Identification of B Cells as a Major Site for Cyprinid Herpesvirus 3 Latency

Cyprinid herpesvirus 3 (CyHV-3), commonly known as koi herpesvirus (KHV), is a member of the Alloherpesviridae, and is a recently discovered emerging herpesvirus that is highly pathogenic for koi and common carp. Our previous study demonstrated that CyHV-3 becomes latent in peripheral white blood cells (WBC). In this study, CyHV-3 latency was further investigated in IgMWBC. The presence of the CyHV-3 genome in IgMWBC was about 20-fold greater than in IgMWBC. To determine whether CyHV-3 expressed genes during latency, transcription from all eight open reading frames (ORFs) in the terminal repeat was investigated in IgMWBC from koi with latent CyHV-3 infection. Only a spliced ORF6 transcript was found to be abundantly expressed in IgM WBC from CyHV-3 latently infected koi. The spliced ORF6 transcript was also detected in vitro during productive infection as early as 1 day postinfection. The ORF6 transcript from in vitro infection begins at 127 bp upstream of the ATG codon 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, both members of the Herpesviridae. This is thefirst report of latent CyHV-3 in B cells and identification of gene transcription during latency for a member of the Alloherpesviridae. IMPORTANCE This is thefirst demonstration that a member of the Alloherpesviridae, cyprinid herpesvirus 3 (CyHV-3), establishes a latent infection in the B cells of its host, Cyprinus carpio. In addition, this is the first report of identification of gene transcription during latency for a member of Herpesvirales outsideHerpesviridae. This is also thefirst report that the hypothetical protein of latent transcript of CyHV-3 contains a consensus sequence with homology to a conserved domain of EBNA-3B from Epstein-Barr virus and ICP4 from herpes simplex virus 1, which are genes important for latency. These strongly suggest that latency is evolutionally conserved across vertebrates.