A considerable body of knowledge has been established in the specialty field of exercise immunology. The effects of different types, intensities and duration of exercise on various parameters of the immune system have been investigated. While consensus is lacking in some areas, there is sufficient agreement to reach some conclusions about the effects of exercise on the immune system.
A basic understanding of the immune system is a prerequisite for a practical interpretation and application of the research on exercise immunology. Thus, this chapter begins with a brief discussion of fundamental concepts in immunology. Subsequent sections discuss the effects of exercise on major parameters of the immune system. Different types, intensities and/or duration of exercise are discussed if such variations produce significantly disparate immune responses.
BASIC CONCEPTS IN IMMUNOLOGY
Natural Immunity
The immune system’s response to antigenic challenge can be categorized as natural (innate) or acquired. Natural immunity provides a first line of defense against infectious agents. It is nonspecific in that the response is the same for different bacteria, viruses and other microbes. Repeated infections with the same microbe (e.g., same strain of bacteria) do not improve the innate immune response.
The natural immune system is composed of several defense mechanisms. Skin and mucus membranes serve as mechanical barriers to microbial invasion. Mucus entraps microbes and contains lysozyme and other proteolytic enzymes. Lysozyme is also found in urine and tears. Urine and tears provide a flushing action for the urinary tract and eyes, respectively. Phagocytic cells (e.g., neutrophils, monocytes, macrophages) ingest (phagocytize) bacteria and kill them. Natural killer cells lyse neoplastic and virally-infected cells. These cells of innate immunity may be considered a second line of defense since the offending agents have penetrated the first line of defense (e.g., skin).
Acquired Immunity
Acquired immunity is the result of a specific immune response to an infectious agent that has penetrated the first line of defense. T cells (thymus-derived) and B cells (bone marrow-derived) are the major lymphocytes involved with acquired immunity. There are subsets of T cells, namely T helper (TH) and cytolytic T (TC) cells. There are also subsets of TH cells, TH1 and TH2. TH1 cells produce cytokines (IL2, IL-12, IFN-g)
that favor cell-mediated immunity, whereas TH2 cells produce cytokines (IL-4, IL-5, IL6, IL-10) that enhance the humoral (antibody) response. B cells have antibodies on their membrane (i.e., surface immunoglobulin or sIg). When the sIg recognizes and binds to a specific antigen such as a bacterial cell (first signal) and a second signal is delivered by a TH2 cell, the B cell may become activated. Upon activation, the B cell will proliferate into a clone of B cells. Different clones develop in response to different antigenic determinants (small parts; 5-7 amino acids, for example) on the antigen*. Some B cells from each clone will become plasma cells that produce antibody that attaches to the antigen (i.e., the antigenic determinants). Bacteria that are coated with antibody are easily phagocytized; thus, the initial phase of the killing process is enhanced. Other B cells from each clone will become memory cells.
Memory cells live for years and become activated very quickly upon exposure to the same strain of bacteria. For most antigenic challenges, TH2 cells are necessary to help
B cells become fully activated. An acquired immune response that involves antibody production is referred to as humoral immunity. The humoral immune response is most commonly employed against pyogenic (pus-producing) bacteria, but antibodies are also produced against viruses, tumor antigens, transplanted tissue antigens, yeasts/fungi, and parasites; however, the humoral response is not capable of eliminating the problem in these latter cases. Cell-mediated immunity is called upon to rid the body of viruses, yeasts/fungi, parasites, tumors, and grafts (i.e., transplanted tissue). *The immune response is not directed against an entire antigen molecule, but rather against small parts of it (5-7 residues, typically amino acids) that serve as antigenic determinants.
