of targeted activation of (class Ia/Ib) PI3K in lesional macrophages.
In general TLR2 is known for the induction of pro-inflammatory responses. For
example blocking TLR2 promotes graft acceptance upon renal transplantation43 and
reduces myocardial infarct sizes44, due to reduced inflammation and tissue necrosis.
In atherosclerosis, studies have shown that administration of Pam3Csk4 (TLR2/TLR1 agonist) and MALP2 (TLR2/TLR6 agonist) result in increased atherosclerotic lesion
formation45–47. Deficiency of either TLR1 or TLR6 was found to have no effect on
atherosclerosis in LDLr-/- mice47. Interestingly, the pro-atherogenic effect of TLR2
in mice was found to be associated to TLR2 signaling in non-bone marrow-derived cells46,48.
In line with our findings, preclinical studies have shown that TLR2 signaling can
also be exploited to reduce inflammation. Intraperitoneal injections of peptidoglycan
and Pam3Csk4 were found to reduce ischaemia/reperfusion injury in a PI3K-
dependent way49. Moreover, intranasal Pam3Csk4 administration suppressed asthma
development50, while intraperitoneal administration reduced type 1 diabetes51, both
via induction of IL-10 responses and Tregs. IL-10-/- mice have been found to have
enhanced Th1 and Th17-like responses52,53, thereby conversely significant amounts
of IL-10 could result in a reduction of Th1 and Th17 responses. Indeed, we observe a significantly decreased induction of splenic Th1 cells and splenic Th17 cells, as well as circulating Th17 cells. The capacity of TLR2-dependent IL-10 production to inhibit
IFN-γ-producing T cells has previously been described16. Moreover, IL-10 is known to
indirectly prevent antigen-specific T cell activation by modulating APCs and directly by
inhibiting T cell expansion10. In fact, we also observed a reduced proliferative capacity
of splenic T cells.
In agreement with earlier observations that IL-10 reduces monocyte adhesion
and recruitment54, we found a significantly reduced expression of adhesion molecules,
ICAM-1 and VCAM-1, as well as CCL2 in the aortic arch, resulting in reduced leukocyte recruitment to the lesions. Both macrophage and T cell presence was dramatically reduced in the aortic arch and the aortic root. A decrease of circulating inflammatory
Ly-6Chi monocytes, which is in line with reduced lesional macrophages, likely resulted
from a decrease in CCL2 and a reduced recruitment of monocytes from bone marrow. In summary, we demonstrate that HK-SA treatment significantly reduces atherosclerotic lesion development by 34%, likely through inducing IL-10-producing
immunoregulatory M2b-like macrophages upon intraperitoneal treatment. Our results
indicate that TLR2/PI3K activation in macrophages via HK-SA could be beneficial for atherosclerosis. Therefore, it will be important to determine which specific components of HK-SA are responsible for the observed protective effect. Eventually, it will be interesting to specifically target lesional macrophages, ideally with purified
Staphylococcus aureus components. We believe our study will help to reassess the role of the TLR2/PI3K pathway in atherosclerosis and will support the discovery and further development of specific molecules to modulate macrophages via TLR2/PI3K pathways for atherosclerosis therapy.
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