Effect of endogenous cortisol levels on natural killer cell activity in healthy humans

The aim of this study was to examine the effects of increasing and decreasing endogenous cortisol levels on natural killer (NK) cell activity in vivo. Normal healthy volunteers participated in the following studies: baseline (n = 27), metyrapone challenge test (n = 10), dexamethasone suppression test (n = 10), and adrenocorticotropic hormone (ACTH) stimulation test (n = 8). Each subject served as his own control for each study. Each subject was tested for NK activity and plasma cortisol levels at 9 a.m., just before the challenge drug administration, and at 10 a.m., except for the dexamethasone study, in which only the 9 a.m. blood was drawn, 10 h after the dexamethasone administration. On the baseline study day, a significant decrease in plasma cortisol levels was found from 9 to 10 a.m. (p <.02) along with a significant increase in NK activity (p <.001). On the metyrapone test day, plasma cortisol levels at 10 a.m. were significantly reduced (p <.005) as expected, while NK activity at the same time point was not affected and was increased to an extent equivalent to the baseline study day. On the dexamethasone test day, plasma cortisol concentrations were significantly decreased (p <.0001) as compared to the same time point on the baseline day, without any significant change in the NK activity. On the ACTH test day, plasma cortisol rose significantly at 10 a.m. (p <.02), with no change in NK activity. We conclude that plasma cortisol alone has no significant effect on NK activity in vivo.