Etiology and triggering factors
Many infectious agents over the years (HHV6, CMV, EBV, enteroviruses, etc.) have been proposed as the initiating factors in CFS pathogenesis. Consistent results regard any specific agent, however, have been lacking. An inability to identify a viral agent does not mean a viral infection has not occurred.
A virus could infect a host, damage its immune or nervous system and then be eliminated, leaving the body in disarray. Non-viral infections that may contribute to CFS pathogenesis include Brucella spp., Mycoplasma spp. andChlamydia pneumoniae.
While no single infectious agent has been implicated, many CFS patients at the onset of their illness report symptoms suggestive of an infectious episode. Many CFS patients also suffered stressful ‘life events’ in the year preceding their illness.
The immune system in CFS
A literature review of CFS immunology indicates three consistent trends; poor cellular function, immune activation and an increased incidence of allergy and auto-immunity. There are also suggestions of a long term shift towards Th2 cytokine dominance.
Poor Cellular Function – The remarkably heterogeneous reports of NK cell number in CFS patients are, the authors suggest, more a function of improperly designed studies that fail to account for confounding factors, than any variability inherent in CFS.
Seven studies have consistently found evidence of impaired NK cell function. The exact mechanism of NK cell inhibition is unclear. A recent model, however, bears great similarity to that posed by the authors. Ogawa et. al. reported a failure in arginine induced NK cell activation in CFS patients but not controls. NO, which is believed to be present in high levels in CFS, is toxic to NK cells.
It is likely, as well, that phagocytosis by NK cells is inhibited in CFS because of calpain and/or caspase induced G-actin cleavage. (The formation of the engulfing ‘pocket’ used to capture pathogens is dependent upon G-actin reorganization).
Another consistent abnormality in cellular function involves depressed responses to phytohemaglutinin (PHA) and pokeweed mitogen (PWM). CFS patients also have decreased levels of IgA, M, D and, in particular, G immunoglobulins. (Immunoglobulins or antibodies are produced by B-cells. They are effective at removing toxins and extracellular viruses and bacteria.) These problems are possibly indicative of a defective humoral response which allows invaders to evade immune defenses.
A guide to natural killer cells
NK cells play a major role in the innate immune defense response that is activated immediately upon infection. NK cells are among the most finely ‘tuned’ cells of the immune system; if by their examination of a cells surface they determine the cell is infected or otherwise damaged, they immediately kill the suspect cell by ‘drilling’ a hole in it, and injecting it with proteolytic enzymes.
They are extremely efficient at eliminating virally effected or cancerous cells. They also secrete cytokines that alert the adaptive branch of the immune system (T, B cells) of an ongoing invasions. They also play an important role in immune regulation and neuroendocrine activation.
NK cell functioning, as was just noted, appears to be inhibited in CFS. In particular NK cells appear unable to produce the substance, perforin, responsible for drilling holes in pathogens.
Low NK cell activity has been associated with cancer, viral infection (including AIDS), depression, and low NK cell syndrome. In AIDS low NK cell activity may result from metabolic defects caused by HIV which result in low L-cysteine and high L-glutamine serum levels. Because T-cells are activated by NK cells, low NK cell activity may result in low T-cell activity and a deficient response to intracellular organisms.
Immune activation, cytokines, allergy and autoimmunity
The reduced NK cell activity, increased allergies and auto-immune manifestations seen in CFS may indicate the presence of an immune response dominated by Th2 (humoral associated) cytokines. The cytokine balance (or imbalance) in CFS is, however, unclear. While several studies have reported altered cytokine levels in CFS patients, just as many have found normal levels.
Several pro-inflammatory cytokines induce symptoms very similar to those found in CFS. The side effects produced during clinical trials of interferon, in particular, indirectly suggest that IFN’s play a role in producing the symptoms found in CFS.
Allergies- The very high rate of allergy found in CFS could result from disruptions in cell mediated immunity that alter cytokine levels. Increased allergy can occur as a result of immune activation in response to infection. (Inflammation can also result in increased viral activity.
