Macrophages in Athersclerosis Flashcards
(42 cards)
What is the primary role of macrophages, and how do they do this?
The identification and degradation of pathogens, cell debris and cancer cells – endocytosing them via pattern recognition receptors and engulfing them in a phagosome, which then merges with the lysosome to degrade the pathogen before exocytosing the remnant material and displaying the antigens.
The activity of this process triggers the release of pro-inflammatory cytokines.
How are macrophages attracted to the endothelium?
Monocytes are attracted to the endothelium by chemokine attractants such as CCL5 and CXCL1, which are released due to the inflammation caused by LDL accumulation and subsequent endothelial activation.
How are macrophages recruited into the endothelium?
The monocytes are recruited via diapedesis by the monocyte P-selectin glycoprotein ligand 1 (PGSL-1) when rolling and their firm adhesion is facilitated by binding the VLA-4 and LFA-1 integrins which dimerise with VCAM and ICAM respectively to mediate internalisation (E-selectin also being involved in these processes).
How does platelet recruitment to the endothelium in atherosclerosis affect macrophage recruitment?
It promotes monocyte-endothelium interactions through deposition of platelet-derived chemokines and its activation of NF-κB signalling, leading to increased expression of VCAM and ICAM.
How does CVD affect the immune cells?
Monocytes can in fact be modified prior to their recruitment by the high levels of plasma cholesterol characteristic of CVD.
How does high cholesterol affect monocytes?
During the production of monocytes from their bone-marrow derived progenitors (HSPCs – haematopoeitic stem/progenitor cells) the cholesterol begins to induce their later phenotype and also increase the numbers of monocytes in circulation.
This is mediated by cholesterol increasing the IL-3 concentration and stimulating the GM-CSF (granulocyte macrophage – CSF) receptor, both of which stimulate proliferation of the monocytes. In ApoE -/- mice monocyte levels are 50% higher than in wildtype.
What happens when monocytes are first internalised into the endothelium?
Once internalised these differentiate into macrophages due to monocyte colony stimulating factor (M-CSF) signalling and proliferate within the plaque.
What is the intended function of the macrophage within the fatty streak?
In the early stages these macrophages serve their intended purpose – clearing the apoptotic cells and LDLs from the lesion or fatty streak.
The LDLs taken up at this stage, even those taken up by scavenger receptors such as SR-A or CD36, are hydrolysed in endosomes and the cholesterol trafficked to the ER via the NPC1/2 pathway.
What effect does the OD of LDL have on the macrophages in the lesion?
Much of this free cholesterol is effluxed via ABCG1, but the large excess leads to esterification by ACAT and storage within the cell to produce foam cells.
Upregulating cholesterol efflux in macrophages is thus a major therapeutic target, as this is the primary mechanism of plaque regression.
What dysregulation occurs in foam cells due to the high lipid content?
The oxysterols and modified PLs from atherogenic lipoprotein uptake (ApoB-LPs) can lead to macrophage apoptosis, or in lesser conditions can enrich the plasma membrane with free cholesterol which activates inflammatory signalling receptors leading to increased cytokine production.
How do foam cells progress the lesion?
This is sufficiently worsened by their uncontrollable oxLDLs uptake (cholesterol loading) to produce enough foam cells to form a lipid lesion core.
New macrophages are still active at this point, but then promote progression of the lesion through degradation of the fibrous cap and thrombotic core formation.
What are cholesterol crystals?
A hallmark of advanced atherosclerosis that forms within lipid cores and is highly pro-inflammatory.
Macrophages attempt to engulf the cholesterol crystals, but this just makes everything worse.
How do cholesterol crystals affect macrophages?
During engulfing they cause endosome damage and subsequent lysosomal disruption, which in conjunction with the subsequent impairment of cathepsin B and L and hence the sterol response network, prevents proper cholesterol management and is hence highly pro-atherogenic.
Also by activating the macrophage inflammasome complex composed of NLRP3, ASC and caspase-1. This leads to increased IL-1β and IL-18 secretion, which act as chemokines to attract more monocytes.
