Comparative Biochemistry and Physiology Part C 123 (1999) 53 – 59 Enhancement of anti-Aeromonas salmonicida activity in Atlantic salmon (Salmo salar ) macrophages by a mannose-binding lectin Christopher A. Ottinger 1 , Stewart C. Johnson *, K. Vanya Ewart, Laura L. Brown, Neil W. Ross Institute for Marine Biosciences, National Research Council Canada, 1411 Oxford Street, Halifax, Noa Scotia, B3H 3Z1, Canada Received 1 September 1998; received in revised form 11 January 1999; accepted 27 January 1999 Abstract We investigated the effects of a calcium-dependent mannose-binding lectin isolated from the serum of Atlantic salmon on Aeromonas salmonicida viability and the anti-A. salmonicida activity of Atlantic salmon macrophages. In the absence of other factors, binding of this lectin at concentrations of 0.8, 4.0 and 20.0 ng ml -1 to virulent A. salmonicida failed to significantly reduce (P 0.05) cell viability. However, binding of the lectin to A. salmonicida did result in significant (P 0.05) dose-dependent increases in phagocytosis, and bactericidal activity. Significant increases (P 0.05) were also observed in phagocyte respiratory burst activity within the lectin concentration range of 4.0 – 20.0 ng ml -1 but the stimulation was not dose dependent at these lectin concentrations. At the lowest lectin concentration tested (0.32 ng ml -1 ), a significant decrease (P 0.05) in respiratory burst was observed. The structure and activity of this lectin are similar to that of mammalian mannose-binding lectins, which are known to play a pivotal role in innate immunity. The presence of this lectin may be an important defense mechanism against Gram-negative bacteria such as A. salmonicida. © 1999 Elsevier Science Inc. All rights reserved. Keywords: Aeromonas salmonicida ; Atlantic salmon; Furunculosis; Innate immunity; Lectin; Macrophage; Mannose binding; Salmo salar 1. Introduction Lectins are sugar-binding proteins that may be intra- cellular, extracellular or membrane bound. In animals, extracellular and soluble lectins recognize specific car- bohydrate arrays on pathogen surfaces and enable the host to identify pathogens as non-self. Once bound to pathogens, lectins can stimulate increased uptake by phagocytosis and complement-mediated cell lysis. In mammals, lectins such as human mannose-binding protein (MBP) are known to play an important role in innate immunity and disease avoidance [15,24,25,31,47]. Lectins have been isolated from serum, plasma, sur- face mucus, egg surfaces and egg components from a wide variety of fish species [18,19]. Generally, little is known about the biological functions of these lectins in fish, although they are thought to play a role in protec- tion against bacterial pathogens and in the prevention of polyspermy. A number of MBPs have been isolated from salmon serum using mannose – agarose beads [4]. One of these was a calcium-dependent lectin, which exists as a disulfide-linked multimer of a polypeptide having a relative molecular mass (M r ) of 17,000. The concentra- tion of this lectin in serum was estimated to be approx- imately 5 g ml -1 . This lectin was demonstrated to bind to the surfaces of the fish bacterial pathogens Vibrio anguillarum and Aeromonas salmonicida [4]. * Corresponding author. Tel.: +1-902-426-2630; fax: +1-902-426- 9413. E-mail address: stewart.johnson@nrc.ca (S.C. Johnson) 1 Current address: National Fish Health Research Laboratory, Kearneysville, WV 25430, USA. 0742-8413/99/$ - see front matter © 1999 Elsevier Science Inc. All rights reserved. PII:S0742-8413(99)00009-2