The innate immune system

9
Innate defences can be classified into three main groups: microbiome. Increasing evidence supports a correlation between
1. Barriers to infection the microbiota and disease development, such as cancer, cardio-
2. Cells vascular disease, diabetes and neurological disorders.1,2 For that
3. Serum proteins and the complement system reason, manipulation of the microbiota, as a potential disease
treatment, has attracted a lot of interest. Faecal microbiota trans-
plantation (FMT), which refers to the introduction of faecal mat-
ter from a donor to the intestinal tract of a recipient, has emerged
BARRIERS TO INFECTION as a potential treatment for various conditions, such as the
Clostridium difficile infection (CDI), cancer and inflammatory
Physical and mechanical bowel disease (IBD), with very promising results (cure rates up to
90% in CDI).3,4 Other approaches include the use of pro- and
Skin and mucosal membranes act as physical barriers to the entry prebiotics, administration of antibiotics and engineering of gut
of pathogens. Tight junctions between cells prevent the majority bacteria.
of pathogens from entering the body. The flushing actions of
tears, saliva and urine protect epithelial surfaces from coloniza-
tion. High oxygen tension in the lungs, as well as body tempera-
CLINICAL NOTES
ture, can also inhibit microbial growth.
In the respiratory tract, mucus is secreted to trap microorgan- Clostridium difficile (C. diff): Hospitalized patients that
isms. They are then mechanically expelled by: are being treated with certain antibiotics (such as
clindamycin, cephalosporins and fluoroquinolones) are
• Beating cilia (mucociliary escalator)
at greater risk of developing a C. diff infection because
• Coughing of the disruption of the patient’s normal gut flora. C. diff
• Sneezing infection causes watery diarrhoea, nausea and
abdominal pain; possible complications include toxic
Chemical megacolon and sepsis. The infection can rapidly spread
between patients in a ward, so adequate hand washing
The growth of microorganisms is inhibited at acidic pH (e.g., in and isolation of infected patients are essential.
  
the stomach and vagina). Lactic acid and fatty acids in sebum
(produced by sebaceous glands) maintain the skin pH between 3
and 5. Enzymes such as lysozyme (found in saliva, sweat and
tears) and pepsin (present in the gut) destroy microorganisms.
CELLS OF INNATE IMMUNITY
Biological (normal flora—the
The cells of the innate immune system consist of:
microbiome)
• Phagocytes
The microbiome is a dynamic collection of organisms (bacteria, • Natural killer (NK) cells
viruses and fungi) that live normally in and on our body (skin, res- • Degranulating cells
piratory system, gastrointestinal and urogenital tract). It consti- • Dendritic cells (DCs)
tutes an essential element of the innate immune system, as it
influences the effectiveness of the immune responses and main-
tains the balance between proinflammatory and antiinflammatory
Phagocytes
responses, promoting immune homeostasis. The microbiome may Phagocytes (macrophages and neutrophils) engulf and then
change in response to various factors, such as stress, diet, medica- destroy pathogens. Macrophages are long-lived sentinel cells sta-
tions and other environmental factors. tioned at likely sites of infection; upon infection, they release
Dysbiosis refers to the loss of the gut homeostasis, resulting in cytokines that recruit the shorter-lived but more actively phago-
a change in the distribution, composition or diversity of the cytic neutrophils.

115
 The innate immune system

Neutrophils (for structure and production, 1

Phagocyte
see Chapter 1: Neutrophils) (neutrophil, Pseudopodia
macrophage or
Neutrophils comprise 50% to 70% of circulating white cells. eosinophil)
Neutrophils are the first cells recruited into the site of inflamma-
tion and in the act of killing pathogens they die; in fact, dead
Pathogen
neutrophils are the major constituent of pus. Neutrophils migrate
from the bloodstream in response to tissue damage, complement
proteins and chemicals released by macrophages (see
Attachment (direct
Chapter 11). As they are phagocytes, they have an important role or via complement)
in engulfing and killing extracellular pathogens. The process of
phagocytosis and the mechanisms of killing are shown in Fig. 9.1.
During inflammation, neutrophil production is stimulated by the
cytokine granulocyte colony-stimulating factor. A high neutro- 2
phil count is part of the acute phase response (see clinical notes
Phagosome
section on ‘Acute Phase Response). Neutrophils can be activated
and recruited either by interleukin (IL)-8 and tumour necrosis
factor (TNF) secreted by macrophages, or by IL-17 secreted by T
cells of the adaptive immune system.
Lysosome

