Heart disease, or more specifically atherosclerosis, is a chronic disease whereby cardiac arteries become partially occluded by the growth of plaques. Narrowed arteries are more likely to trap dislodged blood clots, resulting in blood supply being cut off to a portion of the heart, resulting in oxygen depletion of the cardiac muscle tissue and eventually myocardial infarction or heart attack.
Contrary to common wisdom, dietary fat does not deposit on the arterial wall and "clog your arteries;" plaques grow inside the arterial cell wall and consist of a mix of the smooth muscle cells that naturally line the interior lining of the artery and immune-system cells such as macrophages and lymphocytes. Macrophages, or white blood cells, are the large, amoeba-like cells that form the last line of defense for the immune system. This mix of cells are called foam cells. Foam cells tend become bloated by absorbing large amounts of cholesterol from the blood stream and they form a cyst or lesion which compresses the arterial wall, reducing the effective diameter.
By way of background, there are three times of muscle in the body: cardiac, skeletal (meat muscle, like your bicep), and smooth muscle (such as your scalp or vital organs). Smooth muscle is quite a bit different from the other types in that its cytoskeleton has no deterministic, regular structure. Rather, it just consists of a chaotic jumble of actin filaments. Unlike the other two types, smooth muscle is not normally controlled by the electronic action potentials of the nervous system; rather, it is controlled by the much slower chemical hormonal system.
With time, atherosclerotic lesions may harden by absorbing calcium
from the blood which makes the artery stiffer and tends to result in high blood pressure. This is not unlike how bone tissue is formed by the mineralization of connective tissue, which in turn suggests some sort of hormone dysfunction is in play. Hardened arteries are considered more hazardous than pliable yet still narrow arteries. The likely reason for this is that a narrowed yet pliable artery can distend under pressure to allow a blood clot through, while a calcified artery cannot.
Thus we have three criteria for heart attacks (with some simplification):
- The wall of the cardiac artery has to be sufficiently narrowed so that,
- A dislodged blood clot gets stuck in it, and/or an atherosclerotic lesion ruptures (in the majority of heart attacks anyway), and
- The arterial wall is too stiff to allow the build-up of pressure caused by the obstruction to allow the clot through, resulting in down-stream oxygen deficiency and eventually cell death.
To reduce heart attacks, you can attack any of these three processes. Take the Masai tribesmen of Tanzania
: they have a great deal of atherosclerosis thanks to their milk-based diet, but they don't necessarily suffer heart attacks, likely due to their adequate intake of vitamin K2
I want to talk about the first requisite, atherosclerosis. The question is of course, what causes immune system bodies to form colonies inside the lining of one's arteries? There are two basic possibilities: auto-immune disorder, where the immune system recognizes legitimate tissue as foreign, or actual foreign bodies, such as chronic bacterial or viral infection of the blood vessel. Or both.Enter Chlamydia pneumoniae, bacterium
The idea that atherosclerosis might be caused by chronic infection of the lining of blood vessels is not a new one. In 1999, the American Heart Journal devoted a supplementary issue
to the topic. It is, essentially, an alternative theory for heart disease as compared to the diet-heart hypothesis, whereby diet modifies serum cholesterol levels which in turn causes heart disease by some unknown mechanism. As it contraindicates the standard lipid model, it is considered controversial. The first potential pathogen candidate was cytomegalovirus (aka herpes) but it turned out to be a bust.
