Differential display of grouper iridovirus-infected grouper cells by immunostaining.

Grouper iridovirus (GIV) is one of the most devastating infectious pathogens of aquaculture fish. When infecting a susceptible cell line, such as GK-2, GIV causes antigenic changes in host cellular proteins. To understand the host gene expression characteristics after viral infection, we developed an immunostaining method to screen differentially expressed genes of fish cells in response to GIV infection using phage display complementary DNA libraries. In total, 66 genes were identified from grouper kidney and brain cell lines. These genes are related to replication, transcription, translation, immunity, apoptosis, structure proteins, metabolism, energy, protein modification, and homeostasis. Four dynamic antigenic patterns were observed among these immunocloned genes upon GIV infection. Microarray analysis further confirmed the transcriptional patterns of 80% of the identified genes. This immunostaining screening method provides insights into a host's cellular protein response to viral infection on a translational basis.

Iridovirus Bcl-2 protein inhibits apoptosis in the early stage of viral infection.

The grouper iridovirus (GIV) belongs to the family Iridoviridae, whose genome contains an antiapoptotic B-cell lymphoma (Bcl)-2-like gene. This study was carried-out to understand whether GIV blocks apoptosis in its host. UV-irradiated grouper kidney (GK) cells underwent apoptosis. However, a DNA fragmentation assay of UV-exposed GK cells after GIV infection revealed an inhibition of apoptosis. The UV- or heat-inactivated GIV failed to inhibit apoptosis, implying that a gene or protein of the viral particle might contribute to an apoptosis inhibitory function. The DNA ladder assay for GIV-infected GK cells after UV irradiation confirmed that apoptosis inhibition was an early process which occurred as early as 5 min post-infection. A GIV-Bcl sequence comparison showed distant sequence similarities to that of human and four viruses; however, all possessed the putative Bcl-2 homology (BH) domains of BH1, BH2, BH3, and BH4, as well as a transmembrane domain. Northern blot hybridization showed that GIV-Bcl transcription began at 2 h post-infection, and the mRNA level significantly increased in the presence of cycloheximide or aphidicolin, indicating that this GIV-Bcl is an immediate-early gene. This was consistent with the Western blot results, which also revealed that the virion carries the Bcl protein. We observed the localization of GIV-Bcl on the mitochondrial membrane and other defined intracellular areas. By immunostaining, it was proven that GIV-Bcl-expressing cells effectively inhibited apoptosis. Taken together, these results demonstrate that GIV inhibits the promotion of apoptosis by GK cells, which is mediated by the immediate early expressed viral Bcl gene.