Key Notes
A variety of molecules mediate protection against microbes during the period before adaptive immunity develops. These molecules react with particular structures common to a variety of microbes, and thus with many different microbes that express these structures. Molecules of the innate immune system include complement, acute phase proteins, and cytokines, particularly
interferons and anti-microbial peptides. Some, especially those of the complement system, are vital for adaptive immunity.
The complement system consists of over 20 interdependent proteins, which on sequential activation may mediate protection against microbial infection.
Synthesized by hepatocytes and monocytes, these proteins can be activated directly by microbes through the alternative pathwayand thus have a pivotal role in innate immunity. This system can also be activated through the classical pathwayby antibodies (adaptive immunity) bound to a microbe. On activation, the complement system can: (a) initiate (acute) inflammation;
(b) attract neutrophils to the site of microbial attack (chemotaxis); (c) enhance attachment of the microbe to the phagocyte (opsonization); (d) kill the microbe.
Acute phase proteins are a heterogeneous group of plasma proteins important in innate defense against microbes (mostly bacteria) and in limiting tissue damage caused by infection, trauma, malignancy and other diseases. They include C-reactive protein (CRP), serum amyloid protein A (SAA), and mannose-binding protein (MBP). Acute phase proteins are mainly produced in the liver, usually as the result of a microbial stimulus, or in response to the cytokines IL-1, IL-6, TNFα and IFNγ that are released by activated macrophages and NK cells. These proteins maximize activation of the complement system and opsonization of invading microbes.
Cytokines are small molecules that signal between cells, inducing growth, chemotaxis, activation, enhanced cytotoxicity and/or regulation of immunity.
They are referred to as interleukins if produced primarily by leukocytes, monokines if produced by myeloid cells, lymphokines if produced by lymphocytes, and chemokines if they direct cell migration. Interferons protect against viral infection, activate cells and modulate immunity.
Interferons(IFNs) are produced in response to viral infection and inhibit protein synthesis. Type I IFNs, IFN-alpha (IFNα) and -beta (IFNβ), are produced by many different cells. Type II interferon (IFNγ), mainly produced by Th1 cells and NK cells, induces Th1 responses, increases antigen
presentation, and activates phagocytic and NK cells for enhanced killing.
Lymphokinesare growth factors for lymphocytes and influence the nature of the immune response. IL-2 is made by T cells as a T cell growth factor. IL-3 is Innate molecular
immune defense
The complement system
Acute phase proteins
Cytokines
important in hematopoiesis. IL-4 is produced by Th2 cells and mast cells and is a growth and differentiation factor for Th2 cells and B cells. IL-5, also produced by Th2 cells and mast cells, is important to B cell activation and production of IgA. IL-10, which is produced by Th2 cells and MØ, induces Th2 responses.
Monokineshave activities critical to immune defense and inflammation. IL-1, tumor necrosis factor α (TNFα), and IL-6 activate lymphocytes, increase body temperature, activate and mobilize phagocytes and activate vascular
endothelium. TNFαalso activates MØ. IL-8 is chemotactic for PMNs. IL-12 activates NK cells to produce IFNγ.
Chemokinesare small cytokines produced by many cell types in response to infection or physical damage. They activate and direct effector cells expressing appropriate chemokine receptors to sites of tissue damage and regulate leukocyte migration into tissues. CC chemokines are chemotactic for monocytes, CXC chemokines are chemotactic for PMNs.
Other cytokinesinclude colony-stimulating factors (CSFs) that drive
development, differentiation and expansion of cells of the myeloid series. GM- CSF induces commitment of progenitor cells to the monocyte/granulocyte lineage, G-CSF and M-CSF commitment to the granulocyte or monocyte lineage, respectively. Transforming growth factor β(TGFβ) inhibits activation of MØ and growth of B and T cells. Tumor necrosis factor β(TNFβ) is cytotoxic.
Collectins, a group of carbohydrate-binding proteins, act as opsonins to facilitate the removal and destruction of microbes. Peptide antibiotics, produced by a variety of cells, are able to eradicate bacterial infections.
