Natural killers
NK cells are major effectors of the innate immunity, providing rapid responses to virally infected cells and respond to tumor formation.138,139 Cytokines involved in NK activation include IL-12, IL-15, IL-18, IL-2, and CCL5. NK cells are activated in response to IFNs or macrophage-derived cytokines. NK cells control viral infections by secreting IFN-γ and TNFα.138,139 IFN-γ activates macrophages for phagocytosis and lysis, and TNFα acts to promote direct NK tumor cell killing. NKs express the surface markers CD16 (FcγRIII) and CD56 in humans. A recent study by Huyan et al140 reported the effects of LBPs on primary human NK cells under normal or simulated microgravity conditions. The results demonstrated that LBPs markedly promoted the cytotoxicity of NK cells by enhancing IFN-γ and perforin secretion and increasing the expression of the activating receptor NKp30 under normal conditions. Meanwhile LBPs enhanced NK cell function under simulated microgravity conditions by restoring the expression of the activating receptor NKG2D and reducing the early apoptosis and late apoptosis/necrosis.140 In addition, the antibody neutralization test showed that the complement receptor CR3 may be the critical receptor involved in LBP-induced NK cells activation. These findings indicate that LBPs are potent immune regulators and can promote the immune functions of the public and astronauts during space missions.