Characterizing the effects of acute exercise on Natural Killer cell recruitment and receptor expression in pre-pubertal and post-pubertal youth

Abstract

Natural Killer (NK) cells are recruited into circulation in response to physiological stress such as exercise. In adults, NK deployment and receptor expression are proportional to exercise intensity, and the cytotoxic CD56dim NK subset is preferentially deployed compared to immunoregulatory CD56bright subset. We know much less about the NK response to different exercise stimuli in children; however, pre-pubertal children are less responsive to acute exercise and recover faster than post-pubertal children and adults. The aims of this study were to (1) investigate the effects of exercise intensity and structure on NK recruitment and receptor expression, (2) compare the response among pre-pubertal and post-pubertal children, and (3) assess if factors such as fitness and physical activity were correlated with the magnitude of NK cell response. Healthy, recreationally active, pre- and post-pubertal boys and girls were recruited from the Hamilton community (N=11; 5 pre-pubertal, 6 post-pubertal). At the initial study visit, participants completed an aerobic fitness test to determine V̇O2peak and ventilatory threshold. At the subsequent visits, participants performed one of four cycling structures in a randomized, counterbalanced order, including: high-intensity continuous (HI-CONT), high-intensity intermittent (HI-INT), moderate-intensity continuous (MI-CONT), or moderate-intensity intermittent (MI-INT) exercise. Blood was collected pre-, post-exercise, 30- and 60-minutes into recovery. NK cells, CD56dim, and CD56bright NKs, activating (NKG2D and DNAM1) and inhibitory receptors (NKG2A and KIR2DL2/DL3) were quantified via flow cytometry. Participants were also outfitted with an accelerometer to measure physical activity. Three-way mixed ANOVA were used to examine effects of time, exercise and puberty on NK parameters, with Tukey’s HSD post hoc where appropriate. Pre-pubertal children showed no significant increase in the NK concentration in response to any of the exercise stimuli. Post-pubertal children, increased their NK cell concentration PRE to POST in all exercise stimuli except MI-CONT. Greater increases in NK concentration were seen POST HI-INT (124723 +/- 91596 cells/mL) and MI-INT (109644 +/- 84664 cells/mL), compared to HI-CONT (19931 +/- 1492 cells/mL) and MI-CONT (17082 +/- 9516 cells/mL), respectively (p < 0.001). Only increases in the expression of NKG2A were observed during REC1 (63.6 +/- 13.7 %), and REC2 (64.2 +/- 12.8 %) compared to PRE (57.9 +/- 13.4%) (p < 0.01). However, the density of NKG2D, NKG2A and DNAM1 were all significantly increased at REC2 compared to PRE. Fitness (R2= - 0.742) but not moderate-to-vigorous physical activity (R2= 0.098) or sedentary time (R2= 0.621) were significantly negatively associated with the magnitude of NK cell response. We demonstrated that an acute bout of intermittent cycling (5 min of exercise) leads to greater NK recruitment and a more immunoregulatory NK environment than continuous cycling in post-pubertal children. Contrarily, we showed that pre-pubertal children are not responsive to acute exercise of 30-minutes or shorter. Across our cohort, exercise also upregulated the density of expression of both activating and inhibitory receptors. Future research should examine if the NK cell response to acute exercise is maintained with repeated exercise exposures or exercise training.