**2.2. Role of inflammatory cytokines and chemokines in the HIV-1-triggered endothelial dysfunction**

some structural and regulatory HIV-1 proteins released in the microenvironment by infected cells in driving inflammation and EC dysregulation. This finding highlights the need to target

Chronic inflammation contributes to many leading causes of death, and in particular cardiovascular events have emerged as a clinically significant issue and have become the matter of several studies. HIV-1 infection is characterized by altered immune responses leading to a generalized chronic inflammation and, in particular, to a pro-inflammatory status in the vascular endothelium fostering the development of cardiovascular diseases [1]. A strong correlation between high plasma HIV-1 RNA levels and signs of endothelial dysfunction is known [2], and subclinical signs of atherosclerosis have been found in asymptomatic HIV<sup>+</sup> young men with long-standing HIV-1 disease [3]. As the efficacy of combined antiretroviral therapy (cART) improves and patients live longer, the prevalence of cardiovascular diseases is

protease inhibitors, can cause dyslipidemia, thus contributing to the increased risk for endothelial dysfunction. The high risk of endothelial dysfunction persists even in new-generation antiretroviral drugs era, despite the fact that several adverse metabolic effects (e.g., insulin resistance, dyslipidemia, and hypertension) are abolished [6]. In light of these considerations, the following paragraphs consider three essential factors in the development and pathogenesis of endothelial dysfunction during the natural course of HIV-1 infection: (a) the ability of HIV-1 to promote inflammation, (b) the HIV-mediated damage of endothelium, and (c) the

Chronic activation of the immune system is a peculiar feature of HIV-1 infection. Persistent activation of immune cells is known to gain an elevated pro-inflammatory cytokine/chemokine release contributing to the development of a chronically inflamed microenvironment. HIV-1 virus cycle is dominated by a local replication at the transmission site and in local lymphoid tissues and then dissemination. Virus expansion is associated with a dramatic depletion of

plasma levels of pro-inflammatory cytokines and chemokines. During the early phase of infection, a pro-inflammatory cytokine storm contributes to the control of viral replication but also to the early immunopathology of the infection and to the associated long-term consequences. Many cell types contribute to the release of different pro-inflammatory cytokines and chemokines during HIV-1 infection [7] such as interferon (IFN)-α, tumor necrosis factor (TNF)-α, INFγ, interleukin (IL)-1β, IL-10, interferon gamma-induced protein (IP)-10, IL-15, IL-8, IL-6, IL-18, and monocyte chemoattractant protein (MCP)-1 [8, 9]. Antiretroviral therapy usually controls and even abolishes HIV-1 replication, but does not completely recover immune dysfunction.

T cells, particularly from gut-associated lymphoid tissues and with increased

patients even

capability of HIV-1 structural and regulatory proteins of affecting EC function.

Therefore, immune alteration and inflammation are common features of HIV<sup>+</sup>

**2.1. HIV-1 and inflammatory microenvironment**

individuals [4, 5]. Moreover, many antiretroviral drugs, particularly HIV-1

these viral proteins for therapeutic benefit.

348 Endothelial Dysfunction - Old Concepts and New Challenges

increasing in HIV<sup>+</sup>

memory CD4<sup>+</sup>

under successful cART.

**2. Endothelial dysfunction during HIV-1 infection**

Endothelial dysfunction and vascular diseases such as atherosclerosis and arterial damage are predominantly enhanced during a systemic chronic inflammatory status. Elevated levels of IL-6 have been associated with carotid atherosclerosis and progressive stenosis of the carotid artery, thereby upregulating the lipid uptake in macrophages and inhibiting the activity of lipoprotein lipase [10]. Increased carotid intima-media thickness (IMT) and hypertension are common features of patients with increased plasma levels of IL-18 [11], whereas TNF-α has a key role in promoting atherosclerosis, myocardial ischemia/reperfusion, and heart failure via several mechanisms: increased cholesterol uptake and foam cell formation in macrophages, augmented leukocyte transmigration in subendothelial structures, and increased proliferation and migration of vascular smooth muscle cells [12].

HIV-1 infection generates a systemic chronic inflammatory disorder as a result of continuous alteration of the immune response, contributing to dyslipidemia, EC dysfunction, vascular smooth muscle cell proliferation and migration, and, ultimately, the atherosclerotic plaque formation. The virus itself promotes the release of IL-6, IL-18, and TNF-α, together with IFN-γ, IL-1β, IL-10, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and macrophage colony-stimulating factor (M-CSF) by T cells and monocytes [13].

Liver-synthesized C-reactive protein (CRP) is a member of the pentraxin family factors and is considered a marker for coronary vascular disease and endothelial damage. CRP plasma levels are significantly upregulated in HIV<sup>+</sup> patients and inversely correlated with CD4<sup>+</sup> T lymphocyte count [14], and elevated CRP levels have been associated with an increased risk of myocardial infarction in HIV<sup>+</sup> patients [15]. It is noteworthy that increased levels of IL-6, IL-1, and TNF- α induce CRP, which in turn is able to activate pro-inflammatory cytokines such as IL-6 and M-CSF via a positive feedback loop.

The levels of cell adhesion molecules such as vascular cell adhesion protein 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1) are raised during HIV-1 infection, thus contributing to trans-endothelial migration of immune cells [16].

HIV-1 causes a continuous recruitment of monocytes that migrate across the endothelial barrier in blood vessels, differentiate into macrophages, and produce pro-inflammatory cytokines, thus determining the progressive damage of vessel structures. Furthermore, HIV-1 replicates in macrophages and induces activation and synthesis of several pro-inflammatory cytokines that in turn induce endothelial activation and leukocyte adhesion generating a positive feedback [17].

