Annals of Clinical & Laboratory Science 39:3-16 (2009)
© 2009 Association of Clinical Scientists
Vulnerable Plaque Formation from Obstruction of Vasa Vasorum by Homocysteinylated and Oxidized Lipoprotein Aggregates Complexed with Microbial Remnants and LDL Autoantibodies
Uffe Ravnskov1 and
Kilmer S. McCully2,3
1 Independent Investigator, Magle Stora Kyrkogata 9, 22350 Lund, Sweden; 2 Pathology and Laboratory Medicine Service, Boston Veterans Affairs Healthcare System, West Roxbury, MA; 3 Department of Pathology, Harvard Medical School, Boston MA, USA.
Address correspondence to Kilmer S. McCully, M.D., Veterans Affairs Medical Center, West Roxbury, MA 02132, USA; tel 857 203 5990; fax 857 203 5623; email kilmer. mccully{at}med.va.gov.
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Abstract
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Little attention has been paid to the function of lipoproteins as part of a nonspecific immune defense system that binds and inactivates microbes and their toxins effectively by complex formation. Because of high extra-capillary tissue pressure, aggregates of such complexes may be trapped in vasa vasorum of the major arteries. This complex formation and aggregation may be enhanced by hyperhomocysteinemia, because homocysteine thiolactone reacts with the free amino groups of apo-B to form homocysteinylated low-density lipoprotein (LDL), which is subject to spontaneous precipitation in vitro. Obstruction of the circulation in vasa vasorum, caused by the aggregated complexes, may result in local ischemia in the arterial wall, intramural cell death, bursting of the capillary, and escape of microorganisms into the intima, all of which lead to inflammation and creation of vulnerable plaques. The presence of homocysteinylated LDL and oxidized LDL stimulates production of LDL autoantibodies, which may start a vicious circle by increasing the complex formation and aggregation of lipoproteins. The content of necrotic debris and leukocytes and the higher temperature than its surroundings give the vulnerable plaque some characteristics of a micro-abscess that by rupturing may initiate an occluding thrombosis. This suggested chain of events explains why many of the clinical symptoms and laboratory findings in acute myocardial infarction are similar to those seen in infectious diseases. It explains the presence of microorganisms in atherosclerotic plaques and why bacteriemia and sepsis are often seen in myocardial infarction complicated with cardiogenic shock. It explains the many associations between infections and cardiovascular disease. And it explains why cholesterol accumulates in the arterial wall. Some risk factors may not cause vascular disease directly, but they may impair the immune system, promote microbial growth, or cause hyperhomocysteinemia, leading to vulnerable plaques.
Keywords: vulnerable plaque, lipoprotein aggregates, vasa vasorum, hyperhomocysteinemia, microbial remnants, autoimmunity, oxidized LDL
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Introduction.
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There is general agreement that atherosclerosis begins as an inflammatory process in the arterial wall, and also that rupture of a vulnerable plaque is the starting point for the creation of the occluding thrombus in myocardial infarction and ischemic stroke [1,2]. Therefore, any hypothesis about the cause of atherosclerosis and its consequences must necessarily be able to point to the origin of the inflammation and to explain how a vulnerable plaque is created [3].
According to the current view, the first step is endothelial dysfunction or damage caused by hypercholesterolemia, hyperhomocysteinemia, or other toxic factors in the circulation, allowing the migration of LDL, cholesterol, and monocytes into the arterial wall. LDL is modified by oxidation, leading to an accumulation of T-cells and the production of LDL autoantibodies. Modified LDL is taken up by macrophages that are converted to lipid-laden foam cells, considered as the early lesion of atherosclerosis. The inflammatory process, probably aggravated by antigens from microbes such as Chlamydia, Herpes simplex and Cytomegalo-virus, is followed by smooth muscle cell proliferation and the synthesis of extracellular matrix. The macrophages may become overloaded with lipids and die, resulting in the creation of a vulnerable plaque that by rupturing initiates the formation of an occluding thrombus [4].
