The immune system, the function of which is to protect against infections, comprises two branches: a more primitive one called innate (natural, native) immunity and the more recently evolved one called adaptive (specific) immunity . Innate and adaptive immunity are not two separate compartments but an integrated system of host defense, sharing bidirectional interactions fundamental to both the inductive phase and the effector phase of the immune response. The innate immune system constitutes the first line of host defense during infection and therefore plays a crucial role in the early recognition and subsequent triggering of the proinflammatory response to invading pathogens. The adaptive immune system, on the other hand, is responsible for elimination of pathogens in the late phase of infection, the maintenance of immunological tolerance, and the generation of immunological memory.
The cells of the immune system originate from pluripotent hematopoietic stem cells that give rise to stem cells of more limited potential (lymphoid and myeloid precursors). The immune system functions by means of a complex network of cellular interactions that involve cell surface proteins and soluble mediators such as cytokines.
Cells of Innate Immunity
The innate immune system is the first line of defense against microorganisms and is conserved in plants and animals. It is phylogenetically ancient compared with the more evolved form of immunity, which exists only in vertebrates. The principal components of innate immunity are (1) physical and chemical barriers such as epithelia and antimicrobial substances produced at epithelial surfaces, (2) circulating effector proteins such as the complement components and cytokines, and (3) cells with innate phagocytic activity: neutrophils, macrophages, and natural killer (NK) cells.
Phagocyte surface receptors recognize highly conserved structures characteristic of microbial pathogens that are not present in mammalian cells. The binding of microbial structures to these receptors triggers cells to engulf the bacterium and induces cytokines, chemokines, and costimulators that recruit and activate antigen-specific lymphocytes and initiate adaptive immune responses. Thus, innate immunity not only represents an early effective defense mechanism against infection but also provides the “warning” of the presence of an infection against which a subsequent adaptive immune response has to be mounted. The pivotal role of this compartment in the effector phase of the immune response is discussed later.
Phagocytes
Cells of the phagocyte system originate from a common lineage in the bone marrow, circulate in the blood in inactive form and are recruited and activated in the peripheral tissues in case of infection, tissue injury or other proinflammatory stimuli. Monocytes are the classical example of immature circulating phagocytes, characterized by a granular cytoplasm with many phagocytic vacuoles and lysosomes. Once they enter in the tissues, monocytes mature into macrophages . Macrophages are strategically placed in all organs and tissues where they act as “sentinels” together with dendritic cells. In fact, one of their major roles is to recognize and respond to microbes and to amplify the response against a potentially harmful stimulus. Depending on the tissues in which they are found, macrophages are known with a number of different names: Kupffer cells in the liver, microglial cells in the central nervous system, alveolar macrophages in the airways. These cells are the prototype of the effector cells of innate immunity. Once activated, macrophages initiate a number of crucial events which include phagocytosis and destruction of ingested microbes, and production of proinflammatory cytokines and other mediators of inflammation that lead to further recruitment of cells of innate immunity (monocytes, neutrophils) and provide signals to cells (T and B cells) of adaptive immunity.
Neutrophils, the other major group of phagocytes, are the most abundant type of circulating leukocytes. Their nucleus is segmented into three to five lobules, hence the term polymorphonuclear leukocytes . The cytoplasm is characterized by the presence of two types of granules. Specific granules contain enzymes, including lysozyme, elastase, and collagenase. The azurophilic granules are lysosomes containing enzymes and microbicidal substances. Neutrophils are the first cells that enter the site of infection and represent the prevalent cell type in the early phases of the inflammatory response. Within 1 or 2 days neutrophils are almost completely replaced by newly recruited monocytes–macrophages that represent the dominant effector cells in the later stages of inflammation.
