Dimeric S100A8 in human neutrophils is diminished after phagocytosis.

S100A8 is a major cytoplasmic protein of neutrophils and monocytes/macrophages and has been associated with myeloid cell differentiation and activation. Little is known about its functions or mechanisms of release from neutrophils. We have developed a monoclonal antibody to murine S100A8, which cross-reacts with human S100A8. This antibody, which recognizes the homodimeric form of the protein, detects its expression specifically in human neutrophils and is reactive in formalin-fixed, paraffin-embedded tissues. Using this antibody as well as a commercially available antibody to human S100A8, we show that phagocytic activation of neutrophils, in vivo in acute appendicitis and in vitro following phagocytosis of opsonized zymosan, is characterized by loss of cytoplasmic immunoreactivity for S100A8. In vitro, phagocytosis is associated with rapid diminution of immunostaining without loss of viability. Loss of immunoreactivity for S100A8 may serve as a marker of localized neutrophil activation in tissues.