Intimal Hyperplasia

Adaptive Intimal Hyperplasia

Vascular remodeling that occurs as normal adaptive responses is evident during the developmental process. It is essential to change fetal circulation to the post-fetal configuration. The functional change that directs blood through the pulmonary and systemic circulation is driven by changes in vascular resistance and hypoxia that leads to vasoconstriction of the ductus arteriosus and ductus venosus. These changes occur rapidly to eliminate blood flow through these vessels. The anatomic closure of these structures occurs over the subsequent few weeks. The hypoxia in these excluded vessels stimulates medial smooth muscle cell apoptosis followed by the release of growth factors that simulate intimal hyperplasia and fibrosis that result in the obliteration of these vessels.

During growth and aging, arteries increase in diameter and wall thickness. Between the ages of 20 and 90 years, the arterial intimal medial thickness increases nearly threefold even in the absence of atherosclerosis. ^, While medial thickness remains stable during the aging process, the intimal thickness gradually increases with age. Outward remodeling results in growth in lumen size despite the intimal hyperplasia, adapting to the changes associated with aging. It is thought that lower rates of intimal hyperplasia are consistent with less aging while higher rates suggest accelerated aging. In the presence of disease such as hypertension and diabetes, the intimal thickness and composition of the arterial wall change to include more extracellular matrix and result in increased stiffness.

Arterial Healing Response

Most forms of arterial injury arise from therapeutic interventions such as angioplasty or arterial bypass, which create local areas of vessel wall injury. Angioplasty creates a controlled focal injury to the vessel wall ( Figs. 5.1 and 5.2 ) and is the basis of a popular experimental model of intimal hyperplasia. The immediate response of the vessel to injury is to achieve hemostasis. Endothelial injury and denudation expose the subendothelial matrix which leads to platelet adherence and aggregation. Platelet accumulation occurs rapidly for approximately 8–10 hours after injury. The use of aspirin and other antiplatelet agents following therapeutic vascular interventions is aimed at reducing this early platelet response. Platelet levels on the vessel surface decrease from days 3 to 7. Platelets interact with subendothelial collagen through platelet membrane glycoprotein receptors (GPIb, GPIc/GPIIa, and GPIa/GPIIa), plasma von Willebrand factor, and fibronectin. Glycoprotein IIb/IIIa inhibitor drugs are used to reduce platelet aggregation following percutaneous coronary interventions. ^, The coagulation cascade (tissue factor, factor V, factor VIIa, factor Xa, or α-thrombin) is also activated and contributes to the initiation of intimal hyperplasia. Controlling the coagulation cascade early is key to preventing early stent thrombosis. Following injury, apoptosis (programmed cell death) can be identified in the cells in the arterial intima and media within 1 to 2 hours and abates after 4 hours. It then increases again by day 7 with 50% of medial cells undergoing apoptosis, potentially linked to increased proliferation at that time. By day 14, apoptosis is again markedly decreased.

Following these early events, a robust inflammatory response ensues with the recruitment of polymorphonucleocytes (PMNs) and monocytes to the site of injury by the adherent platelets, activated endothelial cells, and the exposed matrix and smooth muscle cells. There is a sequential expression of inducible cell surface molecules in both endothelial cells and smooth muscle cells after experimental angioplasty. ^, Compared with the normal endothelium, injured and activated endothelial cells express high levels of vascular cell adhesion molecule (VCAM) and intercellular adhesion molecule (ICAM). Within 10 days, smooth muscle cells express both ICAM and MHC class II antigens. The upregulation of these adhesion molecules returns to baseline levels by 30 days post-injury. Chemokines and their receptors participate at every step of the vascular remodeling process. These chemokines signal monocytes to infiltrate into the injured vessel wall, further stimulating inflammation and smooth muscle cell proliferation. The monocyte chemotactic protein (MCP)-1/CC motif receptor 2 (CCR2) axis induces monocyte infiltration and smooth muscle cell proliferation. The RANTES (regulated upon activation, normally T-cell expressed, and presumably secreted) receptors CCR1 and CCR5 also regulate monocyte infiltration and neointimal growth. Reendothelialization and intimal growth is mediated by the chemokine CXCL1, which is augmented by stromal cell-derived factor-1 alpha (SDF-1α) and its receptor CXCR4 that recruit circulating progenitor cells as well as medial smooth muscle cell progenitors to the subintimal location.

