The professional phagocytes of sea bass (Dicentrarchus labrax L.): cytochemical characterisation of neutrophils and macrophages in the normal and inflamed peritoneal cavity.

In order to identify the phagocytic cells of sea bass, the peritoneal leucocyte population of fish injected intraperitoneally with Photobacterium damselae subspecies piscicida was studied by light microscopy using cytocentrifuge preparations stained by the Antonow technique for peroxidase detection. Among the leucocytes present in the peritoneal exudate of the infected fish (macrophages, neutrophils, eosinophilic granular cells, lymphocytes and thrombocytes), macrophages and neutrophils were the only phagocytic cells. Neutrophils were easily distinguished from macrophages in Antonow stained preparations by the pattern of peroxidase positivity. Using ultrastructural cytochemistry, neutrophils were found to have abundant cytoplasmic granules positive for peroxidase and arylsulphatase and were negative for alpha-naphthyl butyrate (ANB) esterase. In contrast, ANB esterase activity was detected in macrophages. These leucocytes were typically negative for peroxidase, but ocasionally, some macrophages with peroxidase or arylsulphatase-positive vacuoles were observed. Both phagocytes had cytoplasmic granules positive for acid phosphatase. Glycogen particles were found in the cytoplasm of the two phagocytic cells, but they were much more abundant in neutrophils. Macrophages were much more abundant than neutrophils in the peritoneal cavity of non-injected sea bass but early after the intraperitoneal injection of bacteria, the number of neutrophils increased quickly and extensively. Higher numbers of intraperitoneally injected bacteria were found inside macrophages as compared to neutrophils because macrophages strongly predominated in the peritoneal population at the time of injection. However, when the bacteria were injected into peritoneal cavities with high numbers of neutrophils (attracted by a previous injection of 12% casein), the percentage of neutrophils with phagocytosed bacteria increased, approaching that of infected macrophages. Taken together, these results show that in sea bass, as in many other organisms, in addition to macrophages, neutrophils are important phagocytic cells, the relative participation of each of the two phagocytes in defense mechanisms against infection depending on the opportunity to encounter the invading infectious agents.