*3.2.2 Host for CP is not only the respiratory epithelial cells*

CP can invade various cell types other than respiratory epithelium (vascular endothelial cells, smooth muscle cells, monocytes, granulocytes, even neural glial cells) and lead to enhanced expression of numerous chemokines [37].

Infection of all these cells leads to an enhanced cytokine expression, which results in a series of events resulting with proinflammatory and proproliferating environment. The ICAM-1 (intracellular adhesion molecule), VCAM-1 (vascular cell adhesion molecule), IL-6, IL-8 cytokine and protein expressions from the endothelial cells, the IL-6, MCP-1 (monocyte chemoattractant protein) from the vascular smooth muscle cells, IL-1ß, IL-6, IL-8, MCP-1, TNFα (tumor necrosis factor α), and MMPs (matrix metalloproteinases) from monocytes are some examples for the increase expression of cytokines and proteins in CP infection. Aggravation of inflammation and vascular cell proliferation induction by these cytokines may have an effect on atherosclerosis development [32, 38–42].

### *3.2.3 Target cell stimulation by CP and activation of signal transduction*

Several receptor systems and signaling pathways are thought to be involved in activation of host cells by CP infection. Stimulation by IL-8, ICAM-1, causes phosphorylation of various kinases, which in turn causes a proinflammatory phenotype in vascular cells [43]. The transcription factor NF-κB that mediates the pro-inflammatory cascades in cells can be activated by CP [44].

Two systems are known to be involved to activation of target cells by CP: Toll-like receptors are the extracellular receptors and Nod proteins are the intracellular receptors.

Toll-like receptor (TLR) 2 and 4 were found to be the essential mediators of CP host cell activation. TLRs are receptors of the immune system, which are responsible for recognition of pathogens. Secretion of cytokines and translocation of NF-κB in dendritic cells are dependent on TLR stimulation. Nod protein system is the intracellular part of the immune system that could be responsible for the cytokine production in chronic infections. Through several kinases finally the NF-κB is activated and immune response is mediated. Various protein kinase systems are activated after contact of CP with the endothelial cells. Mitogen-activated protein kinase family (MAPK) members that are the key elements of proinflammatory, prothrombotic and pro-proliferative responses are the most activated kinases after CP contact. The final

activation of NF-κB is followed by expression of pro-inflammatory mediators: ICAM, VCAM, IL-8, MCP-1, RANTES [30].

These all responses coincide shortly after acute contact by CP. The primary infection of monocytes and the vascular smooth muscle cells resemble persistent infection rather than active infection. The less is known about the changes in the persistent CP infection in which the pro-atherosclerotic signaling cascades supervene [45, 46].

#### **3.3 Atherosclerosis and** *Chlamydia pneumoniae*

#### *3.3.1 Endothelial dysfunction to atherosclerosis*

The atherosclerosis is characterized by deposition of lipids in the artery wall and infiltration of immune cells such as macrophages, T-cell, and mast cells with a surrounding fibrous cap consisting of mainly collagen, which is formed by the vascular smooth muscle cells.

Rudolph Virchow was the first scientist who recognized the inflammatory nature of the atherosclerotic lesions in history; however, his concept of atherosclerosis consisting of the inflammatory process was complicated to comprehend at that century. Eventually, the atherosclerosis was remained to be a concept of a just an arterial cholesterol and thrombotic debris deposition disease in the last century. Thereafter with the discovery of the smooth muscle cell proliferation with the inflammatory cells in the atherosclerotic plaque in 1960s and 1970s, the inflammation was begun to be considered to be a cause of the atherogenesis [47].

Earliest change of atherosclerosis is the dysfunction of the endothelial lining of the lesion-prone areas in the vascular system. The focal permeation, entrapment, and the modification of the circulating lipoprotein particles in the subendothelial space trigger a series of immune reaction including recruitment of circulating monocytes from the circulation. The monocytes differentiate into macrophages and they become foam cells as they resume to phagocyte the modified lipoproteins. These foam cells are the hallmark of early fatty streak lesions. Initiating event in the atherogenic process was assumed to be an injury to the endothelial lining by noxious substance such as oxidized LDL, cigarette smoking contents, hyperhomocysteinemia, altered hemodynamic forces generated in hypertension. However, this type of endothelial injury was failed to be demonstrated in the animal models of natural atherosclerosis. The uncertainty of the evidences of the direct endothelial cell injury in animal studies of natural atherosclerosis and the recent findings that demonstrate the functions and phenotypic modulation of the endothelial cells raised the term endothelial dysfunction in the development of atherosclerosis [48].

