Immunological changes in human skeletal muscle and blood after eccentric exercise and multiple biopsies.

A role of the immune system in muscular adaptation to physical exercise has been suggested but data from controlled human studies are scarce. The present study investigated immunological events in human blood and skeletal muscle by immunohistochemistry and flow cytometry after eccentric cycling exercise and multiple biopsies. Immunohistochemical detection of neutrophil- (CD11b, CD15), macrophage- (CD163), satellite cell- (CD56) and IL-1β-specific antigens increased similarly in human skeletal muscle after eccentric cycling exercise together with multiple muscle biopsies, or multiple biopsies only. Changes in immunological variables in blood and muscle were related, and monocytes and natural killer (NK) cells appeared to have governing functions over immunological events in human skeletal muscle. Delayed onset muscle soreness, serum creatine kinase activity and C-reactive protein concentration were not related to leukocyte infiltration in human skeletal muscle. Eccentric cycling and/or muscle biopsies did not result in T cell infiltration in human skeletal muscle. Modes of stress other than eccentric cycling should therefore be evaluated as a myositis model in human. Based on results from the present study, and in the light of previously published data, it appears plausible that muscular adaptation to physical exercise occurs without preceding muscle inflammation. Nevertheless, leukocytes seem important for repair, regeneration and adaptation of human skeletal muscle. Muscular adaptation to physical stress is of significant importance for normal muscular development and function. Without stimulation from physical activity, muscle tissue will undergo atrophy and decreased functional capacity. In addition, muscular adaptation to physical exercise is indispensable for increased physical performance with training. The mechanisms responsible for maintaining normal muscle function in healthy individuals are largely unknown, as the processes involved in adapting muscle tissue to changes in functional demand have not been clarified. The involvement of several different systems, including the nervous, neuroendocrine, vascular and immune systems, is most probably inevitable (Grounds, 1991; Felten et al. 1993; Ottaway & Husband, 1994; Chambers & McDermott, 1996). Exercise, especially if strenuous and including eccentric muscle contractions, is believed to induce local muscle damage resulting in the release of various substances such as intracellular proteins, cytokines and chemokines, ultimately resulting in an inflammatory response (Shek & Shephard, 1998). This local inflammation may include complement activation, upregulation of adhesion molecule expression on leukocytes and endothelium with subsequent migration and infiltration of positively selected blood leukocytes into the affected tissue (Tidball, 1995). Suggestions have been made that the immune system may play an important role in adaptation to physical stress (Fielding & Evans, 1997), but few studies have yet been conducted to directly investigate this hypothesis. One possible explanation for the lack of investigations may be the lack of applicable analytical methods. Some of the methodological limitations have now been overcome, and numerous studies have investigated the effect of exercise on circulating blood leukocytes regarding absolute number, percentage distribution and in vitro function (Pedersen, 1997). These studies are important for understanding the systemic function of the immune system but give only limited information about local immunological events in peripheral tissue. It may be argued that leukocytes in circulation in fact represent a population of cells on their way towards participation in ongoing tissue surveillance, repair and adaptation. Knowledge regarding the interaction between circulating and tissue leukocytes may be used as a means to understand muscular adaptation to physical exercise, the mechanisms behind the overtraining syndrome in athletes and overuse injuries in conjunction with monotonous work tasks. In a clinical setting, an inflammatory-inducing exercise model can be used in the study of inflammatory muscle diseases as well as in designing exercise protocols for the elimination of some of the disabling symptoms associated with these diseases. The intention of this study was to perform quantitative and qualitative analysis of leukocytes in blood and quantitative analysis of leukocytes in human muscle tissue in response to exercise-induced stress. Based on current knowledge regarding the inflammatory response in tissue, a selection of common leukocyte membrane antigens believed to be involved in the exercise-induced activation of the immune system was investigated (Table 2). Cytokines can be mediators of the inflammatory response in muscle tissue, and some cytokines have previously been detected in muscle tissue from patients with chronic muscle inflammation (Lundberg et al. 1997). Thus, five different cytokines were investigated in this study (Table 2). Because the intention in this study was to investigate inflammatory events, there were concerns raised regarding the effects of multiple biopsies in the same muscle. A control group, which did not exercise, was therefore included to investigate the effects of multiple muscle biopsies on local and systemic immunological variables. Table 2 Staining panel for immunohistochemistry METHODS Subjects Thirteen healthy male subjects with a mean age of 23.9 years (range 19-32) and a mean body mass of 74.9 kg (range 63-95) participated in the study. All subjects were physically active on a regular basis (mean maximal oxygen uptake during concentric cycling (VO2,max) = 3.66 l min−1; range 2.80-4.57). After receiving oral and written information about the study, subjects signed an informed consent and were randomly assigned to either exercise or control groups. The study conformed with the Declaration of Helsinki, and was approved by the Ethics Committee at the Karolinska Institute (Dnr: 97-044).