CLINICAL NOTES
Neutropenic sepsis: Neutropenic individuals are at an
increased risk of serious bacterial infections and sepsis.
3
Neutropenic sepsis is often difficult to detect because
of the decreased immune response to pathogens; any Phagolysosome
suspicion of infection should be investigated, and the
patient treated accordingly. Treatment of neutropenic
sepsis includes fluid resuscitation (in case of Microbial
hypotension) and broad-spectrum antibiotics, along degradation
with antivirals or antifungals, depending on the
suspected offending organism. Close monitoring of
clinical parameters (blood pressure, oxygen saturation,
Fig. 9.1 Phagocytosis. Pseudopodia surround the pathogen. (1)
heart rate and temperature), along with biochemical
They fuse around the organism, producing a vesicle known as a
markers (such as C-reactive protein (CRP) and lactate)
phagosome. (2) Lysosomes fuse with the phagosome to form
is essential for a favourable outcome.
   phagolysosomes. (3) Proteins within the lysosome, and other
granules that fuse with the phagolysosome, lead to degradation
of the organism. The microbial products are then released.
Monocytes and macrophages
Monocytes and macrophages comprise the other major group Macrophages phagocytose and destroy their targets using
of phagocytic cells. Monocytes account for 5% to 10% of the similar mechanisms to neutrophils. The rate of phagocytosis can
circulating white cell count and circulate in the blood for be greatly increased by opsonins, such as immunoglobulin (Ig) G
approximately 8 hours before migrating into the tissues, and the complement protein C3b. Neutrophils and macrophages
where they differentiate into macrophages; these mac- have receptors for these opsonins, which may be bound to the
rophages can live for decades. Some macrophages become antigenic surface. Intracellular pathogens, for example
adapted for specific functions in particular tissues, for Mycobacterium, can prove difficult for macrophages to kill. They
example Kupffer cells in the liver, glial cells in the brain and are either resistant to destruction inside the phagosome or can
osteoclasts in bone. evade by entering the macrophage cytoplasm. For the immune
In comparison to monocytes, macrophages: system to act against these pathogens, T-cell help is required (see
• Are larger and longer lived Chapter 10).
• Have greater phagocytic ability In addition to phagocytosis, macrophages can secrete a num-
• Have a larger repertoire of lytic enzymes and secretory ber of compounds into the extracellular space, including
products cytokines (TNF, IL-8 and IL-1) and hydrolytic enzymes.

