The heat shock protein Gp96 binds to human neutrophils and monocytes and stimulates effector functions

The endoplasmic reticulum (ER)-resident heat shock protein Gp96 is involved in protein folding and is released into the extracellular space after necrotic cell death. In this context, Gp96 has immunostimulatory properties: it activates dendritic cells or macrophages and delivers associated peptides into the antigen presentation pathway, resulting in the induction of specific T-cell responses. The inflammatory response after necrotic tissue damage leads to the recruitment of polymorphonuclear neutrophils (PMNs) and monocytes, allowing them to make their first encounter with Gp96. We therefore investigated whether PMNs and monocytes interact with Gp96. We were able to show that PMNs and monocytes specifically bind fluorescein isothiocyanate (FITC)-conjugated Gp96. The binding of Gp96-FITC was competed by lipopolysaccharide (LPS) or fucoidan, a known inhibitor of scavenger receptors. Interestingly, the binding of LPS-FITC was also competed not only by fucoidan, but by Gp96, suggesting that LPS and Gp96 share a common receptor on PMNs. One important effector function of PMNs is the clearance of an inflammatory site by phagocytosis. We therefore assessed the influence of Gp96 on phagocytic activity using fluorochrome-labeled polystyrene beads. We found a marked enhancement of phagocytosis in the presence of Gp96 and concluded that PMNs not only bind Gp96, but are also activated by it. Additionally, Gp96-stimulated PMNs and especially monocytes release large amounts of interleukin-8, a potent neutrophil-attracting chemokine. In conclusion, we demonstrate that Gp96 specifically binds to and activates PMNs and monocytes, extending the function of Gp96 as a danger signal to additional members of the innate immune system.