Related topics
There are many molecules of the innate immune system which are important in mediating protection against microbes during the period before the develop- ment of adaptive immunity. Although these molecules react with particular structures associated with microbes, they are nonspecific in that they can react with many different microbes that express these structures. The major mole- cules are those of the complement system, acute phase proteins and cytokines, especially the interferons. Most of the molecules which play a role in the innate immune system are also associated with adaptive immunity. Thus, the comple- ment system can be activated by antibodies, and cytokines are involved in acti- vation of antigen-presenting cells critical to triggering T lymphocyte responses.
Cytokines released by macrophages also play a role in acute inflammation.
Thus, the immune response to microbes is continuous with both systems being intimately involved and synergistic. A variety of other molecules are also important to the innate immune system, including the antibiotic peptides.
Innate molecular immune defense
External defenses (A2)
Hemopoiesis – development of blood cells (A5)
Cells of the innate immune system (B1)
Innate immunity and inflammation (B4)
Lymphocytes (C1) B cell activation (E2)
T cell recognition of antigen (F2) T cell activation (F4)
Clonal expansion and development of effector function (F5)
Genes, T helper cells, cytokines and the neuroendocrine system (G5) Immunity to different organisms
(H2)
24 Section B – Cells and molecules of the innate immune system
Other molecules
The complement system is a protective system common to all vertebrates (Topic D8). In man it consists of 20 soluble glycoproteins (usually designated as C1, C2, etc., or as factors, e.g. factor B), many of which are produced by hepatocytes and monocytes. They are constitutively present in blood and other body fluids.
On appropriate triggering, these components interact sequentially with each other (i.e. in a domino-like fashion). This ‘cascade’ of molecular events involves cleavage of some complement components into active fragments (e.g. C3 is cleaved to C3a and C3b) which contribute to activation of the next component, ultimately leading to lysis of, and/or protection against, a variety of microbes.
This system can be ‘activated’ (Fig. 1) directly through the alternative pathway by certain molecules associated with microbes, or through the classical path- wayby antibodies bound to a microbe or other antigen (Topic D8).
The alternative pathway is activated by interaction of C3 with certain types of molecules on microbes or by self-molecules (e.g. CRP, see below) which react with these microbes. Complement component C3 is critical to this interaction and its cleavage into C3a and C3b is the single most important event in the acti- vation of the complement system. More specifically, the alternative pathway depends on the normal continuous low-level breakdown of C3 (Table 1). One of the fragments of C3, C3b, is very reactive and can covalently bind to virtually any molecule or cell. If C3b binds to a self cell, regulatory molecules associated with this cell (Topic D8) inactivate it, protecting the cell from complement- mediated damage. However, if C3b binds to a microbe, Factor B is activated and its cleavage product Bb binds to C3b on the microbe. This C3bBb complex (C3 convertase) is enzymatically active and amplifies the breakdown of addi- The complement
system
B2 – Molecules of the innate immune system 25
C3
Activation leads to
Enhanced phagocytosis
Lysis Inflammation
Convertases
Classical pathway (antibody-mediated) Alternative pathway
(microbe alone)
Fig. 1. The complement system.
Table 1. Sequence of complement activation by the alternative pathway leading to cell lysis
Microbe (M) + C3b M-C3b
MC3b + factor B M-C3b-Bb
M-C3b-Bb + C3b M-C3b-Bb-C3b
M-C3b-Bb-C3b + C5 M-C3b-Bb-C3b-C5b + C5a
M-C3b-Bb-C3b-C5b + C6 + C7 M-C3b-Bb-C3b-C5b-C6-C7 M-C3b-Bb-C3b-C5b-C6-C7 + C8 M-C3b-Bb-C3b-C5b-C6-C7-C8
M-C3b-Bb-C3b-C5b-C6-C7-C8 + C9 M-C3b-Bb-C3b-C5b-C6-C7-C8-C9 Lysis of M
tional C3 to C3b. Equally important, the resulting enzyme cleaves C5 into C5a and C5b, both of which have critical protective functions. C5b is crucial to formation of the ‘membrane attack complex’ (MAC), C5b-C6-C7-C8-C9 which mediates lysis of the microbe. This alternative pathway is important for control of infection in the absence of specific immunity. Thus, many different organ- isms are handled and eliminated as a result of their activation of the alternative pathway.
