Atherosclerosis, a disease characterized by fibroinflammatory lipid
lesions in the arterial wall, is the leading cause of mortality in the
Western world. For decades research has been aimed at dissecting the
pathogenesis of this multifactorial disease. Recently vascular
inflammation induced by both innate and adaptive immune responses has
been shown to be important for the disease development. Innate immunity
is the primary host defense to infections, and elicited by the
recognition of microbial ligands by pattern recognition receptors (PRRs).
Infections have been implicated in the pathogenesis of atherosclerosis
and its complications such as myocardial infarction (MI).
The aim of this thesis was to elucidate the function of innate immunity
in human atherosclerosis. Specific objectives were to investigate
expression and functional relevance of two types of PRRs: Toll-like
receptors (TLRs) and nucleotide oligomerization domain protein 1 (NOD1)
and NOD2, and to characterize the antimicrobial peptide, LL-37, an innate
host defense peptide with effector and regulatory functions in innate
immunity, in the disease.
We investigated the expression of 9 TLRs in human atherosclerotic lesions
and normal vessels. Our data show that endothelial cells of normal artery
express low levels of TLRs. However, TLR1, TLR2 and TLR4, were
upregulated both at the mRNA and protein level in atherosclerotic lesions
compared to normal arteries. TLR2 and TLR4 were expressed by macrophages
and some endothelial cells, and co-localized with activated NF-κB
signaling in the lesions. These findings suggest that TLRs on endothelial
cells constitute the first line of vascular defense against infection,
and may be linked to the inflammatory response in atherosclerosis.
To assess the impact of TLR4-mediated innate immunity on the risk of MI,
the Stockholm Heart Epidemiology Program (SHEEP) was screened for two
functional polymorphisms in the TLR4 gene. Hyporesponsive TLR4 variants
were associated with a higher risk for MI in men. Although the underlying
mechanisms remain to be elucidated, this genetic study indicates that
alteration in TLR4- mediated innate immunity may affect progression of
coronary artery disease.
In addition to TLRs, vascular endothelial cells also express basal levels
of the intracellular PRRs, NOD1 and NOD2. Yet, NOD2 was markedly
increased in atherosclerotic lesions and expressed predominantly by
macrophages and some endothelial cells. NOD2 expression in human
endothelial cells could be induced by proinflammatory cytokines and
activation of NOD2 induced the expression of intercellular adhesion
molecule 1 (ICAM-1) and monocyte chemoattractant protein 1 (MCP-1), two
proteins with important roles in the recruitment and activation of
leukocytes during vascular inflammation. These findings imply that NOD2
might be a key mechanism for sensing intracellular infection in
atherogenesis.
In an effort to identify innate effectors, we analyzed antimicrobial
peptides and found that LL-37 was produced in atherosclerotic lesions,
predominantly by macrophages. Expression of the murine homologue of LL-37
could be induced in macrophages by Chlamydia pneumoniae, a bacterium
associated with cardiovascular disease. Stimulation of endothelial cells
with LL-37 led to dose-dependent induction of a limited set of
proinflammatory genes, signifying a novel proinflammatory pathway
involved in disease development. Although LL-37 exhibits broad
microbicidal activity, this peptide had no microbicidal effect on
Chlamydia pneumoniae.
In summary, our results reveal that the endothelium is the first line
vascular defense of the artery. It is characterized by expression of a
spectrum of PRRs, capable of sensing extracellular as well as
intracellular pathogens. Whether infections contribute to atherosclerosis
remains controversial but the results of this thesis suggest that the
machinery for microbial recognition and effector functions is further
enhanced in atherosclerosis. By identifying a large array of PRRs in
atherosclerosis together with the characterization of the antimicrobial
peptide, LL-37, the work of this thesis hopefully adds novel
understanding of innate immunity in the disease
