In the SENS rejuvenation research view of atherosclerosis, a primary cause is the presence of oxidized lipids in the bloodstream. Rising levels of oxidative stress with aging, with mitochondrial dysfunction as a primary cause, means an increasing number of oxidized lipid molecules. Atherosclerosis begins when these lipids irritate the blood vessel walls, attracting macrophage cells to clean up the problem. The normal process involves macrophages ingesting the problem lipids and either breaking them down or handing them off to high-density lipoprotein (HDL) particles to be carried to the liver where they can be dealt with. Unfortunately, some species of oxidized lipid cannot be processed well by macrophages, and the cells become overwhelmed. They either die or become inflammatory foam cells, making the area of damage worse. The fatty plaques of atherosclerosis that narrow and weaken blood vessels are formed of dead macrophages and the lipids that they should have removed.
The SENS approach is to find ways to break down the problem oxidized lipids, remove them before they can cause harm to the macrophages that are critical to maintenance of blood vessel walls. Some progress in this LysoSENS program for atherosclerosis has been accomplished, mostly focused on 7-ketocholesterol, a particularly harmful species of oxidized lipid. Other groups are starting to pay attention to this line of research, which is a good thing. In the study reported here, scientists used antibodies to thin out a class of oxidized lipid from the bloodstream, and demonstrated that this slows the pace at which atherosclerosis progresses in a mouse model of the condition. This is important evidence that strongly supports the SENS position.
In atherosclerosis, lipids such as cholesterol move in a loop: from the liver to LDL particles, then into atherosclerotic lesions, then taken up by macrophages and passed off to HDL particles, then finally back to the liver. All of the available anti-atherosclerosis technologies interfere in the front half of that loop, the movement of lipids through the bloodstream in LDL particles. They globally reduce cholesterol levels, and that somewhat slows the advance of atherosclerosis. It doesn't do it well, however. Even extremely low cholesterol levels, such as those produced by PCSK9 inhibition, don't significantly reduce existing atherosclerotic plaque - they allow a little reduction, but that is about it. Thus other strategies are needed, and the work here is good evidence for approaches that in some way protect macrophages from oxidized lipids, a methodology that should allow those cells to better clear existing plaque.
Some phospholipids - the molecules that make up cell membranes - are prone to modification by reactive oxygen species, forming OxPL. This event is particularly common in inflammatory conditions such as atherosclerosis, in which artery-blocking plaques form. Prior to this study, researchers were unable to control phospholipid oxidation in a way that would allow them to study its role in inflammation and atherosclerosis.
Researchers engineered mice with two special attributes: 1) they have a gene mutation that makes them a good model for atherosclerosis and 2) they generate a piece of an antibody called E06 that's just enough to bind OxPL and prevent their ability to cause inflammation in immune cells, but not enough to cause inflammation on its own. They fed the mice a high-fat diet.
Here's what happened: Compared to control mice, the mice with E06 antibodies had 28 to 57 percent less atherosclerosis, even after one year and despite having high levels of cholesterol. The antibody also decreased aortic valve calcification (hardening and narrowing of the aortic valves), hepatic steatosis (fatty liver disease) and liver inflammation. E06 antibody-producing mice had 32 percent less serum amyloid A, a marker of systemic inflammation. The E06 antibody also prolonged the life of the mice. After 15 months, all of the E06 antibody-producing mice were alive, compared to 54 percent of the control mice.
Oxidized phospholipids (OxPL) are ubiquitous, are formed in many inflammatory tissues, including atherosclerotic lesions, and frequently mediate proinflammatory changes. Because OxPL are mostly the products of non-enzymatic lipid peroxidation, mechanisms to specifically neutralize them are unavailable and their roles in vivo are largely unknown. We previously cloned the IgM natural antibody E06, which binds to the phosphocholine headgroup of OxPL, and blocks the uptake of oxidized low-density lipoprotein (OxLDL) by macrophages and inhibits the proinflammatory properties of OxPL.
Here, to determine the role of OxPL in vivo in the context of atherogenesis, we generated transgenic mice in the Ldlr-/- background that expressed a single-chain variable fragment of E06 (E06-scFv) using the Apoe promoter. E06-scFv was secreted into the plasma from the liver and macrophages, and achieved sufficient plasma levels to inhibit in vivo macrophage uptake of OxLDL and to prevent OxPL-induced inflammatory signalling.
Compared to Ldlr-/- mice, Ldlr-/-E06-scFv mice had 57-28% less atherosclerosis after 4, 7 and even 12 months of 1% high-cholesterol diet. Echocardiographic and histologic evaluation of the aortic valves demonstrated that E06-scFv ameliorated the development of aortic valve gradients and decreased aortic valve calcification. Both cholesterol accumulation and in vivo uptake of OxLDL were decreased in peritoneal macrophages, and both peritoneal and aortic macrophages had a decreased inflammatory phenotype. Serum amyloid A was decreased by 32%, indicating decreased systemic inflammation, and hepatic steatosis and inflammation were also decreased. Finally, the E06-scFv prolonged life as measured over 15 months. Because the E06-scFv lacks the functional effects of an intact antibody other than the ability to bind OxPL and inhibit OxLDL uptake in macrophages, these data support a major proatherogenic role of OxLDL and demonstrate that OxPL are proinflammatory and proatherogenic, which E06 counteracts in vivo.