The importance of the lymphatic system in vascular disease

The lymphatic system plays a significant role in atherosclerosis. Clearance of oxidized cholesterol from the large and medium-sized arterial wall is dependent upon properly functioning intramural and adventitial lymphatic vessels. This function can be severely impeded by inflammation or augmented to increase lymphatic flow and improve fluidity by lifestyle changes such as diet, exercise, stress modification and toxin avoidance. In order to optimize cardiovascular health, the cycle of oxidation, inflammation and lymphatic dysfunction in the arterial wall must be interrupted by the known and well documented lifestyle modifications. This review cites recent scientific publications that support the importance of the lymphatic system in the reverse cholesterol transport. These works, compilated with those of beneficial integrative modalities, make a strong case for support of this concept. Incorporating the lymphatic system function and those lifestyle changes that affect it, further promotes a unifying concept that includes LDL concentration and density, endothelial dysfunction, intramural inflammation and lymphatic dysfunction, spasm or sclerosis in cardiovascular disease.


Introduction
The lymphatic system is clearly recognized as the immune domain, dealing with infections, tumor cells, waste products and toxins [1][2][3]. It is also critical in fluid balance between the vascular bed and interstitial tissue-since about 10% of the fluid exchange at the capillary bed remains in the tissue and must be returned to the vascular system by way of the lymphatics [4]. Lastly, and often less acknowledged, is its role in returning large molecules, proteins, lipids and lipoproteins to the vascular system from the tissue space. Among other larger molecules, Apolipoprotein A (APO-A), Apolipoprotein B (APO-B),high density lipoprotein cholesterol (HDL-C) and low density lipoprotein cholesterol (LDL-C) and their metabolites egressing the capillary bed are transported to the venous system by way of lymphatics [5].
The errant lipoproteins that randomly enter the intima of the arterial wall through dysfunctional endothelia also must be metabolized and cleared via lymph channels [6][7][8][9]. This presents a problem in the larger and midsized arteries because of the distance to lymphatics, natural barriers and impedance caused by inflammation [5]. Lymphatic function is necessary for the expeditious clearance of cholesterol from the arterial wall [10]. Cholesterol can be found throughout the entire thickness of the arterial wall, (including the adventitia where lymphatics abound) [11]. The lymphatic channels penetrate the large and mediumsized arteries in the middle to outer third of the media [12]. Cholesterol laden foam cells and HDL must penetrate the internal elastic lamina in order to reach these lymph vessels. In the past this was considered unlikely. However, recent research has shown that this indeed occurs [13][14][15][16][17][18][19][20][21][22]. Lymphatic endothelial cells express inflammation-producing proteins and peptides, augmenting the inflammatory cascade [23]. These cells are also responsive to inflammation producing molecules [24]. The lymph channels also contain autonomic nerve endings and contractile smooth muscle fibers making them intimately connected to the neuro-endocrine -systems [1]. Since the lymph system warehouses lymphocytes and is the immune cells' conduit, it exquisitely integrates the endocrine, nervous and immune system for biofeedback, signaling and mind -body interactions [25]. The lymph vessels can dilate or constrict [26]. They have rhythmic pulsation, can go into spasm or become sclerosed [27]. Lymph fluid [4] also has great rheological variability, as it can be more fluid in the sol phase, or more viscous in the gel phase. Its fluidity may be altered by water content and the concentration of formed elements. Its volume is changed by an increase head of pressure as seen in hypertension or anoxia which causes an alteration in capillary permeability with an increase in the filtrate and proteins into the interstitium [1].
Inadequate lymphatic clearance of lipoproteins from the arterial wall as a possible cause of atherogenesis was first postulated 35 years ago [1]. Although this concept could adequately explain the positive correlation with known risk factors for coronary artery disease and the negative correlation with high density lipoproteins and lifestyle modifications, little research was undertaken in this area [14,[28][29][30].The confluence of drainage from the arterial wall lymph with that of the capillary interstices affects the data when measuring the lipoprotein content in lymph vessels. The pathways for LDL, HDL, nascent HDL,APO-A APO-B and other lipoprotein's can be conflated as particles trapped in large and medium sized arterial walls and those transiting from the gut or para-capillary interstitium [31] are admixed. However, recent investigative work has clearly demonstrated the lymphatic systems involvement in reverse cholesterol transport [17][18][19][20][21][22].
