A 32 kDa viral attachment protein of lymphocystis disease virus (LCDV) specifically interacts with a 27.8 kDa cellular receptor from flounder (Paralichthys olivaceus)

Decapod iridescent virus 1 (DIV1) 168L can target cuticle protein 8 from Litopenaeus vannamei

Decapod iridescent virus 1 (DIV1) stands as a significant pathogen affecting crustaceans, posing a grave threat to the shrimp industries in aquaculture dependent nations. Within the Iridoviridae family, the conserved envelope protein DIV1-168L plays a pivotal role in virion entry. Nonetheless, the host factors that interact with 168L remain unidentified. To address this gap, we established a cDNA library derived from Litopenaeus vannamei gill tissue and conducted yeast two-hybrid screening to identify host factors that interact with 168L. Additionally, we performed co-immunoprecipitation assays to verify the interaction between cuticle protein 8 (CP8) and 168L. Expression pattern analysis revealed the presence of CP8 transcripts in the gill and epidermis. Furthermore, immunohistochemistry results demonstrated the expression of CP8 in gill cells and its localization in the gill filament epithelium. Fluorescence analysis indicated that full-length CP8 colocalized with 168L in the cytoplasm of Sf9 cells. Removal of the signal peptide from the N-terminal of CP8 eliminated its concentration in the cytoplasm. Additionally, CP8 expression was significantly inhibited during DIV1 infection. Therefore, our research contributes to a better understanding of the entry mechanism of iridovirids. The GenBank accession number for the DIV1 sequence is MF197913.1.

Extracellular traps in skin lesions infected with lymphocystis disease virus in black rockfish (Sebastes schlegelii)

The lymphocystis disease (LCD), caused by Lymphocystis disease virus (LCDV), is a benign and self-limiting disease described in a many freshwater and marine fish species. Hypertrophic fibroblasts and extensive aggregation of inflammatory cells are characteristics of LCD. In the present study, small animal imaging and ultrastructural investigations were carried out on the lymphocystis nodules of black rockfish (Sebastes schlegelii) naturally infected with lymphocystis iridovirus, to assess pathology, and the exudate with particular attention to the formation of extracellular traps (ETs) in vivo. Ex vivo were examined by nodules sections and primary cells stimulation. By histopathological analysis, the nodules contained infiltrated inflammatory cells and extensive basophilic fibrillar filaments at the periphery of the hypertrophied fibroblasts. ETs were assessed in nodules samples using indirect immunofluorescence to detect DNA and myeloperoxidase. Moreover, LCDV was able to infect peritoneal cells of black rockfish in vitro and induce the formation of ETs within 4 h. In summary, this study proved that ETs are involved in the response to LCDV infection and may be involved in formation of lymphoid nodules. Taken together, the findings provide a new perspective to determine the impact factors on the growth of nodules.

Characterization and function of Japanese flounder (Paralichthys olivaceus) slc2a6 in response to lymphocystis disease virus infection

Slc2a6 is a member of the slc2 family (solute carrier 2 family) and previous reports have indicated its involvement in the inflammatory response. Slc2a6 is regulated by the NF-ĸB signaling pathway. This study investigated the differential expression of slc2a6 in the early embryonic development of Japanese flounder, revealing that the early gastrula stage had the highest level of slc2a6 expression. Moreover, slc2a6 expression was increased in vitro after stimulation by lymphocystis disease virus (LCDV), and in vivo experiments also showed significantly elevated levels in the spleen and muscle tissues following LCDV stimulation. Subcellular localization revealed that Slc2a6 was expressed in both the nucleus and cytoplasm of cells. The pcDNA3.1-slc2a6 overexpression plasmid was successfully constructed; the si-slc2a6 interfering strand was screened and samples were collected. The expression of NF-ĸB signaling pathway-related genes il-1β, il-6, nf-ĸb, and tnf-α was evaluated in overexpressed, silenced, and LCDV-stimulated samples. The results showed that slc2a6 is involved in viral regulation in Japanese flounder by regulating innate immune responses.

Transcriptome analysis reveals molecular mechanisms of lymphocystis formation caused by lymphocystis disease virus infection in flounder (Paralichthys olivaceus)

