|Abstract or Summary
- Cyprinid herpesvirus 3 (CyHV-3), commonly known as koi herpesvirus (KHV), is a member of the Alloherpesviridae and is a deadly pathogen for koi and common carp, Cyprinus carpio. It causes severe gill necrosis and nephritis, dermal ulceration and hemorrhage, and mass mortality of up to 100% of affected fish. Fish that survive KHV infection are latently infected lifelong carriers. Latency is a conserved mechanism among known herpesviruses and is under the control, in part, of viral gene and protein expression.
Our previous study demonstrated that KHV becomes latent in peripheral white blood cells (WBC) of koi. In this study, KHV latency was further investigated in IgM⁺ WBC. The presence of the KHV genome in IgM⁺ WBC was about 20-fold more abundant than in IgM-WBC. To determine if KHV expressed genes during latency, transcription from all 8 open reading frames (ORFs) in the terminal repeat was investigated in IgM⁺ WBC from koi with latent KHV infection. Only a spliced ORF6 was found to be abundantly expressed in IgM⁺ WBC from KHV latently infected koi. The spliced ORF6 transcript was also detected in vitro during productive infection as early as 1 day post-infection. The ORF6 transcript from in vitro infection begins -127 bp upstream of the ATG and ends +188 bp downstream of the stop codon, +20 bp downstream of the polyadenylation signal. The hypothetical protein of ORF6 contains a consensus sequence with homology to a conserved domain of EBNA-3B and ICP4 from Epstein Barr virus and herpes simplex virus 1, respectively and both members of
the Herpesviridae. This is the first report of latent KHV in B cells and identification of gene transcription during latency for a member of the Alloherpesviridae.
To identify and collect an enriched population of KHV⁺ latently infected cells, a nanoflare probe was generated specific to ORF6 RNA and used to separate live KHV latently infected cells from total peripheral white blood cells. Using the nanoflare ORF6 probe, about 1% of peripheral WBC from latently infected koi were identified and collected by their expression of ORF6. When this enriched population of KHV⁺ latently infected cells was examined by RNA-seq, the ORF6 transcript was found to be the only viral transcript that consistently mapped to the KHV reference genome. This study demonstrated that a nanoflare RNA probe could be used to enrich latently infected cells, which can subsequently be used to characterize gene expression during KHV latency.
Little is known about the molecular mechanisms and control of latency for KHV. In this study, the expression of viral protein from ORF6 mRNA was investigated by a polyclonal antibody specific to a synthetic peptide derived from predicted ORF6 protein (anti-ORF6). Using an immunofluorescence assay (IFA), positive staining to the anti-ORF6 was observed in both KHV-infected common carp brain (CCB) cells in vitro and IgM⁺ B cells from koi latently infected with KHV. No IFA staining was observed in uninfected CCB cells nor from IgM-B cells from KHV⁺ latently infected koi. The ORF6 protein expressed during productive infection was detected around 140 kDa, which is bigger than the ~80 kDa predicted protein. ORF6 protein at a similar size as the predicted protein was identified from cloned ORF6 protein in an expression vector pet6XHN transformed in E. coli. Based on an analysis using software GPS-SUMO, 5 potential sumoylation sites were identified in the ORF6 protein sequence. This study demonstrated that ORF6 protein is expressed during KHV latency in koi and may be sumoylated in infected cells.
These works have unveiled molecular strategies of herpesvirus latency for KHV; the identification of a latency associated transcript as well as viral protein expression during latency, which demonstrate conserved mechanisms as other herpesvirus latency programs. Through these discoveries, it is possible to further
investigate the conserved biology of herpesvirus latent infections by using the koi and KHV model for human herpesvirus associated diseases and therapies.