Regarding humoral immunity, a discussion of the different types of antibody molecules produced by humans is necessary to understand some studies that have measured antibody responses to exercise. There are five major classes or isotypes of antibodies. IgG is the most common antibody circulating in peripheral blood, comprising approximately 75% of the total antibody level (about 1,200 mg/dL). IgG is a good opsonin. It also activates complement, a series proteins that ultimately lyse target cells (e.g., bacteria). IgG is the only antibody that crosses the placenta. Thus, newborns are born with an adult level of IgG, which gradually disappears over the next few months. They start making their own antibody at 2-3 months of age. IgG is the major antibody of the secondary immune response (i.e., the response that occurs upon re-exposure to a specific antigen), also called the anamestic or memory response. IgM is the largest antibody molecule, consisting of five monomeric units (IgG is comprised of a single monomeric unit). It can also activate complement, and it is the major antibody of the primary immune response (i.e., the response that occurs upon the first exposure to a specific antigen). IgA circulates in the peripheral blood and is also found in abundance in secretions (e.g., saliva, mucus, breast milk). Secretory IgA is a first line of defense in that it can bind to infectious agents before they have a chance to attach to and infect host cells. IgE and IgD are found in very low concentrations in peripheral blood. IgE is involved in allergies (Type I hypersensitivity reactions) and certain parasitic (helminth) infections. The role of IgD is unknown, but it is found on the surface of many B cells together with IgM.
Cell-Mediated Immunity
Cell-mediated immunity refers to an acquired immune response in which T cells are the major players. Both TH1 cells and cytolytic T cells (TC cells) are typically activated by the offending antigen (e.g., a virus) and a second signal (interaction of other membrane molecules). The TC cells (also called T cytotoxic cells) lyse the infected cells, tumor cells, etc. As stated previously, viruses, yeasts/fungi, parasites, tumors and grafts typically evoke predominantly a cell-mediated immune response, although antibodies are also produced. A pure cell-mediated immune response (no B cells or antibody involved) is uncommon.
Regardless of whether the acquired immune response is humoral or cell-mediated, the offending antigen is usually presented to T cells by an antigen-presenting cell (APC). More specifically, a peptide fragment of the antigen is presented to T cells. Cells that can act as APCs include macrophages, follicular dendritic cells, B cells, Langerhans cells of the skin, and cytokine-stimulated cells (e.g., activated T cells). The peptide is presented to the T cell via interaction with an MHC class I or class II molecule on the APC and the T cell receptor (TCR) on the T cell. In other words, part of the peptide is bound to an MHC class I or II molecule and another part is bound to the TCR. This serves as one signal. The second signal may be provided by interaction of other membrane molecules. Interleukin-1 (IL-1) is a cytokine produced by macrophages and other cells that activates TH cells. Interleukin-2 (IL-2) is a cytokine produced by activated T cells that drives the proliferation and differentiation of T cells. Some antigens, such as lipopolysaccharides, are known as T-independent antigens because they can activate B cells directly without
T cell help, but the immune response is weaker and a memory (anamestic) response does not occur. Note: MHC molecules and TCRs are glycoproteins that are part of the cell membrane. MHC is an acronym for major histocompatibility complex, which is a region on chromsome 6 that codes for these glycoproteins.
Phagocytosis
The phagocytic system consists of neutrophils, monocytes and macrophages. Monocytes become macrophages when they leave the peripheral blood and enter tissues. These phagocytic cells have membrane receptors (i.e., specific molecules on their cell membrane) that recognize and bind to complement proteins (e.g., C3b) and IgG, a type of antibody. Bacteria that are coated with complement and/or IgG antibody will adhere to the phagocytic cells via these receptors. This process is called opsonization and it makes it much easier for phagocytes to ingest bacteria. Once the bacterial cell is ingested, enzymes and other substances are released into the phagocytic vacuole from cytoplasmic granules fused with the vacuole. Some enzymes serve a digestive function while other enzymes and substances contribute to the killing of the bacterial cell. One means of killing bacteria is by generating oxygen radicals (superoxide radical, hydroxyl radical, singlet oxygen) through the hexose monophosphate shunt (HMS). Glucose-6-phosphate dehydrogenase (G6PD) is a very important enzyme for this pathway in that it connects glycolysis to the HMS. In other words, without G6PD the HMS could not function.
