Effects of Exercise on Gene Expression of Inflammatory Markers in Human Peripheral Blood Cells: A Systematic Review

Regular physical activity seems to be one of the most important contributors to prevent disease and promote health. Being physically active reduces the risk of developing chronic diseases such as cardiovascular disease, diabetes, and some types of cancers. The molecular mechanisms are however not fully elucidated. Depending on duration and intensity, exercise will cause disruption of muscle fibers triggering a temporary inflammatory response. This response may not only involve the muscle tissue, but also peripheral tissues such as white blood cells, which are important components of the immune system. The immune system plays a vital role in the development of atherosclerosis, thereby making white blood cells relevant to study when looking at molecular mechanisms induced by physical activity. In this review, we summarize the existing literature on exercise and gene expression in human white blood cells, and discuss these results in relation to inflammation and atherosclerosis.


Introduction
There are substantial epidemiological evidence that regular physical activity (exercise) reduce the risk of developing diseases such as cardiovascular disease [1][2][3][4], type 2 diabetes [2,4,5], and some types of cancers [2,4,6]. Regular physical activity is therefore one of the most important contributors to maintaining health. The mechanisms by which exercise contributes to health are however not fully understood.
An acute bout of exercise, depending on type, intensity, and duration [7][8][9], causes tissue injury, triggering a local and systemic inflammation with a release of both pro-and anti-inflammatory cytokines [10,11], while regularly physical activity seems to attenuate the inflammatory response promoting an anti-inflammatory environment in the body [3,12,13].
The inflammatory response may be studied through changes in circulating biomarkers, such as interleukins, chemokines, and other signaling molecules. When studying possible effects of exercise in relation to atherosclerosis, biomarkers for underlying processes, such as endothelial dysfunction, oxidative stress, and inflammation are relevant (Table 1).
Peripheral white blood cells are important components of the immune system, and the immune system is important in the development of atherosclerosis [14]. White blood cells are constantly interacting with other cells, such as endothelial cells in the arteries [15][16][17], making them relevant for studying the inflammatory process in atherosclerosis [17][18][19].
It is vital for the body to regulate the expression of genes in the process of adapting to changes in the environment, such as exercise [20]. Gene expression studies can be used to get an insight into molecular mechanisms in affected cells, and to reflect the early stages of activation in the immune system. Gene expression studies may therefore be a sensitive tool to characterize the early effects of exercise on immune regulation [12].

This article is part of the Topical Collection on Physical Activity
We have summarized gene expression studies in human white blood cells, including peripheral mononuclear blood cells (PBMCs), lymphocytes and monocytes, but excluding natural killer (NK) cells. Studies, both acute exercise interventions and interventions studies investigating the prolonged effects of exercise, have been included.

Literature Search
A literature search was conducted in a combined search in Ovid Medline and EMBASE in November 2014 and re-ran in February 2015. Medical subject headings (MeSH) combined with words, some of them truncated, in title or abstract (tw) or only in title (ti), was used as follows: leukocytes, Table 1 Common inflammatory markers, included in this review, and their biological functions relevant for atherosclerosis and physical activity Inflammatory markers Gene symbol Function Chemokine (C-C motif) ligand 2 CCL2 Involved in chemotactic activity for monocytes and basophils, binding to CCR2 and CCR4. Chemokine (C-C motif) ligand 3 CCL3 Involved in the acute inflammation by recruitment and activation of leukocytes. Chemokine (C-C motif) ligand 4 CCL4 Involved in the migration of immune cells, a chemoattractant. Chemokine (C-C motif) ligand 5 CCL5 Involved in recruiting leukocytes to inflammatory sites.