The Pattern Recognition Receptors
The innate immune response relies on recognition of evolutionarily conserved structures on pathogens called pathogen-associated molecular patterns (PAMPs), through a limited number of germ line–encoded pattern recognition receptors (PRRs) ( Table 3-1 , Fig. 3-1 ). Among them the family of Toll-like receptors (TLRs) has been studied most extensively. PAMPs are characterized by being invariant among entire classes of pathogens and distinguishable from “self.” This characteristic allows a limited number of germ line–encoded PRRs to detect the presence of many microbial infections. Finally, since PAMPs are essential for microbial survival, mutations or deletions of PAMPs are lethal, reducing the possibility that microbes undergo PAMP mutations in order to escape recognition by the innate system.
| MOLECULAR PATTERN | ORIGIN | RECEPTOR | MAIN EFFECTOR FUNCTION |
|---|---|---|---|
| LPS | Gram-negative bacteria | TLR4, CD14 | Macrophage activation |
| Unmethylated CpG nucleotides | Bacterial DNA | TLR9 | Macrophage, B-cell, and plasmacytoid cell activation |
| Terminal mannose residues | Microbial glycoprotein and glycolipids | (1) Macrophage mannose receptor | |
| Phagocytosis | (2) Plasma mannose-binding lectin | ||
| Complement activation | |||
| Opsonization | |||
| LPS, dsRNA | Bacteria, viruses | Macrophage scavenger receptor | Phagocytosis |
| Zymosan | Fungi | TLR2, Dectin-1 | Macrophage activation |
| dsRNA | Viral | TLR3, RIG-I * | IFN type I production |
| ssRNA | Viral | TLR7/8, MDA5 * | IFN type I production |
| N-formylmethionine residues | Bacteria | Chemokine receptors | Neutrophil and macrophage activation and migration |
| MDP | Gram-positive and Gram-negative bacteria | NOD2, * NALP1 * | Macrophage activation |
Many classes of PPRs are present on the surface of cells of the innate immune system where they act as “tissue sentinels” through the continuous monitoring of peripheral tissues for the possible invasion of microbial pathogens. TLRs are a large class of PPR characterized by an extracellular leucine-rich repeat (LRR) domain and an intracellular Toll/IL-1 receptor (TIR) domain ( Fig. 3-2 ). To date, 13 TLRs have been identified in humans, and they each recognize distinct PAMPs derived from various microbial pathogens, including viruses, bacteria, fungi, and protozoa ( Table 3-2 ).
| TLR | CELLULAR LOCALIZATION | LIGAND (S) | MICROBIAL SOURCE |
|---|---|---|---|
| TLR1 | Cell surface | Lipopeptides | Bacteria, mycobacteria |
| TLR2 | Cell surface | Zymosan | Fungi |
| Peptidoglycans | Gram-positive bacteria | ||
| Lipoteichoic acids | Gram-positive bacteria | ||
| Lipoarabinomannan | Mycobacteria | ||
| Porins | Neisseria | ||
| Envelope glycoproteins | Viruses (e.g., measles, HSV, CMV) | ||
| TLR3 | Endolysosomal compartment | dsRNA | Viruses |
| TLR4 | Cell surface and endolysosomal compartment | LPS | Gram-negative bacteria |
| Lipoprotein | Many pathogens | ||
| HSP60 | Chlamydia pneumoniae | ||
| Fusion protein | RSV | ||
| TLR5 | Cell surface | Flagellin | Bacteria |
| TLR6 | Cell surface | Diacyl lipopeptides | Mycoplasma |
| Lipoteichoic acid | Gram-positive bacteria | ||
| TLR7 | Endolysosomal compartment | ssRNA and short dsRNA | Viruses and bacteria |
| TLR8 | Endolysosomal compartment | ssRNA and short dsRNA | Viruses and bacteria |
| TLR9 | Endolysosomal compartment | Unmethylated CpG DNA | Bacteria, protozoa, viruses |
| TLR10 | Cell surface | Unknown | — |
| TLR11 | Endolysosomal compartment | Profilin and flagellin | Apicomplexan parasites |
| TLR12 | Endolysosomal compartment | Profilin | Apicomplexan parasites |
| TLR13 | Endolysosomal compartment | Bacterial 23S rRNA | Gram-negative bacteria |
Certain TLRs (TLR1, -2, -4, -5, -6, -10) are expressed at the cell surface and mainly recognize bacterial products unique to bacteria, whereas others (TLR3, -7, -8, -9, -11, -12, -13) are located almost exclusively in intracellular compartments, including endosomes and lysosomes ( Fig. 3-2 ) and are specialized in recognition of nucleic acids, with self- versus nonself-discrimination provided by the exclusive localization of the ligands rather than solely based on a unique molecular structure different from that of the host.
The key cell types expressing TLRs are antigen-presenting cells (APCs), including macrophages, dendritic cells (DCs), and B lymphocytes. Ligand binding to TLRs through PAMP–TLR interaction induces receptor oligomerization, which triggers intracellular signal transduction, resulting in the generation of an antimicrobial proinflammatory response that is also able to involve and orient the adaptive immune system.
