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Bacterial infections in CF
The upper airways represent a primary site for the introduction of pathogenic
microorganisms from inspired air. The ciliated epithelium features several powerful
mechanisms for prevention of colonization by inhaled bacteria, thus the lower respiratory
tract usually remains sterile .
Defective mucociliary clearance is associated with the absence or dysfunction of the
Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)
in airway epithelium. This defect plays the key role in the initial bacterial
colonization. CFTR is a chloride channel. The genetic
defects in CFTR (e.g. deltaF508, the most common mutation)
cause reduced secretion of chloride and fluid hydration. Reduced mucociliary clearance,
as well as damaged airway epithelium and excessive secretion of mucins, produce a
biological matrix that facilitates the bacterial growth in biofilm. Mucus plastering
against the airway epithelium flattens cilia and disrupts mucocociliary clearance , , .
Pseudomonas aeruginosa, a Gram negative bacterium, is an opportunistic
pathogen that colonizes instrumented airways, immunocompromised hosts, and individuals
with cystic fibrosis (CF). The idiosyncratic susceptibility in CF airways to respiratory
infection with P. aeruginosa is a severe condition, Over 80% of individuals with
CF suffer from considerable (>75%) morbidity due to chronic lung infection with this
pathogen , , . Mutations in
CFTR are associated with severe lung diseases and are
generally resulted in reduced CFTR protein expression and
function in the apical plasma membranes of the airway epithelial cells that are first
colonized with P. aeruginosa, followed by the progression to infection and severe
inflammation . It is common to see co-infections with other Gram
negative bacteria (Stenotrophomonas maltophilia, Burkholderia cepacia, Haemophilus
influenzae) and certain specific Gram positive bacteria (Staphylococcus aureus) , , , , .
However, the specific molecular and cellular mechanisms of hypersusceptibility of CF
patients to P. aeruginosa infection are not fully elucidated.
P. aeruginosa antigens, such as lipopolysaccharide
(LPS), virulence factor, exoenzyme S
flagellin (Flagellin (P.aeruginosa)) and pilin
(PilA (P.aeruginosa)) are recognized by the surface
receptors asialo-ganglioside GA1 and Toll-like receptors
(TLRs) , , .
Asialo-ganglioside GA1 and TLR2
receptors are increased in cells expressing mutant CFTR and
in areas of regenerating epithelium that are likely present in the inflamed CF airway
, , , . Among the
different TLRs, TLR2 and TLR5
play a major role in signaling epithelial responses to P. aeruginosa in the
TLR2 is the predominant TLR expressed on the apical cell
surface, with other TLRs (TLR3, 4 and 5) residing mainly intracellularly. However, in
inflamed lung following stimulation with bacterial ligands, TLR5
and TLR4 can be mobilized to the apical
Of all the TLRs, TLR2 recognizes the broadest repertoire
of ligands, such as ExoS (P.aeruginosa)
, , or, in conjunction with
TLR1, lipoteichoid acid (LTA)
and Glycopeptide (peptidoglycan, PGN) of gram positive
bacteria , , .
All TLRs induce the canonical pathway of Nuclear factor kappa-B
(NF-kB) activating: Myeloid differentiation primary response
gene 88 (MyD88)/ Interleukin-1 receptor-associated kinases
4, 1 and 2 (IRAK4 and IRAK1/2)/
TNF Receptor-associated factor 6 (TRAF6)/ Mitogen-activated
protein kinase kinase kinase 7 interacting proteins 1 and 2 (TAB1
and TAB2)/ Mitogen-activated protein kinase
kinase kinase 7 (TAK1)/ Mitogen-activated protein kinase
kinase kinase 14 (NIK)/ I-kB kinase complex
(IKK(cat))/ Nuclear factor kappa-B inhibitor
(I-kB)/ NF-kB , , . TLR2 and
TLR4 signaling pathways require an additional
Toll-Interleukin 1 receptor domain containing adaptor protein
(TIRAP) , .
