Cigarette smoke induces inflammation, in part, by activation of Toll like receptor (TLR) 2
and TLR4. TLRs are present on most cells and recognize pathogen associated molecular
patterns (PAMPs) and non-pathogenic molecules, known as danger associated molecular
patterns (DAMPs). Activation of TLRs results in the propagation of a signalling pathway
leading to the de novo synthesis of pro-inflammatory mediators that include CXCL8 and
IL-1β. TLRs initiate inflammation and are involved in tissue repair processes in a number
of diseases such as chronic obstructive pulmonary disease (COPD) and cancer.
Therefore, it is the aim of this thesis to explore the contribution of TLRs in inflammation
and cellular proliferation, as these processes are integral to the pathology in inflammatory
lung disease and cancer.
Activation of IL-1β is a two-step process. The first step synthesises pro-IL-1β and
can be activated by TLR ligands. The second induces inflammasome complex assembly
and results in the cleavage of pro-IL-1β to mature IL-1β by caspase-1. IL-1β is implicated
in the inflammation caused by bacteria during exacerbations in lung disease. Interestingly,
we found a difference in the activation and release of IL-1β from monocytes activated with
the TLR2 ligand, Pam3CSK4 and the TLR4 ligand, LPS. The difference was seen at the
level of inflammasome assembly, where, unlike TLR4, TLR2-induced IL-1β release was
reliant on the hemichannel protein, pannexin-1.
We have previously found that cigarette smoke causes sensitisation of the blood of
smokers to bacterial and viral PAMPs. Cigarette smoke is the major contributing factor in
COPD and it is known that bacterial and viral induced exacerbations play a major role in
the deterioration of lung function in these patients. COPD patients are also more prone to
lung infections than their healthy age-matched equivalents. We therefore hypothesised
that blood from COPD patients is more sensitive to challenge with bacterial and viral PAMPs than blood from age-matched controls. COPD patient’s blood had an increased
sensitivity to challenge with TLR2/1 and TLR3 ligands with respect to IL-1β release.
Regarding CXCL8 release in this system, a greater response to stimulation with TLR2/1,
TLR3, TLR4 agonists and IL-1β was observed in blood from COPD patients. There was a
negative correlation between CXCL8 levels and COPD GOLD staging in response to
TLR4 agonism.
TLR2 has been implicated in the proliferation of cancer cells. There is an emerging
interest in the link between chronic inflammation and cancer. It is well documented that
COPD patients have an increased risk of developing lung cancer, which has poor
prognosis. Our aim was to investigate whether TLRs are a novel drug target for inhibiting
the proliferation of lung cancer cells. TLR2, 3, 4 and 7 agonists had no effect on lung
cancer cell proliferation; however, activation of TLR8 induced apoptosis in these cells.
In conclusion, my thesis shows that TLRs are important receptors involved in the
initiation of inflammation and inhibition of cellular proliferation. We show, for the first time,
that TLR4-induced CXCL8 release in COPD patients may be a good biomarker of disease
severity, and that TLR8 offers a good prospect as an adjuvant in the treatment of lung
cancer.