In addition to transmembrane receptors on the cell surface and in endosomal compartments, there are intracellular (cytosolic) receptors that function in the pattern recognition of bacterial and viral pathogens. These include nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) and the intracellular sensors of viral nucleic acids, such as RIG-I (retinoic-acid-inducible gene I) or melanoma differentiation-associated gene 5 (MDA5), grouped under the term RIG-I-like receptors (RLRs) ( Fig. 3-1 ).
NLRs are a family of about 23 intracellular proteins with a common protein-domain organization but diverse functions ( Table 3-3 ). NLRs are composed of a variable N-terminal effector region consisting of caspase recruitment domain (CARD), pyrin domain (PYD), acidic domain, or baculovirus inhibitor repeats (BIRs), a centrally located NOD (or NACTH) domain that is critical for activation, and C-terminal leucine-rich repeats (LRRs) that sense PAMPs ( Fig. 3-1 ). NOD and NLRP subfamilies are the most characterized among NLRs.
| NAME * | OTHER NAMES | MICROBIAL MOTIFS RECOGNIZED | NLR FAMILY |
|---|---|---|---|
| CIITA | NLRA, C2TA | NLRA | |
| NAIP | NLRB1, BIRC1 | NLRB | |
| NOD1 | NLRC1, CARD4, | GM-tripeptide | NLRC |
| NOD2 | NLRC2, CARD15, BLAU | MDP | |
| NLRC3 | NOD3 | Flagellin from Salmonella, Shigella, Listeria, Pseudomonas | |
| NLRC4 | CARD12, IPAF | ||
| NLRC5 | NOD27 | ||
| NLRP1 | NALP1, CARD7 | MDP | NLRP |
| NLRP2 | NALP2, PYPAF2 | Bacterial RNA, viral RNA, uric acid crystals, LPS, MDP | |
| NLRP3 | NALP3, CIAS1, Cryopyrin, PYPAF3 | ||
| NLRP4 | NALP4, PYPAF4 | ||
| NLRP5 | NALP5, PYPAF8 | ||
| NLRP6 | NALP6, PYPAF5 | ||
| NLRP7 | NALP7, PYPAF3 | ||
| NLRP8 | NALP8, NOD16 | ||
| NLRP9 | NALP9, NOD6 | ||
| NLRP10 | NALP10, NOD8 | ||
| NLRP11 | NALP11, PYPAF6, NOD17 | ||
| NLRP12 | NALP12, PYPAF2, Monarch1 | ||
| NLRP13 | NALP13, NOD14 | ||
| NLRP14 | NALP14, NOD5 | ||
| NLRXI | NOD9 | NLRX |
* According to the Human Genome Organization Gene Nomenclature Committee (HGNC).
The proteins of the NOD subfamily—NOD1 and NOD2—are involved in sensing bacterial molecules derived from synthesis and degradation of peptidoglycan. Whereas NOD1 recognizes diaminopimelic acid produced primarily by Gram-negative bacteria, NOD2 is activated by muramyl dipeptide (MDP), a component of both Gram-positive and Gram-negative bacteria. The NLRP subfamily of NLRs has 14 members, and at least some of these are involved in the induction of the inflammatory response mediated by the IL-1 family of cytokines, which includes interleukin (IL)-1β and IL-18. These cytokines are synthesized as inactive precursors that are cleaved by the proinflammatory caspases, such as caspases 1, 4, 5. These caspases are activated in a multisubunit complex called the inflammasome.