The term "proinflammatory endothelial phenotype" confers to enhanced expression of various effector proteins and cytokines that are responsible for acute and chronic inflammatory responses and disease processes in endothelial cells. In the lesion-prone regions of the arterial vascular tree as a result of endothelial cell activation by the actions of pro-inflammatory cytokines and noxious stimuli, genetic regulation modifications supervene in the endothelial cells primarily driven by the transcription factor NfκB. These include enhanced expression of adhesion molecules such as VCAM-1, ICAM, increased secretion of chemokines, and prothrombotic mediators. The circulating monocytes and T-cells respond to these signals and they migrate into subendothelial space. As a result, the paracrine milieu of cytokines, growth hormones, and reactive oxygen species, which were created by the actions of all activated endothelial cells, smooth muscle cells, monocytes and T-cells all together, *The Probable Role of* Chlamydia pneumoniae *Infection in Acute Stroke DOI: http://dx.doi.org/10.5772/intechopen.109582*

eventuate as a vicious cycle of chronic inflammation. This chronic inflammation establishes the pathophysiological basis of the atherosclerosis [48, 49].

Lesion-prone areas for atherosclerosis are the sites which have and disturbed laminar flow patterns. The sites with low oscillatory endothelial shear stress located near branch point of arteries are most susceptible. The abdominal aorta, coronary arteries, iliofemoral arteries, and the carotid bifurcations are the most affected sites. These predilection sites are characterized by the presence of subendothelial macrophages. Modifications of gene expression in endothelial cells are present in these sites [50].

The most of the lesion-prone areas are the bifurcation sites and the other regions with altered hemodynamics. The absence of an disrupted endothelial lining at these branch points in the detailed morphological studies undermines the arterial injury hypothesis to explain this phenomenon. Similar to the changes in lesion-prone areas *in vivo*, the enhanced endothelial cell turnover, oxidative stress, and the alterations in endothelial cell shape, and changes in cytoskeletal and junctional proteins were demonstrated *in vitro* studies. These findings suggest the hemodynamic forces might have an effect on endothelial cell dysfunction in atherogenesis [48, 51]. The association between hemodynamic forces and the various genes that are important in development of atherogenesis such as hemostasis, thrombosis, growth regulation, and proinflammatory activation was demonstrated in previous studies [52, 53]. These results suggest a presence of a system of biomechanical endothelial gene regulation [48].

#### *3.3.2 Progression of atherosclerosis*

American Heart Association (AHA) defined six lesion types according to atherosclerosis progression.

*Type I lesion*: initial lesions. Intimal thickening and fatty streak lesions are frequent in infants and children. The earliest vascular change is intimal thickening consisting of layers of smooth muscle cells and extracellular matrix with small isolated groups of macrophage foam cells.

*Type II lesions*: include fatty streaks, which are visible as yellow-colored streaks on the intimal surface of arteries. Macrophage foam cells are abundant scattered in smooth muscle cells and proteoglycan-rich intima. T cells are identified in these lesions but they are less numerous than macrophages. Foam cells, easily recognizable by light microscopy, are signs of lipoprotein-driven inflammation occurring in the vascular wall. Xanthomas are harmless and reversible in case of disappearance of the factor that caused their formation. Probably due to maternal risk factors, they are visible in some fetal aortas and infants in the first 6 months of life, but their number decreases in following years. They reappear in lesion-prone areas in adolescence period.

*Type III lesions*: Pathologic intimal thickening. The earliest progressive lesions are primarily composed of layers of smooth muscle cells in a proteoglycan-collagen matrix with an underlying acellular lipid pool rich in hyaluronan and proteoglycans. There is a variable accumulation of macrophages outside the lipid pool. These lesions are found in young adults.

*Type IV lesions:* Atheroma. The lipid core is evident with foam cells.

*Type V lesions:* Fibroatheroma. The lipid core is covered by a fibrous capsule. Necrotic core is present that is made up of cellular debris and this core is covered by a thick fibrous cap consisting of smooth muscle cells in a proteoglycan and collagen matrix. The fibrous cap is critical for the maintenance of the lesion.

*Type VI lesions:* Complicated lesions; intraplaque hemorrhage, fissures, erosions, or thrombosis.