Herpes simplex virus activity, for instance, is increased with sunburn.) Indeed, CFS patients with EBV exhibit higher allergic responses that those without EBV. A ‘remarkably high’ percentage of CFS patients exhibited bronchial hypersensitivity in a recent study.
Infectious agents
The sudden onset, and the flu-like symptoms that often accompany the syndrome, have prompted researchers to investigate a number of infectious agents.
HHV6 (Human Herpes Virus 6)
A far stronger role can be posited for HHV6 in CFS pathology than for any other virus. Antibodies to HHV6 are consistently elevated in CFS. The cytokine patterns found in CFS, and HHV6’s ability to infect NK cells indirectly suggests HHV6 involvement.
Overall these authors believe there is little doubt that HHV6 contributes to CFS symptomology. Because the high rates of HHV6 reactivation found are believed to be the consequence, not the cause, of immune dysregulation, HHV6 is believed to be yet another co-morbid disease. RNase L upregulation was recently correlated in CFS and multiple sclerosis patients with HHV6 infection. (An overview of HHV-6 in CFS can be found here).
Mycoplasmas
Mycoplasmas are bacteria lacking cell walls able to penetrate cells and tissues that are associated with a wide variety of diseases and have the potential to cause many symptoms associated with CFS. Increased infection rates of Mycoplasma spp. over time in CFS patients suggests suggests a tendency to pick up more and more infections as the disease progresses. CFS patients appear to have significantly higher mycoplasma infection rates than the population as a whole.
Mycoplasmas may modulate a host of immune and auto-immune responses, and may promote rheumatic diseases through their interactions with B-lymphocytes. They may also increase rates of opportunistic infections and appear to contribute to disease progression in AIDS and other immunosuppressive diseases. They may also be associated with autoimmune diseases such as MLS, ALS, lupus and others.
The autoimmunity found in these diseases couldresult from the incorporation of pieces of the hosts cell membranes into mycoplasmic structures. Several chronic airway disorders found in CFS (asthma, airway inflammation and bronchial hypersensitivity) are associated with mycoplasmic infection.
CFS patients with mycoplasma infections display a higher degree of RNase L fragmentation. An endonuclease Mycoplasmas overexpress may, in fact, generate fragments that can induce 2-5OAS activation and leave the monomeric RNase L unprotected from cleavage by enzymes. They may also be able to directly cleave RNase L (!).
Chlamydia
Chlamydia are intracellular prokaryotic cells that are frequently associated with fatigue in infected patients. Elevated antibody titers of C. pnuemoniae, a widely recognized pathogen in athereosclerosis, bronchitis, sinustis, asthma, etc. are found in about 25% of CFS patients. Given the widespread and usually asymptomatic nature of C. pnuemonieaeinfection, however, the significance of these numbers needs to be taken with caution.
Chronic Borrelia, Brucella, Rickettsial Infections are often under diagnosed and present with very similar symptoms as CFS. Insensitive tests with high false positive rates complicate diagnostic procedures. Both Borreliosis (Lyme disease) and rickettsial infections have been successfully treated by (sometimes long-term) antibiotic treatments.
Onset, immune changes and infections: an integrated model that explains the symptoms of CFS
Seven different groups of predisposing and onset factors can produce the immune alterations (poor cellular immunity, Th2 dominated immunity) seen in CFS:
(1) Cellular stress – A significant number of CFS patients experienced an onset of their symptoms shortly after blood transfusions or pregnancy. Both events (as well as radiation exposure!) can increase cellular stress.
(2) Acute viral infections can cause immune suppression for long periods of time during which endogenous viruses may reactivate and opportunistic infections can occur.
(3) Toxins such as heavy metals, organophosphates, PCP, etc. can cause immune dysfunction.
(4) Long term physical or mental stress negatively impacts both cellular immunity and cytokine balance and causes shifts in the Th1/Th2 balance. Viral reactivation occurs after high cortisol levels (a stress induced hormone) decrease macrophage function. (Since the macrophages are at the heart of any infectious response this would put quite a damper on the immune system.) How to explain high levels of Th1 cytokines (IFN-y, IL-12) but low levels of Th1 activity (NK cells)? The authors suggest that fragmentation of the STAT I signaling protein is preventing NK cells responding to IFN-y. Thus, while the Th1 system appears to be upregulated in CFS in reality it is not.
