Neutrophil and macrophage responses to inflammation in the peritoneal cavity of rainbow trout Oncorhynchus mykiss. A light and electron microscopic cytochemical study.

The neutrophil and macrophage responses that accompany inflammation were studied in the peritoneal cavity of rainbow trout Oncorhynchus mykiss using light and electron microscopic cytochemistry. Neutrophils of inflammatory peritoneal exudates were alpha-naphthyl butyrate esterase-negative, peroxidase-positive and rich in cytoplasmic glycogen granules. Macrophages were poor in glycogen, esterase-positive and usually peroxidase-negative. Some peroxidase-positive macrophages were due to the transfer to macrophages of neutrophilic peroxidase. The ultrastructural double labelling for glycogen-peroxidase or esterase/peroxidase was most useful for precisely characterising neutrophils and macrophages in the inflamed peritoneal cavities and for correctly labelling peroxidase-positive macrophages. Intraperitoneal injection of casein, Incomplete Freund's Adjuvant (IFA) and live or formol-killed Yersinia ruckeri resulted in a rapid influx of neutrophils, peaking at 24 to 48 h post-injection and reaching values, in the case of live bacteria, 500 x those in the resting, unstimulated peritoneal cavity. Peritoneal macrophages also increased, but the response was slower (peak at 5 d) and with more modest increases in number (7.5 x). Neutrophil and mononuclear cells returned to normal values after 15 d in the case of casein and bacteria, but continued above base values 30 d after the injection of IFA. Conversely, after the injection of phosphate buffered saline, India ink or with sham-injections, very moderate neutrophil and and macrophage responses subsided in a few hours. Phagocytosis of bacteria was studied by light microscopy of preparations stained for peroxidase by a new method which allows for the simultaneous observation of intracellular bacteria and peroxidase staining. When Y. ruckeri was injected into resting peritoneal cavities, bacteria were ingested by the resident macrophages. When the bacteria were injected into cavities with high numbers of neutrophils (due to the previous injection of casein), more neutrophils than macrophages contained bacteria. Results show that the macrophages are the resident phagocytes of the peritoneal cavity of trout, while neutrophils are present in that body cavity in significant numbers only in situations of inflammation and only as long as the inflammation persists.