Differential effects of viral hemorrhagic septicaemia virus (VHSV) genotypes IVa and IVb on gill epithelial and spleen macrophage cell lines from rainbow trout (Oncorhynchus mykiss)

The two most prominent genotypes of viral hemorrhagic septicemia virus (VHSV) are -I in the Northeastern Atlantic region and -IV in North America, but much more is known about the cellular pathogenesis of genotype -I than -IV. VHSV genotype -IV is divided into -IVa from the Northeast Pacific Ocean and -IVb from the Great Lakes and both of which are less virulent to rainbow trout than genotype -I. In this work, infections of VHSV-IVa and -IVb have been studied in two rainbow trout cell lines, RTgill-W1 from the gill epithelium, and RTS11 from spleen macrophages. RTgill-W1 produced infectious progeny of both VHSV-IVa and -IVb. However, VHSV-IVa was more infectious than -IVb toward RTgill-W1: -IVa caused cytopathic effect (CPE) at a lower viral titre, elicited CPE earlier, and yielded higher titres. By contrast, no CPE and no increase in viral titre were observed in RTS11 cultures infected with either genotype. Yet in RTS11 all six VHSV genes were expressed and antiviral genes, Mx2 and Mx3, were up regulated by VHSV-IVb and -IVa. However, replication appeared to terminate at the translational stage as viral N protein, presumably the most abundant of the VSHV proteins, was not detected in either infected RTS11 cultures. In RTgill-W1, Mx2 and Mx3 were up regulated to similar levels by both viral genotypes, while VHSV-IVa induced higher levels of IFN1, IFN2 and LGP2A than VHSV-IVb.