Most PRRs sense not only pathogens but also misfolded/glycated proteins or exposed hydrophobic portions of molecules released at high levels by injured cells; this has therefore been termed damage-associated molecular pattern (DAMP). DAMP molecules, including high-mobility group box 1 protein (HMGB-1), heat-shock proteins (HSPs), uric acid, altered matrix proteins, and S100 proteins, represent important danger signals that mediate inflammatory responses through TLRs or NLRPs, or other specific receptor like RAGE (receptor for advanced glycation end products) after release from activated or necrotic cells. The term alarmins has also been proposed for DAMP molecules. A prototypic DAMP molecule—the nuclear protein HMGB-1—is either passively released by necrotic cells or actively secreted with delay by activated cells. S100A8, S100A9, and S100A12 (also called calgranulins ) are calcium-binding proteins expressed in the cytoplasm of phagocytes and secreted by activated monocytes or neutrophils. Once released from cells, calgranulins exert numerous extracellular functions. Secreted S100A8/A9 complexes bind specifically to endothelial cells and directly activate the microvascular endothelium, leading to loss of barrier function, apoptosis of endothelial cells, upregulation of thrombogenic factors, and an increase of junctional permeability. S100A8/A9 and S100A12 upregulate expression and affinity of the integrin receptor on neutrophils and facilitate their adhesion to fibrinogen and to fibronectin and the adhesion of monocytes to the endothelium in vitro . In addition, the S100A8/A9 complex, as well as S100A8 itself, bind to and signal directly through the lipopolysaccharide receptor complex including TLR4, MD2, and CD14. The binding to both receptors is able to induce the activation of a number of intracellular proinflammatory pathways (see below). The phagocyte-specific calgranulins S100A8, A9, and A12 are secreted by activated phagocytes and bind to PRRs, which mediates downstream signaling and promotes both inflammation and autoimmunity in a number of immune-mediated conditions.
Downstream Effects of the Stimulation of PRRs
Phagocytosis.
The binding of microbes to phagocytes through PRRs initiates the process of phagocytosis of microorganisms and their subsequent destruction in phagolysosomes. The activation of phagocytes through PPRs also induces effector molecules such as inducible nitric oxide synthase and other antimicrobial peptides that can directly destroy microbial pathogens. This is particularly true for polymorphonuclear neutrophils, which are the major contributors on the immediate innate immune response. Their capacity of phagocytosis exceeds that of macrophages, but their capacity to synthesize RNA and proteins is low. Neutrophils are the major source of oxidants, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), which participates in regulation of the immune response and intracellular killing of bacterial pathogens. The main source of ROS in neutrophils is the membrane-bound enzyme complex nicotinamide adenine dinucleotide phosphate (NADPH) oxidase.
Peptides are generated from microbial proteins and presented by professional APCs, such as DCs to T cells to initiate adaptive immune response. Following antigen uptake DCs become activated and migrate to regional lymph nodes to present antigenic peptides in the context of relevant major histocompatibility complex (MHC) molecules. During this process, phagocytosis, upregulation of costimulatory molecules (including CD80, CD86, and CD40, and antigen-presenting MHC molecules), switches in chemokine receptor expression, and cytokine secretion are all events that are regulated through the recognition of pathogens by PRRs expressed on DCs.
The intracellular signaling after stimulation of PPRs.
Upon engagement of TLRs by individual PAMPs, a number of different signaling pathways are triggered. Signal transduction is mediated initially by a family of adapter molecules, which at least in part determines the specificity of the response. Recruitment of one or several adapter molecules to a given TLR is followed by activation of downstream signal transduction pathways via phosphorylation, ubiquitination, or protein–protein interactions, ultimately culminating in activation of transcription factors that regulate the expression of genes involved in inflammation and antimicrobial host defense ( Fig. 3-2 ). TLR-induced signaling pathways can be broadly classified on the basis of their utilization of different adapter molecules, i.e., dependent on or independent of the adapter MyD88 or TIR domain-containing adapter inducing interferon (IFN)-γ (TRIF), and, additionally, their respective activation of individual kinases and transcription factors. Three major signaling pathways responsible for mediating TLR-induced responses include (1) nuclear factor kB (NF-κB), (2) mitogen-activated protein kinases (MAPKs), and (3) IFN regulatory factors (IRFs). Whereas NF-κB and MAPKs play central roles in induction of a proinflammatory response, IRFs are essential for stimulation of IFN production ( Fig. 3-3 ).
Following ligand binding, TLRs dimerize and undergo conformational changes required for the subsequent recruitment of cytosolic TIR domain-containing adapter molecules.
MyD88, is involved in signaling triggered by all TLRs, with the exception of TLR3, and plays a major role in TLR-induced signal transduction. In response to TLR4 stimulation by an appropriate PAMP, MyD88 associates with the cytoplasmic part of the receptor and recruits members of the IL-1 receptor (IL-1R)-associated kinase (IRAK) family. IRAK1 or IRAK2 associate with TRAF6, which catalyzes the synthesis of transforming growth factor-activated protein kinase1 (TAK1) and the IκB kinase (IKK) subunit NF-κB essential modifier (NEMO). TAK1 then stimulates two distinct pathways involving the IKK complex and the MAPK pathway, respectively ( Fig. 3-3 ).
