A monoclonal antibody recognising a surface marker on rainbow trout (Oncorhynchus mykiss) monocytes

The characterisation of a monoclonal antibody (mab 45) reacting with phagoc ytic leucocytes isolated from blood and spleen of rainbow trout (Oncorhynch us mykiss, Walbaum) is described. The surface marker labelled by this mab i s expressed at relative low levels on the membrane of large, nearly nongran ulated trout leucocytes, and having the typical morphology of monocytes in flow cytometry (Kfoury et al., 1999, Fish Pathology, 34, 1-6). No reaction of mab 45 with granulocytes, lymphocytes or thrombocytes was detected. In s pleen and head kidney, large, polymorphonuclear leucocytes were immunostain ed. The mab most strongly recognised an antigen of 48 kDa prepared from tro ut leucocytes of different organs, but not in trout plasma. In an in vitro phagocytosis assay trout monocytes were stained with mab 45 after phagocyto sis of Aeromonas salmonicida labelled with the lipophilic fluorescent cell surface linker PKH26. However, previous binding of mab 45 on trout leucocyt es did not inhibit the phagocytosis of A. salmonicida particles. Using mab 45, the dynamics of monocytes in blood, spleen and peritoneal cav ity could be demonstrated after intraperitoneal injection of trout with ina ctivated A. salmonicida. The described mab serves as a useful tool to inves tigate the involvement of monocytes/macrophages in immune reactions of trou t to a variety of pathogens. (C) 2001 Academic Press.