# Effect of Moderate-Intensity Endurance Exercise on Inflammatory Cytokines in Leukocytes of Dogs
## 4 Discussion
Neutrophphils are also the most abundant effector cells in the immune system and are the cells that first migrate from the bloodstream to the site of tissue inflammation in response to pathogens or inflammation [47]. Caused by stress from exercise, when inflammation produced by tissue and immune cells accumulates, it circulates through the blood to the bone marrow to activate hematopoietic stem cells and release numerous cytokines, such as TNF-\(\alpha\), IFN-\(\gamma\), and IL-6, which are responsible for the spread and regulation of inflammation [48; 49; 50; 51]. Neutrophphils are mobilized through the process of emergency granulocyte formation for the proper regulation of cytokines [52]. Since neutrophils are cells with a short lifespan of 6-8 h [53], a large number of neutrophils are required to be produced [52]. Various prior studies have shown that human and animal models show a decrease in WBCs and neutrophils after endurance exercise [40; 45; 54; 55]. Therefore, the transient decrease in WBCs identified in this study is believed to be the result of these mechanisms. Recently, opinions have been raised that WBCs could be a source of cytokines [56; 57; 58]. Changes in TNF-\(\alpha\), IFN-\(\gamma\), IL-1\(\beta\), and IL-4 were identified in dogs that exercised in this study. Cytokines are divided into T helper 1 (Th1) and Th2 T lymphocytes according to the profile produced [59]. TNF and IFN included in Th1 are activated in intracellular inflammatory responses, whereas IL-4, classified as Th2, is activated in extracellular inflammatory responses [59; 60]. In general, exercise is known to induce an increase in IL-6 [61; 62]. The changes in IL-6 identified in this study are unexpected results, and the reason for this is difficult to know. However, even in a study investigating changes in aerobic exercise (runningand cycling), the changes in IL-6 according to the type of exercise did not coincide with each other [63], and it was reported that the health status of the subjects and the increase in IL-6 were inversely proportional [64; 65; 66]. Considering this, it may have been difficult to confirm a significant increase in IL-6 because healthy dogs participated in this study. Although moderate levels of exercise are known to have a positive effect on the immune system, including reduced upper respiratory tract infections and inflammation [67; 68], strenuous exercise is known to increase pro-inflammatory cytokines, such as TNF-\(\alpha\), IL-1\(\beta\), and IL-6, by reducing immune cell function and natural killer cell response [69; 70; 71]. Traditionally, IL-6 is the fastest-expressed inflammatory cytokine in the circulatory system in stressful situations and is known to promote the production of TNF-\(\alpha\) and IL-1\(\beta\)[72; 73]. On the other hand, in an IL-6 knockout mouse model, TNF-\(\alpha\) was slightly decreased after exercise, and it was shown that the expression of TNF-\(\alpha\) can be inhibited by other pathways as well as through independent pathways by IL-6 [74]. As a result of moderate exercise in healthy young adults for six weeks, similar to the exercise period of this study, TNF-\(\alpha\) was significantly reduced compared to the high-intensity exercise group [75]. In another study, healthy adults performed moderate-intensity endurance exercises for four weeks, and TNF-\(\alpha\) was significantly reduced post-exercise [22]. Given this, dogs exhibiting reduced TNF-\(\alpha\) due to proper levels of exercise intensity fits well with the existing literature. Meanwhile, many CD8\({}^{+}\) T cells are \(\beta\)2-adrenergic, producing IFN-\(\gamma\) and IL-18 [76; 77]. In dogs, as in humans, exercise is generally known to activate epinephrine [78; 79], and increased epinephrine appears to inhibit T cell receptors, reducing CD8\({}^{+}\) T cells [80; 81] and leading to a decrease in IFN and IL-1\(\beta\). The same results were found in humans and horses who performed aerobic exercise [76; 82].