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 Cells of innate immunity 9

Macrophages express a wide array of surface molecules Because the C-type lectin receptors are activated by carbohy-
including: drates on normal host cells, the KIR system acts to prevent NK
• Receptors for complement and the Fc portion of IgG cells from attacking the host. Conversely, a cell that is not
• Pattern recognition molecules (PRMs) expressing class I MHC molecules will not activate the KIR
• Cytokine receptors, for example, TNF-α and interferon-γ system and so would be attacked by NK cells. This is useful in
(IFN-γ) eliminating body cells that have downregulated class I MHC
• MHC and B7 molecules (to activate the adaptive immune because of viral infection (e.g., herpes) or mutation; the lack of
response) MHC I in these infected or mutated cells evades attack from
T cells but this same MHC I deficiency causes NK cells to attack.
Macrophages can be activated by:
NK cells can also destroy antibody-coated target cells irre-
• Cytokines such as IFN-γ spective of the presence of MHC molecules, a process known as
• Contact with complement or products of blood coagulation antibody-dependent cell-mediated cytotoxicity (ADCC). This
• Direct contact with the target through PRM stimulation occurs because killing is initiated by cross-linking of receptors
Following activation, macrophages become more efficient for the Fc portion of IgG1 and IgG3.
phagocytes and have increased secretory and microbicidal activ- NK cells are not clonally restricted, have no memory and are
ity. They are also able to process and present antigen in association not very specific in their action. They induce apoptosis in target
with class II major histocompatibility complex (MHC) molecules, cells (Fig. 9.2) by:
stimulating the adaptive immune system (see Chapter 10). • Ligation of Fas or TNF receptors on the target cells; NK cells
In comparison to neutrophils, macrophages: produce TNF and exhibit Fas ligand (FasL). This initiates a
• Are longer-lived (they do not die after dealing with sequence of caspase recruitment and activation, resulting in
pathogens) apoptosis.
• Are larger (diameter 25–50 µm), enabling phagocytosis of • Degranulation by NK cells, which releases perforins and
larger targets granzymes. Perforin molecules insert into and polymerize
• Move and phagocytose more slowly within the target cell membrane. This forms a pore through
• Retain Golgi apparatus and rough endoplasmic reticulum which granzymes can pass. Granzyme B then initiates
and can therefore synthesize new proteins, including apoptosis from within the target cell cytoplasm.
lysosomal enzymes and secretory products The induction of apoptosis is a crucial tool for the immune
• Can act as antigen-presenting cells (APCs) system. It can be used for targeted killing of infected or mutated
cells and is also a key part of the development of the adaptive
Killing by phagocytes immune system (see Chapter 10).
The process of phagocytosis allows cells to engulf matter that
needs to be destroyed. The cell can then digest the material in a
HINTS AND TIPS
controlled fashion before releasing the contents. The process of
phagocytosis is shown in Fig. 9.1. Fas receptors are found on human cells. Once they bind
to the FasL, they stimulate apoptosis.
  
Natural killer cells
Natural killer cells develop from lymphoid progenitor cells, but
unlike the other types of lymphocytes (T cells and B cells; see Degranulating cells
Chapter 10), their function mainly lies in the innate immune
response. NK cells use cell-surface receptors to identify virally Mast cells and basophils (Chapter 1:
modified or cancerous cells. Similar to macrophages, NK cells do Basophils)
not require T-cell help to kill pathogens but function more effec- Mast cells and basophils have similar functions but are found in
tively when T-helper cells secrete IFN-γ. One set of receptors different locations; basophils comprise <1% of circulating white
activates NK cells, initiating killing; others inhibit the cells: cells, whereas mast cells are resident in the tissues.
• Activating receptors include calcium-binding C-type High concentrations of mast cells are found close to blood
lectins, which recognize certain cell-surface carbohydrates. vessels in connective tissue, skin and mucosal membranes. The
• Inhibitory receptors include killer-cell immunoglobulin-like two types of mast cells – mucosal and connective tissue – differ in
receptors (KIRs). KIRs are members of the Ig gene their tissue distribution, protease content and secretory profiles.
superfamily (see Chapter 10) and recognize class I MHC Mast cells function by discharging their granule contents (such
molecules on other cells. as histamine, serotonin, bradykinin, prostaglandins and

117
 The innate immune system

NK cell

FasL
1 5

TNF-
4

3
Fas

TNF receptor
6
Intracellular
signalling

Apoptosis
Target cell

Fig. 9.2 Mechanism of killing by natural killer (NK) cells. Activation of NK cells in the absence of an inhibitory signal results in
degranulation. (1) Perforins form a pore in the target cell, allowing entry of granzymes (2) (3). Tumour necrosis factor (TNF) produced
by NK cells acts on the target cell’s receptors (4). Fas ligand (FasL) interacts with target cell Fas (5). Intracellular signalling from Fas
TNF receptors and granzymes results in apoptosis (6).

lumens, such as tapeworms, which otherwise would evade

Antigen
immune attack. Mast cell activation releases leukotrienes, which
Mast cell
attract eosinophils to the site of parasitic infection. This plays an
important role in the development of an episode of a type I hyper-
sensitivity, with the mast cells and basophils providing the early
+ phase response and the eosinophils mediating the late phase
Ca2
IgE response. This underlies the allergic response (see Chapter 12).

