The aim of this study was to determine the effects of supervised aerobic exercise training and to compare two forms of exercise prescription (HIIT or MICT) on peripheral T-cell subsets indicative of immunosenescence in a population of older women at high risk of developing breast cancer. The main findings from this study were: 1) HIIT decreased total granulocytes, total CD4 + T-cells, CD4 + naïve T-cells, CD4 + RTE and the CD4:CD8 ratio after 12-weeks training, whereas MICT increased total lymphocytes and CD8 EM T-cells; 2) The change in number of total T-cells, CD4 + naïve T-cells, CD4 + CM T-cells and CD4 + RTE was elevated after MICT compared to HIIT; and 3) changes in VO2max after training, regardless of exercise prescription, was positively related to changes in β2-AR expression on CM subsets of both CD4 + and CD8 + T-cells, and tended to be negatively related to the change in CD8 + EMRA T-cells.
Immunosenescence may increase the risk of developing age-related diseases such as cancer . Changes in the composition of peripheral T-cells are hallmark features of immunosenescence, with ‘older looking’ T-cell compartments (e.g. lower CD4:CD8 T-cell ratios, increased CD8 EMRA T-cells, and fewer naïve and RTE subsets of CD4 + and CD8 + T-cells) being predictive of poor immune responses to vaccination and all-cause mortality in older adults . T-cell profiles associated with immunosenescence were identified in women recently diagnosed with breast cancer and there is emerging evidence that immunosenescence could precipitate cancer occurrence [29, 30]. Exercise is known to increase immune function over the lifespan and concomitantly reduce cancer risk, particularly breast, prostate, colorectal and lung cancer [22, 31]. While cross-sectional studies have provided evidence that exercise can mitigate age-related changes in the peripheral T-cell compartment [5, 19, 19, 21, 22], longitudinal studies are required for us to know if exercise can help rejuvenate an already acquired ‘older looking’ T-cell profile. For the first time, we show here that MICT, but not HIIT, positively alters the peripheral T-cell compartment toward a less senescent phenotype. After training, the change in numbers of total T-cells, CD4 + naïve T-cells, CD4 + CM T-cells and CD4 + RTE were elevated after MICT compared to HIIT. Remarkably, MICT and HIIT tended to evoke divergent effects on the peripheral T-cell pool, with those T-cell subtypes found to increase or be maintained after MICT had conversely decreased after HIIT. The finding that MICT but not HIIT increased the frequency of CD8 + EM cells in circulation could have important implications for anti-tumor immune surveillance as these are the CD8 + T-cell subsets that predominantly infiltrate human breast tumors .
Exercise interventions involving HIIT have been preferred over MICT due to the lower time commitment and comparable improvements in cardiorespiratory fitness and biomarkers of inflammation [33, 34]. However, the present findings indicate that HIIT may not be an effective form of exercise training to evoke positive changes in the frequency of the peripheral T-cell compartment as they relate to markers of immunosenescence and could actually promote a pro-senescent phenotype. For instance, HIIT reduced naïve CD4 + T-cell numbers by ~ 38% whereas MICT tended to increase naïve CD4 + T-cells by ~ 20%. Although IL-7 is known to promote thymic mass and correlates with the frequency of peripheral naïve T-cells and RTEs [35, 36], we surprisingly found no relationships between changes in serum IL-7 and RTEs despite observing a trend for serum IL-7 levels to drop after HIIT but not MICT training. We did find, however, that changes in IL-7 were positively related with changes in the number of circulating CD4 and CD4 naïve T cells expressing the β2-AR in both treatment groups (HIIT + MICT) but not the controls. It has been shown in mice that the β2-AR binds norepinephrine to generate Th1 cells that produce 2- to 4-fold more IFN-γ during an immune response . It is possible, therefore, that IL-7 levels regulated by exercise can influence the generation of naïve CD4 + T-cells that are capable of differentiating into Th1 T-cells via norepinephrine but this requires further investigation. Additionally, HIIT reduced the total number of circulating granulocytes to near significant (p = 0.05) levels whereas MICT did not, indicating a potentially greater anti-inflammatory response promoted by HIIT over MICT. Indeed, a recent pilot study reported improvements in neutrophil function after 10-weeks of HIIT in older adults identified as high risk of developing type 2 diabetes, although this study did not compare HIIT to MICT . Collectively, these data underscore the importance of exercise, mode, intensity, duration and volume when it comes to prescribing exercise for immune and anti-inflammatory benefits in older adults, and in people at high risk of disease.