Further research suggested a better candidate. Chlamydia pneumoniae
(aka Chlamyophila pneunomiae) is the bacterium most commonly associated with heart disease in the literature. As the name suggests, it is one of the sources of pneumonia and other respiratory infections. It has also been associated with Alzheimer's and asthma. It is a relatively recently discovered pathogen (in that the diet-heart hypothesis was formulated before anyone knew it existed), and its responsibility for respiratory infection was only discovered in 1986 (Grayson etl al., 1986
). The association with heart disease was made very quickly (Saikku et al., 1988
), since the hunt for a potential atherosclerotic pathogen had been underway since the early 1980s.Bellard et al. (2003)
lay down the case for C. pneumonaie succinctly,
Exposure to Chlamydia pneumoniae is extremely common, and respiratory infections occur repeatedly among most people. Strong associations exist between C. pneumoniae infection and atherosclerosis as demonstrated by: (i) sero-epidemiological studies showing that patients with cardiovascular disease have higher titres of anti-C. pneumoniae antibodies compared with control patients; (ii) detection of the organism within atherosclerotic lesions, but not in adjacent normal tissue by immunohistochemistry, polymerase chain reaction and electron microscopy and by culturing the organism from lesions; and (iii) showing that C. pneumoniae can either initiate lesion development or cause exacerbation of lesions in rabbit and mouse animal models respectively.
This list is not exhaustive, and it does not note probably the most important point: C. pneumoniae can create foam cells in vitro (i.e. in a Petri dish). C. pneunomiae has been shown to infect the constituent cells in foam colonies (i.e. macrophages and smooth muscle) (Fryer et al., 1997
), inhibit the mechanism whereby cholesterol (LDL) is relinquished (Kalayoglu, 1999
), and oxidize low-density lipoprotein (LDL) via releasing heat shock proteins (Kol et al., 1998
). More on this later.
The physical presence of C. pneunomiae in atherosclerotic lesions has, as previously mentioned, been detected by a wide variety of methods. It's also highly prevalent. Of many studies that have found C. pneumoniae in foam colonies, (Muhlestein et al., 1996
) is probably the most solid. From a population of 90 patients, they found evidence of C. Pneumoniae in 79 % of atherosclerotic plaques, but only 1 of 24 control biopsies.
A basic question then is how does one particular type of bacteria manage to not only evade the immune system, but distort its response in order to cause great harm to the host? (Belland et al., 2003
) explore the mechanism,
Chlamydial growth is biphasic, consisting of two alternating functional and morphological forms (Fig. 1). The elementary body (EB) is the metabolically inert, infectious form of the organism that is capable of transient extracellular survival. EBs bind to as yet undefined host cell receptors, are internalized via a pathogen-specified process and are detectable within a membrane-bound vesicle immediately after entry. This vesicle is capable of interacting with post-Golgi secretory vesicles in ways that allow for the incorporation of host phospholipids [RM: phospholipids are cell membranes, i.e. camouflage] (Hackstadt et al., 1996; 1997). Chlamydiae also block intracellular host cell responses, such as fusion of the pathogen-containing endosome with lysosomes, and thus avoid host cell factors that would be detrimental to intracellular survival. Soon after entry, chlamydiae differentiate from infectious EB to the intracellular replicative form of the organism, referred to as the reticulate body or RB. This differentiation, which is dramatic in terms of altered chlamydial morphology, must reflect an orchestrated sequence of differential gene expression. Transformation of EB to RB results in loss of the disulphide cross-linking of the outer membrane complex, decondensation of the genome and initiation of DNA, RNA and protein synthesis. RB multiplication results in the formation of an intracellular microcolony (termed the inclusion) of chlamydiae.
Ok so that's a wordy quote, but to sum it up in one word it is mimicry. A big area of research in bio-nanotechnology is the development of phospholipid coatings on implanted medical devices to prevent the immune system from recognizing them as foreign and attacking them. This technique is known as, "stealth technology." Here we have an example of an organism that has evolved this technique. <acerbic>But don't you dare eat any eggs, they have cholesterol in them</acerbic>.
It's not clear if C. pneumoniae causes all atherosclerosis but I do believe it causes a majority of such. Another potential source of plaque-forming bacteria is Helicobacter pylori, which among other things, is thought to be responsible for ulcers. Unlike C. pneumoniae, which enters through the lungs, H. pylori typically lives in the gut. A study by Mayr et al. (2000)
found an association between H. pylori antibodies and cardiovascular disease, but only in the low status (i.e. poor) individuals in their population. This study was conducted in Austria, so one wonders what might poor Austrians be eating that would cause them to suffer H. pylori infections?