This suggested chain of events is based mainly on epidemiological observations and experimental models, where vascular changes similar to human atherosclerosis have been produced in rodents with inherited or dietary hypercholesterolemia. However, it conflicts with many clinical, epidemiological, pathological, and experimental observations. There are in particular six disturbing facts:
- The concept that high LDL cholesterol causes endothelial dysfunction is unlikely because there is no association between the concentration of LDL cholesterol in the blood and the degree of endothelial dysfunction [5].
- The concept that endothelial damage leads to influx of LDL cholesterol is unlikely as well, because the atherosclerotic plaques seen in extreme hyper-homocysteinemia caused by inborn errors of methionine metabolism do not contain any lipids in spite of pronounced endothelial damage [6,7].
- No study of unselected individuals has found an association between the concentration of LDL or total cholesterol in the blood and the degree of atherosclerosis at autopsy [8].
- In studies of women and the elderly, hypercholesterolemia is a weak risk factor for cardiovascular disease, or, in most cases, not a risk factor at all [9], although the large majority of cardiovascular deaths occur in people above 65 years of age.
- Among individuals with familial hypercholesterolemia (FH) there is no association between LDL-cholesterol and the prevalence or the progress of cardiovascular disease [10–15]. The higher coronary mortality in young people with FH may instead be due to inherited abnormalities of the coagulation system, often seen in FH and a strong risk factor for coronary heart disease in this population [15,16].
- With one exception [17], an occluding coronary thrombus has never been produced experimentally in rodents by hypercholesterolemia alone [3], indicating that the pathological process in these models may differ from that in human beings.
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Origin of vulnerable plaques.
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In the following discussion we present a new interpretation of the origin of vulnerable plaques that we think is in better agreement with presently available evidence. This interpretation is based on the fact that the lipoproteins function as a nonspecific immune system that binds and inactivates microorganisms and their toxins by complex formation. In the case of a massive microbial invasion, these complexes may aggregate, in particular in the presence of hyperhomocysteinemia, because homocysteine thiolactone causes aggregation and precipitation of thiolated LDL [18]. Complex formation and aggregation may also be enhanced by autoantibodies against thiolated LDL and oxidized LDL. Because of high extra-capillary tissue pressure, the aggregates may be trapped in arterial vasa vasorum, resulting in local vascular ischemia, intramural cell death, and the creation of vulnerable plaques.
Such plaques have many characteristics of a micro-abscess, which, by rupturing, initiates the occluding thrombosis and releases its content of infectious material into the circulation and the myocardium. This suggested chain of events explains why many of the clinical symptoms and laboratory findings in acute myocardial infarction are similar to those seen in infectious diseases. It also explains the frequent presence of microbial remnants in atherosclerotic plaques, the many associations between infections and cardiovascular disease, the similarities between myocarditis and myocardial infarction, and why cholesterol accumulates in the arterial wall.
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The microbial hypothesis.
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A century ago, bacteria and viruses were considered as the main cause of atherosclerosis, a view that was based mainly on post-mortem observations. Thus, Thayer reported a high frequency of arterial lesions in patients who died from typhoid fever and a high prevalence of hardened radial arteries in those who survived [19]. Wiesel found an association between the degree of atherosclerosis in people who had died from an infectious disease and the length of the preceding infection [20], and Osler described the vulnerable plaque as an atherosclerotic pustule [21]. The following statement by Klotz and Manning is typical for the general view at that time: "There is every indication that the production of tissue in the intima is the result of a direct irritation of that tissue by the presence of infection or toxins" [22]. The molecular mechanisms were unknown and because of the chemical composition of advanced atherosclerotic plaques, more recent research has instead focused on cholesterol.