Update to these lesions: type VII lesions, if calcification predominates; type VIII lesions if fibrosis predominates. The type IV lesions (atheroma) can evolve to any of the further stages. The progression does not need to be in a sequential manner [54]. The fate of plaque is determined by the following mechanisms: lipid retention rate, macrophage phenotype, inflammation, apoptosis and necrosis, smooth muscle cell proliferation, arterial remodeling, and stability of fibrous cap. Most of the plaques remain asymptomatic, and some become obstructive, while some of them due to complications of the plaque may elicit acute thrombosis, which present as acute coronary syndromes and stroke [55].

The higher LDL levels induce more progressive disease due to increased amount of lipid retention in the plaque. The modified and oxidized LDL exerts chronic stimulation of the immune system [56].

The phenotype of the recruited macrophages is important for the plaque progression. Macrophages with the M1-like phenotype, possibly *via* binding of modified LDL to the Toll-like receptors, secrete proinflammatory cytokines such as interleukin-1β and tumor necrosis factor-α and enzymes and reactive oxygen products, which promote further modification of LDLs. This type of proinflammatory phenotype also secretes the mediators that were demonstrated to have a role in atherosclerosis. In contrast, the macrophages with M2 phenotype secrete factors such as transforming growth factor and proresolving lipids, which cease the severity of inflammation [55, 57].

Apoptosis and secondary necrosis of foam cells and smooth muscle cells and impaired removal of the apoptotic remnants cause the formation of necrotic core in the atheroma. The enlargement of the necrotic core induces further plaque inflammation [55, 58].

New vessels can develop in the atherosclerotic lesion mainly originating from adventitial vasa vasorum. They provide an alternative entry way for the immunocytes. Intraplaque hemorrhages from these fragile vessels promote inflammation and lead to expansion of the necrotic core [59].

Smooth muscle cells of the plaque are characterized by presence of abundant secretory organelles. The contractile smooth muscle cells of the tunica media can migrate to intima and phenotypic modifications supervene in these cells. These synthetic phenotypes of smooth muscle cells increase in number with the lesion progression. The collagen, elastin, and proteoglycans of the plaque matrix are produced by these cells. Collagen-rich tissue becomes a dominant component of the plaque as the plaque expands [55].

The involved arterial segment tends to remodel in a way that does not allow the compromisation of the luminal area until plaque volume enlarges. This type of expansive remodeling is seen in fibroatheromas, and the extent of the enlargement is correlated with the plaque inflammation and necrotic core. Continued plaque growth with the shrinkage of the local vessel segment results in the stenosis of the vessel segment. This type of contrictive remodeled arterial segments contains lesions rich in fibrous tissue [55, 60].

#### *3.3.3 Acute clinical presentations of atherosclerosis: The vulnerable plaque*

Acute coronary syndromes and the vast majority of strokes are cases caused by luminal thrombi due to plaque rupture or a sudden plaque hemorrhage with or without vasopasms [61, 62]. The atherosclerotic plaque rupture occurs from the site where the cap is thinnest and most infiltrated by the foam cells. Plaque rupture is the most frequent cause of luminal thrombosis [61].

Ruptured plaques contain fewer smooth muscles cells and less collagen when compared with the intact plaques. And these lesions are demonstrated to be heavily infiltrated with macrophages rich in proteolytic activity suggesting the enhanced degradation of extracellular matrix elements. These two concurrent mechanisms leading to loss of supporting elements of the plaque are thought to explain the plaque rupture [63, 64].

Another mechanism leading to the intraluminal thrombus formation is the plaque erosion. The endothelial coating in these lesions is absent; however unlike the ruptured plaque the internal and external elastic lamina and contractile smooth muscle cells are present. The vasospasm of the involved arterial segment was suggested to be as a cause of endothelial damage and resulting thrombosis [65].

These two mechanisms whether the plaque rupture or the plaque erosion result in intraluminal thrombosis are comprised in a concept of a dynamic-active plaque that results in an acute clinical presentation: the vulnerable plaque. This term is used for the plaques to describe a group of histological features that are associated with plaque rupture and subsequent intraluminal thrombosis. The typical rupture-prone vulnerable plaque is the plaque with a thin fibrous cap containing a large necrotic core and infiltrated with abundant macrophages in the cap. Other features of this plaque include neovascularization, plaque hemorrhage, and adventitial inflammation [55, 66].
