Differing cell population structure reflects differing activity of Percoll-separated pronephros and peritoneal leucocytes from barramundi (Lates calcarifer)

Aquaculture of barramundi or Asian sea bass (Lates calcarifer L) is expanding throughout the Asia-Pacific region in both marine and freshwater systems. Incidence of bacterial and viral diseases is high in this species throughout the region yet little is known about the immune system of this highly adaptable euryhaline fish. Ultimately, most pathogens are eradicated by the phagocytic cells, however there is great diversity and plasticity amongst these cell populations in mammals and in fish. To better understand disease processes in barramundi, Percoll-purified leucocyte populations from haematopoietic tissues of the head kidney were compared with populations isolated from the peritoneal cavity morphologically, cytochemically and in terms of the ability to respond to stimulation, using flow cytometry, light microscopy and fluorimetric/luminometric assays. The peritoneal cells comprised predominantly of macrophages and putative mature monocytes whilst the head kidney cells comprised lymphocytes, including immunoglobulin-positive B-lymphocytes, some small monocytes and macrophages. The differing population structures were reflected in the ability of the cells to respond to stimulation with either lipopolysaccharide or phorbol myristate acetate, as the chemiluminescence response of peritoneal cells was 7 to 9-fold higher than head kidney cells. Both populations were capable of being primed by LPS, but the kinetics differed, with optimal priming in peritoneal cells occurring after 6 h exposure whilst head kidney cells required at least 24 h exposure to LPS for optimal priming. Both head kidney and peritoneal populations produced nitric oxide in response to stimulation with LPS and interferon gamma, but again, response was higher in peritoneal cells. The implications of the differing population structures and activities amongst these cells should be considered when developing models for further study of host–pathogen interactions in this increasingly important fish.