of commonly occurring and conserved molecular patterns that are essential products or structural components of microbes.
PAMPs and DAMPs: Patterns for Innate Immune Recognition
Pathogen-Associated Molecular Patterns
The general name given to the targets of innate immune recognition in microbes is pathogen-associated molecular patterns (PAMPs). These structural features or components distinctive for microorganisms are not normally found in the animal host. The best-known example of a PAMP is bacterial lipopolysaccharide (LPS), a ubiquitous glycolipid constituent of the outer membranes of gram-negative bacteria. Another important example is the peptidoglycan structure present as the basic cell wall component in nearly all bacteria. These structures may vary partially from one bacterium to another, but the basic elements are conserved, thus providing the possibility of recognizing a broad array of pathogens by sensing a single or a relatively small number of PAMPs. Many PAMPs that serve as targets of recognition for the innate immune response are now known to be associated with bacteria, fungi, and viruses.
In addition to allowing direct recognition of molecules produced by various microorganisms, the innate immune system is able to respond to the patterns of host-derived molecules released by cells undergoing necrotic death. The molecules recognized are generally referred to as damage-associated molecular patterns (DAMPs), and include multiple different families of proteins, as well as nonproteinaceous substances such as uric acid microcrystals.6,7 Thus, the response to DAMPs can be an indirect response to microbial invasion, or it can be triggered by other types of tissue damage such as ischemia to result in sterile inflammation.
Pattern Recognition Receptors
Recognition of PAMPs and DAMPs is mediated by a collection of germline-encoded molecules known collectively as pattern recognition receptors (PRRs) (Table 18-2). These receptors are host proteins that have evolved, through many millions of years of natural selection, to possess precisely defined specificities for particular PAMPs or DAMPs expressed by microorganisms. The total number of PRRs present in complex vertebrates such as humans is estimated to be several hundred—a number limited by the size of the genome of any animal and the number of genes it can dedicate to immune protection. The human genome, for example, is estimated to contain approximately 20,000 to 35,000 genes, most of which are not related directly to the immune system. This demonstrates one of the strong points of contrast between innate and adaptive immune systems, because the latter can possess in the range of 1014 different somatically generated receptors for foreign antigens in the form of antibodies and T cell receptors. With its much more limited array of receptors, the innate immune system uses the strategy of targeting highly conserved PAMPs that are shared broadly by large classes of microorganisms. Because most pathogens contain PAMPs, this strategy allows the generation of at least partial immunity against most infections.
PRRs are expressed by many cell types, some of which are specialized effector cells of the immune system (e.g., neutrophils, macrophages, dendritic cells, lymphocytes), and others of which are not generally regarded as part of the immune system (e.g., epithelial and endothelial cells). Unlike the T and B cell receptors used for adaptive immune recognition, expression of PRRs is not clonal, which means that all receptors displayed by a given cell type (e.g., macrophages) have identical structure and specificity. When PRRs are engaged by recognition of their associated PAMPs or DAMPs, effector cells bearing the PRRs are triggered to perform their immune effector functions immediately, rather than after undergoing proliferation and expansion, as in the case of adaptive immune responses. This accounts for the much more rapid onset of innate immune responses.
In recent years, considerable progress has been made toward identifying many of the important PRRs involved in the induction of innate immunity. These receptors can be classified into three functional classes: secreted, endocytic, and signaling PRRs (see Table 18-2). In addition, many of the known PRRs can be classified into structurally defined families on the basis of a few characteristic protein domains. Among these, the best known include proteins with calcium-dependent lectin domains, scavenger receptor domains, and leucine-rich repeat domains.
Pattern Recognition Receptors of the Lectin Family
Calcium-dependent lectin domains are common modules of secreted and membrane-bound proteins involved in the binding of carbohydrate structures. A well-characterized PRR belonging to this class is the mannan-binding lectin (MBL), also known as soluble mannose-binding protein, which represents a secreted PRR that functions in initiation of the complement cascade (Figure 18-2).8,9 This protein is synthesized primarily in the liver on a constitutive basis, although its production can be increased as an acute phase reactant following many types of infection. MBL binds to carbohydrates on the outer membranes and capsules of many bacteria, as well as fungi, some viruses, and parasites. Although mannose and fucose sugars bound by MBL can also be found on the surfaces of normal mammalian cells, they are present at too low a density or in the wrong orientation to efficiently engage the lectin domains of MBL. In contrast, the coats of many microorganisms contain an array of these sugars, which allows strong binding of MBL. Thus, in this case, the spacing and orientation of specific carbohydrate residues constitute the PAMP that triggers the activation of innate immunity by MBL. MBL functions as one of a small number of secreted PRRs that can initiate the lectin pathway of complement activation. At least two other soluble proteins with lectin activity in human plasma, known as ficolins (ficolin/P35 and H-ficolin), can also activate this pathway following their interaction with bacterial polysaccharides.10
Figure 18-2 Structure and function of mannan-binding lectin (MBL), a soluble pattern recognition receptor. Left, MBL is a multimer protein structure with multiple carbohydrate-binding lectin domains. Three identical 32-kD polypeptides associate to form a subunit, which then oligomerizes to form functional complexes (the trimeric form consisting of three subunits is illustrated, which is one of several different oligomer sizes that has been observed for MBL). Each polypeptide in the subunit contains an N-terminal cysteine-rich domain, a collagen-like domain, a neck region, and a C-terminal carbohydrate recognition domain. Right, Initiation of the lectin pathway for complement activation by MBL. The carbohydrate recognition domains of MBL bind to carbohydrates that are characteristic of bacterial surfaces. This leads to the recruitment of several other serum proteins, including small MBL-associated protein (sMAP) and the three MBL-associated serine proteases (MASP1, MASP2, MASP3). The protease activity of MASP2 cleaves complement C4 and C2 subunits, generating the C3 convertase (C4bC2a). MASP1 is able to cleave C3 directly. The deposition of C3 cleavage products on the bacterial surface results in opsonization and phagocytosis of the bacterial cell.