An important alteration in lipid metabolism is evident in more than 50% of HIV<sup>+</sup> patients. It likely relies on the upregulation of hepatic fatty acid synthesis and very low-density lipoprotein (VLDL) production, usually triggered by inflammatory cytokines as IFN-γ, TNF-α, and IL-1β [18]. At the same time, the continuous trans-endothelial migration of immune cells and their inhibited reverse transport determines the localization of monocytes inside the vessel wall and promotes the formation of foam cells, the fat-laden macrophages that are implicated in the buildup of an atheromatous plaque [17].

Monocytes, depending on the cytokine/chemokine stimulation, may differentiate into M1 macrophages, which promote inflammation or into M2 macrophages, which are inflammatory resolving cells [19]. In particular, IFN-γ and IL-1β drive monocytes to acquire an M1 profile, whereas IL-4 and IL-13 generate M2 macrophages. HIV-1, by infecting macrophages, polarizes these cells toward the M1 phenotype [20]. This leads to the imbalance of the M1/M2 ratio, a condition necessary for sustaining endothelial dysfunction [21].

**2.3. HIV-1-triggered damage of ECs**

side (gal)-C and its sulfated derivative.

the mechanism for vascular dysfunction.

all vascular repair mechanisms are impaired in HIV<sup>+</sup>

Along with reduced EPC levels, HIV<sup>+</sup>

sustain endothelial dysfunction [27]. HIV<sup>+</sup>

peripheral tissues and viral dissemination [28].

CXCR4 expressions.

HIV-1 is not an endothelium-tropic virus. It displays a narrow tropism predominantly determined by the cell surface receptors required for HIV-1 infection. CD4 and co-receptors are usually essential for HIV-1 to infect cells efficiently. The chemokine (C-C motif) receptor type 5 (CCR5) is the main co-receptor used in vivo, but variants that use another co-receptor, namely, chemokine (C-X-C motif) receptor type 4 (CXCR4), evolve during disease. In vitro, more than a dozen different co-receptors have been identified that support infection of cell lines by different HIV-1 strains. Moreover, HIV-1 particles interact with a range of cell surface receptors via interactions of its envelope glycoprotein gp120 with glycolipid galactocerebro-

HIV-1 capability to infect ECs in vitro depends on the tissue source of ECs and on their functional status. Microvascular ECs from the brain, kidney glomeruli, hepatic sinusoid, and bone marrow may be infected by HIV-1 in the absence of cytolysis [22, 23]. HIV-1 infection of brain ECs has been largely studied for its relevance in neurological diseases. T cell tropic but not brain-derived macrophage tropic HIV-1 strains selectively infect the brain endothelium in vitro, suggesting that T cell tropism may be important for HIV-1 entry through the bloodbrain barrier [22] and spreading in the central nervous system [24]. However, it is important to underline that in vivo studies do not support the presence of replicating virus in ECs. Even if HIV-1 infection of ECs cannot be completely ruled out, this may suggest an indirect action of molecules released in the microenvironment by HIV-1-infected cells at the base of

In the pathophysiology of cardiovascular disease, the damage of ECs assessed by responses to altered blood flow (e.g., flow-mediated dilatation) and differences in the levels of EC specific molecules released in the blood (e.g., von Willebrand factor) represent a hallmark. The equilibrium between the mechanisms of vascular damage and repair plays a crucial role during homeostasis of vascular integrity. Following a blood vessel injury, high levels of circulating ECs (cECs) and microvesicles are released from endothelium, and the reinstatement of the vascular integrity mainly implies activity of endothelial progenitor cells (EPCs), plaque neovascularization, and reverse cholesterol transport [25]. EPCs are key determinants of endothelial dysfunction and show a high predictive value of early vascular disease. Interestingly,

than HIV-1-seronegative subjects [26]. Decrease in the number of EPCs is attributed to HIV-1, which seems to be able to infect these cells because of their chemokine receptor CCR5 and

microvesicles also known as microparticles that are small membranous structures released from ECs during apoptosis, which impair the restoration of physiological conditions and

of high sensitivity C-reactive protein (hsCRP), IL-6, TNF-α, D-dimer, fibrinogen, soluble ICAM, and VCAM, suggesting endothelial activation and damage. These molecules are also responsible for an increased interaction of infected monocytes with ECs, thereby disrupting the integrity of the EC monolayer and promoting extravasation of HIV-1-infected cells into

individuals who have lower EPC levels

Endothelial Cell Dysfunction in HIV-1 Infection http://dx.doi.org/10.5772/intechopen.73023 351

individuals show high plasma levels of EC-derived

patients also exhibit high plasma concentrations

Endothelin-1 (ET-1) is a potent vasoconstrictor that promotes migration and proliferation of smooth muscle cells. HIV-1-triggered secretion of ET-1 promotes a reduction of vascular nitric oxide (NO) production by ECs with the consequent proliferation and migration of smooth muscle cells leading to arterial vasoconstriction.

Altogether, these findings suggest that HIV-1 produces a general inflammatory microenvironment that contributes to dyslipidemia, EC dysfunction, chemotaxis, and vascular smooth muscle cell proliferation and migration. All these conditions are likely to foster endothelial degeneration and atherosclerotic plaque formation (**Figure 1**).

**Figure 1.** HIV-1 capability to promote inflammation, dyslipidemia, and endothelial dysfunction through the activation of different immune cells such as T and B cells, macrophages, and natural killer cells (NK cells).