Phagocytes also generate these unstable forms of oxygen in inflammatory conditions other than just bacterial infections. When tissues such as muscle or tendons are damaged, an inflammatory process is typically activated; however, the resulting tissue fragments may be too large for ingestion by phagocytes. The “frustrated” phagocytes release the contents of their cytoplasmic granules in the damaged area. Free oxygen radicals that are generated damage tissue in the area via oxidation. Consequently, ingestion of antioxidants (e.g., bioflavanoids, vitamins A, C, E) may help minimize inflammation. Interestingly, antioxidants have been purported to curtail atherosclerosis by inhibiting the oxidation of LDL.
Complement Proteins
Complement is a series of proteins that normally circulate in the plasma. A number of substances can activate complement, including IgG and IgM. Typically, when the first complement protein is activated, the remaining complement proteins are sequentially activated ultimately leading to lysis of the target cell (e.g., bacterium). Some of the complement proteins serve other biological functions in addition to cell lysis. For example, C5a, C3a and C4a produce smooth muscle contraction, increase vascular permeability, and induce the release of histamine from mast cells and basophils (i.e., they are anaphylatoxins). C5a is also chemotactic; it attracts neutrophils. Some complement proteins serve as opsonins (e.g., C3b, C4b) as phagocytes and other cells have membrane receptors for these proteins. C3 and C4 are commonly measured complement proteins. A deficiency or defect of one or more of the complement proteins is often associated with an increased frequency of infections.
Membrane Marker Terminology
Since studies on exercise and immunology typically use membrane marker terminology to indicate specific cell types, a discussion of the most common membrane markers is necessary. Standardized nomenclature uses the clusters of differentiation (CD) designation for various proteins on the surface of lymphocytes and other immune cells. CD2 is found on all T cells. It is an adhesion molecule that binds to the LFA-3 receptor (CD58) on other cells (e.g., antigen-presenting cells). The T cell receptor (TCR; no CD designation) is also found on all T cells. Structurally, it resembles the antigen-binding region of antibody molecules. As stated previously, it recognizes protein antigens (peptide fragments) as the T cell interacts with an antigen-presenting cell. CD3 consists of four polypeptide chains that are physically associated with the TCR. It appears to transduce activating signals to the cytoplasm of the T cell when antigen binds to the TCR. Helper/inducer T cells are identified by CD4. About two-thirds of peripheral blood T cells express CD4. CD8 is expressed by about one-third of peripheral blood
T cells and identifies these cells as cytolytic T cells. CD4 and CD8 play an important role in antigen presentation in that CD4 T cells will only interact with APCs that present antigen bound to MHC class II molecules, and CD8 T cells interact only with APCs that present antigen bound to MHC class I molecules.
A key membrane marker of B cells is surface immunoglobulin (sIg), which plays a major role in antigen recognition and activation of the immune response. B cells also carry CD19, CD20 and numerous other membrane proteins. CD16 and CD56 are membrane markers found on natural killer cells. It is noteworthy that while a particular membrane marker may be used to identify a specific cell or subset of cells, that marker may be found on other cells as well. Typically two or more markers are used to identify a specific cell. Many membrane markers have been described for several cell types including lymphocytes and their subsets, monocytes, macrophages, neutrophils, endothelial cells, dendritic cells, and erythrocytes.
Cytokines and Cytokine Receptors
Cytokines are proteins made by cells that affect the behavior of other cells. Cytokines act on specific cytokine receptors on the cells that they affect. Interleukin-1 (IL-1) is produced by macrophages and other cells. It activates T cells and produces fever. Interleukin-2 (IL-2) is produced by T cells and stimulates T cell proliferation. It is also known as T cell growth factor. Interleukin-3 (IL-3) is produced by T cells. It works with colony stimulating factors (CSFs) to stimulate the proliferation and differentiation of hematopoietic cells (e.g., lymphocytes, granulocytes, monocytes, erythrocytes). Interleukin-6 (IL-6) stimulates T and B cell growth and differentiation, acute phase protein production and fever. T cells, macrophages, and endothelial cells produce it.