Cytokines including tumor necrosis factors (TNF), interleukins (IL), lymphokines, monokines, interferons, colony-stimulating factors, and transforming growth factors (TGFs) are produced by both infiltrating inflammatory cells and injured cells of the vessel wall. These cytokines propagate the inflammatory response, cell adhesion, proliferation, migration, and apoptosis. Cytokines are also linked to increased mitochondrial reactive oxygen species production, activation of Ca ²⁺ , and multiple intracellular protein kinase pathways. Cytokines interact with integrins and matrix metalloproteinases (MMPs) to modify extracellular matrix composition, a key component of the developing neointima. The inflammatory response to arterial injury can be modulated by inducing changes in macrophage phenotype with a reduction in intimal hyperplasia. Administration of low-dose inhaled carbon monoxide (CO) for a brief exposure prior to angioplasty injury results in a significantly attenuated intimal hyperplastic response by modifying leukocyte function. In humans, targeted therapy against IL1α showed a trend toward reduced restenosis following SFA interventions. Exogenous granulocyte colony-stimulating factor (GF) increased circulating endothelial progenitor cells (EP) and increased reendothelialization, resulting in reduced vascular inflammation and neointima size.

Medial smooth muscle cells are normally quiescent with <1% in a proliferative state. This increases to >20% within 48 hours after injury. The exact mechanisms by which vascular injury induces and promotes smooth muscle cell proliferation remains an area of investigation. The first phase of smooth muscle cell proliferation appears to be driven by basic fibroblast growth factor (bFGF) released from dead and damaged cells in the injured vessel. Extracellular matrix degradation by matrix metalloproteinases (MMPs) allows the medial smooth muscle cells to migrate into the subintimal space. Smooth muscle cell migration is regulated by receptor tyrosine kinase-linked agonists (PDGF, bFGF, and hepatocyte growth factor) and G-protein coupled receptor agonists (vascular endothelial cell growth factor, chemokines, LPA, thrombin, and uPA). Inhibition of the PDGF receptor PDGFR-β/β with blocking antibodies inhibited intimal hyperplasia in animal models but was not effective in human trials. Circulating mesenchymal stem cells also contribute to the subintimal cells, differentiating into smooth muscle cells.

Once within the subintima, the smooth muscle cells begin to proliferate around day 7 and reach a peak at 14 days before returning to baseline by 28 days when vascular healing is complete. However, proliferation may continue for up to 12 weeks in areas where reendothelialization is delayed. While the total number of neointimal smooth muscle cells stabilizes after 12 weeks, the neointima continues to grow through the elaboration of extracellular matrix (collagen and proteoglycans) by the smooth muscle cells. In human intimal lesions, hyaluron content was inversely related to collagen I and III staining. Little matrix accumulation occurs over the first 1 to 2 months following balloon injury but dramatically increases from 3 to 6 months in restenotic human coronary arteries. As collagen increases in the ECM, the intimal and adventitial smooth muscle cells shift to a contractile phenotype, leading to arterial contraction and “negative” remodeling.

Myofibroblasts represent an important cell population in intimal hyperplasia. A marked infiltrate of myofibroblasts, some derived from circulating mesenchymal stem cells, is observed by day 2 and may represent up to 50% of cells within the intima by day 14. ^, The presence of myofibroblasts is common in wound healing and contributes to wound contraction. A similar phenomenon may occur in the healing vessel. Injured vessels may undergo chronic elastic recoil or negative remodeling, reducing lumen size without increasing neointimal area. Retrieved atherectomy specimens from restenotic lesions showed low proliferative activity but the smooth muscle cells from these lesions exhibited elevated migratory activity and collagen synthesis. These findings support the important role of intimal remodeling in the final determination of luminal diameter. ^,

Traditionally, the regulation of vascular development and response to injury has been attributed to the endothelial cells and the medial smooth muscle cells in an “inside-out” fashion. The adventitia was believed to serve as structural support and transporting nutrients as well as maintaining sympathetic innervation to the vessel wall. It is now recognized as a source of cells that regulate all the layers of the vessel wall, supporting an “outside-in” hypothesis of vascular inflammation and healing. The cells in the adventitia include fibroblasts, smooth muscle cells, adipocytes, pericytes, and resident inflammatory cells. It is rich in stem and progenitor cells that can migrate and differentiate into a variety of cells in the media and neointima during vascular healing and disease. The adventitia is also home to the vasa vasorum, lymphatics and perivascular nerves. These cells and structures are embedded in an extensive network of extracellular matrix rich in collagen, elastic fiber nets, proteoglycans, fibronectins, and tenascin-c. The adventitial collagen fibers protect the vessel from over distention at high pressures while physiologic responses are mediated by the medial elastin layers. Evidence supporting outside-in healing is that early after injury, adventitial myofibroblasts express high levels of signals including monocyte chemoattractant protein-1 (MCP-1) that recruit and activate monocytes to the adventitia. Following arterial injury, adventitial fibroblasts become activated through sonic hedgehog signaling and increased reactive oxygen species production from upregulated NADPH oxidase and leads to increased neointima formation. The adventitial myofibroblasts, while contributing to the neointimal cells, are also important for negative remodeling through vascular contraction. Finally, aging results in increased resident inflammatory cells in the adventitia and may explain the greater susceptibility of aged arteries to atherosclerosis and intimal hyperplasia.