Structure of the zebrafish galectin-1-L2 and model of its interaction with the infectious hematopoietic necrosis virus (IHNV) envelope glycoprotein.

Galectins, highly conserved ß-galactoside-binding lectins, have diverse regulatory roles in development and immune homeostasis and can mediate protective functions during microbial infection. In recent years, the role of galectins in viral infection has generated considerable interest. Studies on highly pathogenic viruses have provided invaluable insight into the participation of galectins in various stages of viral infection, including attachment and entry. Detailed mechanistic and structural aspects of these processes remain undetermined. To address some of these gaps in knowledge, we used Zebrafish as a model system to examine the role of galectins in infection by infectious hematopoietic necrosis virus (IHNV), a rhabdovirus that is responsible for significant losses in both farmed and wild salmonid fish. Like other rhabdoviruses, IHNV is characterized by an envelope consisting of trimers of a glycoprotein that display multiple N-linked oligosaccharides and play an integral role in viral infection by mediating the virus attachment and fusion. Zebrafish's proto-typical galectin Drgal1-L2 and the chimeric-type galectin Drgal3-L1 interact directly with the glycosylated envelope of IHNV, and significantly reduce viral attachment. In this study, we report the structure of the complex of Drgal1-L2 with N-acetyl-d-lactosamine at 2.0 Å resolution. To gain structural insight into the inhibitory effect of these galectins on IHNV attachment to the zebrafish epithelial cells, we modeled Drgal3-L1 based on human galectin-3, as well as, the ectodomain of the IHNV glycoprotein. These models suggest mechanisms for which the binding of these galectins to the IHNV glycoprotein hinders with different potencies the viral attachment required for infection.

Differential virulence mechanisms of infectious hematopoietic necrosis virus in rainbow trout (Oncorhynchus mykiss) include host entry and virus replication kinetics.

Host specificity is a phenomenon exhibited by all viruses. For the fish rhabdovirus infectious hematopoietic necrosis virus (IHNV), differential specificity of virus strains from the U and M genogroups has been established both in the field and in experimental challenges. In rainbow trout (Oncorhynchus mykiss), M IHNV strains are consistently more prevalent and more virulent than U IHNV. The basis of the differential ability of these two IHNV genogroups to cause disease in rainbow trout was investigated in live infection challenges with representative U and M IHNV strains. When IHNV was delivered by intraperitoneal injection, the mortality caused by U IHNV increased, indicating that the low virulence of U IHNV is partly due to inefficiency in entering the trout host. Analyses of in vivo replication showed that U IHNV consistently had lower prevalence and lower viral load than M IHNV during the course of infection. In analyses of the host immune response, M IHNV-infected fish consistently had higher and longer expression of innate immune-related genes such as Mx-1. This suggests that the higher virulence of M IHNV is not due to suppression of the immune response in rainbow trout. Taken together, the results support a kinetics hypothesis wherein faster replication enables M IHNV to rapidly achieve a threshold level of virus necessary to override the strong host innate immune response.

Characterization of the mutant spectra of a fish RNA virus within individual hosts during natural infections.

Infectious hematopoietic necrosis virus (IHNV) is an RNA virus that causes significant mortalities of salmonids in the Pacific Northwest of North America. RNA virus populations typically contain genetic variants that form a heterogeneous virus pool, referred to as a quasispecies or mutant spectrum. This study characterized the mutant spectra of IHNV populations within individual fish reared in different environmental settings by RT-PCR of genomic viral RNA and determination of partial glycoprotein gene sequences of molecular clones. The diversity of the mutant spectra from ten in vivo populations was low and the average mutation frequencies of duplicate populations did not significantly exceed the background mutation level expected from the methodology. In contrast, two in vitro populations contained variants with an identical mutational hot spot. These results indicated that the mutant spectra of natural IHNV populations is very homogeneous, and does not explain the different magnitudes of genetic diversity observed between the different IHNV genogroups. Overall the mutant frequency of IHNV within its host is one of the lowest reported for RNA viruses.

Phylogeography of infectious haematopoietic necrosis virus in North America.

Infectious hematopoietic necrosis virus (IHNV) is a rhabdoviral pathogen that infects wild and cultured salmonid fish throughout the Pacific Northwest of North America. IHNV causes severe epidemics in young fish and can cause disease or occur asymptomatically in adults. In a broad survey of 323 IHNV field isolates, sequence analysis of a 303 nucleotide variable region within the glycoprotein gene revealed a maximum nucleotide diversity of 8.6 %, indicating low genetic diversity overall for this virus. Phylogenetic analysis revealed three major virus genogroups, designated U, M and L, which varied in topography and geographical range. Intragenogroup genetic diversity measures indicated that the M genogroup had three- to fourfold more diversity than the other genogroups and suggested relatively rapid evolution of the M genogroup and stasis within the U genogroup. We speculate that factors influencing IHNV evolution may have included ocean migration ranges of their salmonid host populations and anthropogenic effects associated with fish culture.