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Chapter 2
Pulmonary defence
mechanisms and
inflammatory pathways
in bronchiectasis
B.N. Lambrecht*,#, K. Neyt* and C.H. GeurtsvanKessel*,"
Over recent years there has been a tremendous increase in the
understanding of pulmonary immunity, mostly driven by large
research efforts in understanding the basis of asthma and
chronic obstructive pulmonary disease. Bronchiectasis is well
understood. In this article, an overview of pulmonary defence
mechanisms as well as inflammatory mechanisms is given as a
basis to understand the pathogenesis of bronchiectasis.
Keywords: Bronchiectasis, inflammatory mechanisms,
immunity, pulmonary defence
*Dept of Pulmonary Medicine,
Laboratory of Immunoregulation and
Mucosal Immunology, Ghent
University, Ghent, Belgium.
#
Dept of Pulmonary Medicine, and
"
Dept of Virology, Erasmus
University Medical Center,
Rotterdam, The Netherlands.
Correspondence: B.N. Lambrecht,
Dept of Pulmonary Medicine,
Laboratory of Immunoregulation and
Mucosal Immunology, Ghent
University, De Pintelaan 185, B-9000
Ghent, Belgium, Email
[email protected]
B.N. LAMBRECHT ET AL.
Summary
Eur Respir Mon 2011. 52, 11–21.
Printed in UK – all rights reserved.
Copyright ERS 2011.
European Respiratory Monograph;
ISSN: 1025-448x.
DOI: 10.1183/1025448x.10003210
B
11
ronchiectasis is a chronic disorder characterised by permanent dilatation of the bronchi
accompanied by inflammatory changes in their walls and in the adjacent lung parenchyma.
The pathogenesis is related to recurrent inflammation of the bronchial walls combined with
fibrosis in the surrounding parenchyma. The resultant traction on weakened walls leads to
eventual irreversible dilatation [1]. Bronchiectasis can result from defective pulmonary defence
mechanisms that lead to recurrent, severe and tissue-damaging microbial insults or chronic
bacterial colonisation with persistent inflammation leading to structural changes to the airway
wall. Given the fact that restoration of inflammation and return to immune homeostasis is crucial
in the lung to protect the delicate gas exchange machinery, it is also possible that bronchiectasis
results from defective anti-inflammatory pathways that serve to dampen chronic inflammation.
Therefore, in this chapter we provide a brief overview of lung defence mechanisms and how these
immune defence mechanisms can contribute to chronic inflammation and structural changes to
the airway wall if not properly counter-regulated by anti-inflammatory pathways. The major
inflammatory cell types found in bronchiectasis are neutrophils in the airway lumen causing
purulent sputum and macrophages, dendritic cells (DCs) and lymphocytes in the airway wall [2, 3].
The latter cells often occur in lymphoid aggregates or so-called tertiary lymphoid follicles, and are
typically seen in patients with tubular bronchiectasis and are a major cause of small airway
obstruction [4].
Mechanical and physical pulmonary defence mechanisms
PULMONARY DEFENCE AND IMMUNITY
The inspired air is contaminated with toxic gases, particulates and microbes. The first line of
defence of the lung is made up of the complex physical shape of the conducting upper and lower
airways, causing a highly turbulent airflow that facilitates the impaction, sedimentation and
deposition of particulate matter and microorganisms on the mucosa, followed by the removal of
these deposited particles by the mucociliairy blanket and/or the physical expulsion from the
respiratory tract by sneezing, coughing or swallowing. Reductions in the cough reflex are
associated with increased frequency of respiratory infections, but it is not known at present
whether this would also predispose to development of bronchiectasis [5]. The presence of isolated
middle lobe bronchiectasis and colonisation with nontuberculous mycobacteria (the so-called
Lady Windermere syndrome) has been proposed to be caused by cough suppression [6].
The action of the mucociliary blanket is a dynamic and complexly regulated escalator for bringing
inhaled particles to the throat so that they can be swallowed. Defects in the function of the
mucociliary blanket can cause bronchiectasis. The conducting airways are lined with ciliated
epithelium and the structure and function of the cilia in propulsing mucus has been extensively
studied [7–9]. Genetic defects in the structure of the outer dynein arm proteins that connect
microtubules in cilia are the cause of primary ciliary dyskinesia [10]. Other mutations involve the ktu
gene, which is involved in the assembly of both the outer dynein and the inner dynein arm [11].
Defects in radial spoke head proteins are associated with abnormalities of the central microtubule pair
of the cilium (presence of only one microtubulus rather than two) [10]. Ciliary disturbances
(sometimes associated with situs inversus; Kartagener syndrome) almost always lead to bronchiectasis
and are often also associated with chronic rhinosinusitis. The correct movement of cilia and function
of the mucociliary escalator also depend on the low viscosity of the periciliary fluid layer, physically a
hydrated sol layer, allowing sufficient separation between the apical side of the epithelium and the
viscous mucous blanket covering the cilia. If the periciliary fluid layer is concentrated (i.e. like in cystic
fibrosis (CF)), the periciliary fluid layer becomes thinner and the cilia become entangled in the mucus
layer, thus impeding normal ciliary propulsion of the mucus [12, 13].
Humoral innate immune mechanisms in the lung
Innate immune defences are evolutionary conserved pathways of defence that kill microbes in a
generic pathway, often relying on the recognition and antagonism of common motifs in microbial
proteins or lectins, the so-called pathogen-associated molecular patterns (PAMPs), which are so
crucial for the function of the microbe that their antagonism leads to loss of pathogenicity. Just
like acquired or adaptive immunity, innate immunity consists of a humoral and a cellular part.
