Andrew Gladman takes a brief look at what cytokines are and how they might relate to our disease – exploring some of the research that is indicating their involvement in disease pathways…
In recent years ME/CFS research has turned the spotlight on several areas, such as autoimmunity, the cardiovascular system and the autonomic nervous system.
While it is fair to say that much remains unknown about their function and behaviour, cytokines have a role to play in all these areas and they are being talked about with increasing regularity.
What are cytokines?
The term ‘cytokine’ encompasses quite a few diverse families of proteins, peptides and glycoproteins which are secreted primarily by the different cells of the immune system. However, there are many other cells that release cytokines and that then trigger an immune response.
They act as signalling molecules between cells and fundamentally regulate the hosts’ immune response primarily to infection, inflammation and trauma – and for this reason they are often described as immunomodulating agents.
Once secreted by the cell, cytokines can function in one of two ways. The first, known as autocrine, means that the cytokine binds to a receptor on the cell surface of the same cell secreting the cytokine, triggering further responses within that cell. The second, known as paracrine, is when the secreted cytokine binds to a receptor on the cell membrane of an adjacent cell triggering further responses within that cell.
Types of cytokine
There is still a dispute between biochemists as to what does and does not constitute a cytokine. The following list encompasses quite well the accepted family groups, but whilst these titles are now partly redundant, many of the terms are still commonly used. It should be noted that these classifications are non-exclusive – meaning that a cytokine, for example interlukin 2, can be classed both as an interlukin and a lymphokine:
Interlukins: Initially this term referred to signalling molecules which were primarily targeted towards leukocytes, however many newly discovered cytokines are given this name regardless of targeted cell. This family are for the most part produced by Helper T-cells.
Lymphokines: Produced almost exclusively by lymphocytes, they often aid B-cells in producing antibodies.
Monokines: Produced almost exclusively by Monocytes such as macrophages. Many monokines are also classed as chemokines.
Interferons: Primarily involved in antiviral immune responses, but they can also be secreted in response to a bacterial or parasitic infection and furthermore can help fight tumours.
Chemokines: Mediate the movement of many cells involved in the immune response (chemotaxis). This is important to ensure cells of the immune system migrate to the area of the infection.
Fundamentally, studying cytokine levels in sick people gives us a snapshot into the functional status of that patient’s immune system at a given point in time.
The study of cytokines has consistently helped in the differentiation of pathogenic and autoimmune based diseases, for example, but this is only possible by understanding a little more of the specific function that certain cytokines exhibit and imagining how these cytokines react with cells to produce changes that can be observed on an organism level.
Function of cytokines…
The most important part of a cytokine is not in the binding that it makes to a cell (as briefly described above), but the changes that it has the ability to cause within that cell.
It is worth remembering that even the more in-depth explanation of the effects many of the more common cytokines can have is still something of an oversimplification.
In truth, cytokines are still at the forefront of much research to discover their numerous functions and the different effects they can induce in different cells.
One common misconception I must address at this point is the notion that the presence or finding of cytokines automatically means there is inflammation present.
While this may be true in many instances, certain pro-inflammatory cytokines, for example, can in fact can also exhibit anti-inflammatory properties.
To save space, I have attached a non-exhaustive list that I hope you might find helpful. It details the common function of those cytokines you are likely to encounter in research papers and articles.
It may be worth referring to this list when I now begin to discuss specific cytokine findings in relation to CFS/ME research. But do note that there are many functions of cytokines we still do not yet fully understand.
CFS/ME researchers whose work has indicated key cytokine involvement
Professor Ian Lipkin
Lipkin revealed in a CDC telephone broadcast in September 2013, a range of findings that may relate to the immune profile of ME/CFS patients. Simon covered these exciting developments in his Phoenix Rising article and transcript: Lipkin finds biomarkers not bugs.
While the main purpose of the study was to determine is there was any consistent viral or bacterial agents to be found, it also involved a more extensive examination of cytokine profiles and it is here that the potential biomarkers were discovered.
The main finding was the emergence of two possible subsets within the patient cohort, differentiated by the duration of illness. In those ill for less than three years the disease profile seemed to lean more towards an allergy type response – with increased number of eosinophils.
And, whilst cytokine findings are often very difficult to interpret, Lipkin expressed confidence in the additional finding of a consistent up-regulation in the production of Interlukin 17 within this same subset (IL17 is produced by T-helper cells and has a pro-inflammatory effect while recruiting further neutrophils).
The second subset identified were those who have been ill for a more prolonged period of time. In this group it appears that the ‘allergy’ aspect of the disease had run its course and was replaced by a chronic immune dysregulation phase.
The key findings in this second group were decreases in the serum levels of Interleukin’s 17, 2, 8 and also in TNF-alpha. Inteterlukin 2 is produced by a vast array of cells and has a stimulating effect upon T, B and NK cells; and if the findings outlined in this research hold true, then it is clear to see that a down-regulation in the levels of IL-2 could have a negative effect upon the immune response of a patient.
