Effect of hindlimb suspension simulation of microgravity on in vitro immunological responses

The effects of microgravity on the immune system are largely unknown, but understanding such effects becomes increasingly important as space exploration continues and mission duration increases. Reductions in postflight human T cell reactivity to mitogens is well documented. Similar results have been obtained using a clinostat as an in vitro model of microgravity. In this study, a rat tail suspension model of weightlessness was used to examine in vitro lymphocyte proliferation in response to mitogens. Experiments were designed to uncover potential deficits in events related to proliferation including cell surface protein and IL-2 receptor (IL-2R) expression, interleukin-2 (IL-2) production, and accessory cells. Suspension of rats for 1 week led to a significant depression in [3H]thymidine incorporation by mitogen-stimulated peripheral blood lymphocytes (PBL) but only a small decrease in the proliferation of lymph node lymphocytes and splenocytes. There were no changes in the percentages of cells expressing CD4, CD5, CD8 or immunoglobulin. Moreover, no changes in IL-2 production or IL-2R expression were observed. More esterase-positive macrophages were detected in all lymphatic tissues of suspended rats, but there was no corresponding increase in the percentage of cells bearing the macrophage markers OX41 or OX42. This increase in the number of macrophages may be related to the observed suppression of lymphocyte proliferation. The tissue specificity of the decrease in mitogen activation indicates that there may be a compartmentalized response in the rats tested in the hindlimb suspension model.