12
Humoral innate defence mechanisms are elaborate in the lung and consist of lactoferrin, lyzozyme,
defensins, complement, cathelicidins and collectins [14]. These molecules can be produced by
airway structural cells or by recruited innate immune cells such as neutrophils and macrophages
(see later). Lactoferrin chelates Fe2+ molecules that are crucial for the growth of some bacteria but
also stimulates the function of neutrophils. Lyzozyme degrades Gram-positive cell walls. Defensins
are made by neutrophils (a-defensins) and epithelial cells (b-defensins). They serve to make pores
in bacterial cell walls, and thus are truly antibacterial peptides but also neutralise viruses and fungi
and recruit DCs via activation of the CCR6 chemokine receptor on these cells [15]. The proper
function of defensins depends on the correct salt concentration in the airway surface liquid [16].
Thus, in CF patients defensin function against Staphylococcus aureus is defective, possibly
explaining the susceptibility to colonisation, although this theory has also been questioned. LL37 is
a well-known airway cathelicidin that is also salt sensitive and has broad antimicrobial activity but
also has effects on innate and adaptive immune cells [17]. Surfactant protein A and D are
collectins that opsonise bacteria and viruses such as influenza. A closely related collectin family
member is mannose binding lectin (MBL), it is not secreted into the lung lining fluid but is an
important circulating factor that can activate the complement cascade. Deficiency of MBL is a
cause of recurrent bacterial infections and could be a cause of bronchiectasis. Low MBL levels in
CF patients and other forms of bronchiectasis are also associated with a more rapid decline in lung
function [18].
Cellular innate immune mechanisms in the lung
The cellular arm of innate immunity in the lung is primarily made up of alveolar macrophages and
recruited neutrophils (fig. 1). Alveolar macrophages serve an important function in the
phagocytosis, killing and/or neutralisation of inhaled particulate antigens. Resident alveolar
macrophages continuously encounter inhaled substances due to their exposed position in the
alveolar lumen. These cells are packed with enzymes, metabolic products and cytokines that are
vital to defence of the alveolar space but can potentially damage the alveolocapillary membrane.
To avoid collateral damage to type I and type II alveolar epithelial cells (AEC) in response to
harmless antigens, they are kept in a quiescent state, producing few inflammatory cytokines [19].
It has been estimated previously, that the pool of alveolar macrophages can handle up to 109
B.N. LAMBRECHT ET AL.
Stimulus
Ingestion by
alveolar
macrophages
Direct triggering
of epithelial cells
Activation of
dendritic cells
Secondary
neutrophil
influx
TNF-α, IL-1,
G-CSF, GM-CSF,
chemokines (IL-8)
13
Figure 1. When a pathogen enters the lung, it triggers both epithelial cells, macrophages and dendritic cells.
The epithelial cells make chemokines that subsequently attract neutrophils that help in phacocytosing the
pathogens. All recruited cells together with epithelial cells then make cytokines and growth factors that further
enforce innate immune responses to the pathogen by further recruitment of inflammatory cells. TNF: tumour
necrosis factor; IL: interleukin; G-CSF: granulocyte colony-stimulating factor; GM-CSF: granulocyte-macrophage
colony-stimulating factor.
14
PULMONARY DEFENCE AND IMMUNITY
intratracheally injected bacteria before there is spill-over of bacteria to DCs and before adaptive
immunity is induced [20]. Elegant studies have demonstrated that in vivo elimination of alveolar
macrophages using clodronate filled liposomes lead to overt inflammatory reactions to otherwise
harmless particulate and soluble antigens [21], but also to an increased sensitivity to bacterial,
fungal and viral infection. In their exposed position, alveolar macrophages serve as the first line of
defence against inhaled pathogens not only by directly acting as the main phagocytes, but also as
an important producer of pro-inflammatory chemokines, cytokines and lipid mediators; bioactive
mediators that recruit other cell types to the lung.
In contrast to alveolar macrophages that reside in the lung and serve as an immediate line of
innate defence against inhaled pathogens, neutrophils are recruited within minutes following
inoculation of microbes into the lung. The main function of neutrophils is phagocytosis and
killing of microbes, particularly fungi such as Aspergillus sp. and Pneumocystis jeroveci. They can
also kill microorganisms through the release of a-defensins and lyzozyme. Neutrophil killing
function depends on oxidative enzymes such as those of the NADPH oxidase system and
myeloperoxidase. Chronic granulomatous disease is caused by missense, nonsense, frameshift,
splice or deletion mutations in the genes for p22(phox), p40(phox), p47(phox), p67(phox)
(autosomal chronic granulomatous disease) or gp91(phox) (X-linked chronic granulomatous
disease), which result in variable production of neutrophil-derived reactive oxygen species [22].
Neutrophil extravasation is also a highly organised process requiring the rolling, arrest and
diapedesis of cells on the vessel wall. Defects in certain integrins, selectins or their activator can
cause defective neutrophil recruitment and cause recurrent pulmonary infections [23]. Once
recruited, neutrophils can also further enhance more neutrophil recruitment through production
of cytokines (interleukin (IL)-1, tumour necrosis factor (TNF)-a and IL-6) as well as through
release of calcium binding proteins of the S100 family (S100A8, A9 and A12) that act on the RAGE
(receptor for advanced glycation end products) receptor.