CXCL16
Involved in the migration of immune cells, a chemoattractant.
Endothelial nitric oxide synthase NOS3 Involved in the generation of NO in blood vessels, regulating vascular tone, and platelet aggregation. GATA binding protein 3 GATA3 T cell-specific transcription factor involved in the regulation of T cell development.
Glutathione peroxidase GPX Involved in the detoxification of hydrogen peroxide. Heat shock 27 kDa proteinassociated protein 1 HSPBAP1 Involved in stress resistance; actin organization, and translocation from the cytoplasm to the nucleus.
Heat shock 70 kDa protein 1A HSPA1A Involved in stress resistance; stabilizing proteins against aggregation and mediates the folding of newly translated proteins. Heat shock 70 kDa protein 6 HSPA6 Involved in stress resistance; protein folding, stabilization, and shuttling functions in response to stress. Inducible nitric oxide synthase NOS2 Involved in immune response and important in cellular signaling, produces NO. Interferon gamma IFNG Involved in the regulation of immune and inflammatory response, promotes Th1 differentiation. Interleukin 1 receptor antagonist IL1RN Inhibits the activity of IL1A/IL1B, and modulates a variety of interleukin 1-related immune and inflammatory responses. Interleukin 1 receptor-like 1 IL1R1 Involved in cytokine-induced immune and inflammatory response. Interleukin 10 IL10 Downregulates the expression of Th1 cytokines, enhances B cell survival, proliferation, and antibody production, able to block NF-κB activity. Interleukin 13 IL13 Immunoregulatory cytokine that plays a role in B cell maturation and differentiation, downregulates macrophage activity inhibiting the production of pro-inflammatory cytokines and chemokines. Interleukin 1a, interleukin 1b IL1A/IL1B Proliferation and maturation of lymphocytes, involved in inflammation and acute-phase response. Interleukin 4 IL4 Pleiotropic cytokine involved in T cell and macrophage differentiation and modulate the differentiation to Th2. Interleukin 6 IL6 A pleiotropic cytokine that plays important roles in inflammation and the acute-phase response. Interleukin 8 IL8 Involved in the acute inflammatory response, a chemoattractant. Matrix metallopeptidase 9 MMP9 Involved in the breakdown of extracellular matrix and tissue remodeling. NADPH oxidase NADPH oxidase Involved in the vascular superoxide production.