T cells, NK cells and other leukocytes produce interferons (IFN-a, IFN-b, IFN-g). Interferons exhibit anti-viral properties, activate macrophages, and increase the expression of MHC molecules. Tumor necrosis factors (TNFs) are produced primarily by macrophages, T cells, B cells and NK cells. They contribute to the inflammatory response, activate cells, and kill tumor (and other) cells. Transforming growth factor ß (TNF-ß) is produced by T and B cells, macrophages and other cells. This cytokine suppresses the immune system at the systemic level, but stimulates immune and inflammatory responses at the local level [4]. Colony-stimulating factors promote the growth and differentiation of various cell lines.
EFFECTS OF EXERCISE ON THE IMMUNE SYSTEM
Before discussing the effects of exercise on the immune system, problems with assessing immune function need to be addressed. These issues make interpretation of the findings of studies challenging and may account for some differences among studies evaluating the same or similar immune responses to exercise. To begin with, the type, intensity, frequency, and duration of exercise vary among the different studies. There are also significant differences between animal and human studies. For example, invasive sampling can be performed on animals, whereas human studies typically rely on immune cells obtained from peripheral blood only. Additional differences among studies include age, gender, heredity, diet, life style, and initial fitness level of subjects. There are also different techniques employed for measuring immune parameters and variations in the way in which results are reported. Nonetheless, some generalizations can be made.
Cell Counts
The total white cell count increases during and after a bout of exercise most likely due to demargination (detachment from the endothelial lining). Whether or not increased production and release from the bone marrow occurs may be debatable [3], although hematologists consistently attribute the increase to demargination [1]. Other sources such as the lung, spleen, and GI tract have also been reported [3]. The increase in the leukocyte count is due primarily to an increase in neutrophils, although lymphocytes and monocytes are increased as well [6]. Exercise intensity, duration and/or fitness level may play a role in the degree of leukocytosis [2]. Both the neutrophil and lymphocyte count are normal in athletes [2] suggesting that training does not alter these cell counts. Some studies have reported decreased numbers of NK cells and monocytes during periods of intense training [2,6]. Mechanisms that may be involved with exercise-induced leukocytosis include increased catecholamines, which increase cardiac output and modify adhesion molecules [6], and alterations in cytokine levels that influence the expression of adhesion molecules.
Cell Function
Conflicting results have been reported on neutrophil function, which may be due to different exercise protocols and the particular function being measured. Brief high intensity, prolonged submaximal and intense exercise have been reported to increase oxidative burst activity in neutrophils [2], an indication of enhanced microbicidal activity. Various exercises have induced increased neutrophil activation as inferred by degranulation [6]. A number of neutrophil functions at rest and after exercise tend to be lower in athletes compared to nonathletes [2]. On the other hand, some studies have reported no change or an increase in certain functions (e.g., adhesion) in response to moderate training, whereas heavy training tends to decrease neutrophil function [6].
Studies on the effects of exercise on lymphocyte function have also produced inconsistent results; however, MacKinnon [2] states that acute exercise and exercise training may activate lymphocytes. It is not known if this is due to selective recruitment of activated cells into the circulation or because cells are activated during exercise or both. MacKinnon [2] cites studies that suggest both brief and very prolonged intense exercise induce activation of T cells.
Lymphocyte proliferation may be stimulated or not affected by brief moderate exercise, whereas intense or prolonged exercise may suppress proliferative responses. Studies noted by Shephard [6] depict a similar pattern, although there are too many contradictory results to reach any firm conclusions. NK cells typically show increased cytolytic activity during brief and prolonged exercise, but a consistent postexercise pattern has not emerged from the data [2,6], although prolonged exercise appears to produce postexercise suppression of NK cytolytic activity [2]. The increased NK activity during exercise may by due simply to an increased number of NK cells in the circulation. Although there are some inconsistencies among studies that evaluated lymphocyte function in response to training, most studies have reported no difference in lymphocyte activity between athletes and nonathletes suggesting that training does not alter lymphocyte function. T cell function and B cell function have also been assessed via cytokine production and antibody production, respectively. Cytokine and antibody responses to exercise are discussed in the next section. Studies of monocyte/macrophage function have produced disparate results for both acute exercise and training effects. Despite the inconsistent studies, it is feasible that moderate exercise may have a beneficial effect on macrophage function, while exhaustive exercise may have a suppressive effect [6].