Induction of innate immune responses in the lung
The above mechanisms of innate defence act in a coordinated fashion. Although a single aspect of
the innate defence system can be triggered directly through recognition of foreign PAMPs, the
innate defence mechanisms are often induced simultaneously via triggering of common receptors
on both phagocytes (for cellular defences) and epithelial cells (for inducing the production of
humoral innate defence mechanisms). The most famous pattern recognition receptors belong to
the family of Toll-like receptors (TLR)1-11, NOD-like receptors, RIG-I-like receptors and C-type
lectin receptors [24]. These receptors recognise particular conserved PAMPs on specific groups of
microbes. The archetypical TLR4 is expressed at the cell surface and recognises the Gram-negative
cell wall component lipopolysaccharide, whereas TLR2 recognises peptidoglycan and TLR5
recognises bacterial flagellin. The endosomal TLR receptors TLR3 recognise double-stranded RNA,
TLR7 and TLR8 single-stranded RNA and TLR9 unmethylated CpG motifs [24]. The exact cellular
localisation and downstream signalling mechanisms of these pathways have been studied
extensively over the past few years and several clinical primary immunodeficiency syndromes have
been brought back to deficiencies in one of the signalling intermediates of these pathways.
Deficiency of IRAK4, a critical intermediary in TLR4 signalling causes recurrent bacterial
infections, particularly at a young age [25]. Deficiency of the C-type lectin receptor dectin-1 or the
downstream signalling intermediate molecule CARD9 causes immunodeficiency to candida and
P. jeroveci, most probably due to reduced induction of T-helper cell (Th)17 responses [26].
Conversely, over activity of these signalling cascades, for example caused by small polymorphisms
in or mutations of negative regulators of these pathways are associated with auto-immunity and
overzealous inflammatory pathways. As one example, polymorphisms in the ubiquitin editing
enzyme TNF-a-induced protein 3 (TNFAIP3, also known as A20), cause hypersensitivity of TLR
and cytokine receptors and are often found in patients with systemic lupus erythematosus [27].
Our own unpublished data also show that genetic deficiency of A20 in epithelial cells causes severe
mucosal inflammation in response to inhalation of intrinsically harmless proteins, but it is
unknown at present how this could be implicated in the regulation of inflammatory pathways
relevant to bronchiectasis.
Adaptive cellular immunity
Like innate immunity, adaptive or acquired immunity consists of a cellular and a humoral arm.
Cellular adaptive immunity is made up of different types of T-lymphocytes, whereas humoral
immunity is made up of B-lymphocytes and plasma cells and their secreted product;
immunoglobulins (Ig).
DCs are potent antigen presenting cells that have emerged as key regulators of adaptive immunity
(see [28] for a more detailed review on the biology of lung DC function). The general function of
lung DCs is to recognise and pick up foreign antigens at the periphery of the body, and
subsequently migrate to the draining mediastinal lymph nodes where the antigen is processed into
immunogenic peptides and displayed on major histocompatibility complex (MHC)I and MHCII
molecules for presentation to naı̈ve T-cells. In fact, these cells should be seen as specialised cells of
the mononuclear phagocyte system, which have evolved from the cells of the innate immune
system to control adaptive immunity that came later in evolution [29]. DCs express all the pattern
recognitions receptors shared with phagocytes of the innate immune system, yet at the same time
also have the machinery to talk to T-cells and B-cells and relay information about the type of
antigen to these cells, so that a tailor-made adaptive response is induced and long-term memory is
initiated. As these cells respond to many noxious stimuli from both the outside world (PAMPs)
and from within (danger-associated molecular patterns) and at the same time closely
communicate with lung structural cells such as alveolar epithelial cells, endothelial cells and
fibroblasts, it has been proposed that they could be crucial players in many lung diseases,
particularly where T-cell responses are involved in initiation of maintenance of the disease [30].
Very recently the first case reports of patients presenting with defects in the DC system have been
reported. These DC-deficient patients are at risk of severe viral skin infections and pulmonary
infections with atypical mycobacteria, which also leads to bronchiectasis [31, 32]. Our own
experiments employing DC-deficient mice have elucidated a crucial role for these cells in the
induction of antiviral immunity to influenza virus, via induction of both CD4 and CD8 T-cell
responses [33]. Similar conclusions have been reached in models of tuberculosis and bacterial lung
infections [34]. Conversely, DCs are also heavily involved in maintaining immunopathology in
which T-cells play a predominant role, the best example being the mucosal inflammation seen in
asthma and chronic obstructive pulmonary disease (COPD) [35]. In humans with bronchiectasis,
as well as in a rat model of bronchiectasis, there is an increased infiltration of the airway wall with
DCs [2, 3]. The airways of patients with diffuse panbronchiolitis, a disorder of the small
bronchioles that can also lead to bronchiectasis, contain increased numbers of DCs that have a
clearly activated phenotype, while treatment with neomacrolides reduces the antigen presenting
capacities of these DCs [36, 37].
B.N. LAMBRECHT ET AL.
Induction of adaptive cellular immunity by DCs
Constituents of adaptive cellular immunity
15
Adaptive cellular immunity consists of defined subsets of CD4+ Th cells and CD8+ cytotoxic Tcells. Once DCs transport their antigenic cargo to the draining lymph nodes, they induce the
proliferation and differentiation of naı̈ve T-cells into particular types of T-cell responses (fig. 2).
