Induction of the Mx protein of rainbow trout Oncorhynchus mykiss in vitro and in vivo with poly I:C dsRNA and infectious hematopoietic necrosis virus

Expression of the Mr protein of rainbow trout was analyzed after induction by poly I:C dsRNA and infectious hematopoietic necrosis virus (IHNV). Poly I:C dsRNA-treated rainbow trout gonad (RTG-2) cells expressed Mx protein that was detectable by immunoblot analysis at 24 h post induction. Increased expression was observed at 48 h and then declined by 72 h post-induction. In contrast, IHNV was not an efficient inducer of Mr protein in cells in culture. An immunocytochemical assay was also established to detect Mr proteins after transient transfection of chinook salmon embryo (CHSE-214) cells with trout Mr cDNA expression clones. Using these same techniques, endogenous Mr protein was detected in RTG-2 cells induced with 5 and 50 mu g ml(-1) of poly I:C dsRNA at 48 h post induction. In vivo, the appearance of Mr protein in rainbow trout after infection with IHNV (isolated at Rangen Research, Hagerman, ID, USA) was demonstrated in the immunoblots of kidney extracts of 4 out of 4 fish at 2 d post infection. Immunohistochemical staining of the kidney tissue from 3 out of 3 rainbow trout fry infected with the RB-76 strain of IHNV (isolated at Rounded Butte Hatchery, OR, USA) confirmed the production of Mr protein in the kidney tubules at 2 d post infection. These results suggest that Mr is a useful marker for the induction of fish interferon.