Cytokines
MacKinnon [2] discusses several issues concerning the significance of cytokine changes in response to exercise. Cytokines are normally present at extremely low concentrations and are rapidly cleared from the blood and other body compartments. Although the pro-inflammatory cytokines (IL-1, IL-6) are released during and after exercise, the responses appear to be subtle and have not been consistently observed. Furthermore, interpretation of changes in cytokine levels is difficult due to the large number of cytokines and their diverse actions.
Vigorous and prolonged exercise or eccentric exercises typically produce an inflammatory reaction. Urinary excretion of IL-1ß, IL-6, and IFN-g has been noted following prolonged exercise [2,5]. Interestingly, these four cytokines were chronically elevated in the urine of trained versus untrained persons. Pedersen [5] has proposed a model to explain the possible relationship between cytokines and muscle damage. He suggests that eccentric exercise damages muscle leading to necrosis and inflammation. Inflammatory cytokines are produced which augment the inflammatory response. Neutrophils accumulate in the muscle followed by macrophages. Cytokines induce the macrophages to produce prostaglandins (e.g., PGE2), which bring about muscle pain.
In contrast, Malm [3] suggests that cells do not migrate to skeletal muscle after exercise. He also reports that muscle adaptation to exercise may occur in a non-inflammatory fashion. The immune system may affect skeletal muscle adaptation via interactions between leukocyte and endothelial cell adhesion molecules and release of cytokines and growth factors. Additionally, Malm [3] indicates that moderate exercise enhances a
TH1-type cytokine response, which should boost protection against viral infection, while strenuous exercise augments TH2 cytokines and, thus, promotes protection against bacterial infection. Clearly, more research is needed to determine the cytokine response to exercise.
Immunoglobulins (Antibodies)
Both MacKinnon [2] and Shephard [6] cite studies that report substantial decreases in salivary IgA after intense and/or prolonged exercise, but little or no effect after moderate exercise. Generally, serum immunoglobulins either do not change or increase slightly after various types of exercise. Both salivary and serum immunoglobulins either do not change or increase after moderate training, but decrease with intense training [2,6]. While MacKinnon [2] notes that athletes undergoing intense training can still mount an appropriate serum antibody response, Shephard [6] observes that decreases in salivary IgA have been associated with an increased prevalence of upper respiratory tract infections (URTIs).
Pedersen [5] has proposed an open window hypothesis to at least partly explain the increased incidence of URTIs in elite athletes. The hypothesis asserts that the immune system is enhanced during moderate and severe exercise, but suppressed following intense long-duration exercise. The immunosuppression includes a decreased number of lymphocytes, decreased natural killer cell cytotoxicity, and decreased secretory IgA levels in mucosa. The period of immunosuppression following intense long-duration exercise is referred to as the “open window.” It is during this time that infectious microbes, particularly viruses, may take advantage of the opportunity to establish an infection. This window of opportunity may be longer and more pronounced in elite athletes. Pedersen [5] also proposes a potential benefit of this period of immunosuppression. If tissue damage results from the intense exercise, the immune system could be exposed to new antigens. The immunosuppression may serve to prevent autoimmune reactions.
Overtraining
Overtraining involves physiological, psychological, and immunological factors. This discussion will focus on the immune factors, particularly the reactions to tissue injury and the similarity of these reactions to sepsis. The information from this section is derived from Shephard [6]. A single bout of exhaustive exercise, especially eccentric exercise, can cause an inflammatory reaction in the active muscle, which resembles the process of sepsis. The affected muscle is infiltrated by neutrophils followed by macrophages. Free radicals (unstable forms of oxygen) accumulate as the neutrophils and macrophages are activated. Inflammatory cytokines and acute phase reactants are released. Complement and the coagulation and fibrinolytic pathways are activated. Given these inflammatory events, icing the affected muscle and ingestion of antioxidants (bioflavanoids, vitamins A, C, and E) should help control the inflammation and, consequently, reduce muscle soreness.