However, in addition to and in accordance with the older findings, much epidemiological, clinical, laboratory, and experimental evidence has more recently been reported, suggesting that infectious processes may play a role in cardiovascular disease [23–27]. Cardiovascular mortality increases during influenza epidemics [28]. A third of patients with acute myocardial infarction or stroke have had an infectious disease immediately before onset [29]. Bacteriemia and periodontal infections are associated with an increased risk of cardiovascular disease [30,31]. Serological markers of infection are often elevated in patients with cardiovascular disease and are also risk factors for such diseases [32]. A role of infectious agents is suggested by the narrowing of the coronary arteries seen in children who died from an infectious disease [33] and from thickening of carotid intima-media on high-resolution ultrasound in those who survived [34].
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The lipoprotein immune system.
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A normal serum factor is able to neutralize the hemolytic effects of streptolysin-S, and, for this reason, the factor was named antistreptolysin-S and was previously considered to be an antibody. However, this concept was questioned in 1939 by Todd et al, who found that this serum factor did not behave as a normal antibody because its titer fell below normal values in patients with rheumatic fever at the peak of the clinical symptoms [35]. A few years later, Stollerman and Bernheimer also found that, in contrast to the antistreptococcal antibodies, the antistreptolysin-S titer did not rise above its normal level during convalescence [36]. At the same time, Humphrey discovered that antistreptolysin-S was located within the lipid fraction of the blood [37]. Stollerman et al identified antistreptolysin-S as a phospholipoprotein complex [38]. Since then, at least a dozen research groups have established that antistreptolysin-S is identical with the lipoproteins and constitutes a nonspecific host defense system, able to bind and inactivate not only streptolysin-S, but also other endotoxins and several virus species [39–55] (Table 1
). In rodents, cholesterol is mostly transported by high-density lipoprotein (HDL), and in these species HDL has the main protective effect [42,43], whereas human studies have generally found that all lipoproteins participate in the nonspecific defense system.
Most investigators have identified the immunoprotective role of the lipoproteins by demonstrating inhibition of the biological effects of various microorganisms and endotoxins, such as hemagglutination, hemolysis, the cytokine response of human monocytes, and virus replication. Skarnes first suggested that the lipoproteins also form complexes with microbial products [39]. By using immunodiffusion with anti-endotoxin and serum from various rodents that had been injected with Salmonella enteridis endotoxin, he demonstrated lipoprotein-positive staining and esterase activity on the precipitation lines.
Using crossed immunoelectrophoresis, Freudenberg et al found that the HDL peak of rat plasma changed position after injection with various lipopolysaccharides (LPS); they concluded that the effect was due to the formation of a complex between LPS and HDL [42]. By separating a mixture of rabbit plasma and LPS from Salmonella minnesota by column chromatography with sepharose linked with LPS antibody, Ulevitch et al found that the eluate from the bound material contained both LPS and apoprotein A1, the major protein of rabbit HDL [43]. There is strong evidence that human lipoproteins complex with microbial components as well. By electron microscopy (EM) Bhakdi et al found that the inactivation of Staphylococcus aureus alpha-toxin by purified human LDL led to oligomerization of 3S native toxin molecules into ring structures of 11S hexamers that adhered to the LDL molecules [44].
Lipoproteins also form complexes with viruses. Huemer et al found that all lipoprotein subclasses were able to bind purified Herpes simplex virus, as demonstrated by EM, enzyme-linked immunoabsorbence assay, and column chromatography [47]. Superti et al confirmed that all human subclasses of lipoproteins were able to inhibit the infectivity and hemagglutination by SA-11 rota-virus, and complex formation was visualized by EM [51].
The lipoprotein immune system may be particularly important in early childhood as, in contrast to antibody-producing cells, this system works immediately and with high efficiency. For instance, human LDL inactivated up to 90% of Staphylococcus aureus alpha-toxin [44], and it inactivated an even larger fraction of bacterial lipopolysaccharide (LPS) [48]. In agreement with these findings, hypocholesterolemic rats injected with LPS had a markedly increased mortality compared with normal rats, which could be ameliorated by injecting purified human LDL [54]. On the other hand, hypercholesterolemic mice challenged with LPS or live bacteria had an eightfold increase of LD50, compared with normal mice [55].
