Apoptosis of gut-associated lymphoid tissue in rainbow trout Oncorhynchus mykiss after incubation with Candida albicans and bacterial lipopolysaccharide.

Until now a few studies have been carried out on the gut lymphoid system in fish despite its protective role in the host. Here, we have evaluated the effects of Candida albicans (Ca) and lipopolysaccaride (LPS) on the pyloric and terminal segments of gut in the rainbow trout Oncorhynchus mykiss. In particular, data show that both Ca and LPS are able to cause apoptosis of intestinal lymphoid cells as detected by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) procedure. These findings suggest a further modality of gut response in fish to environmental antigens.

Lymphoreticular system in fish: erythrocyte-mediated immunomodulation of macrophages contributes to the formation of melanomacrophage centers.

In fish, melanomacrophage centers (MMC) represent the equivalent of germinal centers of birds and mammals. They consist of aggregates of macrophages filled with fragments of cells, mainly erythrocytes, and various pigments such as hemosiderin, lipofuscin, and melanin. The kidney, the spleen, and the liver contain many MMC either moving freely or encapsulated in the context of the lymphoreticular tissue. Here, we have demonstrated for the first time that MMC exist in the kidney, spleen, and liver of the rainbow trout Salmo gairdneri Richardson by using traditional stainings and cytoenzymatic methods: periodic acid-shiff, alpha-naphthyl acetate esterase, and peroxidase. Finally, in the view of the already described peripheral interaction between fish erythrocytes (FE) and macrophages (MO), we postulate that these cellular aggregates (rosettes) travel to distant organs, thus contributing to the formation of MMC in the kidney, spleen, and liver.