PTGS2
An enzyme responsible for formation of prostanoids, involved in inflammation.
Superoxide dismutase 1 SOD1 Involved in the anti-oxidative defense destroying free superoxide radicals in the body. Superoxide dismutase 2 SOD2 Involved in the anti-oxidative defense destroying free superoxide radicals in the body. TNF receptor-associated factor 6 TRAF6 Involved in signal transducing in NF-kappa B pathway.
Toll-like receptors 2, 4, and 7 TLR2, 4, and 7 Involved in recognition of pathogen-associated molecular patterns (PAMPs), mediate the production of cytokines necessary for the development of effective immunity. Transforming growth factor beta TGFB Involved in proliferation, differentiation, adhesion, and migration. Tumor necrosis factor alpha TNF Prototypical pro-inflammatory cytokine, plays a central role in inflammation, immune system development, and apoptosis. mononuclear/ or exp lymphocyte subsets/ or exp b-lymphocytes/ or exp t-lymphocytes/ or monocytes/ or (leukocyte* or m o n o n u c l e a r o r m o n o c y t e s o r l y m p h o c y t e * o r macrophage*).tw. AND exp exercise/ or exp physical endurance/ or (exercise* or physical activit* or athlete*).tw. or training.ti. or Athletes/ AND gene expression/ or exp transcription, genetic/ or gene expression regulation/ or downregulation/ or gene amplification/ or protein modification, translational/ or protein processing, post-translational/ or rna processing, post-transcriptional/ or transcriptional activation/ or up-regulation/ OR (gene expression or transcriptom or rna or mrna).tw. exp RNA AND exp Cytokines/ or exp Inflammation/ or exp Immune System Phenomena/ or (cytokine* or interleukin* or inflammat* or immun*).tw. The search was limited to humans with: not animals (including studies were data on humans and animals were reported separately) and to articles published in English, Danish, Norwegian, or Swedish.
After removing duplicates, 565 papers were identified. Only original papers and papers including invention studies with leukocytes, including PBMCs, monocytes, lymphocytes, and dendrite cells, were included. Studies were excluded if only whole blood was analyzed, no gene expression data were presented, or the intervention included giving food supplements in combination with exercise. Studies including NK cells only were not included. Neither were papers including subjects on a weight reduction program. Using these criteria, the number of relevant articles was 78. Another 46 papers were excluded after reading the abstract/full article. Additional two articles, identified from the reference lists of the already included papers, were found to be relevant and included in the review. After a re-run of the search, three new articles were included in the review. In total, 37 papers are included in the review as shown in Fig. 1.  Table 2a summarizes the effect of an acute bout of exercise on gene expression in white blood cells from 28 studies. In 26 of the studies, the subjects performed an acute bout of endurance exercise, while in the remaining two studies subjects performed an acute bout of strength exercise. In 18 of the studies, PBMCs were investigated, while in the other studies leukocytes (seven studies), lymphocytes (two studies), and monocytes (one study) were investigated.
Ullum et al. were one of the first to publish a paper where gene expression in PBMCs in response to exercise was studied [21]. They measured gene expression of interleukin (IL) 1A, IL1B, IL6, and tumor necrosis factor alpha (TNF) before and after an ergometric bicycle exercise, but did not find any effect on gene expression after exercise. These results were largely supported by Moldoveanu et al. [25], Bernecker et al. [35], and Natelson et al. [22]. Ostrowski et al. concluded slightly differently when showing that PBMC gene expression of interleukin 1 receptor antagonist (IL1RN) and IL1B were upregulated after a marathon race [23]. Xiang et al. showed an upregulation of messenger ribonucleic acid (mRNA) levels 1 week after a marathon race for several genes, among them IL4, GATA binding protein 3 (GATA3), chemokine (C-C-motif) receptor (CCR)4, CCR3, and CCR2 [37]. A change in the Th1/Th2 ratio was observed from pre-to post-marathon as interferon gamma (IFNG)/IL4 ratio and T cell-specific T-box transcription factor T-bet (TBX21)/GATA3 ratio decreased. In agreement with Xiang el al. [37], Ulven et al. showed an upregulation of GATA3 mRNA expression in PBMCs after ergometric cycling [36]. mRNA expression of IL1B, chemokine (C-X-C motif) ligand 16 (CXCL16), IL8, prostaglandinendoperoxide synthase 2 (PTGS2), and TBX21 were upregulated, while toll-like receptor 2 (TLR2) mRNA expression was downregulated after exercise. Li et al. found an increase in the mRNA gene expression of the anti-inflammatory cytokine IL13 after one hour tai chi [32].
Toll-like receptors (TLRs) play an important role in the immune system by recognizing and initiating an inflammatory response to dangerous molecules, possibly leading to the transcription of cytokines and chemokines [55,56]. Nickel et al. investigated how a marathon race affected the expression of TLRs in lean subjects exercising regularly compared to lean subjects and obese subjects exercising less regularly [33]. They found differences in mRNA expression of TLR4 and TLR7 between the groups.
There is some evidence that a reduced level of nitric oxide (NO) or an elevated level of superoxide (O 2 − ) increase the risk of cardiovascular disease. The expression and activity of inducible nitric oxide synthase (NOS2) relative to endothelial nitric oxide synthase (NOS3) is important in the regulation of inflammation [31]. Jenkins et al. compared the mRNA expression of several genes related to the antioxidant defense system in sedentary and physically active males after a treadmill test [31]. In CD34 − PBMCs, NOS3 gene expression increased in the sedentary group compared to the active group after exercise. No change was seen in the expression level of NOS2 between groups. Niess et al. found an increase in the expression of NOS2 after a marathon race, but not after a graded treadmill test [26].
The abovementioned studies were performed using reverse transcription polymerase chain reaction (RT-PCR). When using RT-PCR, only a limited number of genes can be analyzed at the same time. With a whole genome transcriptomic approach, it is possible to analyze thousands of genes at the same time.
Connolly et al. showed that 311 genes were altered in PBMCs from baseline to immediately after a cycle ergometer workout, 552 genes were regulated from end of exercise and 60 min into recovery while 292 genes were regulated between baseline and 60 min into recovery [38]. The majority of the genes upregulated from baseline to end of exercise were related to inflammation and stress. From end of exercise and 60 min into recovery, an upregulation of nuclear receptor subfamily 4, group A, member 2 (NR4A2) and regulator of G protein signaling 1 (RGS1) were observed. The mRNA levels of IL6 and IL10 were not affected by the exercise at any time point.
Radom-Aizik et al. investigated how a bout of ergometric cycling altered gene expression in PBMCs in early and late puberty females [13] and males [12]. They observed that there were differences in gene expression between genders and pubertal phase. Genes commonly regulated by exercise in all groups were related to growth, apoptosis, inflammation, and tissue repair.
Even though these studies are differently designed and the exercise programs are of different duration and intensities, exercise seems to have an influence on PBMCs. Genes regulated in these studies are associated with stress, inflammation, and tissue repair.