(5) Estrogen – Situations that cause high estrogen levels such as pregnancy and other physiological and pathological situations can shift the Th1/Th2 balance towards Th2. Even in the presence of Th1 cytokines such as IFN-y and IL-12, high estrogen levels can illicit a Th2 response.
(6) Infections that are eliminated slowly can shift the Th1/Th2 balance. A Th2 environment that prevails during pregnancy may give intracellular organisms a better chance to invade.
(7) Having an atopic environment (having a lot of allergies) favors the development of stealth organisms.
A verbal explication of a diagram presented in the book may be helpful here. Five agents (parasites and bacteria, pregnancy, long standing physical or mental stress, increased apoptosis rates, and an atopic or allergic (Th2) constitution) are believed to cause at least a temporary shift in the Th1/Th2 balance.
Toxins and viruses can also lead to compromised immunity and poor cellular function. Either of these conditions, the authors believe, leave the immune system vulnerable, if sometimes only for a short while, to the invasion of intracellular organisms.
Once CFS is present the intracellular and opportunistic infections that jump-started the RNase L system open the window of vulnerability wider by contributing to a Th2 shift. As does increased apoptosis.
These things may jumpstart the RNase L system but what deregulates it? The arrows in the diagram suggest that two things can; intracellular invasions or ‘cellular stress’. The authors posit that these two processes have one thing in common in CFS patients; they both produce ‘poor ds (ss) RNA inducers’.
These poor RNA inducers are either very small fragments of RNA (<25 bp’s (base pairs I believe) OR they have ‘low oligo (C). (Oligo © is a component of 2-5A. ???).
There is some new information here! The authors noted in chapter two that p69, a 2-5OAS isoform that produces 2-5A trimers (the good guys) needs dsRNA that are at least 25 bp’s long to become activated. RNA’s of less than 25 bp’s are believed to activate the (bad) p100 isoform. Here we learn that ‘cellular stress’ (a rather large category) may be the source of the small pieces of RNA.
In Chapter Two the authors suggested that the mystery RNA may originate in the mitochondria, nucleus or microsomes. Microsomes are minute cellular structures consisting of ribosomes and broken bits of endoplasmic reticulum and mitochondria that can be obtained during a centrifuge process.
Here the authors leave open the possibility that intracellular infections may also produce the small sized RNA particles. Or low levels of certain sections of the oligoadenylate polymers that make up 2-5A may predispose 2-5OAS to produce dimers rather than trimers. Since 2-5A is produced out of ATP does this suggest a problem with ATP production?
Two factors increase RNase L activity; the fragmentation of RLI removes RNase L’s inhibitor and increased viral infections/reactivations increase the presence of dsRNA. It is notable that the waxing and waning of the flu-like symptoms seen CFS that are so suggestive of viral reactivation, are associated with increased RNase L activity and fragmentation as well as increased levels of cytokines and lymphotoxins.
The increased activity of the 37-kDa RNase L fragment causes what the authors suggest maybe one of the most significant ramifications of RNase L dysfunction; a channelopathy that may involve as many as 10 ABC transporters. (Until now a possible channelopathy in CFS has received only intermittent interest. Three papers have, however, explored this subject in the last year; see the updates Chapter four.)
The authors suggest that one of the ankyrin domains found in the 37-kDa fragment is able, because of its close resemblance to RLI, to bind with membrane proteins and disrupt ion channel functioning. Malfunctioning ABC transporters have the potential to cause a (truly fantastic) array of symptoms.
They include night sweats, hypersensitivity to chemicals and drugs, depression (due to low tryptophan uptake), CNS abnormalities, immunosuppression, Th2 dominance, anemia, etc. Decreased potassium alone can cause fatigue, sleep disturbances, muscle weakness, weakness of the respiratory muscles, metabolic acidosis, bronchial hyperactivity, low blood volume, bladder problems, PMS, etc. (!).
The down-regulation of the RNase L inhibitor (RLI) and the subsequently increased RNase L activity adds another layer to the illness by inhibiting protein synthesis, reducing hormone production and causing resistance to hormones (via destruction of mRNA), reducing cell proliferation and decreasing mitochondrial ATP production.
