The pro-inflammatory gene, macrophage colony stimulating factor (MCSF), has been shown in a murine model to play a critical role in early atherosclerosis formation and to completely overcome the effects of profound hypercholesterolemia observed in the presence or absence of the MCSF gene.

Crossing LDL receptor knockout hypercholesterolemic mice with atherosclerotic lesions with mice lacking MCSF completely prevented atherosclerosis development. Further, knocking out one of the MCSF alleles resulted in a nearly 90% inhibition of atherosclerosis despite the profound cholesterolemia, and actually augments the hypercholesterolemia. MCSF are responsible for growth, maturation, survival and functional stimulation of the monocyte and macrophage lineage.

Rupture of the fibrous cap can be considered a structural failure in which the collagen and matrix components in the fibrous cap are lost or degraded causing thinning of the cap, making it vulnerable to disruption either spontaneously or by an extrinsic or intrinsic trigger. Two seminal features associated with plaque rupture are the presence of a relatively large lipid rich core and the presence of an inflammatory cell infiltrate, often at the sites of rupture prone regions and under thinned out portions of the fibrous cap. Most of inflammatory cell infiltrate is comprised of monocyte-derived macrophages and foam cells and to a lesser extent it contains activated T lymphocytes and degranulated and activated mast cells. The presence of inflammatory cell infiltrate is an indicator of vulnerability to rupture and thrombosis formation.

Thinning of the cap is a prelude to its subsequent rupture. Matrix dysregulation can contribute to thinning of the cap, as it is predominantly comprised of collagen and other matrix proteins. A combination of reduced matrix synthesis, with smooth muscle cell the primary responsible cell, and increased matrix breakdown, with monocyte macrophage the primary responsible cell, comprise this dysregulation. Studies show that the inflammatory cell may play a critical role in this matrix dysregulation.

Evidence of ongoing collagen breakdown has been shown by Libby, supporting the hypothesis that there is increased collagen breakdown in the lipid rich vulnerable atherosclerotic lesions. The mechanism for this increased breakdown has been attributed to a family of matrix degrading metalloproteinases (MMP). It is not clear which one(s) is responsible for matrix protein in atherosclerotic plaque. Serine proteases such as cathespin-S are also present in human atherosclerotic lesions. The proteases are predominantly produced by the macrophage and foam cells in the lesions, and to a lesser extent by the smooth muscle cells and endothelial cells lining the microvasculature.

MMP gene expression in the macrophage and smooth muscle cell may be stimulated by a number of factors including oxidative stress. Low concentrations of oxidized LDL induce MMP gene expression in the macrophage. Co-localization with increased circumferential stress and expression of MMP-1 in human atherosclerotic lesions has been show by Lee.

Cell-cell interaction activates T lymphocytes, particularly through ligation of CD 40 and through elaboration of interferon gamma, and may be mediators of MMP expression. Tenascin, an MMP produced by macrophages, is capable of inducing the MMP-9 gene in the macrophage, as shown by Shah. Infectious agents and their components, particularly chlamydia pneumonia, also may be able to induce MMP in macrophages.

Tenascin is a very complex macromolecule produced by the macrophage. Shah's laboratory demonstrated that Tenascin mRNA in atherosclerotic lesions co-localizing with macrophage immunoreactivity and with Tenascin protein immunoreactivity. Tenascin has the ability to induce MMP-9. So, the same molecule that the macrophage is producing is also capable of upregulating the proteolytic arsenal in the macrophage. They have also shown that the epidermal growth factor ligand (EGF-L) of Tenascin can be cleaved by MMP. The exposed EGF-L is a powerful of inducer of apoptosis in the vascular smooth muscle cells. Recently they demonstrated that atherosclerotic lesions can demonstrate in situ cleavage of Tenascin and exposure of the EGF-L domain. This lends plausibility to the concept that macrophages may not only induce breakdown of collagen and matrix proteins through elaboration of proteases, but may also prevent synthesis of collagen by releasing mediators, such as Tenascin, that can cause apoptosis of the smooth muscle cells.

A study by Shah in collaboration with Fuster examined the thrombogenecity of components of the vessel wall using an ex vivo flow chamber. The materials are placed into the flow chamber and blood with labeled platelets is allowed to flow. The extent of thrombus formation on various substrates is measured to create an index of thrombogenecity of these components. The lipid gruel, the extracellular lipid rich core, was the most thrombogenic component of human atherosclerotic plaque, as it is particularly rich and impregnated with tissue factor. Tissue factor is a pro-coagulant protein which upon exposure to circulating blood activates the thrombin cascade by activating Factor X to Xa, which then cleaves pro-thrombin to thrombin allowing it to recruit platelets and induce fibrin formation.

Tissue factor, a major factor in creating thrombogenecity, comes from the macrophages. Shah and other investigators have shown that carotid atherosclerotic plaques stained for macrophages and tissue factor are co-localized. It has also been shown that dying and dead macrophages can impregnate the lipid core with tissue factor, thus making the lipid core highly thrombogenic and accounting in part for the fact that lipid rich plaques are often the substrate for subsequent thrombus formation. Again, the inflammatory cell plays a critical role in determining the thrombogenecity of the atherosclerotic lesion.

An intriguing emerging hypothesis is that some infectious agents, either directly or indirectly, also may be inducers of inflammation. Chlamydia pneumonia has been implicated as one of the agents that may create a pro-inflammatory in the atherosclerotic plaque.

Tull like receptor-4 (TLR-4) has recently been identified by Shah to be one of the receptors by which lipopolysaccharide is able to induce a pro-inflammatory cascade. TLR-4 activates the intracellular inflammatory cell cascade involving Nf6 B activation. TLR-4 is expressed in lipid-rich plaques, and macrophages are the major source of the TLR-4 in these plaques. This is provides another potentially interesting target for investigation and possibly activation.