Gene Expression Studies in Leukocytes-Endurance Training
Heat shock proteins (HSP) have important functions as molecular chaperons and are produced in response to different stressful stimulus. It has also been shown that HSPs are able to function as powerful cytokines [57] by binding to TLR2 and TLR4 [51,57].  [29].
Subjects already participating in leisure activities performed a strenuous treadmill exercise, followed by a moderate treadmill exercise two weeks later in a study performed by Buttner et al. [39]. The mRNA levels of several genes were upregulated in both types of exercise, but there were differences depending on training intensity. Upregulated genes belonged to pathways associated with inflammation, stress signaling, electrolyte and substrate transport, extracellular matrix, and transcription factors.
Training intensity was also studied by Sakharov et al. who showed that 310 genes were upregulated in skiers after performing an exhausting treadmill test (RTE), while 69 genes were upregulated in the same subjects after performing a moderate treadmill test (MT) [43]. Sixty-four of the genes were identical in the RTE and MT, indicating a greater change in gene expression in response to a strenuous exercise.
These studies show that an acute bout of exercise alters gene expression in leukocytes and that exercise intensity might influence the level of inflammatory markers. The HSP may also play an important role in the acute response to exercise, possibly by inducing TLRs [51].

Gene Expression Studies in Monocytes and Lymphocytes-Endurance Training
Ferrer et al. wanted to assess the effects of swimming on the pro-and antioxidant system of lymphocytes [28]. They found an increase in gene expression of B cell CLL/lymphoma 2 (BCL2) and uncoupling protein (UCP)-3 after one hour swimming. No changes were seen in the expression of catalase, glutathione catalase (GPX), or peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PPARGC1A).
Heme oxygenase 1 (HO-1) is suggested to have both immune-protective and anti-inflammatory properties [27]. Thompson et al. performed a cross-over study were invited subjects ran for 75 min, and the following test day were sitting calmly in the laboratory. The HO-1 mRNA expression level increased after exercise, while no changes were seen when resting in the laboratory [27]. The results from Thompsons' study [27] was confirmed by Sureda et al. who showed that HO-1 gene expression increased after moderate-and highintensity endurance training in soccer players [30].
Thomas et al. investigated if exercise was associated with an activation of peroxisome proliferator-activated receptor gamma (PPARG) signaling in monocytes in response to an acute bout of exercise [34]. The mRNA expression level of thrombospondin receptor (CD36), nuclear receptor subfamily 1, group H, member 3 (NR1H3) and ATP-binding cassette, subfamily A, member 1 (ABCA1) were upregulated while the expression levels of PPARG, PPARGC1A, cholesteryl ester transfer protein (CETP), and lecithin-cholesterol acyltransfer-ase1 (L-CAT) were unchanged. These genes are key regulators of lipid and energy metabolism, which also are closely linked to inflammation. Radom-Aizik et al. found that the mRNA levels of TNF, TLR4, and CD36 were downregulated after an acute bout of exercise in monocytes [45] contradicting the results of Thomas et al. [34].
In the studies mentioned above, several genes known to be involved, or related to the immune system, have been investigated, both in males and females, young and elderly, and at different exercise intensities. All factors appear to have an effect on the outcome measured [59]. Few studies have been performed investigating the effects of gender and age. Some more studies have been performed studying differences in training intensities, making Sakharov et al. hypothesize that passing the anaerobic threshold is responsible for the differences seen in gene expression in white blood cells between low-and highintensity activities [43].

