Dimeric S100A8 in human neutrophils is diminished after phagocytosis

S100A8 is a major cytoplasmic protein of neutrophils and monocytes/macropha ges and has been associated with myeloid cell differentiation and activatio n. Little is known about its functions or mechanisms of release from neutro phils, We have developed a monoclonal antibody to murine S100A8, which cros s-reacts with human S100A8, This antibody, which recognizes the homodimeric form of the protein, detects its expression specifically in human neutroph ils and is reactive in formalin-fixed, paraffin-embedded tissues. Using thi s antibody as well as a commercially available antibody to human S100A8, we show that phagocytic activation of neutrophils, in vivo in acute appendici tis and in vitro following phagocytosis of opsonized zymosan, is characteri zed by loss of cytoplasmic immunoreactivity for S100A8, In vitro, phagocyto sis is associated with rapid diminution of immunostaining without loss of v iability. Loss of immunoreactivity for S100A8 may serve as a marker of loca lized neutrophil activation in tissues.