Hudgins et al demonstrated that high-molecular weight lipoproteins not only bind LPS, but lipoproteins disappear from the general circulation in infected human beings [56]. They injected a small dose of LPS in normal volunteers and demonstrated the expected rise of the usual inflammatory markers and a fall of total cholesterol, LDL-cholesterol and apo-B, whereas concentrations of HDL-cholesterol and apo-A1 were unchanged.
The formation of complexes between lipoproteins and microbial products may lead to aggregation of lipoprotein particles. In case of a massive invasion of microorganisms, the size of such aggregates, especially those composed of the high-molecular weight VLDL and LDL, may impede their passage through capillary networks, in particular the vasa vasorum of the artery walls, because of high extra-capillary tissue pressure. Indeed, aggregated lipid structures similar to the size of LDL have been demonstrated by electron microscopy in the extracellular space beneath fatty streaks [57].
Recent reviews [58,59] summarized the evidence that both LPS and lipoteichoic acid (the Gram-positive counterpart of LPS) form aggregates in solution. In addition, sphingolipids interact with bacterial toxins, and all lipoproteins isolated from animals treated with LPS contain high levels of sphingolipids (ceramide), which promote lipoprotein aggregation.
An unsettled question concerns the nature of the process that converts macrophages into lipid-laden foam cells, one of the main factors in production of atherosclerotic lesions. Normally excess cellular uptake of cholesterol is counteracted by down-regulation of the LDL receptor, indicating that another pathway must be responsible for foam cell formation. According to the current view, oxidized LDL cholesterol in the arterial wall is taken up by the scavenger receptor of macrophages, allowing an unlimited uptake of cholesterol, independent of the LDL receptor. However, macrophages also take up aggregated LDL by phagocytosis after modification by vortexing or by digestion with phospholipase C [60]. LDL that is modified by complex binding with microbial products is also taken up by the same process, because in vitro experiments have shown that LPS from Chlamydia pneumoniae [61] and also from several periodontal pathogens [62] is able to convert macrophages to foam cells in the presence of human LDL.
A direct attack of microorganisms or their products on the endothelium, as often suggested, seems unlikely, as demonstrated by Madjid et al [63]. In a post-mortem study of 27 patients with coronary atherosclerosis, 14 of whom had had a systemic infection within two weeks before death, luminal coronary thromboses and myocardial infarction were found in 5 of the infected patients. They found that the number of macrophages in the infected group was much greater in the adventitia than around the plaques, whereas no difference was noted in the uninfected control group, which suggests that the microbes arrive via vasa vasorum. In agreement with this view, Guyton et al found that extracellular lipid deposits are almost entirely located deep within the intima, close to the vasa vasorum and well below most of the foam cell lipid [57]. This finding opposes the view that the lipid-rich core region of plaques originates primarily from the debris of dead intimal foam cells, but the finding agrees with the spontaneous atherothrombosis observed in genetic double knockout mice [64]. These thrombi were demonstrated on the surface of atherosclerotic lesions similar to human vulnerable plaques, accompanied by marked medial degeneration and invasion of inflammatory cells into the adventitia.
During the oxidative breakdown of microbial material inside macrophages, cholesterol is partially oxidized and returned to the liver by HDL, and the cholesterol content of fibrous plaques is not higher than in normal arterial tissue [65]. Indeed, several HDL processes that are able to convert oxidized LDL cholesterol to free cholesterol have been identified [66]. Also, esterified cholesterol may be converted to free cholesterol by microbial processes [67] and deposited as extracellular cholesterol crystals found deep within the intima [57].
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Hyperhomocysteinemia and autoimmunity.
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Homocysteine thiolactone, the reactive cyclic anhydride of homocysteine, reacts with free amino groups of protein to form peptide-bound homocysteine [68]. The process of homocysteinylation of proteins is termed thiolation, because this reaction produces a free sulfhydryl group within the peptide-bound homocysteine molecule. Homocysteine thiolactone reacts with the free amino groups of apoB protein of LDL [69]. When an increased concentration of homocysteine thiolactone reacts with human LDL, the thiolated LDL becomes aggregated and subject to spontaneous precipitation [18]. LDL aggregates are phagocytosed by cultured human macrophages, forming foam cells with greatly increased cholesterol and cholesterol ester content.