Several of the soluble lectin-type PRRs also play an important role in the opsonization of microbes by binding to their surfaces and directing them to receptors on phagocytic cells. Among these are two pulmonary surfactant proteins, SP-A and SP-D, which similarly recognize and bind to the surface sugar codes of microbes in the respiratory tract.11 These molecules are similar in structure to MBL, having both collagen-like and lectin domains, and together they constitute a family of soluble PRRs known as collectins. Another family of soluble PRRs that performs a similar function in plasma is the pentraxins, so called because they are formed by the association of five identical protein subunits.12 This family includes the acute phase reactants C-reactive protein (CRP) and serum amyloid P protein (SAP), along with a number of so-called long pentraxins, which have an extended polypeptide structure with homology to the classic short pentraxins (i.e., CRP and SAP) only at their carboxy-terminal domains. Long pentraxins are expressed in a variety of different tissues and cells, and their specific functions are mostly unknown. However, the long pentraxin PTX3 has been shown to play an important, nonredundant role in resistance to fungal infection in mice, and recent studies indicate that PTX3 is essentially a functional ancestor of antibodies that recognizes microbes and promotes their clearance through complement activation and phagocytosis.13
In addition to these soluble proteins, a large number of membrane-bound glycoproteins with lectin domains are known to exist; some of them participate in innate immunity by serving as endocytic PRRs for the uptake of microbes or microbial products14,15 (Figure 18-3). One of the most extensively studied of these is the macrophage mannose receptor (MMR).16 Although originally identified on alveolar macrophages and known to be expressed on macrophage subsets throughout the body, this receptor is also expressed on a variety of other cell types, including certain endothelia, epithelia, and smooth muscle cells. The MMR is a membrane-anchored, multilectin domain–containing protein that mediates the binding of a broad range of pathogens, leading to their internalization via endocytosis and phagocytosis. Although the major function of the MMR appears to be directing the uptake of its ligands, evidence suggests that this receptor may be capable of signaling to modify macrophage functions following receptor engagement.17 Another member of this receptor family, the β-glucan binding cell-surface lectin known as dectin-1, has a role in the modulation of inflammation in a mouse model of infection-induced arthritis.18
Figure 18-3 Endocytic pattern recognition receptors of the scavenger receptor and lectin families. Left, Illustrations of three members of the scavenger receptor family. These are trimeric complexes of type II transmembrane polypeptides that have their N-terminals positioned in the cytoplasm and their C-terminals in the extracellular space. Three distinct extracellular structural domains are indicated: (a) the scavenger receptor cysteine-rich (SRCR) domain (absent in SR-A II), which has no currently known function; (b) the collagen-like domain, which is implicated in the binding of polyanionic ligands; and (c) the α-helical coiled-coil domain (absent in macrophage receptor with collagenous structure [MARCO]), which is believed to assist in receptor trimerization. Right, Two examples of multilectin domain endocytic pattern recognition receptors—macrophage mannose receptor (MMR) and DEC-205. Distinct extracellular domains in these receptors include (d) a cysteine-rich N-terminal domain, (e) a fibronectin-like domain, and (f) multiple calcium-dependent (C-type) lectin domains that bind various carbohydrate ligands.
(Reproduced in part from Peiser L, Mukhopadhyay S, Gordon S: Scavenger receptors in innate immunity, Curr Opin Immunol 14:123, 2002.)
