Simulated microgravity, psychic stress, and immune cells in men: observations during 120-day 6 degrees HDT

Because 6 degrees head-down tilt (HDT) is an established method to mimic lo w gravity on earth, the aim of the present study was to determine the effec ts of 120-day HDT on psychic stress and peripheral blood immune cells in si x healthy male volunteers. Psychological state was assessed by a current st ress test, and cortisol was measured in saliva. During HDT, all volunteers developed psychic stress, and the diurnal rhythm of cortisol secretion was significantly altered. In addition, urine excretion of dopamine and norepin ephrine increased. The innate part of the immune response was activated, as evidenced by the increase in the expression of beta (2)-integrins on polym orphonuclear leukocytes and a rise in the number of circulating natural kil ler (NK) cell lymphocytes. The ratio of T-helper to T-cytotoxic and T-suppr essor cells decreased, whereas no changes in T and B lymphocytes were obser ved. Plasma levels of interleukin-6 increased significantly and returned to basal levels after the end of the HDT period. Thus 6 degrees HDT appears t o be a valid model to induce psychic stress and neuroendocrine-related chan ges in the immune system, changes that might also be encountered by astrona uts and cosmonauts during long-duration spaceflights.