It was suggested [18] that thiolation of LDL would also alter its antigenic properties and lead to autoantibody formation. Ferguson et al showed that thiolated LDL is immunogenic in rabbits, producing a polyclonal antibody recognizing thiolated LDL [70]. Antibodies to N-thiolated serum albumin were demonstrated in patients with coronary heart disease [71,72]. Thiolated LDL is present in human serum at low concentration (0.04–0.1%), but autoantibodies to human thiolated LDL have not been reported [73].
The possibility that autoantibodies against thiolated LDL may play a role in the creation of atherosclerosis is suggested by other observations. Hyperhomocysteinemia, a potent risk factor for atherosclerosis, is found in autoimmune diseases, such as lupus erythematosus, rheumatoid arthritis, Behcet’s disease, inflammatory bowel disease, and myelodysplastic syndrome [74]. These diseases all are characterized by increased susceptibility to vascular disease and activation of immunity and inflammation. Homocysteine activates cytokines and pro-inflammatory molecules, such as IL-1beta, IL-6, IL-12, IL-18, IL-1 receptor antagonist, C-reactive protein (CRP), adhesion molecules (P-selectin, E-selectin, ICAM-1), and metalloproteinases (MMP-9). Homocysteine up-regulates reactive oxygen species, leading to NF-kappaB activation [74]. CRP binds oxidized LDL and oxidized phospholipids, enhancing phagocytosis to form foam cells [75].
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Oxidized LDL and autoimmunity.
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Oxidized LDL (OxLDL) has long been considered as the main culprit in atherosclerosis. OxLDL stimulates the production of autoantibodies, but the role of anti-OxLDL has been controversial because its titer does not reflect or predict cardiovascular disease [76–80]. We envision that anti-OxLDL antibodies may aggregate and participate in the obstruction of vasa vasorum. Therefore, the reason the titer of anti-OxLDL does not reflect cardiovascular disease may be that the expected increased level of anti-OxLDL in patients with cardiovascular disease is counteracted by a decrease in anti-OxLDL level because of the accumulation and aggregation of circulating anti-OxLDL within vasa vasorum of arteries. In support of this concept, Schumacher et al found that patients with acute myocardial infarction and a marked elevation of plasma creatine kinase had a significant decrease of anti-OxLDL during the acute phase, whereas this phenomenon was not seen in patients with only a minor elevation of creatine kinase [81]. Su et al found an inverse association between the concentration of anti-OxLDL and progress of atherosclerosis in hypertensive patients, measured as change of the maximum carotid intima-media thickness, suggesting that anti-OxLDL is protective against atherogenesis [82]. This interpretation may be correct in healthy, non-infected people without hyperhomocysteinemia. However, the association may also be explained by the disappearance from the circulation of anti-OxLDL immune complexes by their aggregation with LDL within vasa vasorum, because the association was significant for IgM subclasses only, and the much larger size of such complexes may render them more susceptible to aggregation. This interpretation may also explain the recent finding that low levels of IgM antibodies against phosphorylcholine, a component of inflammatory phospholipids known to cause OxLDL-related immune reactions, are associated with a greater risk of ischemic stroke [83].
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Creation of the vulnerable plaque.
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Obstruction of the vasa vasorum by aggregated lipoprotein complexes may increase the vulnerability of the cells that they nourish and lead to cell death because of localized ischemia of the vascular wall. Vasa vasorum may rupture, and the aggregated LDL particles with their load of microbial products will enter the arterial wall. These products may include living microorganisms, because viable Chlamydia pneumoniae have been cultured from atherosclerotic plaques by Ramirez [84] and Jackson et al [85]. Probably this is a common phenomenon, because Maass et al identified viable Chlamydia pneumoniae in 11 of 70 atheromas, whereas none was present in 17 non-atherosclerotic control samples [86]. The presence of Chlamydia pneumoniae in human coronary plaques was confirmed by electron microscopy [87,88]. These organisms were also demonstrated within adventitia by immunohistochemical staining and polymerase chain reaction (PCR) for microbial DNA, presumably arriving via monocytes migrating from vasa vasorum [89]. Other living microorganisms may be present as well, but to our knowledge no successful isolations from human plaques have been reported.