Icing will slow the delivery of neutrophils and monocytes/macrophages to the site by reducing blood flow. It will also slow enzyme reactions that proceed optimally at body temperature, thus slowing the entire inflammatory process. Antioxidants will counteract the oxygen radicals. Since antioxidants must be at the site to be effective, ingestion must occur prior to the initiation of exercise and should continue throughout the training period (i.e., daily dosages). While these measures will help control inflammation, they will not prevent the many symptoms associated with overtraining, nor is it likely that muscle soreness can be entirely prevented during extreme training periods.
INFECTIOUS DISEASE
The relationship between exercise and susceptibility to infection involves several factors, including the type of infection, the quality and quantity of exercise, and the timing of exercise relative to the course of the disease process [6]. Regular moderate exercise enhances immune function, while prolonged intense exercise can suppress immune function. The effect of moderate versus intense prolonged exercise on specific immune parameters (cell number and function, antibodies) has been described above.
After reviewing several human and animal studies, Pedersen [5] provided the following conclusions regarding exercise and infectious disease:
- Exercise or training prior to inoculation of an infectious agent (especially viruses) tends to reduce susceptibility to infectious disease.
- Moderate exercise training during an infection does not affect the outcome of the disease.
- Exhaustive exercise during an infection generally enhances the severity of the infection. This is likely related to exercise-associated immunosuppression (i.e., open window hypothesis).
- The nature of the infectious microbes and the site of infection may play a role.
The higher incidence of upper respiratory tract infections after intense exercise may be due in part to suppression of natural immunity and possibly direct suppression of the secretory immune system by the cold and dry air effects on the local mucosa. Moderate exercise enhances the immune system; intense exercise suppresses the immune response.
EXERCISE-INDUCED ASTHMA
Shephard [6] provides a definition of exercise-induced bronchospasm as a decrease in forced expiratory volume of at least 10%. It typically develops within 5 to10 minutes after starting to exercise and may resolve in 30 to 90 minutes of rest or while still exercising. The usual trigger appears to be inhalation of cold, dry air via the mouth. Mucosal drying may trigger the release of pharmacologic mediators from mast cells, which bring about the bronchospasm. The mediators include histamine, leukotrienes, kinins, serotonin, prostaglandins, thromboxanes, and cytokines. They produce vasodilation, increased vascular permeability, smooth muscle contraction, and activate other cells. Following the bronchospasm a refractory period develops which lasts for several hours. During this period bronchospasm is not easily provoked, possibly due to inhibitory prostaglandins. There is little known about the effects of training on bronchospasm, but improved fitness should decrease the ventilatory effort needed for exercise and should reduce the degree of cooling and drying of the bronchial mucosa. Prophylaxis involves avoiding of known allergens before exercise, wearing a facemask in cold weather to humidify air and treating respiratory infections promptly. The main treatment is an aerosolized ß-2 agonist, which inhibits smooth muscle contraction and mediator release. Unfortunately for sufferers, ß-2 agonists are banned in many athletic competitions. Antihistamines, calcium channel blockers and other drugs may also be effective.
CANCER
The role of the immune system in preventing and fighting cancer is not entirely clear [4]. The immune system may be involved primarily in malignancies of viral origin. Viral antigens are the most immunogenic molecules on tumors. An immune response that protects against (oncogenic) viral infections may be the primary manner in which the immune system protects against cancer.
Generally, moderate regular exercise seems to decrease the risk of colon cancer and possibly breast cancer and cancer of the female reproductive tract [5,6]. Possible immune mechanisms for cancer protection that result from training include increases in the number and/or activity of macrophages and natural killer cells and their regulatory cytokines. There are a number of secondary effects of exercise that may also help protect against cancer [6]. A healthy lifestyle tends to be adopted by those who exercise regularly. Transit of food through the intestines is likely increased by exercise, which would decrease exposure to carcinogens. Exercise tends to decrease body fat and obesity has been associated with an increased cancer risk. Improved circulation may facilitate interaction of immune cells with tumors.