Discrete types of Th cells provide crucial help for different parts of the innate and adaptive
immune response [38]. Th1 cells make interferon (IFN)-c and mainly provide help to monocytic
cells, including macrophages and DCs, thus enforcing killing of intracelullar pathogens, and at the
same time enforcing opsonisation of these through provision of B-cell help. Conversely, Th2 cells
make IL-4, IL-5 and IL-13 providing help to eosinophils, mast cells and basophils to eliminate
IL-4
Th2
Gata-3
c-maf
STAT6
IL-10
TGF-β
Th0
Treg
IL-4
IL-5
IL-13
TNF-α
IL-10
TGF-β
Foxp3
IL-6
TGF-β
IL-1
IL-23
IL-12
Th17
IL-17
IL-22
RORγ
STAT3
Th1
T-bet
STAT4
STAT1
IFN-γ
TNF-α
complex helminths, and at the same
time induce IgG1 and IgE from Bcells to arm the basophils and mast
cells with effector potential. For a
long time since the original description of the Th1/Th2 concept, it has
been unclear which subtype of TAnti-inflammatory
Suppresses lymphocytes cell help was important for induProfibrotic?
cing neutrophilic responses and
protection from extracellular pathogens such as fungi. This gap has
Antifungal
been breached recently by the disStimulates neutrophils
covery of the cytokines IL-17 and
Autoimmunity
IL-22 which are produced by Th17
cells that induce neutrophilic inflammation and production of defensins
Intracellular pathogens
Stimulates macrophages by epithelial cells and are important
Stimulates lgG2a
for clearance of fungi and extracelDelayed hypersensitivity
lular bacteria [39].
Antihelminthic
Stimulates eosinophils
Stimulates lgE, lgG1
Allergy
The precise signals that induce
different types of Th lineage-comtheir secreted cytokines. When a T-helper cell (Th) type 0 encounters
mitment of naı̈ve T-cells has been
antigen on a DC, it will be induced to differentiate into various
intensely studied [38]. Antigen premutually exclusive cell fates. Each T-cell differentiation programme is
senting cells can provide different
controlled by transcription factors such as Gata-3, forkhead box P3
levels and quality of signal one
(Foxp3), RAR-related orphan receptor gamma (RORc) or T-bet,
which enforce Th cell lineage choice. Eventually Th cells emerge
(peptide-MHC), signal two (cothat are specialised for performing various antimicrobial tasks.
stimulatory molecules) and signal
IL: interleukin; Treg: T-regulatory cell; TGF: transforming growth
three (instructive cytokines) to
factor; TNF: tumour necrosis factor; IFN: interferon; Ig: immunoglonaı̈ve T-lymphocytes upon antigen
bulin; STAT: signal transducer and activator of transcription.
encounter and triggering of their
pattern recognition receptors [29]. When stimulated through the unique T-cell receptor (TCR),
naı̈ve CD4+ T-cells differentiate into Th1 cells in the presence of high amounts of IL-12. IL-12
instructs Th1 development via activation of signal transducer and activator of transcription (STAT)4
and the lineage instructing transcription factor T-bet. IL-17 producing cells are induced when
exposed to a cocktail of cytokines including transforming growth factor (TGF)-b, IL-6, and IL1a/b,
while IL-23 further enhances the proliferation of these cells. The Th17 lineage specific transcription
factor RAR-related orphan receptor ct enforces Th17 characteristics in naı̈ve T-cells, and is induced
by the cocktail of cytokines instructive to their development. The mechanisms leading to Th2 cell
differentiation in vivo are still poorly understood, but in most instances require a source of IL-4 to
activate the transcription factors STAT6 and GATA-3, and a source of IL-2, IL-7 or thymic stromal
lymphopoietin to activate the transcription factor STAT5 [40–44]. Despite the overwhelming
evidence that IL-4 is necessary for most Th2 responses, DCs were, however, never found to produce
IL-4 and it was therefore assumed that Th2 responses would occur by default, in the absence of
strong Th1 or Th17 instructive cytokines in the immunological DC T-cell synapse, or when the
strength of the MHCII-TCR interaction or the degree of co-stimulation offered to naı̈ve T-cells was
weak [45–48]. In this model, naı̈ve CD4 T-cells were the source of instructive IL-4. In an alternative
view, IL-4 is secreted by an accessory innate immune cell type, such as natural killer T-cells,
eosinophils, mast cells or basophils, that provide IL-4 in trans to activate the Th2-differentiation
programme [49]. In the lung allergic response to house dust mite allergen, we have recently found
that basophils help DCs to induce Th2 immunity by providing an important, but not essential source
of IL-4 [50].
PULMONARY DEFENCE AND IMMUNITY
Figure 2. T-cell polarisation induced by dendritic cells (DCs) and
16
Lung DCs are also essential in instructing the selection and expansion of CD8 cytotoxic T-cells
that recognise virus-infected cells, cells infected with intracellular bacteria and tumourally
transformed cells via presentation of endogenous cellular antigen on the MHCI complex [33]. An
important conceptual point is that DCs do not have to be infected themselves to perform this task,
but can phagocytose virally infected or transformed cells and use the process of cross-presentation
to present the exogenous antigen into their MHCI loading machinery. Once activated by DCs and
CD4 T-cell help, cytotoxic T-cells can lyse and kill infected cells in a process requiring granzyme
and/or perforin, or kill target cells in a FasL- and/or TNF receptor-like apoptosis inducing liganddependent manner, causing apoptotic cell death in targets [51].
Humoral immune mechanisms in the lung
Humoral immunity plays a predominant role in protection from severe infections with
encapsulated bacterial strains. Antibodies are well known for their neutralising effects on
secondary infections and this is the principle of most vaccinations against childhood infections.
During a primary infection, however, antibodies, some of which have broad-spectrum specificity
(so-called natural antibodies), also have the capacity to activate complement and opsonise
bacterial cell walls and capsules, thus facilitating clearance of the pathogens. Antibodies of the IgA
and IgG class are actively secreted into the airway lumen via the action of the polymeric Ig
receptor. Airway luminal IgA is an important defence against viral entry. Maybe the most
prevalent cause of bronchiectasis is deficiencies in humoral immunity, such as common variable
immunodeficiency (CVID), a group of disorders characterised by low to absent Ig and various
degrees of T-lymphocyte abnormalities [18, 58]. CVID can be caused by mutations in the proteins
involved in T–B-cell communication such as ICOS, BAFF, TACI and APRIL [59, 60]. This is a
rapidly evolving field and it is only a matter of time before all these mutations can be diagnosed on
a routine basis.
