Effects of microgravity and other space stressors in immunosuppression and viral reactivation with potential nervous system involvement

Space exploration exposes astronauts to a variety of gravitational stresses. Exposure to a reduced gravity environment affects human anatomy and physiology. Countermeasures to restore homeostatic states within the human body have begun. The pathophysiological effects of exposure to microgravity, on the neurological system, are, however, still not clear. NASA has scheduled deep space exploration of extraterrestrial locations such as the Moon and Mars in the 2030s. Adverse health effects related to the human exposure to microgravity from previous, relatively shorter missions have been documented. A lengthy deep space travel to Mars could be overburdened by significant adverse health effects. Astronauts demonstrate a significant increase in the number of many types of circulating white blood cells (neutrophils, monocytes, T-helper cells, and B-cells) but a decrease in natural killer cells. It is unclear whether these changes are due to increased production or decreased clearance of these cells. In this review, viral reactivation in astronauts will be discussed, including the occurrence of clinical cases before, during, or after spaceflight and their management during and after flight. Studies on models used in spaceflight studies such as the AKATA cells (an immortalized B-cell line derived from a Japanese patient with Burkitt's lymphoma, a tumor induced by Epstein-Barr virus) and other cell lines which shed these latent viruses, will be reviewed with specific reference to gravitational changes, radiation, and spaceflight-induced immune suppression.