Natural killer cell activity during head-up tilt-induced central hypovolemia in humans

Changes in immunological competent blood cells were evoked in seven humans during passive head-up tilt (50° anti-Trendelenburg's position maintained until appearance of presyncopal symptoms). Blood samples were collected after 60 min of rest, when presyncopal symptoms appeared during tilt, and 105 min after tilt-down. Natural killer (NK) cell activity increased during head-up tilt due to a three to four-fold increase in CD16⁺ NK cells in blood. In support NK cell activity boosted with interferon-α and interleukin 2 (IL-2) rose in parallel with unboosted NK cell activity. This effect on NK cells disappeared during recovery but was not suppressed when compared to prevalues. Lymphocyte concentration also increased during head-up tilt. Concentrations of CD3⁺ and CD4⁺ T cells were almost stable during head-up tilt, whereas the percentage of CD3⁺ T cells in relation to blood mononuclear cell (BMNC) concentration decreased, due to a diminished percentage of CD4⁺ T cells and the marked simultaneous increase in the percentage of CD16⁺ NK cells. Although changes in the BMNC composition occurred, the proliferative responses of BMNC following stimulation with phytohemagglutinin, purified derivative of tuberculin, or IL-2 did not change significantly. We conclude that head-up tilt induced marked changes in subpopulations of BMNC, especially the CD16⁺ NK cells, as they were recruited to the blood.