B.N. LAMBRECHT ET AL.
Several defects in adaptive immunity are associated with increased susceptibility to lung infection
and can be an important risk factor for later development of bronchiectasis. Defects in the IL-12/
IFNcSTAT1 axis are a well-known risk factor for mycobacterial infections and invasive
Salmonellosis [52]. Defects in the IL-23//Th17 axis are associated with increased risk of fungal
infections and P. jeroveci infections [53]. Patients with sporadic or autosomal dominant forms of
the hyper IgE syndrome (Job’s syndrome when associated with connective tissue abnormalities)
have mutations in STAT3, and hence deficient differentiation of Th17 cells [54, 55]. These patients
are at risk for severe recurrent Staphyloccal infections, pneumatocoeles and mucocutaneous
candidiasis. In recessive forms of the hyper IgE syndrome, mutations in DOCK8 have been
described, and these patients are similarly at risk for recurrent sinopulmonary infection and have
defects in Th17 generation [56]. The few biopsy studies that have been performed in
bronchiectasis have seen increased infiltration of the bronchial wall with CD4 and CD8 T-cells.
The neutrophilic inflammation seen in CF and other forms of bronchiectasis is typically associated
with the increased presence of Th17 cells [57]. In bronchiectasis associated with allergic
bronchopulmonary aspergillosis, one has also observed increased numbers of Th2 cells, thus
explaining the association with sputum eosinophilia.
Organised lymphoid structures and bronchiectasis
17
The organised accumulation of lymphocytes in lymphoid organs serves to optimise both
homeostatic immune surveillance, as well as chronic responses to pathogenic stimuli [61]. During
embryonic development, circulating haemopoietic cells gather at predetermined sites throughout
the body, where they are subsequently arranged in T- and B-cell specific areas, leading to the
formation of secondary lymphoid organs, such as lymph nodes and spleen. In contrast, the body
has a limited second set of selected sites that support neo-formation of organised lymphoid
aggregates in adult life. However, these are only revealed at times of local, chronic inflammation
when so-called tertiary lymphoid organs (TLO) appear. Just like in lymph nodes and spleen, areas
of TLO are characterised by formation of specialised high endothelial venules and the organised
production of chemokines leads to cellular organisation of T-cells and B-cells in discrete areas. In
humans, TLO has been observed in the joint and lung of rheumatoid arthritis [62], around the
airways of COPD patients [63] and in the thyroid [64]. Certain infectious diseases are also
accompanied by the formation of TLO. Influenza virus infection of the respiratory tract leads to
formation of inducible bronchus-associated lymphoid tissue (iBALT) that supports T- and B-cell
proliferation and productive Ig class switching in germinal centres [65, 66]. Tertiary lymphoid
follicles or iBALT is frequently seen in tubular bronchiectasis, and the close association with
bronchi might explain the obstruction of small bronchioles and airway obstruction that is often
seen. This is certainly the case in rheumatoid arthritis-associated bronchiectasis, in which
bronchial obstruction is often caused by strongly enlarged TLOs that impinge on the lumen of the
airway, an entity known as follicular bronchiolitis by pathologists and reflecting the presence of
B-cell follicles inside TLO structures [62]. Formation of TLO could be the result of chronic
colonisation of bronchiectatic airways by microbes, and indeed it has been proposed that latent
adenoviral infection is a cause of follicular bronchiectasis [4]. However, in one school of thought,
TLO formation can also be seen as a source of self-specific autoantibodies and a reflection of an
underlying auto-immune component of the disease. In TLO associated with rheumatoid arthritisbronchiectasis, one has indeed seen the production of pathogenic antibodies to citrullinated
proteins [62].
18
PULMONARY DEFENCE AND IMMUNITY
Anti-inflammatory pathways
With its large surface area, the lung is a portal of entry for many pathogens as inhaled air is
contaminated with infectious agents, toxic gases and (fine) particulate matter. At the same time,
inhaled microbes and toxic substances can gain easy access to the bloodstream across the delicate
alveolar–capillary membrane. Innate and adaptive immune defence of this vulnerable barrier is not
easy and needs to be tightly controlled as too much oedema, inflammation and cellular
recruitment will lead to thickening of the alveolar wall and will jeopardise the diffusion of oxygen
vital to life. Considering the large surface area of the respiratory epithelium and the volume of air
inspired on a daily basis it is remarkable that there is so little inflammation under normal
conditions, suggesting the presence of regulatory mechanisms that act to protect the gas-exchange
mechanism. Even following severe bacterial or viral infection, a return to homeostasis is the usual
outcome. Understanding the conditions by which lung immune homeostasis is regulated might be
crucial to advance our insight into the pathogenesis of inflammatory lung diseases such as
bronchiectasis. One type of cell that has received particular attention in suppressing immune
responses in the lung is the alveolar macrophage. Alveolar macrophages adhere closely to AECs at
the alveolar wall and are separated by only 0.2–0.5 mm from interstitial DCs. In macrophagedepleted mice, the DCs have a clearly enhanced antigen presenting function [67]. When mixed
with DCs in vitro, alveolar macrophages suppress T-cell activation through the release of nitric
oxide (mainly in rodents), prostaglandins, IL-10 and TGF-b. Alveolar macrophages also express
CD200R, an inhibitory receptor that regulates the strength of innate immunity to inhaled
pathogens. Another cell type that has received a lot of attention is the regulatory T-cell (Treg).