Elastase, a proteolytic enzyme found in polymorphonuclear cells (granulocytes) can cause damage to epithelial cells (found in skin, lungs, stomach, intestines, etc.) as well as increased mucus production and other mucus abnormalities.
Elastase, (which may be an agent of RNase L fragmentation) activates a substance, bradykinin, that causes bronco-constriction. The authors speculate that increased elastase in CFS could effect the elastin fibers in connective tissues, and cause several conditions commonly found in CFS; (hernias, mitral valve prolapse, pulmonary hypertension, exertional dyspnea (difficulty breathing), tachynpea (rapid breathing), chest pain and light-headedness).
A recent study indicated that the bronchial hyperresponsiveness commonly seen in CFS patients appears to be associated not with elastase activity but with excessive cytotoxic T cell activity.
PKR’s increased activity in CFS results in increased apoptosis as well as increased nitric oxide (NO) production through PKR’s stimulation of NF-kB. One of the many things NO does is relax smooth muscles causing the blood vessels to dilate and sending immune system components to the site of injury.
The authors suggest that chronic NO stimulation can, in conjunction with the low blood volume and increased sympathetic activity (increased heart rate, decreased smooth muscle contraction), explain some of the vascular problems in CFS.
Finally, decreased NK cell activity and p53 fragmentation and the apoptotic inhibition seen in a subset of CFS patients, may contribute to increased cancer rates in CFS patient. ?A recent study found no evidence of increased cancer (or other) deaths in CFS.
Conclusion
The basis of CFS lies in persistent infections and an acquired immune disregulation. (An evocative and no doubt intentional acronym. The many and very varied symptoms found in CFS are indeed reminiscent of the different systemic problems found in AIDS.)
Small fragments of ds (ss) RNA dysregulate RNase L and upregulate the PKR systems. One consequence of a dysregulated RNase L system appears to be an acquired channelopathy which may cause many symptoms found in CFS. Fluctuating cytokine levels, and increased RNase L and PKR activity will account for many of the other symptoms found in CFS.
Extra – a layman’s guide to Type II interferons
IFN-y figures in a very wide range of activities. It is of particular interest in CFS because it is central to activation of the arm of the immune system (Th1) that appears to be inhibited in CFS. IFN-y ramps up the alert level of antigen presenting cells by enhancing the expression of the (MHC) molecules they use to display foreign antigens on.
Antigens are anything – viruses, bacteria, and chemicals- that provoke an immune response. IFN-y also plays a major role in immune cell growth and differentiation by activating cytotoxic T-cells (Tc) (which kill virally infected cells), by increasing natural killer (NK) cell activity, and by activating monocytes/macrophages (sentries of immune system). IFN-y differentiation of dendritic cells – the major antigen presenting cells – triggers the adaptive immune response (ThI) response which seems to be deficient in many people with CFS.
Upon contact with an infected or damaged cell macrophages secrete two cytokines (IL-1 and 12) that alert T-cells of the possible danger. When the precursors of T cells come across macrophages or other APC’s, they search its surface to see if it displays a harmful antigen.
If it does, and the T-cell has already been primed by the presence of IL-12, then the T-cell is activated and grows and then splits apart into several T-cells and secretes IFN-y and TNF-a. The cytokines further activate the macrophages to seek out and destroy the invaders of the antigen they displayed.
IL-12 not only stimulates the development of more ThI cells but also prompts NK cells to produce more IFN-y. The IFN-y produced acts to, among other things, suppress the development of Th2 cells.
IFN is the one of several immune system mediators NK cells produce. The poor NK cell function found in CFS could directly result in low ThI differentiation. IFN-y also induces nitric oxide production in macrophages. Nitric oxide disregulation may be a key feature of CFS.)
What happens if the signal for IFN-y is not getting through? Macrophages are not activated and the invader slips by the first arm of the immune system. As cells become infected the RNase L/PKR systems are activated, and as Dr. Cheney puts it, they grind away waiting for the cavalry (T and B cells) to come, but it doesn’t because they never got the message.