The major functions of the complement system are:
● Initiation of (acute) inflammation by direct activation of mast cells (C3a, C5a).
● Attraction of neutrophils (chemotaxis) to the site of microbial attack (C5a).
● Enhancement of the attachment of the microbe to the phagocyte (opsoniza- tion) (C3b).
● Killing of the microbe activating the membrane attack complex (lysis) (C9).
Acute phase proteins are important in innate defense against microbes (mostly bacteria and protozoa) and in limiting tissue damage caused by microbial infec- tion, trauma, malignancy and other diseases, e.g. rheumatoid arthritis. They are also important in tissue repair. These molecules include C-reactive protein (CRP), complement components, opsonic proteins such as mannose-binding protein (MBP), metal-binding proteins and protease inhibitors. The major acute phase proteins, CRP and serum amyloid protein A (SAA), have similar struc- tures and are termed pentraxins, based on the pentagonal association of their subunits. CRP, which was named based on its ability to react with the C-protein of pneumococcus, is composed of five identical polypeptides associated by noncovalent interactions. MBP binds residues of mannose on glycoproteins or glycolipids expressed by microbes in a form different from that on mammalian cells. Its binding properties permit it to interact with a variety of pathogens.
These proteins, mainly produced by the liver, can either be produced de novo (e.g. CRP is increased by as much as 1000-fold within a few hours), or are present at low levels and rapidly increase following infection (fibrinogen). They are produced by hepatocytes in response to the cytokines IL-1, IL-6, TNFαand IFNγ released by activated macrophages and NK cells. IL-6 is important in enhancing production of acute phase proteins.
Acute phase proteins have several functions (Table 2), the most important being to maximize activation of the complement system and opsonization of invading microbes, and to limit tissue damage caused by these microbes. CRP Acute phase
proteins
26 Section B – Cells and molecules of the innate immune system
Table 2. Acute phase proteins and their functions
Protein Function
C-reactive protein (CRP) Binds to bacterial phosphoryl choline, activates complement through C1q, acts as an opsonin
Serum amyloid A (SAA) Activates complement (through C1q), acts as an opsonin
Mannose binding protein (MBP) Binds to mannose on bacteria, attaches to phagocyte MBP receptors (opsonization), activates complement via classical pathway (Topic D8) Complement components Chemotaxis, opsonization and lysis (Topic D8)
Metal binding proteins Removal of essential metal ions required for bacterial growth
Fibrinogen Coagulation factor
α1 anti trypsin, α1 anti chymotrypsin Protease inhibitors
binds to a wide variety of microbes and on binding activates complement through the alternative pathway, causing C3b deposition on the microbe (opsonization) and thus ultimately its phagocytosis by phagocytes expressing receptors for C3b. MBP binding to microbes also initiates complement activa- tion and subsequent opsonization mediated by C3b, but in addition it directly opsonizes these organisms for phagocytosis. In addition, metal-binding proteins inhibit microbial growth, and protease inhibitors limit tissue damage by neutralizing lysosomal enzymes released from phagocytes.
Both CRP and SAA, as well as having complement activation properties, bind to DNA and other nuclear material from cells, helping in their clearance from the host. Quantitation of CRP in the serum of patients with inflammatory diseases (e.g. rheumatoid arthritis) is used as a way to assess the inflammatory activity of the disease. High levels of CRP signify a high level of disease activity.
Cytokines Cytokines are small molecules, secreted by cells in response to a stimulus. They may have an effect on the cell that produces them and are critical to signaling between cells, with each cytokine often inducing several different biological effects. Many different cells release cytokines, but each cell type releases only certain of these molecules. Cytokines may induce growth, differentiation, chemo- taxis, activation, and/or enhanced cytotoxicity. Moreover, it is not uncommon for different cytokines to have similar activities and for many cytokines, some with opposing activities, to be released by a particular stimulus. Thus, the resulting biological effect is a factor of the sum of all of these activities.