Natural Tregs express high levels of CD25 and express the lineage specific transcription factor
Foxp3 [68]. These cells are generated in the thymus and have a natural reactivity for self antigens
as well as some foreign antigens, and mainly suppress autoimmunity [69]. Induced Tregs are
generated when DCs encounter self antigen in the periphery or upon chronic immune stimulation.
It is assumed that these induced Tregs serve to dampen overt immune activation to stimuli that
cannot be fully eliminated, a typical example being chronic helminth infections or mycobacterial
infections [70]. As bronchiectasis is a disorder of chronic inflammation accompanied by microbial
colonisation, it is very likely that increased Tregs are found inside lesions, although this has not
been formally addressed. It is also possible that failure of Treg function at a certain stage of the
disease contributes to ongoing inflammation, which might ultimately progress to fibrosis. In this
regard it is a striking observation that Tregs also make TGF-b as part of their suppressive
programme. TGF-b might be at the crossroads of immunoregulation and fibrosis initiation.
Immune regulation might also stem from changes in stromal cells of the airways, such as epithelial
cells. Airway epithelial cells play a predominant role in deciding whether or not an acute or
chronic stimulus like endotoxin is recognised or not [71]. Epithelial cells express many pattern
recognition receptors and the sensitivity of these can be regulated through negative regulators of
signalling. Finally, some epithelial derived cytokines, such as IL-37, have an intrinsically antiinflammatory effect on innate immunity in the lung [72]. It is currently unknown if defects in
these counter-regulatory mechanisms are involved in the maintenance of inflammation in patients
with bronchiectasis.
Conclusion
There has been great progress in our knowledge of innate and adaptive immune responses in the
lung. Immune defects in innate and adaptive cellular and humoral immunity can all lead to
bronchiectasis. In contrast to other obstructive airway diseases, such as asthma and COPD, we
have not yet fully grasped the immunopathogenesis of chronic inflammation in this disorder.
Support statement
None declared.
References
1. Katzenstein AL. Nonspecific inflammatory and destructive diseases. In: Katzenstein AL, ed. Katzenstein and
Askins’s Surgical Pathology of Non-Neoplastic lung disease. Philadelphia, W.B. Saunders, 1997; pp. 422–425.
2. Lapa e Silva JR, Guerreiro D, Noble B, et al. Immunopathology of experimental bronchiectasis. Am J Respir Cell
Mol Biol 1989; 1: 297–304.
3. Silva JR, Jones JA, Cole PJ, et al. The immunological component of the cellular inflammatory infiltrate in
bronchiectasis. Thorax 1989; 44: 668–673.
4. Bateman ED, Hayashi S, Kuwano K, et al. Latent adenoviral infection in follicular bronchiectasis. Am J Respir Crit
Care Med 1995; 151: 170–176.
5. Barber CM, Curran AD, Fishwick D. Impaired cough reflex in patients with recurrent pneumonia. Thorax 2003;
58: 645–646.
6. Reich JM, Johnson RE. Mycobacterium avium complex pulmonary disease presenting as an isolated lingular or
middle lobe pattern. The Lady Windermere syndrome. Chest 1992; 101: 1605–1609.
7. Cowan MJ, Gladwin MT, Shelhamer JH. Disorders of ciliary motility. Am J Med Sci 2001; 321: 3–10.
8. de Iongh RU, Rutland J. Ciliary defects in healthy subjects, bronchiectasis, and primary ciliary dyskinesia. Am J
Respir Crit Care Med 1995; 151: 1559–1567.
9. Santamaria F, Montella S, Tiddens HA, et al. Structural and functional lung disease in primary ciliary dyskinesia.
Chest 2008; 134: 351–357.
10. Zariwala MA, Knowles MR, Omran H. Genetic defects in ciliary structure and function. Annu Rev Physiol 2007; 69:
423–450.
11. Omran H, Kobayashi D, Olbrich H, et al. Ktu/PF13 is required for cytoplasmic pre-assembly of axonemal dyneins.
Nature 2008; 456: 611–616.
12. Matsui H, Grubb BR, Tarran R, et al. Evidence for periciliary liquid layer depletion, not abnormal ion
composition, in the pathogenesis of cystic fibrosis airways disease. Cell 1998; 95: 1005–1015.
13. Matsui H, Randell SH, Peretti SW, et al. Coordinated clearance of periciliary liquid and mucus from airway
surfaces. J Clin Invest 1998; 102: 1125–1131.
14. Bals R, Hiemstra PS. Innate immunity in the lung: how epithelial cells fight against respiratory pathogens.
Eur Respir J 2004; 23: 327–333.
19
Statement of interest
B.N. LAMBRECHT ET AL.
B.N. Lambrecht is supported by grants from Fonds voor Wetenschappelijk Onderzoek Flanders
(Odysseus Program), European Research Council (ERC) starting grant and Multidisciplinary
Research Platform (GROUP-ID consortium) of University of Ghent, Ghent, Belgium. K. Neyt is
supported by a fellowship of FWO Flanders.
PULMONARY DEFENCE AND IMMUNITY
20
15. Yang D, Chertov O, Bykovskaia SN, et al. b-defensins: linking innate and adaptive immunity through dendritic
and T cell CCR6. Science 1999; 286: 525–528.
16. Garcia JR, Krause A, Schulz S, et al. Human b-defensin 4: a novel inducible peptide with a specific salt-sensitive
spectrum of antimicrobial activity. FASEB J 2001; 15: 1819–1821.
17. Shaykhiev R, Sierigk J, Herr C, et al. The antimicrobial peptide cathelicidin enhances activation of lung epithelial
cells by LPS. FASEB J 2010; 24: 4756–4766.
