Altered cytokine production by specific human peripheral blood cell subsets immediately following space flight

In this study, flow cytometry was used to positively identify the specific lymphocyte subsets exhibiting space flight-induced alterations in cytokine production. Whole blood samples were collected from 27 astronauts at three points (one preflight, two postflight) surrounding four space shuttle missions. Assays performed included serum/urine stress hormones, white blood cell (WBC) phenotyping, and intracellular cytokine production following mitogenic stimulation. Absolute levels of peripheral granulocytes were significantly elevated following space flight, but the levels of circulating lymphocytes and monocytes were unchanged. Lymphocyte subset analysis demonstrated a decreased percentage of T cells, whereas percentages of B cells and natural killer (NK) cells remained unchanged after flight. Nearly all the astronauts exhibited an increased CD4/CD8 T cell ratio. Assessment of naive (CD45RA(+)) vs. memory (CD45RO(+)) CD4(+) T cell subsets was ambiguous, and subjects tended to group within specific missions. Although no significant trend was seen in absolute monocyte levels, a significant decrease in the percentage of the CD14(+)CD16(+) monocytes was seen following space flight in all subjects tested. T cell (CD3(+)) production of interleukin-2 (IL-2) was significantly decreased after space flight, as was IL-2 production by both CD4(+) and CD8(+) T cell subsets. Production of interferon-gamma (IFN-gamma) was not altered by space flight for the CD8(+) cell subset, but there was a significant decrease in IFN-gamma production for the CD4(+) T cell subset. Serum and urine stress hormone analysis indicated significant physiologic stresses in astronauts following space flight. Altered peripheral leukocyte subsets, altered serum and urine stress hormone levels, and altered T cell cytokine secretion profiles were all observed postflight. In addition, there appeared to be differential susceptibility to space flight regarding cytokine secretion by T cell subsets. These alterations may be the result of either microgravity exposure or the physiologic stresses of landing and readaptation to unit gravity. Future studies, including in-flight analysis or sampling, will be necessary to determine the cause of these alterations.