Although neither MICT or HIIT significantly altered the number of CD8 EMRA T-cells, we did find a near significant inverse relationship between the changes in absolute VO2max (ml/min) and number of CD8 + EMRA T-cells (p = 0.055). We previously reported inverse relationships between VO2max and highly differentiated (e.g. KLRG1+/CD28-/CD57+) CD8 + T-cells associated with immunosenescence in healthy men . These cross-sectional findings have been corroborated by other groups, in addition to the observation that VO2max is positively associated with the composition of naïve CD4 + and CD8 + T-cells and RTEs in peripheral blood [19, 35]. Collectively, these findings indicate that an exercise training program may have to evoke discernible changes in aerobic fitness to reduce the frequency of late-stage differentiated T-cells from the periphery. A recent study reported a decrease in ‘senescent’ CD57 + CD8 + T-cells and an increase in naïve CD8 + T-cells in peripheral blood following 6-weeks strength endurance training in older women seropositive to cytomegalovirus . The positive shifts in T-cell frequency reported here and by Dinh et al., bolstered by previous randomized controlled trials showing increased immune responses to vaccination after a period of exercise training [41, 42], provide robust evidence that exercise training is capable of rejuvenating an already acquired senescent phenotype to evoke meaningful changes in overall immune function. Moreover, while evidence is beginning to show that patients newly diagnosed with cancer have immunosenescent profiles and that exercise can extend survival during treatment for breast cancer and other solid tumors [43, 44], whether these exercise-induced changes in the frequency of T-cell subsets indicative of immunosenescence can lower the risk of developing breast cancer remains to be determined. A recent long-term follow up study of > 50,000 women found no relationship between circulating numbers of CD4 + or CD8 + T-cells and risk of developing breast cancer , although it is important to note that this study did not consider T-cell subset composition (e.g. naïve, CM, EM and EMRA) or functionality, which can be altered considerably without greatly affecting total CD4 + or CD8 + T-cell numbers [46–49]. It will also be important to consider how exercise-induced shifts in T-cell subsets can influence prognosis in breast cancer patients on active treatment and at different stages of disease. For instance, in patients with metastatic breast cancer, a higher frequency of circulating naïve CD4 + and CD8 + T-cells is associated with poorer prognosis in patients treated with high-dose paclitaxel but not cyclophosphamide containing regimens , while higher numbers of late-stage differentiated CD8 + T-cells have been associated with shorter progression-free survival and overall survival [30, 51].
The mechanisms by which exercise training can alter the composition of the peripheral T-cell compartment have not been fully determined. Exercise may limit the age-related expansion of late-stage differentiated T-cells by helping to exert better control over latent viral infections (e.g. CMV), or by progressively removing these cells by increasing their exposure to pro-apoptotic stimuli over time [39, 52]. Increased apoptosis of late-stage differentiated T-cells by exercise has been hypothesized to promote the mobilization of ‘new recruits’, facilitated by an increase hematopoiesis and muscle-derived cytokines, such as IL-7, that can promote maintenance of thymic mass and increase production and development of naïve T-cells . As previous studies found positive associations between the levels of muscle-derived cytokines such as IL-7 and IL-15 in plasma with the numbers of naïve and RTE T-cells in the blood of older endurance trained athletes [1, 19, 53], we investigated here whether changes in T-cell frequency were associated with changes in the levels of circulating IL-7, IL-15, IL-6, and ostenectin after the 12-week training intervention. IL-15 is an important myokine for optimal memory T-cell responses , increases T-cell antitumor immunity , and general T-cell activation and function , and has been shown to be highly expressed in muscle . IL-6, on the other hand, is thought to be an overall pro-inflammatory cytokine  responsible for increasing chronic morbidity and aging , but its release from skeletal muscle during exercise plays an anti-inflammatory role  and has also been shown to facilitate tumor infiltration of exercise-mobilized NK-cells [61–63]. Osteonectin is a myokine that has been shown to inhibit tumorigenesis in colon cancer and to also potentially play a role in repairing damaged skeletal muscle . Here, we found that the change in CD4 T-cells and CD4 N T-cells were negatively related to changes in IL-6 and osteonectin after HIIT but not MICT, indicating a potential role for osteonectin and IL-6 in the maintenance of peripheral naïve T-cells in response to exercise training. We also found that changes in CD8 EMRA T-cells were inversely correlated with serum IL-15 levels for the MICT but not the HIIT group. More research is needed to identify potential causative roles for these myokines in regulating the peripheral T-cell compartment after exercise training.
The β2-AR has been shown to play an important role in the activation, mobilization and redistribution of immune cells with an increased ability to infiltrate tumors in response to exercise. In murine models of cancer, exercise has been shown to promote CD8 + T-cell and NK-cell infiltration to tumors and suppression of tumor growth via a catecholamine and β2-AR dependent mechanism [63, 65, 66]. Potential increases in β2-AR expression with exercise training might help contribute to a more effective anti-tumor response. While neither MICT or HIIT affected β2-AR expression, we did find a positive association between the changes in absolute VO2max and β2-AR expression on the surface of central memory subsets of both CD8 + and CD4 + T-cells. There was also a trend for a positive relationship between the changes in β2-AR expression on CD8 + CM T cells and plasma osteonectin levels. The β2-AR has also been shown to modulate memory CD8 T-cell function , making the β2-AR a potential target in mediating aging related immunosenescence via exercise training.
Despite this being the first randomized controlled trial to show that an aerobic exercise training intervention can positively alter the frequency of T-cell subsets indicative of immunosenescence in a population of older women at high risk of developing breast cancer, we do acknowledge several limitations. These include the small sample size, lack of an endpoint measure of global immune competency (e.g. systemic challenge with a vaccine or experimental antigen), and the correlative nature of several of our findings. We also did not control for dietary intake and focused solely on a population of women at high risk for breast cancer who were living with overweight/obesity. As adiposity is known to influence immune cell function and phenotype [68–71], our interpretations of these findings must be taken with caution when applied to the lean older adult population.