FcRI
HINTS AND TIPS
During severe allergy (anaphylaxis, see Chapter 12:
Anaphylaxis), mast cell tryptase levels increase for a
few hours (median half-life 90 minutes). This is a useful
diagnostic test for anaphylaxis. However, the immediate
Degranulation diagnosis of anaphylaxis is based on clinical symptoms
and examination (breathlessness, chest tightness, face
Fig. 9.3 Activation of mast cells by immunoglobulin E (IgE). or tongue swelling, difficulty swallowing, anxiety,
IgE, produced by plasma cells, binds via its Fc domain to dizziness/loss of consciousness). Tryptase testing may
receptors on the mast cell surface. Cross-linking of these be done after treating the allergic reaction
receptors by an antigen causes an influx of calcium ions (Ca2+) (approximately 4–6 hours) in order to confirm the
into the cell. Calcium ions cause a rapid degranulation of diagnosis (symptoms of anaphylaxis similar to other
inflammatory mediators from the mast cell. FcεRI, High-affinity medical emergencies, such as acute myocardial
IgE receptor. ischaemia and exacerbation of asthma).
  
leucotrienes). Degranulation is triggered by cross-linking of high-
affinity receptors for the Fc portion of IgE (Fig. 9.3). Cross-linkage
results in an influx of calcium ions into the cell, which induces Eosinophils Chapter 1: Eosinophils
release of pharmacologically active mediators from granules. This Eosinophils comprise 1% to 3% of circulating white cells and are
mechanism allows mast cells to attack larger organisms living in found principally in tissues. They are derived from the
118
 Soluble proteins 9

colony-forming unit for granulocytes, erythrocytes, monocytes with the adaptive immune system, initiating a long-lasting, anti-
and megakaryocytes (CFU-GEMM) and their maturation is gen-specific response.
similar to that of the neutrophil (see Chapter 1). They are impor- Dendritic cells are also responsible for maintaining immune
tant in the defence against parasites and cause damage by extra- homeostasis under a steady state, by protecting against the T-cell
cellular degranulation. Their granules contain major basic recognition of self-antigens. This is mediated by the
protein, cationic protein, peroxidase and perforin-like mole- DC-induction of immunosuppressive T-regulatory cells (Tregs)
cules. Peroxidase generates hypochlorous acid, major basic pro- that prevent the initiation of an immune response against self- or
tein damages the parasite’s outer surface (as well as host tissues) nonpathogenic environmental antigens.
and cationic protein acts as a neurotoxin, damaging the parasite’s
nervous tissue.

Dendritic cells SOLUBLE PROTEINS

Dendritic cells constitute the major APCs, serving as an essential The soluble proteins that contribute to innate immunity
link between the innate and the adaptive immune system. As (Table 9.1) can be divided into antimicrobial serum agents and
cells of the innate immune system, they are able to detect invad- proteins produced by cells of the immune system.
ing pathogens due to their expression of molecules sensing for
pathogen-derived components (such as the Toll-like and NOD-
like receptors). However, instead of destroying them directly
Acute phase proteins
(like phagocytes), they internalize and process pathogen-derived The acute phase response is a systemic reaction to inflammation,
proteins yielding antigenic peptides, which are then loaded onto where macrophages and monocytes release cytokines IL-1, IL-6,
the MHC-class I or II molecules in order to be presented to naïve IFN-γ and TNF. These cytokines alter the production of certain
T cells. These cells are then activated and induced to become proteins, the acute phase proteins (APPs), by the liver. APPs are
effector (cytotoxic) cells. DCs also express co-stimulatory mole- defined as proteins whose plasma concentration is altered
cules (such as CD80 and CD86), potentiating the T-cell (increased or decreased) by at least 25% during an inflammatory
responses and produce stimulatory cytokines (such as IL-12) that process. APPs with a positive impact include the CRP,
are required for T-cell activation. This way DCs communicate ceruloplasmin, serum amyloid A, fibrinogen, a1-antitrypsin,

Table 9.1 The soluble proteins of innate immunity

Protein Notes
Secreted agents Lysozyme Bactericidal enzyme in mucus, saliva, tears, sweat and breast milk
Cleaves peptidoglycan in the cell wall
Innate antimicrobial Lactoferrin Iron-binding protein that competes with microorganisms for iron, an
serum agents essential metabolite
Complement Group of 20 proenzymes activation leads to an enzyme cascade, the
products of which enhance phagocytosis and mediate cell lysis. Alternative
pathway can be activated by nonspecific mechanism
MBL Activates the complement system
C-reactive protein Acute phase protein, produced by the liver
Binds C-polysaccharide cell wall component of bacteria and fungi
Activates complement via classical pathway
Opsonizes for phagocytosis
Proteins produced by IFN-α Produced by virally infected cells
cells of the innate system
IFN-β Induces a state of viral resistance in neighbouring cells by:
Inducing genes that will destroy viral DNA
Inducing MHC class I expression
IFN-γ Mainly produced by activated NK cells. Activates NK cells and macrophages
IFN-γ, interferon-γ; MBL, mannan-binding lectin; MHC, Major histocompatibility complex; NK, natural killer.

