Cellmediated immunity and T cell changes

The more recent studies of lymphoid cells confirmed a modest depression in cell-mediated immunity after both short and long (over 1 month) space flights. Inducer and cytotoxic T cell populations declined. Decreased numbers of both CD4~ and CD8~ lymphocytes were found in postflight blood samples of 30 shuttle astronauts. Humans were also found to have postflight reductions in phytohemagglutinin-triggered blast transformation of cultured blood lymphocytes. After prolonged stays in space, the proliferative index of cosmonaut T cells showed reductions in xenogeneic graft-versus-host reactions.

Human NK cells showed reduced numbers and cytotoxic activities after both long and short space flights. NK cells from the marrows and spleens of rats flown in space showed significantly inhibited in vitro cytotoxicity for target cells, although their binding to these cells appeared to be unaffected. The surface expression of some cytokine receptors was reduced in rat NK cells.

Delayed-type dermal hypersensitivity (DTH) reactions showed reductions, or even disappeared, during protracted stays in space. When tested during space missions, DTH reactions were consistently reduced after the fourth day of weightlessness. Inflight measurements of DTH were facilitated by the use of a multitest skin puncture device that simultaneously injected predetermined doses of seven recall antigens and one control.

Cell-mediated immunity was also studied in rats. After short flights, rats showed decreased numbers of blood CD4- and CD8" lymphocytes. The proliferative response of rat spleen cells to mitogens or to interleukin 2 (IL-2) was inhibited after 1-2 week flights. After 10 days in space, the phenotype of rat lymph node or spleen lymphocytes showed several tissue-specific differences, including an increased percentage and fluorescent intensity of splenocyres which expressed CD4^ and CDS' markers, as well as the integrins EFA-la and )3. In contrast, these markers remained unchanged, or showed decreases, in lymph node cells from the same rats.

The response to concanavalin A of cultured cells from lymph nodes (but not spleens) of space-flown rats was depressed, and could not be restored by supplementation with either IL-1 or IL-2. However, these cultured cells showed normal responses to other mitogens, normal production of IL-2, and normal expression of surface markers. Because some of the immunological effects of microgravity appeared to be lymphoid organ specific, it was postulated that the findings may have been influenced by variations in lymphocyte trafficking, brought about by body fluid compartment shifts associated with micro-gravity.

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