Role of aerobic exercise training on tumor‐infiltrated immune cells
Authors: Voltarelli Vanessa A. , Amano Mariane T. , Camargo Anamaria A. , Brum Patricia C. 
Published: May 2022

Background

Despite the remarkable treatment progress in the last decades, cancer remains the second leading cause of death in the world. Therefore, strategies capable of attenuating tumor initiation, progression and aggressiveness are important for reducing incidence and mortality caused by cancer. Considering this, it is well known that aerobic exercise training (AET) promotes several beneficial adaptations in the body, and it has emerged as a possible strategy to attenuate tumor progression. However, the mechanisms underlining the beneficial effects of AET on tumor progression are still unclear. Aim: Therefore, this study intended to evaluate whether AET could positively modify the tumor microenvironment, favoring infiltration of “antitumor” immune cells, and/or reducing the amount of “protumor” immune cells, consequently attenuating tumor progression.

Methods

To test this, Balb/c mice were submitted to a moderate intensity AET (60% of maximal aerobic capacity, 5 days/week, 1 hour/day, in a treadmill) for 1 month before being inoculated with 1x106 CT26 colon carcinoma cells (subcutaneous injection). The AET protocol was maintained during tumor progression, and the last exercise session was performed 48 hours before animals were sacrificed (9 days after tumor cells inoculation). Tumor volume and body weight were measured daily. Experimental groups were divided into control (healthy sedentary mice), CT26 SED (sedentary tumor‐bearing mice) and CT26 TR (trained tumor‐bearing mice). Tumors were harvested at day 9 and digested with collagenase IV and DNAse. After that, tumor leukocytes were separated by a Percoll gradient. Tumor‐infiltrated dendritic cells, macrophages, natural killer cells (NK), and T lymphocytes were measured by flow cytometry. Statistical analysis: Anova One‐way, Duncan post hoc, p<0.05. Ethical committee approval: CEUA EEFE‐USP 2017/02.

Results

Tumor‐bearing mice submitted to AET presented an attenuated tumor progression when compared with CT26 SED group, with significant reduction in tumor volume and mass (ex vivo). AET significantly increased the total amount of tumor‐infiltrated leukocytes (CD45+), which included an increase in the percentage of NK cells, total, activated and effector memory CD8+ T cells, and effector memory CD4+ T cells in CT26 TR when compared to CT26 SED. Although there were no significant changes in the tumor amount of total and activated CD4+ T cells, the CD4+/CD8+ ratio was significantly reduced in CT26 TR comparing with CT26 SED mice, suggesting that the balance between CD4+ and CD8+ T cells in trained mice tumors favored a cytotoxic/effector T lymphocyte profile. Moreover, the percentage of tumor‐infiltrated regulatory T cells (Tregs) in CT26 TR was significantly decreased when compared to CT26 SED mice. No significant changes were found for dendritic cells and macrophages (types M1 and M2).

Conclusion

AET increases tumor‐infiltrated amounts of cytotoxic/effector immune cells, such as NK and CD8+ T cells, while decreasing the percentage of Tregs in a colon cancer animal model. These data suggest that AET can attenuate tumor growth by modulation of tumor‐infiltrated immune cells number and profile.