Head-down tilt bed rest and immune responses

Head-down tilt bed rest (HDT) is used as a model for studying the physiolog ical changes occurring in weightlessness during spaceflight. In the present study, eight volunteers were subjected to a strict HDT of -6 degrees for 4 2 days. Blood samples were obtained 37 and 13 days before, at days 13, 34, and 41 during, and 12, 33, and 47 days after HDT. FACScan analysis was used to determine cell subpopulations. Plasma was used to quantify various circ ulating hormone levels. Whole blood and reconstituted blood were stimulated with various activators such as phytohaemagglutinin-P (PHA), PHA combined with phorbol-12-myristate 13-acetate (PMA), anti-CD2, anti-CD3, and lipopol ysaccharide. Supernatants were collected and analysed for the interleukins IL-1 beta, IL-2, IL-6, and IL-10, interferon-gamma (IFN-gamma) and tumour n ecrosis factor-alpha (TNF-alpha). The total number of T lymphocytes and mon ocytes did not change significantly, whereas the number of polymorphonuclea r cells increased during HDT. The percentage of CD2(+) and CD3(+) cells was increased at day 35 of HDT. The percentage and total number of natural kil ler cells (CD2(+)/CD3(-)/CD56(+)) was increased 12 days before and 14 days after HDT TNF-alpha secretion did not change significantly during HDT, IL-2 , IL-10 and IFN-gamma were increased at day 34 of HDT. IL-1 beta levels wer e increased before and during HDT compared to post-HDT measurements. No sig nificant changes were observed in plasma immunoglobulin, complement factors and other factors of the inflammatory system. Prolactin levels increased s lightly but significantly at day 35 of HDT, thyreotropin and growth hormone levels remained virtually unchanged. Cortisol decreased slightly but signi ficantly over the entire duration of the study. The changes observed during HDT do not indicate that the immune system is blunted, and these changes d o not seem to correlate with the duration of HDT. Taken together these resu lts show that a HDT does not reproduce the changes in immune responses obse rved after spaceflight.