What factors promote foam cell apoptosis in the lesion core?
Prolonged ER stress in the overloaded foam cells, as well as the often hypoxic conditions, lack of growth factors and activation of death receptors (IFN and TLR3/4 signalling), triggers apoptosis of the foam cells.
What factors prevent foam cell apoptosis in the lesion core?
In some cells this is prevented by the ER stress activating cell survival pathways including NF-κB, Akt, p38 kinases and autophagy, thus reducing necrotic core formation. This is, however, inhibited by the uptake of LDLs via scavenger receptors.
How does macrophage/foam cell apoptosis impact formation of the necrotic core and lesion progression?
Macrophage apoptosis itself does not cause necrosis, but defective clearance of the products by the remaining macrophages leads to secondary necrosis and formation of the necrotic core.
Macrophage cell death in early lesions can actually be beneficial, decreasing the inflammation and number of foam cells.
How do macrophages promote plaque rupture?
In the advanced plaque macrophages and the products of their apoptosis contribute to SMC cell death (via TNF-α and NO secretion and activation of the Fas pathway) and degradation of the extracellular matrix.
This weakens the fibrous cap promoting plaque rupture, and also promote further inflammation and thrombosis.
The defective clearance of apoptotic cells also reduces anti-inflammatory TGFβ signalling, which signals SMCs to reduce collagen production thus further weakening the cap.
How do macrophages promote degradation of the extracellular matrix when they weaken the fibrous cap?
By secretion of macrophage derived MMPs (MMP2, 9, 13 and 14) and serine and cysteine proteases (neutrophil elastase and cathepsins L, K and S).
What therapeutic strategies aim to prevent LDL invasion of the intima?
Many CVD drugs (including statins) aim to prevent formation of the lesions by reduction of plasma atherogenic lipoproteins, reducing retention of them in the intima.
Others target HDLs or mimic their effects to prevent lipoprotein retention, decrease endothelial cell activation and reduce LDL oxidation. However, these treatments only tend to be administered in later stage CVD after presentation of symptoms.
What therapeutic strategies aim to regress or ameliorate the lesions? What is a primary limitation to these?
Some aim to reduce the number of macrophages in the lesions, either by promoting macrophage emigration or by reducing monocyte entry through neutralisation of chemokines, their receptors, or the adhesion molecules using antibody and gene therapies.
Novel strategies include increasing macrophage cholesterol efflux thus reducing foam cell formation by increasing HDL concentration or upregulating efflux transporters such as ABCG1.
Targeting Delivery to the Plaque is a problem.
What methods are being used to target drugs to the lesions?
This is an emerging technology, with potential strategies including atherosclerosis directed nanoparticles, encapsulated macrophage targeted-siRNA treatments (allowing for knockdown of genes in macrophages only) and stent-based delivery.
Only 2-3% of the lesions actually lead to symptomatic plaques, so targeting these over the other ones is also important.
Targeting the macrophage processes involved in advanced plaque progression is a strategy.
What therapies are used to target the macrophage processes involved in advanced plaque progression?
• Pro-inflammation resolution mediators
o Lipoxins
o Resolvins
o IL-10R activators
• MerTK inhibitors
o Involved in recognition of apoptotic cells
• LXR activators
o Increases MerTK expression but also ABCA1/G1
• PPARδ activators
What are the main cellular markers used to define monocyte types?
Two main cellular markers exist for human monocytes, allowing them to be classified by their expression of them or lack thereof. These are cell differentiation factors 14 and 16 (CD14 and CD16).
CD14 is also called the LPS (lipopolysaccharide) receptor and CD16 the FcyIII receptor.
How can monocyte cellular markers be used to classify them into subsets?
There are three potential combinations of these, given that one or both of them must be being expressed.
These are denoted CD14++CD16-, CD14++CD16+ and CD14+CD16++ depending on the relative levels of expression (minus signs denoting lack of expression, one plus sign low level expression and two for high level).