Interlukin 8 is also produced by quite a large variety of cells and acts as a chemokine, aiding in the recruitment of neutrophils. A down-regulation in Il-8 is also therefore going to have a negative impact upon the immune competency of a patient.
But perhaps the most interesting finding for me was the observed down-regulation of TNF-Alpha. TNF-A is known universally as a strongly pro-inflammatory cytokine, and finding it to be decreased is therefore quite odd – if we were to assume ME/CFS is an inflammatory disease. Though it is worth noting that TNF-A has many other functions aside from stimulating inflammation, including activation of the endothelium and also inducing the release of other cytokines.
As with all research, this data would need to be replicated before being considered reliable, thought there do appear to be several promising and very unexpected leads among the preliminary revelations. It’s clear that such cytokine findings in the absence of any infectious agent point quite strongly towards an ongoing process of disruption involving the immune system.
Perhaps autoimmunity will be more clearly linked to ME/CFS in the future, and this apparent change over time in cytokine activity could be an indicator of this: it is well known that many diseases such as Lupus and Multiple Sclerosis change over time, with disease activity often flaring and remitting quite unpredictably and causing disruption to functional ability. I speculate, but maybe what we are seeing here is the first observation of such a process in our own condition?
Dr Nancy Klimas
There have been many studies exploring cytokine levels within CFS/ME but one such study that I wanted to highlight, came from Klimas and focused upon the plasma cytokine levels of female patients.
The interesting aspect of this study for me, was the wide array of cytokines studied, some 16 in total, but the downside was the sample size. While 40 patients may seem adequate, much larger samples are really now required for research into CFS/ME to better gain wider acceptance.
However, Klimas found numerous cytokines were elevated, including interlukins 1, 4, 5, 6 and 12. Interlukin 1 is pro-inflammatory and stimulates endothelial activation. This might be seen as something of a challenge to the results talked about above, where TNF-A was found to be down-regulated – TNF-A also stimulates endothelial activation – but perhaps this likely indicates a greater degree of cytokine dysfunction.
Interlukin 5 interestingly recruits eosinophils, as discussed above, where it appeared that eosinophils might play a larger role during the initial illness period and then decrease their involvement over time – unfortunately we do not know the illness duration for patients from the Klimas study.
Interlukin 5 also stimulates an isotype switch to IgA and Interlukin 4 also stimulates an isotype switch, but to IgE. With the two isotype switching cytokines acting in opposition to one another it would also indicate dysregulation of the immune system.
Three cytokines in this patient sample were found to be decreased: Interlukins 8, 13 and 15. Interlukin 8 was also found to be down-regulated by the Lipkin study, this is therefore a reasonably promising finding and deserves further investigation.
Interlukin 13 functions primarily as an anti-parasitic agent, but it also stimulates another isotype switch to IgE, and this additional down-regulation adds to the evidence of immune system irregularity. And finally, Interlukin 15 stimulates NK cell and cytotoxic T-cell proliferation also dampening down immune system activity.
Contradictory to the Lipkin studies preliminary announcement above, the Kilmas study found TNF-A to be at normal levels. However, and whilst we await publication of the Lipkin paper (due by the end of March 2014 it is believed), given the sample sizes of each study, the Lipkin results appear to show greater reliability.
From the evidence presented, it is clear that there is much to be learnt about the disease process of ME/CFS from cytokine findings alone. The vast array of dysfunction and dysregulation strongly imply an ongoing process, one that is likely self perpetuating.
As discussed in relation to the work of Professor Lipkin, there has never been a universally acknowledged infectious cause of ME/CFS, and the numerous infectious agents in patients might appear to be a product of the dampened immune response rather than a causative problem.
We therefore have to look beyond an infectious cause and perhaps consider a self perpetuating autoimmune process. Other autoimmune diseases such as Lupus have similar cytokine profiles to those outlined in this article – there is however still a wide degree of variation even within a well defined disease such as Lupus.
In Lupus patients with significant thrombocytopenia of the skin, TNF-Alpha is often measurably increased relative to the normal range, whereas in Lupus patients with significant lymphandeopathy or nervous system involvement, often TNF-Alpha serum levels are within normal range. Demonstrating the difficultly cytokines can present even within a single, well-defined disease such as Lupus. It is therefore easier to acknowledge the challenge such findings present to researchers.
While cytokine findings are an important piece of the greater ME/CFS puzzle, it must be remembered that they are only that. Cytokines have a tendency to fluctuate wildly from one day to another, even in otherwise healthy patients.
They are also prone to changing prior to the symptomatic onset of an infection – therefore in a small sample size, a simple cold in 2 or 3 patients can drastically change the results of a study. Having said all this, the findings from the two pieces of research I have chosen to feature, should not I think be understated. They present firm evidence of an immunological problem in ME/CFS patients and provide new roads and avenues for further exploration and research.
If replicated and proved consistent, not only do these abnormalities have the potential to prove a useful bio-marker, they also go some way to aiding our understanding of the disease mechanism at the heart of the disease and perhaps even aiding in the development of treatments in the future.