Indirect evidence of a role of living micro-organisms in the creation of vulnerable plaques was presented by Grattan et al [90]. They found graft failure because of accelerated atherosclerosis in two-thirds of 91 cardiac transplant patients infected with Cytomegalovirus, but only in one-third of 209 non-infected patients.
With a healthy immune system, the micro-organisms may be eliminated, new capillaries will enter the lesion, and reparative processes will convert the dead tissue into a stable, fibrous plaque. But in case of an insufficient clearing of the microorganisms and the ensuing inflammatory response, cell death may accelerate and impede repair processes, creating a vulnerable plaque, the preferential site for occluding thrombi [91]. The suggested chain of events is illustrated in Fig. 1 and in a flow chart (page 10).
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Fig. 1. Development of the vulnerable plaque. The small globules inside the vasa vasorum and in the vulnerable plaque represent lipoproteins; the black dots represent microorganisms, endotoxins, anti-OxLDL autoantibodies, and anti-thiolated-LDL autoantibodies; the large globules at the basal part of the vulnerable plaque and inside the macrophages represent lipid droplets. The right capillary represents the situation in a normal healthy artery; there are only a few microbes and the lipoproteins are able to traverse the capillary lumen without adherence or obstruction. The left capillary represents the situation in an artery with a severe microbial invasion; microbial products and autoantibodies stick to the lipoproteins, which aggregate and obstruct the capillary lumen, leading to local ischemia, microbial growth, and inflammation. A monocyte enters the plaque from the arterial lumen by diapedesis between endothelial cells; another monocyte enters the plaque via vasa vasorum, leading to formation of foam cell macrophages within the plaque. In the case of an intact immune system, the inflammatory area heals and becomes converted to a fibrous plaque. In the case of an insufficient immune system, microorganisms escape into the tissue and create a microabscess, the vulnerable plaque.
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Clinical and pathological observations.
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According to our hypothesis, LDL-cholesterol does not enter the artery through the endothelium as suggested previously, but via the capillary web of vasa vasorum in and around the arterial walls. Oxidation of LDL does not take place before LDL has entered the macrophage but occurs after phagocytosis, as part of a normal physiological process explaining why attempts to prevent cardiovascular disease by antioxidants have been largely unsuccessful.
Some reasons for considering the vulnerable plaque to be a type of micro-abscess are that more than one plaque may occur simultaneously [91,92], and their temperature is higher than that of the surrounding tissue [93]. Whereas neutrophilic polymorphonuclear leukocytes, the hallmark of pyogenic infections, are rare in stable plaques, they are always found in and around the core of vulnerable plaques, and there are just as many neutrophils in the intact as in the ruptured plaques [94], contradicting the assumption that their presence is secondary to rupture.
Our interpretation explains the clinical and laboratory similarities between myocardial infarction and myocarditis [95], and it explains the frequent occurrence of bacteriemia and sepsis in myocardial infarction complicated with cardiogenic shock [96]. It explains why fever, diaphoresis, leukocytosis and elevation of inflammatory markers in the blood, including CRP, the classical symptoms of an infectious disease, are common findings in myocardial infarction. Chronic elevation of CRP in patients with atherosclerosis is a risk factor for myocardial infarction. Our interpretation agrees with the almost constant finding of polymorpho-nuclear leukocytes in the myocardium in acute myocardial infarction, as well as in infarctions of other organs. It also explains a recent report of Chlamydia pneumoniae antigens within cardiomyocytes of patients with fatal myocardial infarction [97], an observation needing corroboration from future studies.