Gene Expression Studies in PBMCs After Acute Resistance Exercise
Only two studies have been looking at gene expression in response to an acute bout of resistance exercise, both using a microarray approach. Carlson et al. recruited healthy men to perform 30 min of resistance training [40]. Several genes were regulated in response to the exercise, and the greatest transcriptional changes were seen in genes related to immune response, cellular communication, and matrix remodeling. These results are largely supported by Storey et al. who found that 202 genes, primarily involved in cell-to-cell signaling, immune cell trafficking, organism survival, cell cycle, and cell death, were regulated after strength exercise [46].
The study of Carlson et al. [40] and Storey et al. [46] indicate that also strength exercise gives an acute inflammatory response. Given the differences in muscle work between endurance and strength exercise, one might expect a different inflammatory response, although some of the same pathways appear to be regulated as indicated by these two studies. Table 2b summarizes the effects of prolonged exercise on gene expression in white blood cells from nine studies, seven of which were performed using PBMCs and two using leukocytes. In studies using strength exercise as the intervention (four studies), mRNA expression level was measured before and after an acute bout of exercise, both at baseline and after the intervention period. In the remaining studies, gene expression was measured once before and once after the training period.

Gene Expression Studies in PBMCs and Leukocytes-Endurance Exercise
PBMC gene expression levels of IL6 and TNF in elite and recreational gymnasts were investigated by Tringali et al. [52]. The gene expression levels of IL6 and TNF were higher in elite gymnasts than in recreational gymnasts after half a year with exercise. Tringali et al. also showed that the IL6 gene expression level was higher in pre-pubertal girls than in girls having reached the menarche, and that the ratios of IL6/IL10 and TNF/IL10 were higher in elite gymnasts than in recreational gymnasts [52].
Eight weeks of a low-intensity training program elicited an increase in gene expression of PPARGC1A, peroxisome proliferator-activated receptor gamma, coactivator 1 beta (PPARGC1B), IL4, and CD 14 molecule (CD14) in leukocytes, while the mRNA expression levels of IL6, CXCL2, TNF, and CCL2 were downregulated in a study performed by Yakeu et al. [48].
Dias et al. used a microarray approach when discovering that 211 gene transcripts related to immune function, cell cycle processes, development, and growth were regulated in PBMCs after 18 weeks of endurance training. One hundred and fifty-two gene transcripts were upregulated while 59 were downregulated [53]. Thompson et al. took another perspective when they examined the effect of an exercise intervention followed by two weeks without training [15]. Fifty-three gene transcripts, primarily involved in cell cycling, cell-mediated immune response, and cell-to-cell signaling and interactions, were differently regulated after the exercise period. After a period of detraining, the expression levels of 22 of the 53 genes were unaltered while the expression of 31 genes returned to baseline.
Even though the abovementioned studies are performed using different subjects (male and female, young and elderly), the results indicate that regular and moderate endurance exercise seems to lower some of the pro-inflammatory markers and/or promote an anti-inflammatory profile in the body.

