In vivo neutralization of Cryptobia salmositica metallo-protease by α2-macroglobulin in the blood of rainbow trout Oncorhynchus mykiss and in brook charr Salvelinus fontinalis

Abstract

Cryptobia salmositica infects both rainbow trout Oncorhynchus mykiss and brook charr Salvelinus fontinalis, but it does not cause disease (e.g. anaemia) in brook chart. The natural anti-protease, α2-macroglobulin (α2-M), was found in the blood of rainbow trout and brook charr, but it had much higher activity in brook charr. Under in vivo conditions, α2-M of trout and charr were neutralized by C. salmositica extracellular protease The onset and persistence of anaemia in infected rainbow trout were related to low levels of α2-M. Infected brook charr did not develop anaemia as α2-M remained high during the infection. Secreted metallo-protease of C. salmositica was detected in the blood of infected rainbow trout when its α2-M was low and the fish was anaemic; however, metallo-protease was not detected in infected brook charr. We suggest that host α2-M plays an important role in defense against cryptobiosis by inhibiting the activity of metallo-protease secreted by the pathogenic C. salmositica.