Respiratory burst activity of ayu (Plecoglossus altivelis) neutrophils : comparison with other fresh water fish

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Neutrophils of vertebrates undergo a respiratory burst activity (RBA) as a defense mechanism against bacterial infections. We report here that ayu (Plecoglossus altivelis) have unusually high RBA even when they are in a healthy condition. Kidney (major hematopoietic organ in teleost fish) and blood leukocytes were obtained from ayu, rainbow trout (Oncorhynchus mykiss), carp (Cyprinus carpio), eel (Anguilla japonica), and pond smelt (Hypomesus nipponensis). Neutrophil RBA was measured by flow cytometry using dihydrorhodamine after stimulation with phorbol myristate acetate (PMA). The amount of RBA of neutrophils from both blood and kidney was significantly higher in ayu than in the other species (e.g., the fluorescence intensity of ayu blood neutrophils was about 3 to 7 times higher than that from trout and carp, and that of ayu kidney neutrophils was 2 to 19 times higher than that of rainbow-trout, carp, eel, and pond-smelt). This unique character of ayu neutrophils was invariable even … More at different ages, locations, and sex-maturation stages.To understand this unique character of ayu neutrophils, the RBAs of resting and inflammatory neutrophils of ayu and carp were compared. Inflammation was induced in the peritoneal cavity by injecting killed-bacteria. The inflammatory neutrophils were obtained from peritoneal cavity. Resting neutrophils were obtained from blood and kidney of non-injected fish.In carp, the RBA of inflammatory neutrophils was much higher than that of resting neutrophils. On the other hand, in ayu no significant difference was observed. The RBA of neutrophils was already high in the kidney stock. The process of inflammation did not further enhance RBA. In addition to PMA, other stimulants (zymosan, opsonized-zymosan, and zymosan treated serum) were used to measure RBA. Even with these stimulants, the RBA of inflammatory neutrophils was always higher than that of kidney neutrophils in carp. On the other hand, no significant difference was observed in ayu. These results indicate ayu neutrophils have spontaneously activated characteristics with the respect to the ROS generation even in the resting state.At this point, it is not clear why ayu have such unique neutrophils with respect to the ROS generation. One possibility is that the high respiratory burst activity is related to the short life span of ayu (essentially one year). With such a short life span, it is likely that the innate immunity by spontaneously activated neutrophils is more important than acquired immunity. Less