To some extent cytokines can be grouped by the cell populations that secrete them. Monokines are cytokines secreted by cells of the myeloid series (mono- cytes, macrophages) and lymphokines are cytokines secreted primarily by lymphocytes, although some cytokines are produced by both lymphocytes and myeloid cells. The term interleukin (IL) is often used to describe cytokines produced by leukocytes, although some interleukins are also produced by other cell populations. A group of small heparin-binding cytokines, chemokines, direct cell migration, and may also activate cells in response to infectious agents or tissue damage. Interferons are produced by a variety of cells in response to viral infection.
It is important to note that the same cytokine can be made by several differ- ent cell populations. For example, IFNαis made by most if not all nucleated cells in response to viral infection. IFNγis produced both by Th1 cells and by NK cells. IL-1 is produced by macrophages, B cells and nonimmune keratinocytes. Many different cell types make IL-6, several make IL-4, etc.
Moreover, the same cytokine can induce different functions in different cell types. For example, TNFαcan promote the proliferation of B cells but activate killing mechanisms in other cell populations. IFNγactivates macrophages to kill intracellular microbes, induces B cells to switch their antibody class to IgG and induces endothelial cells to increase expression of MHC class II molecules.
Interferons
Interferons are pro-inflammatory molecules which can mediate protection against virus infection, and are thus particularly important in limiting infection during the period when specific humoral and cellular immunity is developing.
They can be divided into two groups, type I IFN (IFNαand IFNβ) and type II IFN (IFNγ) also called immune IFN (Table 3).
B2 – Molecules of the innate immune system 27
IFNαand IFNβ are produced by many different cells in response to viral or bacterial infections, especially by intracellular microbes. At least 12 different, highly homologous species of IFNαare produced, primarily by infected leuko- cytes as well as by epithelial cells and fibroblasts. In contrast, a single species of IFNβ is produced, normally by fibroblasts and epithelial cells. The proinflam- matory cytokines IL-1 and TNFαare potent inducers of IFN-α/β secretion, as are endotoxins derived from the cell wall of Gram-negative bacteria.
The receptor for both IFNαand IFNβ is the same and found on most nucle- ated cells. Binding of IFNαand IFNβ to this receptor inhibits protein synthesis and thus viral replication as a result of the induction of the synthesis of inhibitory proteins and of preventing mRNA translation and DNA replication.
In addition, these interferons inhibit cell proliferation, increase the lytic activity of NK cells and induce increased expression of MHC class I and other compo- nents of the class I processing and presentation pathway leading to induction of antigen-specific cytolytic T lymphocyte (CTL) responses against virally infected cells. Induction of MHC class I is also important for protection of uninfected cells from killing by NK cells (Topic B1). The importance of IFN-α/β in innate defense against viral infections is indicated by animal studies in which treat- ment of virus-infected mice with antibodies to IFN-α/βresulted in death.
In contrast to the broad and rather nonspecific antiviral activity of IFN-α/β, IFNγ is primarily a cytokine of the adaptive immune system, as it is important not only for antiviral activity but also plays a major role in regulation of the development of specific immunity and in activation of cells of the immune system. Produced pri- marily by Th1 cells and NK cells, IFNγplays a critical role in induction of Th1 immune responses. That is, early in the development of a specific immune response, IFNγis involved in inducing Th0 cells to differentiate to Th1 cells which make more IFNγand provide help for development of CTL responses and for IgG antibody production. In addition, Th1 cells or CTLs responding to peptides pre- sented in MHC molecules produce IFNγwhich acts both locally and systemically to activate monocytes, MØ, and PMNs which are then better able to kill intracel- lular pathogens. In particular, IFNγincreases the expression of Fc receptors for IgG on macrophages and PMNs (Topic D8) as well as MHC Class II expression on a wide variety of cells. This enhances the phagocytic function of these cells as well as the antigen-presenting capabilities of professional antigen-presenting cells.