Pattern Recognition Receptors of the Scavenger Receptor Family
The scavenger receptor family contains a broad range of structurally diverse cell surface proteins that are expressed most prominently on macrophages, dendritic cells, and endothelial cells19 (see Figure 18-3). Although they were originally defined by their ability to bind and take up modified serum lipoproteins, they also bind a wide range of other ligands, including bacteria and some of their associated products. Multiple members of this family have been implicated as PRRs for innate immunity. These include the scavenger receptor A (SR-A) and a related molecule called the macrophage receptor with collagenous structure (MARCO).20 Both of these molecules contain a scavenger receptor cysteine-rich domain in the distal ends of their membranes and a collagen-like stalk with a triple-helical structure. Both are known to bind bacteria, and SR-A also binds well-known PAMPs such as lipoteichoic acids and LPS.21,22 Mice that have been made deficient in SR-A by targeted gene disruption show increased susceptibility to infections caused by a variety of bacteria, thus providing strong evidence of the role of scavenger receptors in protective immunity, most likely through the activation of innate immune mechanisms.23,24 Members of the class B scavenger receptor family, including CD36 and SR-BI/CLA-1, have also been found to recognize a variety of pathogen-derived molecules.19 Although these members of the scavenger receptor family clearly function as endocytic PRRs in the uptake of microbes, their potential to serve as signaling receptors has not yet been established. However, several scavenger receptors play a co-receptor role in the signal transduction process mediated by members of the Toll-like receptor (TLR) family (discussed later), most likely by capturing specific ligands and transferring them to adjacent TLRs.19
Pattern Recognition Receptors with Leucine-Rich Repeat Domains
Leucine-rich repeat domains (LRRs) are structural modules found in many proteins, including PRRs involved in signaling the activation of innate immunity. Molecules in this class include, most notably, the family of mammalian Toll-like receptors (TLRs), which are membrane-bound signal-transducing molecules that play a central role in the recognition of extracellular and vacuolar pathogens.25 Two families of cytoplasmic LRR-containing receptors have also been identified; they play a prominent role in the innate immune recognition of PAMPs expressed by intracellular pathogens. These include the families of caspase activation and recruitment domain (CARD) proteins and of pyrin domain proteins.26 Molecules are closely related in structure and function to proteins found in invertebrates and plants involved in pathogen resistance, highlighting the ancient origin of these pathways for host defense; they appear to have been recognizably conserved throughout approximately 1 billion years of evolution.
The first member of the Toll family to be discovered was the Drosophila Toll protein, which was identified as a component of a signaling pathway that controls dorsoventral polarity during development of the fly embryo.27 The sequence of Toll showed it to be a transmembrane protein with a large extracellular domain containing multiple tandemly repeated LRRs at the N-terminal end, followed by a cysteine-rich domain and an intracellular signaling domain (Figure 18-4). A role for Toll in immune responses was suggested by the observation that its intracellular domain shows homology to the mammalian interleukin-1 receptor (IL-1R) cytoplasmic domain.28 This association was later confirmed in studies showing that Toll was critical for the antifungal response in the fly, linking this pathway for the first time to innate immunity.29 Identification of Drosophila Toll eventually led to a search for similar proteins in mammals; this effort has been richly rewarded, yielding a family of 10 Toll-like receptors (TLRs) in humans and 12 in mice.30 Among these, TLR1 through TLR9 are conserved between mice and humans, TLR10 is present only in humans, and TLR11 through TLR13 are expressed only in mice.30 All these molecules contain large extracellular domains with multiple LRRs, as well as intracellular signaling domains known as Toll/IL-1R, or TIR, domains.31 Many of these TLRs have been linked to innate immune responses against various PAMPs of different microorganisms.32
Figure 18-4 Toll-like receptors (TLRs) and associated proteins. Left, TLR4 is a transmembrane polypeptide present in the plasma membrane as a homodimer. The TLR4 polypeptide has three distinct extracellular regions: (a) an N-terminal flanking domain; (b) the leucine-rich repeat (LRR) region, which contains 21 leucine-rich motifs and is thought to be directly involved in binding to lipopolysaccharide (LPS) and other ligands; and (c) a C-terminal flanking cysteine-rich domain. The cytoplasmic domains of TLR4 and all other TLRs have homology to the human interleukin (IL)-1 receptor and are designated Toll-IL-1 receptor (TIR) domains. The extracellular portion of TLR4 associates with at least two other proteins, CD14 and MD-2, which are involved in ligand recognition. The intracellular TIR domains associate with multiple adapter proteins (MyD88, TIRAP/MAL, Tollip), which link the receptor complex to kinases that activate signaling cascades. For TLR4, and probably for most other TLRs as well, activation of the IL-1 receptor–associated kinase (IRAK) is an important step leading to release of the active form of transcription factor nuclear factor κB (NFκB). In addition, signaling through TLR4 leads to signal transduction through the activation of mitogen-activated protein kinases (MAPKs), double-stranded RNA-binding protein kinase (PKR), and other members of the IRAK family such as IRAK-2. Right, A different set of ligands is recognized by TLR2, which functions as part of a heterodimeric complex with other TLRs such as TLR6. The TLR2-TLR6 complex shares many features with the TLR4 complex in terms of its associated proteins. However, the TIR domain of TLR2 also appears to recruit phosphatidylinositol-3-OH kinase (PI3 kinase, p85 and p110 subunits) and the membrane-associated GTPase Rac1, which allows the activation of other signaling molecules such as the serine-threonine kinase Akt. Thus, although the major signaling pathways activated by different TLRs are similar or identical (i.e., activation of NFκB and MAPKs), it is likely that each TLR complex has subtle differences in its secondary pathways of signal transduction. These differences may lead to partially overlapping but distinct outcomes in response to ligands recognized by different TLR complexes.
(Adapted from Underhill DM, Ozinsky A: Toll-like receptors: key mediators of microbe detection, Curr Opin Immunol 14:103, 2002.)