Gene Expression Studies in PBMCs-Resistance Exercise
Jiménez-Jiménez et al. investigated the inflammatory response in PBMCs in elderly men before and after an eccentric training program [47]. Following both the first and the second bout of exercise, an increase in gene expression of NOS2 and PTGS2 were observed. IL6 mRNA increased after the first bout of exercise only. An attenuation of the acute inflammatory response after the training period for NOS2 and PTGS2 following the second exercise bout were observed.
Fernandez-Gonzalo et al. investigated how gene expression levels of CD14 and TLR4 were regulated in response to a bout of eccentric exercise performed before and after a training program in men [49] and women [50]. The mRNA expression levels of TNF and TNF receptor-associated factor 6 (TRAF6) were also studied in males and females, respectively. The acute bout increased the gene expression of CD14 and TLR4 in both male and female. The gene expression levels of TNF in men and TRAF6 in females were also increased after the first bout of exercise. The training period did not influence the acute response of CD14 or TLR4 mRNA gene expression, but the mRNA levels of TNF and TRAF6 were attenuated in males and females, respectively, after the second bout of exercise compared to the first bout in trained subjects.
When investigating the gene expression of IL10 and TNF after a resistance exercise program, Rodriguez-Miguelez et al. found an increase in the gene expression level of IL10 in the training group compared to the control group [51]. The authors concluded that resistance exercise may represent an effective tool to lower the pro-inflammatory status through an increased IL10/TNF-ratio.
Gano et al. found that the mRNA expression levels of advanced glycosylation end product-specific receptor (AGER), neutrophil cytosolic factor 1 (NCF1), and CCL2 were downregulated after an exercise period, improving the inflammatory/oxidative gene expression profile after a training period [16].
In all the studies using strength exercise, an acute bout of exercise has been included both at baseline and after exercise. With this design, the investigators will get valuable information about how one subject reacts to an acute bout of exercise while being trained compared to being untrained. This might be a valuable approach to study the molecular adaptions to exercise.
There are few studies performed investigating the effects of prolonged exercise on white blood cells and inflammation. The studies performed indicate that both resistance and endurance exercise promote an anti-inflammatory environment, or attenuate the acute response seen after a period of regular exercise.

Discussion
Epidemiological studies have shown that regular physical activity improves health. The molecular mechanisms behind the beneficial effects have however not been completely elucidated. A number of factors such as individual genetic variability, different exercise protocols, the heterogeneous nature of exercise itself, and other lifestyle factors influence the effects observed [5]. Furthermore, exercise exerts a number of effects on the body such as improving insulin sensitivity and lipid profile in addition to lowering blood pressure [5]. All these factors influence each other making it hard to understand the complex interactions.
Exercise causes damage to the muscle resulting in disarrangement in fiber structures, loss of fiber integrity, and leakage of muscle protein. Trying to restore homeostasis, several repair processes starts, involving inflammation, resolution, muscle repair, and finally regeneration. The balance between pro-and anti-inflammatory cytokines and other signaling molecules seems to be important for the outcome of the repair and regenerating process [60].
It has been recognized that the muscle is an endocrine organ being able to influence other organs such as the immune system [61]. The immune system is also directly involved in the cellular and molecular events in the muscle after exercise by recruitment of macrophages, neutrophils, and lymphocytes participating in the clearance of necrotic tissue and producing signaling molecules [60]. It is therefore plausible to believe that exercise may induce a response in PBMCs [24]. It is also possible that PBMCs are influenced by the exercise itself, since the immune system is directly involved in the repair processes of the muscle after exercise [36]. Studying how exercise affects PBMC gene expression to elucidate the molecular mechanisms of exercise is therefore highly relevant.
Exercise seems to induce responses both in the innate and in the adaptive immune system. It appears to influence both signaling molecules and transcription factors, as shown in many of the studies included in this review. The innate and the adaptive immune systems mutually affect each other, even though the first reaction most likely occurs in the innate immune system. The response to exercise seems to be closely regulated and both pro-and inflammatory cytokines and signaling molecules are released.
TLRs are important constituents of the innate immune system. They are located at the cell surface and may be stimulated by endogenous molecules that potentially arise during exercise, such as HSPs [57] and interleukins [56]. An activation of the TLR may further elicit an immune response resulting in an upregulation of pro-inflammatory cytokines and chemokines, involving the NF-κB and MAPK-pathways.
Nickel et al. [33], Ulven et al. [36], and Fernandez-Gonzalo et al. [49,50] investigated the gene expression levels of TLRs after exercise. Their results are somewhat conflicting, but show that gene expression levels of TLRs are affected by exercise. The explanation for the different results might be due to the type of exercise performed, the training intensities, and the training status of the individuals-all shown to influence the inflammatory response [62].
NF-κB is a transcription factor regulating a large number of genes, not only related to immune response, but also to cell survival, differentiation, and proliferation. NF-κB is expressed in all cells and is involved in both the innate and the adaptive immune system [63]. The pro-inflammatory effects of NF-κB are well known, but there are also evidence indicating an effect of NF-κB signaling in the resolution of an inflammatory response [64]. There is some evidence that NF-κB play an important role in the development of atherosclerosis [65].
NADPH oxidases play an important role in the innate immune system. NADPH oxidases are enzymes producing superoxide (O 2 − ), which again may produce reactive free radicals (ROS), thus being a possible contributor to atherosclerosis. It is hypothesized that the NCF1 play a role in TNF signaling and that impairing its expression may lower the oxidative and inflammatory status [67]. Gano et al. showed that the gene expression levels of NCF1 and NADPH oxidase 1, subunit gp91 phox (NOX2) were downregulated in PBMCs after two months of brisk walking [16], supporting this hypothesis.
Superoxide dismutases (SODs) are important molecules in the body's defense against O 2 − and free radicals [68]. SODs are able to catalyze the O 2 − radical into regular oxygen or hydrogen peroxide (H 2 O 2 ) and further to H 2 O. Jenkins et al. investigated changes in mRNA levels of SODs in sedentary and endurance-trained athletes after a treadmill test [31], and found an increase in SOD1 and SOD2 gene expression in CD34 − cells after exercise in the sedentary group. There are two main classes of lymphocytes that are important in the adaptive immune responses. B lymphocytes are primarily responsible for the antibody responses, while T lymphocytes are responsible for the cell-mediated immune responses [69].
T lymphocytes may be divided into subsets such as T helper 1 (Th1) cells or the humoral/antibody T helper 2 (Th2) cells. Th1 cells primarily produce pro-inflammatory cytokines, while Th2 cells primarily produce anti-inflammatory cytokines [70,71]. These responses are well-documented, but they are not the only cytokine pattern possible [70,72]. It seems like the Th1-Th2 decision is important for proper immune function [70] and that the NF-κB pathway is involved in the regulation of this differentiation [20,63].
When comparing studies looking at the inflammatory response to exercise, it is important to compare subjects at the same physical level. Capomaccio et al. [73] and Fehrenbach et al. [24] found different expression levels of IL6 and heat shock protein 27 kDa-associated protein 1 (HSPBAP1) in highly trained athletes compared to lightly trained and untrained subjects at rest. Different baseline values may cause a different inflammatory response after exercising. Comparison among different cells types should also be done with caution as different cell types might respond differently to exercise [74].
There is also evidence indicating that the immune function and the response to exercise is altered with age [5,75] and across puberty [12,76]. Elderly people also seem to have a higher basal level of inflammatory markers than younger people [16,51,77].

