In this thesis I examined the role of circulating monocytes and lung macrophages in the
pathogenesis of the early fibrotic, progressive fibrotic and resolution phases of
pulmonary fibrosis.
Pulmonary fibrosis with destruction of lung architecture and consequent respiratory
failure and death represents a massive worldwide health burden. Although idiopathic
pulmonary fibrosis (IPF) is the archetypal and most common cause of lung fibrosis,
numerous respiratory diseases can progress to pulmonary fibrosis, and this usually
signifies a worse prognosis. Importantly, the incidence and prevalence of IPF continue
to rise and it remains one of the few respiratory conditions for which there are no
effective therapies.
The mechanisms resulting in pulmonary fibrosis are controversial. Early work in the
1980s and 1990s suggested that lung macrophages were important. However, at the turn
of the 21st century there was a shift to a belief that pulmonary fibrosis resulted from
aberrant wound healing as a consequence of repetitive epithelial injury from an as yet
unknown cause. However, with the ever expanding knowledge of the importance of
macrophages in other fibrotic conditions such as the kidney and liver, the potential
importance of macrophages in pulmonary fibrosis has become more pertinent.
Using an in vivo depletional strategy in several murine models of lung fibrosis, in
conjunction with human studies, I sought to characterise the role of circulating
monocytes and lung macrophages in the pathogenesis of pulmonary fibrosis.
I have established that circulating monocytes and lung macrophages are not critical for
the development of early lung fibrosis. In contrast, circulating monocytes and lung
macrophages are important during the progressive fibrotic phase of lung fibrosis.
Furthermore, my data suggest that the pro-fibrotic alternatively activated macrophages
may be the sub-class of macrophages that mediate this fibrogenic effect. In addition and
in contrast, I have established that lung macrophages are required for the resolution of
fibrosis. This finding is in keeping with important work performed in the field of liver
fibrosis.
There is an ever increasing literature examining the role of matrix metalloproteinases
(MMPs) during tissue fibrosis and repair. My work has suggested that during lung
fibrosis there may be compartmental specific functions of MMPs that regulate lung
fibrogenesis, although more work is required before this exciting finding can be properly
defined