Cell Tissue Res (1995) 282:41-48 Cell&Tissue Research 9 Springer-Verlag 1995 Immune-complex trapping in the splenic ellipsoids of rainbow trout (Oncorhynchusmykiss) A. Espenes, C.MeL. Press, B.H. Dannevig, T. Landsverk Department of Morphology, Genetics and Aquatic Biology, Norwegian College of Veterinary Medicine, P.O. Box 8146, Dep. 0033, Oslo, Norway Received: 24 January 1995 / Accepted: 5 May 1995 Abstract. Rainbow trout (Oncorhynchus mykiss), im- munised with horseradish peroxidase, were given horse- radish peroxidase intravenously, and the trapping of anti- gen in the spleen was followed 1, 24, and 48 h after in- jection. After 1 h, the localisation of horseradish peroxi- dase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colo- calisation of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and sug- gested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very lit- tle horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in associa- tion with large cells with prominent cytoplasmic exten- sions. In nonimmunised fish given horseradish peroxi- dase intravenously, antigen was not detected in ellip- soids. Thus, the observed difference between immunised and nonimmunised trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary im- mune response. Key words: Ellipsoids - Spleen - Immune complexes - Immunohistochemistry - Oncorhynchus mykiss (Tele- ostei) Introduction The morphological and functional features of splenic el- lipsoids have been studied in a wide range of verte- brates, including mammals (Schweigger-Seidel 1863; Mills 1926; Blue and Weiss 1981), birds (Olfih and Glick 1982), reptiles (Kroese and Van Rooijen 1982), and fish (Yoffey 1929; Graf and Schlfins 1979; Pulsford et al. 1982; Quesada et al. 1990; Espenes et al. 1995). The splenic ellipsoids are terminations of arterioles that possess specialised, cuboidal endothelial cells and are Correspondence to: A. Espenes surrounded by a sheath of stellate reticular cells and macrophages supported by a matrix containing reticular fibres (Weiss 1988; Zapata and Cooper 1990). These studies identify the ellipsoids as a phylogeneticatly con- served element in the spleen. However, a clearly defined functional role has not yet been established. The constit- uents of the ellipsoidal cuff have been reported to range from lymphocytes in man (Buyssens et al. 1984) to lip- ids in the cat (Dustin 1975). The participation of ellip- soids in the clearance of blood-borne particulate material has been demonstrated for exogenous substances, such as bacteria in the carp (Maas and Bootsma 1982), colloi- dal carbon in the chicken (White 1981), and polystyrene microspheres in the rainbow trout (Espenes et al. 1995). This functional feature has been attributed to a perme- able endothelium (Dustin 1975; Brown et al. 1993) and the Surrounding dense reticular and cellular meshwork (Blue and Weiss 1981). The ellipsoidal deposition of soluble antigenic molecules has been investigated in the immunologically naive chicken and was found to occur only after the beginning of antibody production against the injected antigen (White and Gordon 1970). When a soluble antigen was given in combination with specific antibodies, there was a rapid deposition of antigen in the ellipsoids (White et al. 1975). These observations sug- gested that antigens were trapped in ellipsoids in the form of immune complexes. In teleosts, only a few studies have investigated the fate of soluble antigens. Ellis (1980) introduced bovine serum albumin into plaice and found that the antigen could be detected extracellularly, probably in the form of immune complexes, in the ellipsoidal walls from day 3 to day 37 after intraperitoneal injection. Similar results were obtained in the carp immunised against human gamma globulin. Up to 5 weeks after intraperitoneal im- munisation with human gamma globulin, the antigen showed an ellipsoidal localisation (Secombes and Man- ning 1980). After a second intraperitoneal immunisation of carp with human gamma globulin, Secombes et al. (1982) observed trapping of the antigen in the ellipsoids from day 1 and over the next 1-2 weeks.