Concluding Remarks and Future Perspectives
The effects of exercise on the body are multitudinous. Even though the study designs, the groups included, and the types of exercise used in the studies included in this review varies, it seems reasonable to conclude that exercise has an effect on cells of the immune system. Genes regulated after exercise are involved in inflammation, cellular communication, signal transduction, cellular protection, growth, and repair.
Overall, these studies show that an acute bout of exercise induces an immediate pro-inflammatory response, but that changes also occur in some of the anti-inflammatory markers. Prolonged and regular physical activity seems to promote an anti-inflammatory environment or attenuating the acute response to exercise, possibly reducing the risk of developing inflammatory-related diseases such as atherosclerosis [14,78].
More research, preferably long-term standardized mechanistic studies, is needed to understand the impact of the observed change in gene expression, both in the muscles and in other organs, and to elucidate the complex interaction between white blood cells and muscle inflammatory response.
Financial Disclosure KBH and SMU have received research grant from Tine SA, Mills DA and Olympic Seafood.

Compliance with Ethics Guidelines
Conflict of Interest Gyrd Omholt Gjevestad is employed in TINE SA. Kirsten Holven reports grants from TINE BA, grants from Mills, grants from Olympic seafood, outside the submitted work. Stine Marie Ulven reports grants from TINE SA, grants from Mills, grants from Olympic seafood, outside the submitted work. This work was supported by grants from the Throne Holst Foundation for Nutrition Research; the University of Oslo, Oslo; and Akershus University College of Applied Sciences.
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