18. Gregersen S, Aalokken TM, Mynarek G, et al. Development of pulmonary abnormalities in patients with common
variable immunodeficiency: associations with clinical and immunologic factors. Ann Allergy Asthma Immunol
2010; 104: 503–510.
19. Holt PG. Inhibitory activity of unstimulated alveolar macrophages on T-lymphocyte blastogenic responses. Am
Rev Respir Dis 1978; 118: 791–793.
20. MacLean JA, Xia WJ, Pinto CE, et al. Sequestration of inhaled particulate antigens by lung phagocytes: a
mechanism for the effective inhibition of pulmonary cell-mediated immunity. Am J Pathol 1996; 148: 657–666.
21. Thepen T, Van Rooijen N, Kraal G. Alveolar macrophage elimination in vivo is associated in vivo with an increase
in pulmonary immune responses in mice. J Exp Med 1989; 170: 494–509.
22. Kuhns DB, Alvord WG, Heller T, et al. Residual NADPH oxidase and survival in chronic granulomatous disease.
N Engl J Med 2010; 363: 2600–2610.
23. Moser M, Bauer M, Schmid S, et al. Kindlin-3 is required for b2 integrin-mediated leukocyte adhesion to
endothelial cells. Nat Med 2009; 15: 300–305.
24. Kawai T, Akira S. The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors. Nat
Immunol 2010; 11: 373–384.
25. Ku CL, Picard C, Erdos M, et al. IRAK4 and NEMO mutations in otherwise healthy children with recurrent
invasive pneumococcal disease. J Med Genet 2007; 44: 16–23.
26. Ferwerda B, Ferwerda G, Plantinga TS, et al. Human dectin-1 deficiency and mucocutaneous fungal infections.
N Engl J Med 2009; 361: 1760–1767.
27. Musone SL, Taylor KE, Lu TT, et al. Multiple polymorphisms in the TNFAIP3 region are independently associated
with systemic lupus erythematosus. Nat Genet 2008; 40: 1062–1064.
28. Lambrecht BN, Hammad H. Biology of lung dendritic cells at the origin of asthma. Immunity 2009; 31: 412–424.
29. Banchereau J, Steinman RM. Dendritic cells and the control of immunity. Nature 1998; 392: 245–252.
30. van Rijt LS, Jung S, Kleinjan A, et al. In vivo depletion of lung CD11c+ dendritic cells during allergen challenge
abrogates the characteristic features of asthma. J Exp Med 2005; 201: 981–991.
31. Bigley V, Haniffa M, Doulatov S, et al. The human syndrome of dendritic cell, monocyte, B and NK lymphoid
deficiency. J Exp Med 2011; 208: 227–234.
32. Vinh DC, Patel SY, Uzel G, et al. Autosomal dominant and sporadic monocytopenia with susceptibility to
mycobacteria, fungi, papillomaviruses, and myelodysplasia. Blood 2010; 115: 1519–1529.
33. GeurtsvanKessel CH, Willart MA, van Rijt LS, et al. Clearance of influenza virus from the lung depends on
migratory langerin+CD11b- but not plasmacytoid dendritic cells. J Exp Med 2008; 205: 1621–1634.
34. Jiao X, Lo-Man R, Guermonprez P, et al. Dendritic cells are host cells for mycobacteria in vivo that trigger innate
and acquired immunity. J Immunol 2002; 168: 1294–1301.
35. Lambrecht BN, Hammad H. The role of dendritic and epithelial cells as master regulators of allergic airway
inflammation. Lancet 2010; 376: 835–843.
36. Ishida Y, Abe Y, Harabuchi Y. Effects of macrolides on antigen presentation and cytokine production by dendritic
cells and T lymphocytes. Int J Pediatr Otorhinolaryngol 2007; 71: 297–305.
37. Todate A, Chida K, Suda T, et al. Increased numbers of dendritic cells in the bronchiolar tissues of diffuse
panbronchiolitis. Am J Respir Crit Care Med 2000; 162: 148–153.
38. Zhu J, Yamane H, Paul WE. Differentiation of effector CD4 T cell populations (*). Annu Rev Immunol 2010; 28:
445–489.
39. Ouyang W, Kolls JK, Zheng Y. The biological functions of T helper 17 cell effector cytokines in inflammation.
Immunity 2008; 28: 454–467.
40. Kopf M, Le Gros G, Bachmann M, et al. Disruption of the murine IL-4 gene blocks Th2 cytokine responses. Nature
1993; 362: 245–248.
41. Le Gros G, Ben-Sasson SZ, Seder R, et al. Generation of interleukin 4 (IL-4)-producing cells in vivo and in vitro:
IL-2 and IL-4 are required for in vitro generation of IL-4- producing cells. J Exp Med 1990; 172:
921–929.
42. Paul WE, Zhu J. How are T(H)2-type immune responses initiated and amplified? Nat Rev Immunol 2010; 10:
225–235.
43. Seder RE, Paul WE, Davis MM, et al. The presence of interleukin 4 during in vitro priming determines the
lymphokine-producing potential of CD4+ T cells from T cell receptor transgenic mice. J Exp Med 1992; 176:
1091–1098.
44. Zheng W, Flavell RA. The transcription factor GATA-3 is necessary and sufficient for Th2 cytokine gene
expression in CD4 T cells. Cell 1997; 89: 587–596.
45. Constant S, Pfeiffer C, Woodward A, et al. Extent of T cell receptor ligation can determine the functional
differentiation of naive CD4 T cells. J Exp Med 1995; 182: 1591–1596.
B.N. LAMBRECHT ET AL.
