A reliable method for enrichment of neutrophils from peripheral blood in barramundi (Lates calcarifer).

Neutrophils are a short-lived, terminally differentiated, innate immune cell, that are critical first responders during infection. Research into neutrophil-pathogen interactions in fish has primarily employed cells derived from the pro-nephros and nephros. Since these sites are also the location of neutrophil and other immune cell development, there may be some ambiguity in maturation and functional ability of these cells, and difficulty in differentiating the effects of neutrophils from those of macrophages and monocytes. In contrast, peripheral blood circulating neutrophils are mature and ready to respond, thus it may be more physiologically relevant to use these cells for immune studies when evaluating interactions with blood-borne pathogens. The enrichment of tropical, euryhaline fish blood cells cannot follow classic mammalian enrichment methods for several reasons: Fish have nucleated red blood cells (RBC's), a high number of reticulocytes, a very low number of granulocytic leukocytes and an osmotic tolerance, rendering techniques such as water lysis ineffective. Enrichment of neutrophils, while minimizing RBC contamination, is imperative for studies where luminescence or fluorescence signals may be confounded by background from an overabundance of RBC's. We have optimized a method for enriching neutrophils from peripheral blood, with an initial settlement step employing 6% dextran (Mr 450,000–650,000), for 30–60 min at room temperature, followed by density separation on an 8-step Percoll density gradient. This method provides a cell suspension comprising 20–50% neutrophils, free of contamination from reticulocytes. These are then suitable for luminometric or fluorometric downstream analyses.