APOPTOSIS IN PHAGOCYTOTIC CELLS OF LYMPHOID-TISSUES IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) FOLLOWING ADMINISTRATION OF CLODRONATE LIPOSOMES

Macrophage function has been studied in vivo by using liposomes that c ontain dichloromethylene-bisphosphonate (clodronate liposomes) to depl ete macrophage subpopulations. In the present study, the effects of in travenously injected clodronate liposomes on the head kidney and splee n of the rainbow trout (Oncorhynchus mykiss) were studied from 1 h to 7 days after injection. The rapid trapping of liposomes in the splenic ellipsoids was followed by depletion of ellipsoidal sheath macrophage s and accumulation of particulate material and IgM in the ellipsoidal wall, findings illustrating the importance of ellipsoidal macrophages in the clearance of filtered substances trapped in the reticular matri x of the ellipsoidal wall. A reduced reactivity for acid phosphatase i n the spleen and ultrastructural evidence of cell death in phagocytoti c cells of the head kidney and spleen supported the selective effect o f clodronate liposomes on macrophages in rainbow trout. Apoptotic bodi es were prominent ultrastructural features in tissues collected from c lodronate-liposome-treated rainbow trout. The increased presence of ap optotic cells in clodronate-liposome-treated trout compared with trout given liposomes containing phosphate-buffered saline was confirmed by using terminal deoxynucleotidyl-transferase-mediated deoxyuridine-tri phosphate nick-end-labelling of cells with extensive DNA fragmentation . The characterization of liposome-mediated macrophage depletion in fi sh provides a useful model for further investigation of piscine macrop hage function.