119
 The innate immune system

haptoglobin, hepcidin, ferritin, procalcitonin, complement com-

ponents and others; those with a negative impact include albu- CLINICAL NOTES
min, transferrin and transthyretin. The acute phase response: The acute phase response
Clinically, the APP levels are particularly useful, as they indi- provides us with chemical markers of inflammation that
cate the presence (and intensity) of an inflammatory process and can be measured. In a child presenting with abdominal
can be used for the diagnosis and monitoring of disease activity pain, a CRP level can aid the clinician in their diagnosis.
or as prognostic markers. However, none of the APPs are spe- A normal CRP can allow more conservative
cific, nor can they be used to distinguish between infection and management, whereas a raised CRP would indicate an
other causes of inflammation. CRP and erythrocyte sedimenta- inflammatory response and necessitate urgent
treatment, such as surgery in differentiating the
tion rate (ESR) constitute the most widely used APPs. The
abdominal pain of constipation from that of
change in plasma concentration of APPs is accompanied by appendicitis. The ESR takes more time than CRP to
fever, leucocytosis, thrombocytosis, catabolism of muscle pro- become elevated – CRP rises within 2 hours of the insult
teins and fat deposits. Symptomatically, this change contributes and falls within a day after the insult resolves, making it
to what is described as severe fatigue –‘malaise’. a more sensitive and therefore preferable test (Fig. 9.4).
The neutrophil count is often used to monitor
C-reactive protein inflammation; both neutrophil count and CRP level are
simple, cheap and fast to measure.
Levels of CRP rise within hours of tissue injury or infection. The
actions of CRP are outlined in Table 9.1. CRP elevation can be For these reasons the neutrophil count and CRP are
slight (e.g., cerebrovascular accident), moderate (e.g., myocardial often used, in conjunction with clinical signs, to see if
inflammation is taking place and when it has recovered.
infarction) or marked (e.g., bacterial infections).
The white cell count and ESR are useful markers
measured in chronic inflammatory diseases. Some
hospitals now measure plasma viscosity instead of
HINTS AND TIPS
ESR. However, ESR levels are still crucial for the
Tumour necrosis factor-α, IFN-γ, IL-1 and IL-6 released diagnosis and monitoring of rheumatologic conditions
by macrophages and monocytes stimulate the liver to such as giant cell (temporal) arteritis and systemic lupus
produce the APPs. erythematosus.
  
  

Erythrocyte sedimentation rate

The ESR is an indirect measure of the acute phase response and
is defined as the rate at which the red cells in blood fall when
C-reactive protein
CRP/ESR levels (arbitrary units)

placed in a vertical tube. It is especially representative of the

concentration of fibrinogen, although other constituents of the
blood may influence its levels, such as the α-globulins.
Elevated ESR is associated with acute and chronic infection,
ESR
anaemia, pregnancy, renal disease, multiple myeloma and obe-
sity, whereas decreased levels can be found in hypofibrinoge-
naemia, heart failure, cachexia, extreme leucocytosis and
abnormalities of red cells (such as sickle cells, spherocytosis,
acanthocytosis).

HINTS AND TIPS

0 7 14 21
In chronic inflammation, high CRP and ESR persist. The Time (days)
resulting catabolism of muscle and fat may lead to
severe weight loss. Fig. 9.4 Comparison of the change in C-reactive protein (CRP)
   levels and erythrocyte sedimentation rate (ESR) following an
inflammatory stimulus.