ExoS (P.aeruginosa) was
shown to induce Tumor necrosis factor alpha (TNF-alpha)
production through activation of both TLR2 and
TLR4. The ability to activate cells expressing
TLR2 was attributed to the C terminus of
ExoS, whereas the ability to activate
CD14 complex was attributed to the N terminus of
ExoS , . In addition, P.
aeruginosa has been shown to signal through TLR4/
MD-2/ CD14 complex with its
LPS moiety , . Although
TLR4 is expressed in airway epithelial cells, it does not
appear to be prominently involved in signaling of P. aeruginosa presented at the apical
surface of airway epithelial cells , , , . The regulation of MD-2 expression under
pathological conditions is also proposed to determine the airway epithelial responses to
Flagellin (P.aeruginosa)  and
PilA (P. aeruginosa)  bind bacteria to the
host cell glycolipid receptor, asialo-ganglioside GA1.
TLR2 forms a receptor complex with
asialo-ganglioside GA1 and activates
NF-kB signaling and Interleukin-8
(IL-8) production , , . TLR5 also recognizes Flagellin
from P.aeruginosa and stimulates a similar signaling cascade , . Expression of Interleukin-6
(IL-6) and IL-8 are increased
in CF epithelial cells in response to stimulation by P. aeruginosa antigens,
which may contribute to the excessive inflammatory response in CF , , .
TLR2 can also mediate Beta-defensin
2 expression via NF-kB in response to
bacterial antigens in the human airway epithelia . The antimicrobial
activity of defensins is compromised by changes in airway surface liquid composition in
lungs of CF patients, therefore contributing to the bacterial colonization in the lung.
It has been demonstrated that Beta-defensin 2 is susceptible
to degradation and inactivation by the host cysteine proteases Cathepsin
B, Cathepsin L, and Cathepsin S
. Cathepsins are not present in the healthy lung. In
chronic lung diseases, such as CF and emphysema, overexpression of cathepsins may lead to
accelerated degradation of beta-defensins, thereby favoring bacterial infection and
CFTR promotes a rapid expression of Fas ligand (TNF
superfamily, member 6) (FasL) and Fas (TNF receptor
superfamily, member 6) (FasR),
as well as an apoptotic response to P. aeruginosa infection, whereas cells with
deltaF508 CFTR show little apoptosis and delayed FasL and
FasR expression .
Mucins are among the most abundant polymers in CF airways. P. aeruginosa has
mucin-specific adhesins that mediate interactions between bacterial cells and mucins.
Flagellin (P. aeruginosa) appears to play a prominent role
in an interaction with Mucin1 (MUC1) .
Dehydrated mucus present in CF generates a unique environment in which bacteria are
confined spatially. This increases the local concentration of autoinducers, leading to
accelerated formation of biofilm , , .
Moreover, MUC1 is overexpressed in CF compare with a health
airway , , , probably in a
NF-kB-dependent manner . By an unknown
mechanism, MUC1 can suppress Flagellin
(P.aeruginosa)-induced TLR5 signaling , .
P. aeruginosa infection causes Interleukin 1 receptor, type I
activation , . Rapid release of IL-1
beta (most probably NF-kB-dependent) in respiratory epithelial cells in
response to P. aeruginosa is enhanced in the presence of functional
CFTR, but not deltaF508 CFTR . In response
to IL-1 beta CF airway epithelial cells induce
NF-kB-dependent Chemokine (C-C) ligand 20
(CCL20) and IL-8 production
One well-studied component of bacterial killing, Nitric
Oxide, is defective in CF . Epithelial cells with abnormal
CFTR activity have reduced inducible nitric oxide synthase
(iNOS) expression . Abnormalities in CF
reduce both NF-kB and IFN-gamma
signaling components that are necessary for complete iNOS
expression. Active signal transducer and activator of transcription 1
(STAT1), necessary for both
iNOS and Interferon regulatory factor 1
(IRF1) expression, was found to be bound to the Protein
inhibitor of activated STAT1 (PIAS1), resulting in reduced
IRF1 and iNOS expression in CF
epithelial cells .
P. aeruginosa antigens can induce cytotoxicity and facilitate progress of
infection in CF airway. Surfactant pulmonary-associated proteins A and D
(SP-A and SP-D) are cleaved by
zinc-metalloprotease elastase LasB (P. aeruginosa) . Degradation of SP-A and
SP-D occurs in the cystic fibrosis airway environment, and
this degradation eliminates many normal immune functions of this proteins , .
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