NF-κB exists in an inactive form in the cytoplasm physically associated with its inhibitory protein inhibitor of NF-κB (IκB). Upon inflammatory stimuli, IκB is phosphorylated and degraded releasing NF-κB dimers, which translocate to the nucleus. Phosphorylation of IκB is performed by IκB kinase (IKK). NF-κB binds to promoters or enhancers of target genes in the nucleus leading to increased transcription and expression.
MAPKs are an important kinase family involved in rapid downstream inflammatory signal transduction resulting in activation of several nuclear proteins and transcription factors. MAPK pathways are activated through sequential phosphorylations, beginning with activation of MAPK kinase kinase (MAPKKK), which phosphorylates and activates MAPK kinase (MAPKK), which in turn activates MAPK by phosphorylation. MAPK pathways include p38, JNK, and ERK. The pathways p38 and JNK phosphorylate and activate transcription factors such as ATF-2 and AP-1, which are necessary for the upregulation of several proinflammatory molecules. Cytosolic pattern recognition receptors like NLRs and intracellular sensors of viral nucleic acids (RIG and DAI) exert the same function played by the membrane-bound PRRs. For example, the stimulation of NOD1 or NOD2 by bacterial-derived peptidoglycan fragments results in the activation of NF-kB and MAPKs, which drive the transcription of numerous genes involved in both innate and adaptive immune responses.
Innate immune cells can be activated through interaction with a number of other cells and soluble factors of the immune system as a response of pathogen recognition. Thus macrophage and neutrophils are activated by immune complexes and complement fragments through the binding with immunoglobulin and complement receptors expressed on their surface ( Fig. 3-2 ). The functional activity of macrophages upon stimulation of PPRs is highly influenced by the environmental conditions and by the interaction with other cells. In response to signals derived from microbes, damaged tissues, or activated lymphocytes, it has been suggested that macrophages may develop into one of two different states: the classically activated M1 phenotype and the alternatively activated M2 phenotype ( Fig. 3-4 ). T helper (Th)1-related cytokines such as IFN-γ, as well as microbicidal stimuli, polarize macrophages to an M1 phenotype. M1 macrophages produce high levels of IL-12 and IL-23 and other molecules engaged in inflammatory, microbicidal, and tumoricidal activities ( Fig. 3-4 ). In contrast, Th2 cytokines such as IL-4 and IL-13 polarize macrophages to an alternatively activated (or M2) phenotype, characterized by immunomodulatory mediators such as IL-10, IL-1 decoy receptor, and IL-1 receptor antagonist (IL1Ra). M2 macrophages dampen inflammation, promote tissue remodeling and repair, help in parasite clearance and tumor progression, and possess immunoregulatory functions ( Fig. 3-4 ).
The Inflammasome and Its Role in the Secretion of IL-1β
The role of some cytosolic PRRs is complementary to that of membrane-bound TLRs for the activation of the inflammatory response: an example is the role of some NLRP proteins in the activation and secretion of the active form of IL-1β ( Fig. 3-4 ). Unlike most cytokines, IL-1β (together with IL-18 and IL-33) lacks a secretory signal peptide and is externalized by through a nonclassical pathway, arranged in two steps. TLR ligands such as LPS induce gene expression and synthesis of the inactive IL-1β precursor (pro-IL-1β). The activation of caspase-1 then catalyzes cleavage of pro-IL-1β to the 17kd active form. The protein complex responsible for this catalytic activity is termed the inflammasome . The inflammasome is composed of the adapter ASC (apoptosis-associated speck-like protein containing a CARD), pro-caspase-1, and an NLR family member (such as NLRP1, NLRP3 or Ipaf [Ice protease-activating factor]). Oligomerization of these proteins through CARD/CARD interactions results in activation of caspase-1, which cleaves the accumulated IL-1 precursor, resulting in secretion of biologically active IL-1. A growing number of NLR proteins have been shown to have the capacity to activate caspase-1, each recognizing different danger signals or PAMPs through their respective receptors ( Fig. 3-5 ).
