Uptake of neutrophils and neutrophilic components by macrophages in the inflamed peritoneal cavity of rainbow trout (Oncorhynchus mykiss)

The interaction between neutrophils and macrophages in peritoneal inflammatory exudates in rainbow trout injected intraperitoneally with several bacterial and non-bacterial phlogistic agents was studied by light and electronmicroscopic histochemistry. The transfer of neutrophilic components to macrophages was demonstrated by the presence in macrophages of myelo-peroxidase, an enzyme of the neutrophil not produced by rainbow trout monocytes and macrophages. Two mechanisms observed for the transfer were phagocytosis of intact neutrophils and phagocytosis of apoptotic bodies released from inflammatory neutrophils. Neutrophils with phagocytosed bacteria were also found to be ingested by macrophages, with the consequent transfer of neutrophilic components and bacteria to the macrophages. Two other mechanisms of transfer are considered. One involves the intake by the macrophage of neutrophilic granules or granular molecules released by lysing or degranulating neutrophils, respectively. The other may be through gap junctions which were observed between adhering inflammatory neutrophils and macrophages.