Differential growth of U and M type infectious haematopoietic necrosis virus in a rainbow trout-derived cell line, RTG-2. 
Infectious haematopoietic necrosis virus (IHNV) is one of the most important viral pathogens of salmonids.  In rainbow trout, IHNV isolates in the M genogroup are highly pathogenic, while U genogroup isolates are significantly less pathogenic.  We show here that, at a multiplicity of infection (MOI) of 1, a representative U type strain yielded 42-fold less infectious virus than an M type strain in the rainbow trout-derived RTG-2 cell line at 24 h post-infection (p.i.).  However, at an MOI of 10, there was only fivefold difference in the yield of infectious virus between the U and M strains.  Quantification of extracellular viral genomic RNA suggested that the number of virus particles released from cells infected with the U strain at a MOI of 1 was 47-fold lower than from M-infected cells, but U and M virions were equally infectious by particle to infectivity ratios.  At an MOI of 1, U strain intracellular viral genome accumulation and transcription were 37- and 12-fold lower, respectively, than those of the M strain at 24 h p.i.  Viral nucleocapsid (N) protein accumulation in U strain infections was fivefold lower than in M strain infections.  These results suggest that the block in U type strain growth in RTG-2 cells was because of the effects of reduced genome replication and transcription.  The reduced growth of the U strain does not seem to be caused by defective genes, because the U and M strains grew equally well in the permissive epithelioma papulosum cyprini cell line at an MOI of 1.  This suggests that host-specific factors in RTG-2 cells control the growth of the IHNV U and M strains differently, leading to growth restriction of the U type virus during the RNA synthesis step.