Fatty streaks are not necessarily the precursors of atherosclerotic plaques. Fatty streaks are present in the fetus and are more frequent in early than late childhood [98,99], presumably reflecting a normal and reversible response to infections.
Hydrodynamic pressure is usually cited as the reason that atherosclerosis is localized only within systemic arteries. This explanation is probably correct, not because the arterial pressure damages the endothelium, but because the lipoprotein complexes are trapped more easily in vasa vasorum of the systemic arteries where the tissue pressure is much higher than within vasa vasorum around the veins and the pulmonary arteries. By the same reasoning, atherosclerotic plaques are localized to areas of the intimal surface where the hydrodynamic forces, turbulence of blood flow, and tissue pressure are especially high, namely at the branching points of arteries, within tortuous arteries, and within coronary arteries that are compressed by myocardial contractions. Whereas normal pulmonary arteries are generally free of atherosclerosis, they develop atherosclerotic intimal plaques in various conditions that lead to pulmonary hypertension. Current concepts of the anatomy and physiology of vasa vasorum [100] emphasize that these vessels are functionally end arteries, supplying the media to a depth where blood flow and patency are compressed by pressure transmitted from the arterial lumen.
The predilection for plaques within systemic arteries also contradicts the idea that microbes attack the endothelium directly, because if this were so, atherosclerosis would be just as common in veins. Also the focal occurrence of atherosclerotic lesions is in better accordance with a microbial genesis, because if elevated LDL cholesterol were the most important cause, atherosclerosis should be a more generalized disease.
The increased incidence of cardiovascular events found after treatment with rofecoxib and other non-steroidal anti-inflammatory drugs [101] contradicts the idea that atherosclerosis is caused by the inflammation itself, but it is in accord with an infectious origin of atherosclerosis, where inflammation is a necessary step for healing. The ability of HMG-coenzyme A reductase inhibitors (statins) to prevent cardiovascular disease, in spite of their non-steroidal anti-inflammatory properties, is probably attributable to their other pleiotropic effects, including the enhancement of fibrinolysis and nitric oxide production, and the inhibition of platelet activation.
An apparent contradiction to our interpretation is that prevention of cardiovascular disease by antibiotics has been largely unsuccessful. However, in these trials patients have usually received a single antibiotic, chosen because it was effective against Chlamydia pneumoniae, the organism that has been studied most intensively, and the trials have been of relatively short duration.
Chlamydia pneumoniae is not the only microbe that is found in atherosclerotic plaques. Ott et al identified fragments from >50 different microbial species within atherosclerotic plaques, but not a single one in normal arterial tissue [102]. On average, each patient had microbial remnants from 12 different species; some patients had more, some had fewer [102], and other investigators have found various virus species as well [103–105]. It is highly unlikely that a single antibiotic could eliminate >50 different microbial species. It is not even likely that antibiotics could eliminate Chlamydia pneumoniae, because this species is able to survive inside living cells, where they are resistant to the effects of antibiotics [106]. Furthermore, antibiotics are generally ineffective against viral infections. Whether the total burden of multiple microbial invasions or the effect of a single pathogen is the key to progression remains to be determined [107].
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Evidence that high cholesterol is protective.
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Since LDL participates in the immune system, high plasma cholesterol concentrations should be an advantage to survival, not a risk. There is much evidence that high cholesterol is protective against infectious diseases. Plasma cholesterol levels have been found to be inversely associated with total mortality in the elderly and with mortality from respiratory and gastrointestinal diseases [9], most of which have an infectious origin. Cholesterol levels are also inversely associated with mortality after post-operative abdominal infections, inversely associated with the risk of being admitted to hospital because of an infectious disease, and inversely associated with the risk of contracting HIV and AIDS [9].
The protective effect of plasma cholesterol levels is also supported by observations in patients with inherited disorders of cholesterol metabolism. Before the year 1900, when infectious disease was the commonest cause of death, the life span of people with 50% risk of having familial hypercholesterolemia (FH) was longer than in the general population [108]. The frequent and severe infections in children with the extremely low cholesterol levels that are found in Smith-Lemli-Opitz syndrome are alleviated by addition of cholesterol to the diet [109].
