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

The characterisation of a monoclonal antibody (mab 45) reacting with phagocytic leucocytes isolated from blood and spleen of rainbow trout (Oncorhynchus mykiss, Walbaum) is described. The surface marker labelled by this mab is expressed at relative low levels on the membrane of large, nearly nongranulated 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 spleen and head kidney, large, polymorphonuclear leucocytes were immunostained. The mab most strongly recognised an antigen of 48 kDa prepared from trout leucocytes of different organs, but not in trout plasma. In an in vitro phagocytosis assay trout monocytes were stained with mab 45 after phagocytosis of Aeromonas salmonicida labelled with the lipophilic fluorescent cell surface linker PKH26. However, previous binding of mab 45 on trout leucocytes did not inhibit the phagocytosis of A. salmonicida particles. Using mab 45, the dynamics of monocytes in blood, spleen and peritoneal cavity could be demonstrated after intraperitoneal injection of trout with inactivated A. salmonicida. The described mab serves as a useful tool to investigate the involvement of monocytes/macrophages in immune reactions of trout to a variety of pathogens.