NLRP3 has been ascribed a role in recognition of adenosine triphosphate (ATP), uric acid crystals, viral RNA, and bacterial DNA. These stimuli play a crucial second hit for the secretion of IL-1β ( Fig. 3-6 ). Indeed, monocytes stimulated with LPS alone release approximately only 20% of IL-1β. A second stimulus, such as exogenous ATP, strongly enhances proteolytic maturation and secretion of IL-1β. ATP-triggered IL-1β secretion is mediated by P2X7 receptors expressed on the surface of monocytes. Notably, knockout mice deficient in cryopyrin cannot activate caspase-1 upon LPS and ATP stimulation, resulting in lack of IL-1β secretion. Mutations in the cryopyrin gene in humans are associated with diseases characterized by excessive production of IL-1β, called cryopyrinpathies , which belong to the group of the autoinflammatory diseases (see also Chapter 47 ).
Recent evidence suggests a role for oxidative stress in the activation of the NLRP3-inflammasome. The exposure of human monocyte to PAMPs and DAMPs induces oxidative stress in the cells through the production of ROS. The extent to which ROS accumulate in the cells is determined by the antioxidant systems that enable cells to maintain redox homeostasis. Under normal conditions, these systems balance the constitutive generation of ROS. Both events—oxidant and antioxidant—are required for the secretion of IL-1β after DAMP or PAMP triggering ( Fig. 3-6 ).
An additional pathway of activation of NLR3 inflammasome has been recently identified. Increased intracellular Ca2+ and decreased cellular cyclic AMP (cAMP) are able to induce NLRP3 activation through calcium-sensing receptor (CASR). Ca2+ or other CASR agonists (gadolinium or R-568) activate the NLRP3 inflammasome in the absence of exogenous ATP, whereas knockdown of CASR reduces inflammasome activation in response to known NLRP3 activators.
Dendritic Cells
DCs are specialized APCs that originate from the bone marrow and play a critical role in the processing and presentation of antigen to T cells during the adaptive immune response ; they can be considered as a bridge between innate and adaptive immunity. At the immature stage of development, DCs act as sentinels in the epithelia of peripheral tissues (skin, gastrointestinal, and respiratory systems) continuously sampling the antigenic environment. These cells are morphologically identified by their extensive membrane projections. Recognition of microbial or viral products through PRR on the surface of phagocytes initiates the migration of DCs to lymph nodes where they mature (express costimulatory molecules) to present antigen to T cells.
DCs control or influence many aspects of T-cell responses; this is further elaborated in Chapter 4 . For example, under the control of DCs, helper T cells acquire the capacity to produce powerful cytokines such as IFN-γ to activate macrophages to resist infection by facultative and obligate intracellular microbes (Th1 cells); or IL-4, -5, and -13 to mobilize white cells that resist helminths (Th2 cells); or IL-17 to mobilize phagocytes at body surfaces to resist extracellular bacilli (Th17 cells) ( Fig. 3-7 ). Alternatively, DCs can guide T cells to become suppressive by making IL-10 (T regulatory cells) or by differentiating into FOXP positive regulatory T cells (see also Chapter 4 ).
Plasmocytoid DCs (pDCs) are a distinct subtype of DCs that display the unique capacity to secrete large amounts of type I IFN (α/β) in response to certain viruses and other microbial stimuli (they are also called plasmacytoid interferon producing cells ). Viral nucleic acids, as well as self-nucleoproteins internalized in the form of immune complexes, trigger TLR7 and TLR9 expressed by pDCs, leading to type I IFN production. Plasmocytoid DCs have been implicated in several autoimmune conditions including systemic lupus erythematous (see Chapter 23 ) and juvenile dermatomyositis (see Chapter 26 ).
Natural Killer Cells
Natural killer (NK) cells are large lymphocytes characterized by the presence of cytoplasmic granules containing proteins with proteolytic activities (perforin, granzymes) that lack antigen-specific receptors but are able to kill abnormal cells such as some tumor cells and virus-infected cells. Activation of NK cells is regulated through activating and inhibitory cell surface receptors. The inhibitory receptors bind to self-class I MHC molecules, which are normally expressed on the surface of the majority of cell types ( Fig. 3-6 ). The ligands for activating receptors are only partially known. The engagement of both inhibitory and activating receptors results in a dominant effect of the inhibitory receptors. The infection of host cells, for example, by some viruses, leads to the loss of class I MHC from their surface and exposes these cells to the exclusive activity of activating receptors ( Fig. 3-8 ). Once activated, NK cells release the contents of their granules. Perforin creates pores in target cell membranes, and granzymes enter into the cells through the perforin pores, inducing the death of target cells by apoptosis, with the same mechanism of cytolysis used by CD8 cytotoxic T cells.
