Time course of cellular and molecular regulation in the immune system in altered gravity: Progressive damage or adaptation ?

We summarized the current knowledge about adaptation processes of isolated immune cells, animal models and the human body to altered gravity conditions. Many studies indicate an adaptation reaction of the immune system to the new microgravity environment, at least for the T cell system. Animal and human studies indicated adaptation processes starting after two weeks and continuing until 6 month or longer, which was reflected by cytokine concentrations in blood plasma or in stimulation assays. Adaptive reactions regarding IFN-γ, TNF-α and IL-2 concentrations were detected after 12 days spaceflight in animal studies and after 2–4 months in human studies, whereas adaptive reactions regarding IL-4, IL-6, IL-8 and IL-10 were found after 6 months spaceflight. Cellular studies were performed mainly as short-term studies, and only a few studies addressed alterations longer than 3 days. However, cross validation between studies is often not possible or indicated conflicting results. Many in vitro studies, mostly done with T lymphocytes, demonstrated extensive cellular and molecular alterations. In contrast, long-term studies with animals and humans are completely lacking this dramatic picture of short-term cellular effects, which indicates a very efficient adaptation process, partially evidenced by new steady state of adaptive response in the human immune system after weeks until months. Therefore, we assume that the human body and its cells are equipped with a robust and efficient adaptation potential when challenged with low gravitational environments.