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Immune system functional testing of athletes at University of Stellenbosch
Carine Smith, Kathy Myburgh
Because of the ever-increasing high standards set in the sporting world today,
athletes have to keep increasing their training efforts in order to excel at
their chosen sport. This increased stress to the body (both physical and
psychological) may affect many aspects of the normal functioning of the body,
including the immune system. Based on epidemiological data (reviewed in
Calabrese et al. 1996 and Nieman 1998) , it seems that while moderate exercise
may serve as a booster to the immune system, excessive exercise may result in an
increased risk to the athlete of suffering upper respiratory tract infections.
As a result, the Exercise Science Division of the Department of Physiological
Sciences at the University of Stellenbosch became interested in investigating
the physiological effects of exercise on the immune system.
 When we investigate the status of the immune system, the first tests we do
are a full blood count and differential white cell count. These tests tell us
whether there are enough white blood cells of each required type present. However, this does not tell us how efficiently these cells will
respond to a pathogenic challenge. Therefore the numbers of the different types
of white blood cells are not always a good indicator of immune ability and the
numbers of immune cells available should always be looked at in conjunction with
their capacity to react to a challenge.
There is more than one way in which to determine a white blood cell's
reaction capacity. One can stain the cells for cell markers that indicate an
activated status - these are molecules expressed on the cell surface whenever a
cell commits itself to react to a challenge. Alternatively, one can measure the
concentrations of the substances the cells secrete in response to a pathogen or
stimulus.
We employed the latter method for several studies. We were interested in
investigating the release of the cytokine interleukin-6 (IL-6) by immune cells
of athletes. IL-6 is released by many cells in the body, such as immune cells,
endothelial cells, fibroblasts, and contracting muscle cells. Of the immune
cells, the monocytes and lymphocytes, the so-called mononuclear immune cells,
are the main secretors of IL-6. IL-6 has several functions in the systemic
immune response, which include a major role in stimulating the release of acute
phase proteins from the liver, and inducing fever. It also
plays a role in the endocrine response to stress, including indirectly
stimulating cortisol release by increasing ACTH release, and directly stimulating the adrenal cortex to produce
cortisol. Therefore, IL-6 release could be a valid marker of
immune ability in athletes who are training hard and are under a lot of
competitive stress.
However, since so many cells will secrete IL-6, it is difficult to determine
the source of the cytokine in vivo. Therefore, a method which allows for testing
of the function of the cells of interest outside the body (in vitro), may give a
clearer indication of the ability of those specific cells, as it eliminates
interference from other possible sources. We used a
technique developed earlier with which we can measure the in vitro interleukin-6 secretion in whole
blood cultures in response to stimulation by a bacterial endotoxin (E.coli lipopolysaccharide
[LPS]).
The testing procedure consists of two parts: In part one, a whole cell
culture is prepared by adding whole blood to two test tubes, with added
nutrients to ensure that the cells remain viable. Then the stimulus (E.coli
LPS) is added to the one tube, but not to the other. The test tubes, which now
contain a "cell culture", is then incubated at 37 °C (body
temperature) for a few hours, to allow the immune cells to react to the
pathogen. The second part of the process is to measure the amount of IL-6
released into the culture tubes. This is done by a simple ELISA technique. The
reason for having one tube with, and one without E.coli LPS added, is
that some immune cells may already be activated in the body and others may
become activated spontaneously during the testing procedure. The tube without
added E.coli LPS will indicate how many cells spontaneously secreted IL-6
in the absence of an added pathogenic challenge. We then subtract this amount
from the concentration of IL-6 in the tube with added E.coli LPS, to get
the concentration of pathogen-induced IL-6. Performing this test on different
athletes, or at different times of an athlete's training season, can give us an
indication of how stressful the exercise is to the immune system, and whether or
not the immune cells can still react to a different challenge despite this
stress.
The research group at the Department of Physiological Sciences at the
University of Stellenbosch, used this assay in a cross-sectional study of the
functional ability of immune cells from rugby players, triathletes and sedentary
people (accepted for publication in the European Journal of Physiology, 2002).
In short, we found that the immune cells of professional rugby players and
highly trained triathletes responded more poorly to the E.coli LPS
challenge than those of the sedentary subjects.
There is more than one possible explanation for this. Firstly, if the body is
exposed to the stress of exercise for a long time, the immune cells may become
less sensitive to stressors (which may be exercise or a pathogen). An
alternative explanation is that the cells are reacting to the stress of exercise
chronically, so that they are unable to respond further when a real pathogenic
challenge occurs.
If the first explanation is true, the cells would still be able to respond
well to a new challenge if the challenge was great enough. But, if the second
explanation is true, the implication is that the response could be inadequate to
defend the athlete against developing illness. We are currently testing these
two possibilities using an actual ultra-endurance race as the challenge. We
suspect that the subjects will show a poor response to E.coli LPS before
the race, because of all the training done in preparation for the race. During
ultra-endurance races, other researchers have shown that
E.coli LPS actually leaks into the circulation from the gut. Thus the
race itself can act as a pathogenic challenge which may influence the response
to an additional pathogenic challenge added to the blood in vitro after the
race.
In conclusion, being able to determine the functional capacity of the immune
cells rather than simply the numbers of the different sub-types, opens doors to
many new questions.
More Information
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