120
 Soluble proteins 9

The complement system receptors. Furthermore, opsonization stimulates the adaptive

immune system through antigen presentation, leading to the
The complement system (so-called because its actions are com- generation of memory cells. Another goal of the complement
plementary to the function of antibody) is, in fact, much older in is the expansion of the inflammatory response, through the
evolutionary terms than antibody and is equally as important. generation of the peptides C3a and C5a, known as
Complement is a collection of over 20 serum proteins, pro- anaphylatoxins.
duced mainly by the liver, that are always at high levels in the The three pathways that activate the complement system
blood of the healthy individual. The reason for the large number are the classical, the alternative and the lectin. All pathways
of proteins is to allow amplification; many of the components of result in the activation of the complement component C3 to
complement are proenzymes that, when cleaved, activate more C3 convertase, which leads to the engagement and activation
complement. This is similar to the amplification of clotting fac- of a common terminal pathway, called the ‘membrane attack
tors in the coagulation cascade seen in Chapter 6. complex’ (MAC). An overview of the complement system is
The primary goal of the complement is the rapid elimina- given in Fig. 9.5. To simplify, the complement pathway is
tion of a pathogen, which is accomplished through deposi- divided into three steps: attachment (initiation), activation
tion of C3b peptides on the target (opsonization). This and amplification (C3 convertase formation) and the forma-
promotes its phagocytosis by cells that bear complement tion of MAC.

Classical pathway Lectin pathway Alternative pathway

Antigen–antibody Mannan–binding lectin Microbial surfaces
complexes binds some
encapsulated bacteria

Activates CI Activates MASP Stabilizes activation of C3

Activation of factor B by
factor D

Activated C4 and C2

C3 convertase

Common pathway

C3a, C5a C3b

(+C4a)

Recruits phagocytes Formation of MAC Binding to receptors

Degranulation of mast
cells and basophils

Lysis of pathogens Opsonization

and cells (anucleate cells are
more susceptible)

Fig. 9.5 Overview of the complement system. Cell lysis by complement is caused by formation of the membrane attack complex
(MAC). This is formed when C5b, C6, C7, C8 and C9 bind together to form a 10-nm pore in the cell surface. MASP, MBL-associated
serine protease.

121
 The innate immune system

The classical pathway enables the production of large quantities of C3b, thus pro-
The classical pathway is triggered by the presence of antibodies ducing a major amplification step in the complement
on the surface of a pathogen. pathway.
IgM is particularly good at activating complement as it is a
pentamer (has five Fc portions): Effectors of complement
• Fc activates C1 C5 is now cleaved into C5a and C5b. C5b then triggers the activa-
• C1 activates C2 and C4 tion of C6–C9. These form the MAC. The MAC attacks patho-
• C2 and C4 activate C3 (C3 convertase) gens by inserting a hole in their cell membrane; the pathogen
then dies via osmotic lysis. The MAC appears to be the only way
the immune system has of killing one family of bacteria, the
The alternative pathway Neisseria (a family that includes meningococcus and
The alternative pathway does not require the presence of an anti-
gonococcus).
body or prior contact with a pathogen to function. It can be trig-
The cleaved fragments C3a and C5a are anaphylatoxins which
gered when autoactivated C3 (‘C3 tickover’) is amplified by the
are chemoattractants for other immune cells which follow the
presence of a molecule embedded in the membrane of an invad-
concentration gradient to the infection. Complement also
ing pathogen or by the lack of a complement inhibitory protein.
opsonizes bacteria as macrophages have receptors for C3b.
C3 is an unstable molecule and without inhibition, spontaneously
These functions are summarized in Table 9.2.
breaks down to the very reactive C3b. C3b then binds factor B,
which undergoes proteolytic cleavage by factor D, creating the
fragments Ba and Bb. The latter binds to the alternative pathway Inhibitors of complement
C3 convertase (C3bBb), generating an amplification loop, which As previously discussed, complement can activate spontaneously
leads to a large deposition of C3b on the target. through the alternative pathway (so-called C3 tickover, which
refers to the constant autoactivation of a small amount of C3).
This offers a surveillance mechanism by constantly monitoring
The lectin pathway the environment for possible pathogens. Complement is regu-
The lectin pathway is activated by two PRMs, the ficolins and the lated by inhibitory molecules which are necessary to prevent
collectins. The latter is comprised of the mannan-binding lectin complement-mediated damage of healthy cells. There are nine
(MBL), which is a normal component of serum, that binds to complement inhibitors which act at various levels throughout the
carbohydrates found on the cell wall of certain bacteria and fungi pathway:
(e.g., Salmonella, Neisseria, Candida albicans). MBL also binds to
• Membrane cofactor protein, complement receptor type 1,
MBL-associated serine proteases (MASP), which bear structural
C4b-binding protein and factor H: these prevent assembly
homology to the C1 complex. MASP then acts on C4 and C2 to
of C3 convertase
generate the C3 convertase of the classical pathway.
• Decay accelerating factor (CD55): this accelerates decay of
C3 convertase
C3 convertase • C1 inhibitor: inhibits C1
With the production of C3 convertase, all three pathways con- • Factor I and membrane cofactor protein: cleave C3b and C4b
verge. C3 convertase has enzymatic effects against C3 and • CD59 (protectin): prevents the formation of the MAC