21
46. Jankovic D, Kullberg M, Caspar P, et al. Parasite-induced Th2 polarization is associated with down-regulated
dendritic cell responsiveness to Th1 stimuli and a transient delay in T lymphocyte cycling. J Immunol 2004; 173:
2419–2427.
47. Lambrecht BN, De Veerman M, Coyle AJ, et al. Myeloid dendritic cells induce Th2 responses to inhaled antigen,
leading to eosinophilic airway inflammation. J Clin Invest 2000; 106: 551–559.
48. Stumbles PA, Thomas JA, Pimm CL, et al. Resting respiratory tract dendritic cells preferentially stimulate T helper
cell type 2 (Th2) responses and require obligatory cytokine signals for induction of Th1 immunity. J Exp Med
1998; 188: 2019–2031.
49. Ben-Sasson SZ, Le Gros G, Conrad H, et al. Cross-linking Fc receptors stimulate splenic non-B, non-T cells to
secrete interleukin 4 and other lymphokines. Proc Natl Acad Sci USA 1990; 87: 1421–1425.
50. Hammad H, Plantinga M, Deswarte K, et al. Inflammatory dendritic cells – not basophils – are necessary and
sufficient for induction of Th2 immunity to inhaled house dust mite allergen. J Exp Med 2010; 207: 2097–2111.
51. Hufford MM, Kim TS, Sun J, et al. Antiviral CD8+ T cell effector activities in situ are regulated by target cell type.
J Exp Med 2011; 208: 167–180.
52. Holland SM. Interferon c, IL-12, IL-12R and STAT-1 immunodeficiency diseases: disorders of the interface of
innate and adaptive immunity. Immunol Res 2007; 38: 342–346.
53. Bai H, Cheng J, Gao X, et al. IL-17/Th17 promotes type 1 T cell immunity against pulmonary intracellular
bacterial infection through modulating dendritic cell function. J Immunol 2009; 183: 5886–5895.
54. Milner JD, Brenchley JM, Laurence A, et al. Impaired T(H)17 cell differentiation in subjects with autosomal
dominant hyper-IgE syndrome. Nature 2008; 452: 773–776.
55. Minegishi Y, Saito M, Tsuchiya S, et al. Dominant-negative mutations in the DNA-binding domain of STAT3
cause hyper-IgE syndrome. Nature 2007; 448: 1058–1062.
56. Zhang Q, Davis JC, Lamborn IT, et al. Combined immunodeficiency associated with DOCK8 mutations. N Engl J
Med 2009; 361: 2046–2055.
57. Dubin PJ, McAllister F, Kolls JK. Is cystic fibrosis a TH17 disease? Inflamm Res 2007; 56: 221–227.
58. Sweinberg SK, Wodell RA, Grodofsky MP, et al. Retrospective analysis of the incidence of pulmonary disease in
hypogammaglobulinemia. J Allergy Clin Immunol 1991; 88: 96–104.
59. Bayry J, Hermine O, Webster DA, et al. Common variable immunodeficiency: the immune system in chaos. Trends
Mol Med 2005; 11: 370–376.
60. Grimbacher B, Hutloff A, Schlesier M, et al. Homozygous loss of ICOS is associated with adult-onset common
variable immunodeficiency. Nature Immunol 2003; 4: 261–268.
61. Cupedo T, Mebius RE. Cellular interactions in lymph node development. J Immunol 2005; 174: 21–25.
62. Rangel-Moreno J, Hartson L, Navarro C, et al. Inducible bronchus-associated lymphoid tissue (iBALT) in patients
with pulmonary complications of rheumatoid arthritis. J Clin Invest 2006; 116: 3183–3194.
63. Hogg JC, Chu F, Utokaparch S, et al. The nature of small-airway obstruction in chronic obstructive pulmonary
disease. N Engl J Med 2004; 350: 2645–2653.
64. Marinkovic T, Garin A, Yokota Y, et al. Interaction of mature CD3+CD4+ T cells with dendritic cells triggers the
development of tertiary lymphoid structures in the thyroid. J Clin Invest 2006; 116: 2622–2632.
65. GeurtsvanKessel CH, Willart MA, Bergen IM, et al. Dendritic cells are crucial for maintenance of tertiary lymphoid
structures in the lung of influenza virus-infected mice. J Exp Med 2009; 206: 2339–2349.
66. Moyron-Quiroz JE, Rangel-Moreno J, Kusser K, et al. Role of inducible bronchus associated lymphoid tissue
(iBALT) in respiratory immunity. Nat Med 2004; 10: 927–934.
67. Holt PG, Oliver J, Bilyk N, et al. Downregulation of the antigen presenting cell function(s) of pulmonary dendritic
cells in vivo by resident alveolar macrophages. J Exp Med 1993; 177: 397–407.
68. Hori S, Nomura T, Sakaguchi S. Control of regulatory T cell development by the transcription factor Foxp3.
Science 2003; 299: 1057–1061.
69. Watanabe N, Wang YH, Lee HK, et al. Hassall’s corpuscles instruct dendritic cells to induce CD4+CD25+
regulatory T cells in human thymus. Nature 2005; 436: 1181–1185.
70. Grainger JR, Smith KA, Hewitson JP, et al. Helminth secretions induce de novo T cell Foxp3 expression and
regulatory function through the TGF-b pathway. J Exp Med 2010; 207: 2331–2341.
71. Hammad H, Chieppa M, Perros F, et al. House dust mite allergen induces asthma via toll-like receptor 4 triggering
of airway structural cells. Nat Med 2009; 15: 410–416.
72. Nold MF, Nold-Petry CA, Zepp JA, et al. IL-37 is a fundamental inhibitor of innate immunity. Nat Immunol 2010;
11: 1014–1022.