The lack of an association between the degree of cholesterol lowering and outcome that were found in clinical and angiographic trials [8] could be explained if the benefits from HMG-coenzyme A reductase inhibitors (statins) were due to their pleiotropic effects and not to their inhibition of the cholesterol synthesis. Even if the lowering of LDL cholesterol by these drugs were unimportant, there should have been an exposure-response relationship between LDL-cholesterol and outcome, because both the pleiotropic effects and cholesterol lowering are caused by inhibition of the mevalonate pathway. A more complete blockage of the mevalonate pathway should result in stronger pleiotropic effects and a more pronounced lowering of cholesterol, and vice versa. As this was not the case, the findings imply that high cholesterol is protective and that its lowering therefore counteracts exposure-response. This view is in accordance with the trial findings and our present interpretation of these findings.
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Similar events in other arteries.
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If an imbalance between the microbial burden and the immune system contributes to coronary heart disease, other parts of the artery system should be affected as well, and this seems to be true. Stroke and myocardial infarction commonly occur in the same patient, and vulnerable plaques in the carotid arteries are the starting point of thrombosis in cerebral infarcts [110]. In a consecutive study of the common iliac, common carotid, and renal arteries of 49 patients who died in a hospital, those with a history of cardiovascular events had 2–4 times more intimal macrophages and a denser network of vasa vasorum in all of the arteries than atherosclerotic patients without cardiovascular events [111]. Foam cells have been identified adjacent to Bruch’s membrane of the retina, where their number increases with the age of patients [112]. Foam cells are also found in sclerotic glomeruli [113,114]. In addition, adipose tissue, skin, and muscle specimens from people over age 70 have about 25% more cholesterol than those from people age 30, and tendon specimens have several hundred percent more [115].
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Conclusions.
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Our interpretation of the origin of vulnerable plaques explains the molecular, cellular, and tissue processes resulting in atherosclerosis and cardiovascular disease. Promoting factors may not necessarily act by damaging the arterial wall directly, but rather by inhibiting the immune system, by facilitating microbial growth, by causing hyperhomocysteinemia, and by promoting complex formation and aggregation of homocysteinylated lipoproteins. Our interpretation is in accord with several of the classical risk factors. Hyperhomocysteinemia is found in B vitamin deficiency, smoking, hypertension, hypothyroidism, renal failure, and aging, all classical risk factors for cardiovascular disease [116]. Mental stress, a well-known risk factor for cardiovascular disease, stimulates production of cortisol, and an excess of cortisol, either from Cushing’s disease of the adrenal glands or from medical therapy, promotes infections. Furthermore, mental stress, hostility, and anger increase the concentration of homocysteine in blood [117,118], potentially promoting aggregation of LDL particles [18]. Many infectious diseases are more prevalent in smokers and diabetics. The suggestion that excess iron is a risk factor for vascular disease [119] is also in accordance with our interpretation, because bacterial growth is stimulated by the presence of free iron [120]. Therefore, attempts to prevent cardiovascular disease and prolong life may be more successful if we understand the fallacies of the lipid hypothesis [121] and determine what is harmful to the immune system and what may strengthen it.
Our interpretation satisfies Karl Popper’s definition of a scientific hypothesis, because it is susceptible to falsification:
- We anticipate that viable microorganisms and endotoxins in the arterial wall are located within developing vulnerable plaques.
- We anticipate that arteries of germ-free, normocholesterolemic animals should have fewer foam cells and fatty streaks than their conventionally reared litter mates.
- A blood culture should be taken in all patients with unstable angina or myocardial infarction, and we anticipate that if it is positive, the course of the disease should be improved with an appropriate antibiotic.
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Acknowledgement
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We thank Charles F. Foltz, Medical Media Service, VA Medical Center, West Roxbury MA, for assistance in preparing the figure.
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Abstract
Introduction.