Table 9.2 Functions of complement

Function Notes
Cell lysis Insertion of MAC causes lysis of gram-negative bacteria
Nucleated cells are more resistant to lysis because they endocytose MAC
Inflammation C3a, C4a, C5a cause degranulation of mast cells and basophils
C3a and C5a are chemotactic for neutrophils
Opsonization Phagocytes have C3b receptors, which means phagocytosis is enhanced when pathogens are coated
in C3b
Solubilization and Complement prevents immune complex precipitation and solubilizes complexes that have already
clearance of immune been precipitated. Complexes coated in C3b bind to CR1 on red blood cells. The complexes are then
complexes removed in the spleen
MAC, Membrane attack complex.

122
 Innate immune system pattern recognition molecules 9

Deficiency in any one of these inhibitory components can seen how they function to defend the body. These are found in
result in disease: solution in the serum and are classified as collectins, being com-
• Hereditary angioedema: deficiency of C1 inhibitor leads to posed of collagen-like and lectin portions. Lectins are any protein
unbalanced, spontaneous activation of the early that binds sugar molecules, usually on the surface of bacteria; for
complement pathway, causing life-threatening swellings. example, MBL binds to the sugar mannose.
• Atypical haemolytic-uraemic syndrome: genetic deficiency in Nucleotide-binding and oligomerization domain (NOD) is a
a complement inhibitor called Factor I leads to activation of pattern recognition molecule found across the body, including in
the late complement cascade, leading to red cell destruction. epithelial cells in the gut. It recognizes certain bacterial cell wall
• Paroxysmal nocturnal haemoglobinuria (PNH): It is caused components and stimulates an immune response. The NOD gene
by a mutation in the PIGA gene that leads to a deficiency in is mutated in some individuals with Crohn disease.
the GPI-linked complement regulators (CD55 and CD59) on Toll-like receptors (TLRs) are a family of about a dozen
the membrane of haematopoietic cells, rendering them PRMs. When they bind their ligand, they send a signal to innate
susceptible to complement-induced cell lysis. Intravascular immune system cells which then secrete cytokines. They have a
haemolysis, increased tendency for thrombosis and bone few important clinical roles. In sepsis, TLR-4 is stimulated by vast
marrow failure constitute the hallmark features of the disease. amounts of lipopolysaccharide found on bacterial cell walls. This
causes release of large amounts of TNF from macrophages,
Therapeutic complement inhibition which in turn activates nitrous oxide synthase causing a fall in
It is now evident that the complement system constitutes a key player blood pressure and organ perfusion.
in maintaining and regulating immune reactions. Its contribution to Drugs designed to bind and stimulate TLRs are being used in
a variety of autoimmune and inflammatory diseases makes it a very situations where it is helpful to stimulate a more powerful
attractive candidate for therapeutic intervention. The approval of the immune response, for example, in some vaccines and cancer
first complement inhibitor, eculizumab (an antibody against comple- treatments.
ment component C5), in 2007 was a major breakthrough in the field,
which changed the outcome in PNH and other diseases.
CLINICAL NOTES
To date, various therapeutic agents have been developed in an
effort to inhibit the complement, such as: Sepsis: Sepsis is when the body’s immune response to
- Inhibitory monoclonal antibodies: C5 inhibitors infection becomes dysregulated, putting the body at
risk of organ failure and death. This can be caused by
(eculizumab, ravulizumab, etc.)
infection, most commonly bacterial but also viral. The
- Small molecule inhibitory peptides: target C5 (nomacopan), role of overactivation of the innate immune system in
Factor B (iptacopan) and Factor D (danicopan) sepsis has been detailed earlier, with the example of
- Aptamers (single-stranded DNA or RNA molecules): siRNA Toll-like receptor–4 receptor activation leading to a fall
for C5 inhibition (cemdisiran) in blood pressure.
- Recombinant complement inhibitors It is important to identify and treat sepsis as fast as
However, inhibition of the complement pathway comes with possible; Fig. 9.6 shows a typical scenario in which a
side effects, such as an increased risk of encapsulated bacteria patient develops sepsis. Sepsis should be suspected in
(particularly N. meningitidis). For that reason, patients who are any patient that develops signs of systemic infection
due to be started on such therapy are required to be appropriately (tachycardia, tachypnoea, pyrexia, hypotension,
decrease in blood oxygen saturation, decreased level of
vaccinated prior to treatment commencement.
consciousness) and has a likely source of infection
A number of other complement inhibitors are currently (urinary tract infection, respiratory tract infection, septic
under investigation in an effort to minimize the side effects (such arthritis, meningitis, etc.). The ‘Sepsis 6’ describes the
as the breakthrough haemolysis in PNH) and offer a better qual- package of management that should be started
ity of life (e.g., by using a subcutaneous instead of an intravenous immediately and aims to prevent organ failure and death:
formulation) to patients requiring such treatment. 1. Give high-flow oxygen
2. Take blood cultures and consider other source
cultures
INNATE IMMUNE SYSTEM PATTERN 3. Give intravenous antibiotics
4. Give a fluid challenge (e.g., 500 mL 0.9% NaCl)
RECOGNITION MOLECULES 5. Measure serum lactate and other blood tests (full
blood count, CRP, urea and electrolytes, blood gases)
Pattern recognition molecules are required for the detection and 6. Monitor urine output
elimination of pathogens. We have already come across the rec-   
ognition molecules MBL and C1b in the complement system and

