Simulated microgravity ‘disarms’ human Natural Killer cells and suppresses cytotoxic activity against tumor target cells

Abstract

Maintaining immune system integrity is of paramount importance for astronauts embarking on long-duration spaceflight missions. We have shown that Natural killer (NK) cell function is impaired during spaceflight; however, the mechanisms responsible for this effect are unknown. We exposed primary human blood NK cells to 12 h of simulated microgravity (SMG) using a rotating wall vessel zero gravity cell culture analog. Compared to static and vertical axis rotational controls, SMG (horizontal axis rotation) was found to decrease subsequent NK-cell cytotoxic function in 1G against allogeneic target cell lines of leukemia (K562), multiple myeloma (U266), B-lymphoma (721.221) and HLA-E transfected lymphoma (221.AEH) origin. Flow cytometric analysis revealed that SMG had little effect on the surface expression of a wide-range of NK-cell activating and inhibitory receptors, but did decrease intracellular perforin expression. Using cytometry by time of flight (CyTOF), SMG was found to lower NK-cell degranulation as determined by CD107a expression following co-incubation with K562 target cells in 1G. Moreover, SMG reduced NK-cell expression of the pro-inflammatory cytokines TNF-alpha and IFN-gamma in response to K562 co-culture at 1G. These results indicate that microgravity may be involved in spaceflight-associated reductions in NK cell function. Specifically, pre-exposure to SMG appears to disrupt NK-cell killing by ‘disarming’ them of cytolytic granules and impairing their ability to secrete effector cytokines when confronted with tumor target cells in the 1G environment.