IFNγ, which is crucial for macrophage function, enhances macrophage killing of intracellular bacteria and parasites probably as a result of its stimulation of their production of reactive oxygen and reactive nitrogen intermediates.
28 Section B – Cells and molecules of the innate immune system
Table 3. The interferons
Type I (IFN-α/β) Type II (IFNγ)
Chromosomal location 9 12
Origin All nucleated cells, especially NK cells and Th1, γδ and fibroblasts, macrophages and CD8 T cells
dendritic cells
Induced by Viruses, other cytokines, some Antigen-stimulated T cells intracellular bacteria and
protozoans
Functions Antiviral, increases MHC class I Antiviral, increases MHC I expression, inhibits cell and II expression, activates
proliferation macrophages
Lymphokines
A variety of cytokines are produced by lymphocytes and lymphocyte subsets (Table 4), many of which are growth factors for lymphocytes and/or influence the nature of the immune response. As an example, IL-2 is made by T cells as a critical autocrine growth factor that is required for proliferation of T cells, espe- cially Th0 and Th1 cells and CTL. On activation (as a result of the interaction of their antigen receptor complexes with antigenic peptide in MHC molecules on APCs) these T cells make IL-2 for secretion and at the same time IL-2 receptors with which to bind and be stimulated by the secreted IL-2. In the absence of IL-2 and/or its receptor, many antigen-specific T cells do not expand, severely compromising immune responses.
IL-3 is involved in the growth and differentiation of a variety of cell types as a result of its synergistic activity with other cytokines in hematopoiesis. IL-4 is produced by Th2 cells and mast cells and is a growth and differentiation factor for Th2 cells and B cells, and can induce B cell class switch to IgE antibodies.
IL-4 is important in influencing the nature of the immune response, as it can induce the development of Th2 cells from Th0 cells and can inhibit the develop- ment of Th1 responses (Table 4). Thus, IL-4 is not only involved in B cell growth, but it can also influence the B cell and its subsequent plasma cells to produce IgE antibody (Topic D3). IL-5 is also produced by Th2 cells and mast cells and is important to B cell activation and in induction of B cell class switch to IgA antibody. It also has a role in eosinophil growth and differentiation. IL- 10, which is produced by Th2 cells and MØ, induces B cell activation and Th2 responses and inhibits Th1 responses, perhaps by enhancing IL-4 production and/or by suppressing MØ activity and production of IL-12, a Th1-stimulatory cytokine.
Monokines
This group of cytokines (Table 4) has many different local and systemic activi- ties that are critical to immune defense. In addition, these pro-inflammatory
B2 – Molecules of the innate immune system 29
Table 4. Representative lymphokines and monokines
Cytokine Produced by Activity
IL-1 MØ, epithelial cells Activates vascular endothelium; tissue destruction; increased effector cell access; fever; lymphocyte activation; mobilization of PMNs; induction of acute phase proteins (CRP, MBP)
IL-2 T cells Proliferation of T and NK cells
IL-3 T cells, thymic cells Proliferation and differentiation of hematopoietic cells
IL-4 Th2 cells, mast cells B cell activation and proliferation; induces Th2 IgE responses and inhibits Th1 responses
IL-5 Th2 cells, mast cells Eosinophil growth, differentiation; B cell activation, induces IgA responses IL-6 T cells, MØ Lymphocyte activation; fever; induction of acute phase proteins
IL-8 Mo, MØ, Fb, Kr Increases tissue access for, and chemotaxis of PMNs
IL-10 Th2 cells, MØ B cell activation; suppression of MØ activity; induces Th2 and inhibits Th1 responses
IL-12 B cells, MØ Induces Th1 and inhibits Th2 responses; activates NK cells IFNγ T cells, NK cells MØ and PMN activation; induces Th1 and inhibits Th2 responses TNFα MØ, T cells Activates vascular endothelium; fever; shock; increases vascular
permeability; induces mobilization of metabolites
Monocytes (Mo), macrophages (MØ), endothelial cells (En), fibroblasts (Fb), keratinocytes (Kr), neutrophils (PMNs), chondrocytes (Co).