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Fig. 9.6 (A and B) A 30-year-old patient is admitted with cerebral oedema following a road traffic collision. He is treated with high-
dose corticosteroids, intubated, has a urinary catheter inserted and has multiple venous cannulas inserted. While recovering he
develops a productive cough with a respiratory rate of 23 breaths per minute, has a temperature of 38.2°C, a systolic blood pressure
of 96 mmHg and a pulse of 124 beats per minute. This patient has sepsis. Many factors in his initial treatment predispose him to this.
A chest radiograph shows abscesses typical of Staphylococcal pneumonia, suggestive of a source of infection. The ‘Sepsis 6’
should be started immediately and a senior review sought. (From Helbert M. Immunology for Medical Students. 3rd ed. Philadelphia,
PA: Elsevier; 2016).

Chapter Summary

• The innate immune system provides a broad defence against pathogens. It is made up of
barriers to infection, cellular defences and soluble proteins.
• Barriers to infection can be physical, for example, skin, chemical (e.g., stomach acidity) or
biological (e.g., gut flora).
• Cells of the innate immune system are phagocytes (e.g., macrophages, neutrophils), NK
cells and degranulating cells (e.g., mast cells, eosinophils). Phagocytes generally target
extracellular pathogens, NK cells kill virally infected or cancerous host cells, and
degranulating cells generally target multicellular pathogens (e.g., worms).
• APPs and the complement system make up the soluble protein fraction of the innate
immune system.
• APPs are produced in response to infection and contribute to the innate immune
defence in many ways. They are often measured to aid in diagnosing infection (e.g.,
CRP).
• Complement proteins directly attack pathogens and alert the immune system to their
presence. The complement system is activated via multiple pathways, either by
bacterial cell wall components or antibody bound to pathogens.
• PRMs enable the innate immune system to detect pathogens. Overstimulation of PRMs
by large amounts of pathogen contributes to the symptoms of sepsis.

MLA Conditions MLA Presentations

  Sepsis

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