Altered gravity, especially microgravity affects cellular functions of immune cells and can result in immune dysfunction for long-term, manned spaceflight and space exploration. The underlying mechanism, however, of sensing and responding to the gravity alteration is poorly understood. Here, a rotary cell culture system (RCCS) bioreactor was used to elucidate the effects of simulated microgravity on polymorphonuclear neutrophils (PMN)-like HL-60 cells. Alteration of cell morphology, up-regulation of (nitric oxide) NO production, enhancement of interleukin-6 (IL-6), interleukin-8 (IL-8), and monocyte chemotactic protein 1 (MCP-1) secretion, and diversity of cellular adhesion molecule expression were observed for the cells cultured in RCCS, leading to the up-regulated inflammatory immune responses and host defense. It was also indicated that such alterations in biological responses of PMNs mediated the reduced rolling velocity and decreased adhesion of PMN-like HL-60 cells on endothelial cells under shear flow. This work furthers the understandings in the effects and mechanism of microgravity on PMN functions, which are potentially helpful for optimizing the countermeasures to immune suppression in the future long-term, manned spaceflight.