Foam cell

Fat laden M2 macrophages seen in atherosclerosis

Foam cells, also called lipid laden macrophages, are a type of cell that contain cholesterol. These can form a plaque that can lead to atherosclerosis and trigger myocardial infarction and stroke. Foam cells are fat laden cells with a M2 macrophage like phenotype. They contain low density lipoproteins (LDL) and can only be truly detected by examining a fatty plaque under a microscope after it is removed from the body. Despite the connection with cardiovascular diseases they might not be inherently dangerous.

Foam cell formation is triggered by a number of factors including the uncontrolled uptake of modified low density lipoproteins (LDL), the upregulation of cholesterol esterification and the impairment of mechanisms associated with cholesterol release. Monocytes are then able to penetrate the arterial wall as a result of impaired endothelial integrity which increases permeability. Once in the sub endothelium space, inflammation processes induce the differentiation of monocytes into mature macrophages. Once internalized, scavenged lipoproteins are transported to endosomes or lysosomes for degradation, whereby the cholesteryl esters (CE) are hydrolyzed to unesterified free cholesterol (FC) by lysosomal acid lipase (LPL). Free cholesterol is transported to the endoplasmic reticulum where it is re esterified by ACAT1 (acyl CoA: cholesterol acyltransferase 1) and subsequently stored as cytoplasmic liquid droplets. These droplets are responsible for the foamy appearance of the macrophage and thus the name of foam cells. Macrophages within the atherosclerotic legion area have a decreased ability to migrate, which further promotes plaque formation as they are able to secrete cytokines, chemokines, reactive oxygen species (ROS) and growth factors that stimulate modified lipoprotein uptake and vascular smooth muscle cell (VSMC) proliferation. VSMC can also accumulate cholesteryl esters. Foam cells form the fatty streaks of the plaques of atheroma in the tunica intima of arteries. Foam cells are not dangerous as such, but can become a problem when they accumulate at particular foci thus creating a necrotic centre of atherosclerosis. If the fibrous cap that prevents the necrotic centre from spilling into the lumen of a vessel ruptures, a thrombus can form which can lead to emboli occluding smaller vessels. The occlusion of small vessels results in ischemia, and contributes to stroke and myocardial infarction, two of the leading causes of cardiovascular related death. Foam cells are very small in size and can only be truly detected by examining a fatty plaque under a microscope after it is removed from the body, or more specifically from the heart. Detection usually involves the staining of sections of aortic sinus or artery with Oil Red O (ORO) followed by computer imaging and analysis; or from Nile Red Staining. In addition, fluorescent microscopy or flow cytometry can be used to detect OxLDL uptake when OxLDL has been labeled with 1,1′ dioctadecyl 3,3,3′3′ tetra methylindocyanide percholorate (DiI OxLDL). Autoimmunity occurs when the body starts attacking itself. The link between atherosclerosis and autoimmunity is plasmacytoid dendritic cells (pDCs). PDCs contribute to the early stages of the formation of atherosclerotic lesions in the blood vessels by releasing large quantities of type 1 interferons (INF). Stimulation of pDCs leads to an increase of macrophages present in plaques. However, during later stages of lesion progression, pDCs have been shown to have a protective effect by activating T cells and Treg function; leading to disease suppression.

Foam cell degradation or more specifically the breakdown of esterified cholesterols, is facilitated by a number of efflux receptors and pathways. Esterified cholesterol from cytoplasmic liquid droplets are once again hydrolyzed to free cholesterol by acid cholesterol esterase. Free cholesterol can then be secreted from the macrophage by the efflux to ApoA1 and ApoE discs via the ABCA1 receptor. This pathway is usually used by modified or pathological lipoproteins like AcLDL, OxLDL and βVLDL. FC can also be transported to a recycling compartment through the efflux to ApoA1 containing HDLs (high density lipoproteins) via aqueous diffusion or transport through the SR B1 or ABCG1 receptors. While this pathway can also be used by modified lipoproteins, LDL derived cholesterol can only use this pathway to excrete FC. The differences in excretory pathways between types of lipoproteins is mainly a result of the cholesterol being segregated into different areas.

Foamy macrophages are also found in diseases caused by pathogens that persist in the body, such as Chlamydia, Toxoplasma, or Mycobacterium tuberculosis. In tuberculosis (TB), bacterial lipids disable macrophages from pumping out excess LDL, causing them to turn into foam cells around the TB granulomas in the lung. The cholesterol forms a rich food source for the bacteria. As the macrophages die, the mass of cholesterol in the center of the granuloma becomes a cheesy substance called caseum.

Foam cells may form around leaked silicone from breast implants. Lipid laden alveolar macrophages, also known as pulmonary foam cells, are seen in bronchoalveolar lavage specimens in some respiratory diseases.