In order to better understand this process, it is worthwhile to review cardiovascular pathophysiology and the role that inflammation, diet, exercise, stress and environmental toxins play in the genesis of atherosclerosis. LDL is the primary transporter of cholesterol to the tissues at the capillary level. There, it is an integral component for the manufacture of cell membranes, hormones, pro-vitamins, bile salts and other necessary metabolites [3]. Both the LDL and HDl pass across the peripheral microvascular endothelium by passive ultrafiltration [32]. However, in the large and midsized arteries, LDL-cholesterol [33] (especially the small dense particles) transgresses endothelium which has become dysfunctional or denuded [8,9], thus losing its integrity. This endothelial dysfunction is a consequence of turbulence, hypertension, aging, inflammation and metabolic disorders. The amount of cholesterol deposition in the intima is predicated by the serum level and density of LDL, the extent of endothelial damage, [6][7][8][9]34] inflammation, lymphatic clearance and transit time. Upon passing into the intima, the LDL rapidly becomes oxidized [35] initiating an endothelial inflammatory process that involves selectins, vascular cellular adhesion molecules and other pro-inflammatory components thus attracting monocytes which are transformed into macrophages after migration into the intima. The inflammatory process is further enhanced by their secretion of pro-inflammatory cytokines and chemokines [10]. The inflammatory process can be initiated by other than at oxidized LDL. Radical oxygen species, infection, homocysteine and Apo-B particles are examples of these initiators [36][37][38][39]. In an attempt to minimize this inflammation, the macrophages engulf the cholesterol and unless they can transfer the cholesterol to Apo-A HDL or migrate through the arterial wall, create further inflammation and deposition of cholesterol. The efficiency of cholesterol clearance from the sub intimal space is a key factor in the development of atherosclerosis [40]. The foam cells in the inflamed arterial wall manufacturer chemokines, cytokines and enzymes which immobilizes the macrophages and increase fibroblasts and collagen matrix further slowing down the cholesterols progression to the lymphatics. The discoid HDL is not able to transfer the cholesterol from the foam cells due to myeloperoxidase that is expressed by the macrophages [41]. This egress is further delayed by the expression of neuropeptides like netrin-1, Sema 3A, neuropeptide Substance P, and neuropeptide Y [42,43]. These neuropeptides create inflammation and prevent cholesterol transport. Netrin-1 is a neural guidance cue, secreted by the foam cells in the atheroma and is a powerful chemoattractant and smooth muscle cell recruiter and retardant to macrophage exit from the arterial wall to the lymphatics [44]. This suggests then, that foam cells play a critical but reversible role in atherogenesis [45]. Lipid laden macrophages can remove significant cholesterol amounts if unimpeded, but migration is inhibited by reactive oxygen species created by NADPH oxidase in the presence of oxidized LDL [35,44]. This creates a loss of plasticity of the macrophages cellular actin cytoskeleton. However, antioxidants like N-acetylcysteine, resveratrol and other NADPH oxidase inhibitors can restore normal plasticity [45]. Cholesterol clearance is also accomplished by transfer to nascent HDL through the adenosine triphosphate-binding cassette, ABC A1, transporter mechanism [41]. The work of Nordestgaard [28] shows that the majority of HDL cholesterol that entered the arterial wall, passes through the internal elastic lamina. This route is also supported by studies that show a higher percentage of cholesterol laden HDL in the lymph then in the plasma [13][14][15][16]. Increasing inflammation causes dysfunction of HDL, increasing tissue cellularity and viscosity, immobility and death of the foam cells initiating greater inflammation and creation of an atheroma. The longer the arterial wall is exposed to these inflammation-inducing molecules, the greater the tissue destruction and progression of atherosclerosis [13][14][15][16]39,46,47]. The vicious cycle of oxidation, inflammation and delayed lymphatic clearance creates an intriguing model to explain some of the negative and positive correlations to atherosclerosis. Lymphatic flow is increased with exercise, deep breathing, stress modification and a diet high in vegetables and is decreased with a sedentary lifestyle, a pro-inflammatory diet with reactive oxygen species, and stress [1]. These factors correlate both negatively and positively with the incidence of atherosclerosis [1,48].
There is much evidence for the cardiovascular protective effect of a diet high in fruits and vegetables [49][50][51][52]. Plants contain the lymph stimulating polyphenols and flavonoids. Lymphagogic products are essentially compounded flavonoids like Daflon, a combination of two flavonoids -hesperidin and Diosmin. These flavonoids reduce the expression of the ICAM-1, L-Selectin and VCAM-1 besides increasing the intensity and frequency of lymphatic contraction and increasing the total number of lymphatic capillaries. This results in the decrease of adhesion, migration, and activation of leukocytes, leading to lowering of prostaglandin's PGE2 and PGF2a and the reduction of radical oxygen species [53][54][55][56][57][58]. Vitamin D down regulates the inflammation of macrophages and monocytes and slows the ingress of monocytes and dendritic cells into the intima [59][60].
Exercise is essential for optimum cardiovascular health. Exercise considerably increases lymphatic circulation, positively affects antiinflammatory markers and reduces pro-inflammatory ones [61,62]. On its way to the venous system, the majority of lymph flows through the thoracic duct in the chest. Lymph flow through the chest cavity is significantly increase by rapid deep breathing [1] which creates larger diaphragmatic excursions, increases the negative intrathoracic pressure and stimulates lymphatic flow [63]. Changes of physical activity maximize lipid clearance and circulating dendritic and T cells which have been shown to be increasingly significant in cardiovascular disease [64][65][66][67]. Physical and mental stress are significant in the production of inflammation and atherosclerosis [25,68]. Stress increases the epinephrine and cortisol levels and chronic stress can lead to lymphatic sclerosis [68,69].
Incorporating the importance of the clearance of cholesterol and inflammatory metabolites in the arterial wall by the lymphatic system can adequately explain how lifestyle changes such as diet, exercise, stress modification and environmental toxin clearance can significantly improve cardiovascular health [70,71]. It also helps explain how mindbody interactions and biofeedback can also be of benefit through immuno-neuro-endocrine pathways. Incorporating the lymphatic system into this paradigm allows us to forward "The Unifying Concept of Atherosclerosis" [72] (Figure 1). This concept proposes that atherosclerosis is begun by initiating agents in the blood. These can be oxidized cholesterol, oxidized apoprotein's, homocysteine, bacterial and viral infections (i.e Heliobacter Pylori, dental caries, Chlamydia or Cytomegalic Inclusion Virus) [35][36][37][38] that enter the intima through a dysfunctional endothelial lining (e.g. elderly, hypertensives, diabetics). These create an inflammatory milieu which is countered by antiinflammatory homeostasis which includes the recruitment of antioxidants, anti-inflammatories, and enzymes (glutathione peroxidase, catalase and superoxide dismutase). Lymphatic clearance ensues and can be enhanced by positive lifestyle changes or impeded by neuropeptide production and persistent, unresolved chronic inflammation. Including the lymphatic system in an overview of atherogenesis can help us understand the disease process and more importantly, have greater confidence in recommending and incorporating positive lifestyle changes for improved outcomes in cardiovascular disease (Figure 1).