Other important activities of NK cells include their ability to recognize (via Fc receptors) and destroy antibody-coated cells (a process called antibody-dependent cell-mediated cytotoxicity [ADCC]), and to produce high amounts of IFN-γ, a potent stimulator of macrophage activity, as well as tumor necrosis factor (TNF)-α, granulocyte macrophage colony-stimulating factor (GMCSF), and other cytokines and chemokines. Production of these soluble factors by NK cells in early innate responses influences the recruitment and function of other hematopoietic cells. In turn, activated macrophages produce IL-12, a potent inducer of NK cell IFN-γ production and cytolytic activity ( Fig. 3-8 ).
Two major subsets of NK cells are found in human subjects according to their level of expression of CD56, namely CD56 dim and CD56 bright . CD56 dim NK cells represent the 90% of NK cells in peripheral blood. They are fully mature and mediated cytotoxicity response. In contrast, CD56 bright cells are more immature and play a major role in the cytokine production. Moreover, this immature subpopulation is better able to leave the circulation and constitute the majority of NK cells found in lymphoid organs. Familial hemophagocytic lymphohistiocytosis (FHL) is a genetically heterogeneous disorder caused by mutations in genes involved in the secretory lysosome-dependent exocytosis pathway, and it is clinically characterized by a hyperinflammatory syndrome usually triggered by viral infections. Children with autoimmune diseases, especially systemic juvenile idiopathic arthritis (sJIA), may develop a clinical syndrome closely resembling FLH called macrophage activation syndrome (MAS). Even if patients with MAS have normal or reduced NK function, reduced expression of perforin and heterozygous mutations in one FHL-related genes have been observed (see also Chapter 16 ).
Fibroblasts
Fibroblasts, together with cartilage cells, bone cells, and fat cells, belong to the family of connective-tissue cells. All of these cells are specialized in the secretion of collagenous extracellular matrix (ECM) and provide mechanical strength to tissue by providing a supporting framework to the ECM itself. Connective-tissue cells play a central part in repair mechanisms. Tissue fibroblasts may play an active role in the effector arm of the inflammatory response and in immune mediated diseases. During inflammation, proinflammatory cytokines produced by tissue macrophages activate tissue fibroblasts to produce cytokines, chemokines, prostaglandins (PGE 2 ), and proteolytic enzymes such as metalloproteinases. The failure to switch off activated tissue fibroblasts has been proposed as a possible mechanism leading to chronic inflammation, through the persistent overexpression of chemokine and proinflammatory cytokines, and consequent continuous recruitment of leukocytes within tissues. The late mechanisms play a crucial role in the pathogenesis of scleroderma (see Chapter 27 ).
Connective tissue contains a mixture of distinct fibroblast lineages including “mature” fibroblasts with a lesser capacity of transformation, and immature fibroblasts (called also mesenchimal fibroblasts) that are capable of differentiating into several different cell lineages. Moreover, fibroblast precursors with a multipotent character also circulate in blood and, due to their similarity with stromal cells of bone marrow, are called mesenchymal stem cells.
Molecules of Innate Immunity
The Complement System
The complement system consists of several normally inactive plasma proteins, which, after activation under particular conditions, interact to generate products that mediate important effector functions, including promotion of phagocytosis lysis of microbes and stimulation of inflammation. Activation of complement involves sequential proteolytic steps that generate an enzymatic cascade similar to that of the coagulation system.
There are three major pathways of complement activation ( Fig. 3-9 ). The alternative pathway is related to the direct binding of one of the complement proteins, C3b to microbial cells. The classical pathway involves a more sophisticated mode of activation, in which a plasma protein, C1 binds to the C H 2 domains of immunoglobulin (Ig) G or to C H 3 domains of IgM that have bound antigen. The same proteins involved in the classical pathway can be activated in absence of antibodies by plasma proteins (mannose-binding lectin [MBL] or ficolins) ( lectin pathway). Activation of the lectin pathway occurs through direct recognition of carbohydrate or acetylated PAMPs by MBL and ficolins, respectively, in association with MBL-associated serine proteases (MASPs).
