Improved apoptosis detection in ovine neutrophils by annexin V and carboxyfluorescein diacetate staining.

Neutrophil apoptosis is critical for final resolution of the inflammation in the tissues and for maintenance of neutrophil homeostasis under normal condition. An early hallmark of apoptotic cells is translocation of phosphatidylserine (PS) residues, normally located in the inner leaflet of cellular membrane, to the external cell surface; exposed PS is recognized by specific PS receptors on disposing cells. Here we report an improved procedure to detect neutrophil apoptosis by simultaneous staining for exposed PS with Cy3-labeled annexin V (Cy3) and for membrane integrity with the vital dye 6-carboxyfluorescein diacetate (6-CFDA) based on the APOAC apoptosis detection kit (Sigma). Spontaneous apoptosis was evaluated in ovine neutrophils cultured ex vivo for 18 h. We investigated the multiple parameters involved in the assay, i.e. the type of fixative (methanol, paraformaldehyde, or no fixation) and the type of slide (coated with Vectabond, polylysine or Parafilm((R))). Results indicated that both the adhesion to the slide and the fixation can modify neutrophil functional status and morphology, which result in misleading apoptosis detection. In order to minimize these artifacts, we have developed an improved APOAC assay procedure, staining cells while in suspension and using Parafilm((R)) coated slides.