The same paper also found odds ratios for: IgA type antibodies to the C. pneumoniae bacteria, elevated C-reactive protein levels in the blood, and (clinical) chronic respiratory infection. Now, odds ratio
is a funny, non-intuitive statistic, but we can directly compare it to odds ratios found in other studies.Table 1: Odds ratios for various diseases of affluence.
Top quartile LDL cholesterol developing
coronary artery disease within six years.
(El Harchaoui et al., 2007)
Any one of:
- IgA antibodies to C. Pneumoniae.
- Elevated C-reactive protein levels.
- Clinical chronic respiratory infection.
Two of above.
Three of above.
Smokers developing Lung Cancer,
(Doll & Hill, 1956)
As you can see, three out of three is quite a strong association, far stronger than the cholesterol testing that is the most common method of screening for heart disease risk in medicine today. It is actually getting close to that of smoking and lung cancer, which is the gold-standard for causation. If you break it down individually, the strongest of the three criteria is chronic respiratory infection (OR of 3.8), followed by C-reactive protein (OR of 2.4). Since C. Pneumoniae antibodies has the poorest odds-ratio, while the chronic conditions are much higher, we can probably surmise that being infected once isn't going to cause atherosclerosis, in the same sense that over-drinking once is not going to cause fatty liver disease. Heart disease is a chronic condition, caused by chronically applied vectors (i.e. diet and environment). For the same reason, antibiotics were found to be ineffective in treating atherosclerosis: they are effective for acute infection, but in the long run they cause as many problems as they solve, since many of the bacteria in our bodies are beneficial and help out-compete teh nasties [sic].Heat Shock Proteins
Another issue, that I eluded to above, is what exactly is causing inflammation in this case? Medicine has identified oxidized-LDL as a major danger factor, but is this a cause or just a symptom?
One potential candidate is a class of proteins that are produced by cells under stress, collectively known as heat shock proteins
. The name is not well chosen, heat shock proteins should be termed temperature stress proteins. They are produced by cells that are under elevated temperatures (or many other forms of environmental stress) and they protect the other functional machinery of the cell from future elevated temperatures. They are considered to be a part of the general inflammation response of the body, although they tend to operate on a small, single-cell (i.e. autocrine) scale.
Foam cell colonies produce one particular type of heat shock protein, HSP60
, in large amounts.
This particular heat shock protein is usually associated with mitochondria, the energy factories of cells, but it's also known to interfere with apoptosis
, or programmed cell death. Apoptosis is the way in which the body normally disposes of broken or old cells. Elevated levels of HSP60 prevent apoptosis from occurring (Gupta and Knowlton, 2005
One study on 1003 Chinese men found an odds ratio of 2.3 for atherosclerosis by simply being in the top half of the population for HSP60 levels in the blood (Zhang et al., 2008
, full-text link on Pubmed is broken and is available here
). Those in the top quartile for HSP60 levels had an odds ratio of 4.87, which higher yet than the range usually seen for c-reactive protein, which is the standard marker for inflammation.
The high odds ratio with c-reactive protein has been seen as one of the supporting features for the slowly evolving, mainstream view that inflammation and not blood lipids are responsible for heart disease. But is inflammation the cause or symptom again, and what causes localized inflammation of the blood cell wall anyway? The fact that arterial foam cells produce heat shock proteins and a bevy of other inflammatory markers in quantity suggests that atherosclerosis causes inflammation, rather than the other way around
. Indeed, studies have claimed that the chlamydial heat shock protein 60 (cHsp60) oxidizes LDL particles in a test tube (Kalayoglu et al., 1999
). This is big deal, since the actual mechanism whereby LDL oxidizes isn't known. For example, see Steinberg, 1997
First, patients and animals totally lacking the LDL receptor nevertheless accumulate cholesterol in foam cells much the same way as do patients and animals with normal LDL receptors; second, the two cell types in lesions that give rise to cholesterol-laden foam cells (the monocyte/ macrophage and the smooth muscle cell) do not accumulate cholesterol in vitro even in the presence of very high concentrations of native LDL (3,4). This paradox could be resolved if circulating LDL underwent some form of modification and if the modified form, rather than native LDL itself, then served as the ligand for delivery of cholesterol to developing foam cells.
There are no paradoxes in medicine, just an inadequate understanding of nature. That, and a heaping load of bias. If it is the heat-shock proteins that are causing much of the trouble, then we have some idea as to why foam cells are sustained
by the body. The heat shock proteins produced by these bacteria closely mimic the same heat shock protein produced by the arterial wall (Hsp60). Heat shock proteins are one of the basic lego blocks of living cells, and there's not a great deal of variation between those HSPs produced by highly evolved human cells compared to say, yeast. The fact that HSP60 inhibits programmed cell death is another fascinating thread to pull on. Another possibility that occurs to me is that real bacterial infection may be transformed over a long time into an auto-immune disorder, even if
the original bacteria have died off. More research is needed to assess just what the half-life of C. pneumoniae is in foam cell colonies.Known correlations with heart disease
So how are these bacteria getting through the mucous tissues and into the blood stream? The correlation between smoking and heart disease is explained nicely by this hypothesis. Smoking compromises the lungs, leading to C. pneumonia or some other form of infection, which in turn results in atherosclerosis.
One might expect then that other chronic conditions that break down the walls of the mucous membrane/exterior environment barrier could also lead to atherosclerosis: dietary components that cause leaky gut, periodontal disease, and possibly fungal infections. Alternatively, substances that suppress immune system function could be associated with cardiovascular disease.
Other chronic diseases that results in a breakdown of the bloodstream-mucus-environment barrier should also show increased rates of heart disease. The two most obvious choices to me are celiac disease (where wheat gluten destroys the lining of the intestines) and periodontal disease (of the gums in the mouth). For celiac disease, there doesn't seem to have been any research on the topic. Of course, it would be ethically impossible in a clinical setting to find undiagnosed celiac patients and follow them to assess heart disease. Once celiacs are diagnosed, their treatment is straight-forward (i.e. don't eat wheat or casein). However, perio is more difficult to resolve and the results on periodontal disease seem to be consistent, with perio causing a small (relative risk 1.24-1.34) yet consistent and significant increase in heart disease average over many studies (Cronin, 2009
Similarly, substances or deficiency of certain materials in the diet that suppress or deform the immune system response could have an impact on heart disease. The most obvious would be insufficient vitamin D intake.Conclusion
When it comes to the diseases of Western civilization/diet, we always must ask the question, what is the mechanism?
A top-down approach is simply insufficient because separating correlation and causation in living organisms is so difficult and one is consistently tempted to simplify the details rather than break them down into first principles. Hence the enormous failure of pursuing cholesterol as a cause
of heart disease, instead of recognizing it as a symptom. The false path of the cholesterol hypothesis has to rank of one of the greatest scientific blunders of all time and is indirectly responsible for the premature deaths of millions.
On the other hand, the chronic infection theory of heart disease is internally consistent with the data that are available to us. We know that C. pneumoniae is a common form of respiratory infection, and that once in the bloodstream it can infect smooth muscle and macrophage cells both in vivo
and in vitro
. We know that C. pneumoniae can disrupt the cholesterol metabolism of foam cell colonies in vitro. The chameleon nature of C. pneumoniae illustrates how foam colonies can be be persistent in the face of the immune system and morph an acute infection into a chronic condition. I just don't see any gaping holes in the theory. We still need to explain why
C. pneumoniae (and other bacteria like H. pylori) affects some individuals and not others, but the how
is reasonably explained and justified.Author's note: I started writing this post on May 13th, 2009.