Lymphocystis disease is frequently prevalent and transmissible in various teleost species worldwide due to lymphocystis disease virus (LCDV) infection, causing unsightly growths of benign lymphocystis nodules in fish and resulting in huge economic losses to aquaculture industry. However, the molecular mechanism of lymphocystis formation is unclear. In this study, LCDV was firstly detected in naturally infected flounder (Paralichthys olivaceus) by PCR, histopathological, and immunological techniques. To further understand lymphocystis formation, transcriptome sequencing of skin nodule tissue was performed by using healthy flounder skin as a control. In total, RNA-seq produced 99.36%-99.71% clean reads of raw reads, of which 91.11%-92.89% reads were successfully matched to the flounder genome. The transcriptome data showed good reproducibility between samples, with 3781 up-regulated and 2280 down-regulated differentially expressed genes. GSEA analysis revealed activation of Wnt signaling pathway, Hedgehog signaling pathway, Cell cycle, and Basal cell carcinoma associated with nodule formation. These pathways were analyzed to interact with multiple viral infection and tumor formation pathways. Heat map and protein interaction analysis revealed that these pathways regulated the expression of cell cycle-related genes such as ccnd1 and ccnd2 through key genes including ctnnb1, lef1, tcf3, gli2, and gli3 to promote cell proliferation. Additionally, cGMP-PKG signaling pathway, Calcium signaling pathway, ECM-receptor interaction, and Cytokine-cytokine receptor interaction associated with nodule formation were significantly down-regulated. Among these pathways, tnfsf12, tnfrsf1a, and tnfrsf19, associated with pro-apoptosis, and vdac2, which promotes viral replication by inhibiting apoptosis, were significantly up-regulated. Visual analysis revealed significant down-regulation of cytc, which expresses the pro-apoptotic protein cytochrome C, as well as phb and phb2, which have anti-tumor activity, however, casp3 was significantly up-regulated. Moreover, bcl9, bcl11a, and bcl-xl, which promote cell proliferation and inhibit apoptosis, were significantly upregulated, as were fgfr1, fgfr2, and fgfr3, which are related to tumor formation. Furthermore, RNA-seq data were validated by qRT-PCR, and LCDV copy numbers and expression patterns of focused genes in various tissues were also investigated. These results clarified the pathways and differentially expressed genes associated with lymphocystis nodule development caused by LCDV infection in flounder for the first time, providing a new breakthrough in molecular mechanisms of lymphocystis formation in fish.

Identification and characterization of an envelope protein 168L in Cherax quadricarinatus iridovirus (CQIV)

Cherax quadricarinatus iridovirus (CQIV), a new member of family Iridoviridae, mainly infects the shrimps and crayfish with a high mortality rate. Previous gel-based LC-MS/MS study on CQIV has identified 30 structural proteins. In this study, one of the structural proteins, CQIV-168L, was selected for further analysis. RT-PCR and Western-blotting (WB) detection revealed that the transcript and the protein appeared late during infection of C. quadricarinatus cells and that the transcript was blocked by viral DNA replication inhibitor, indicating that CQIV-168L is a late expression gene. The specific antiserum against CQIV-168L was raised and immunofluorescence analysis showed that CQIV-168L was localized in the cytoplasm and associated with virus factories. Western-blotting (WB) assay suggested that CQIV-168L antiserum bound specifically to a 57-kDa protein in both the intact virions and the envelope fraction. As revealed by immunogold labeling, CQIV-168L was a component of the viral envelope. Findings in this work help to further understand the structure and entry mechanism of CQIV.

Peripheral Blood B-Lymphocytes Are Involved in Lymphocystis Disease Virus Infection in Flounder (Paralichthys olivaceus) via Cellular Receptor-Mediated Mechanism

Previous studies imply that peripheral blood leukocytes (PBLs) may play an important role in systemic lymphocystis disease virus (LCDV) dissemination, but whether the PBLs are susceptible and permissive to LCDV infection and the dissemination mechanism need to be clarified. In this study, LCDV was firstly confirmed to infect the PBLs in flounder (Paralichthys olivaceus) in vivo, and to replicate in PBLs in vitro. Subsequently, the 27.8 kDa receptor protein (27.8R), a functional receptor mediating LCDV infection in flounder gill cells, was shown to locate on the cell membrane of PBLs and co-localize with LCDV in PBLs, while blocking of the 27.8R via pre-incubation of anti-27.8R MAb with the PBLs could obviously inhibit LCDV infection, revealing the 27.8R as a receptor for LCDV entry into PBLs. Multicolor fluorescence imaging studies verified that IgM+ and IgD+ B-lymphocyte were involved in LCDV infection. In the sorted IgM+ B-cells, 27.8R+ and LCDV+ signals were simultaneously observed, and LCDV copy numbers increased with time, indicating that IgM+ B-cells expressed the 27.8R and were permissive to LCDV infection. Furthermore, the dynamic changes of IgM+, 27.8R+, LCDV+ and LCDV+/IgM+ PBLs were monitored during the early phase of LCDV infection. It was found that the percentage of IgM+ B-cells in PBLs clearly declined first and then increased, suggesting LCDV infection facilitated damage to B-cells, whereas the amounts of 27.8R+ and LCDV+ PBLs, as well as LCDV-infected IgM+ B-cells, showed an opposite trend. These results proved that IgM+ B-lymphocytes could be infected by LCDV via a receptor-mediated mechanism and support viral replication, which provided novel insights for the first time into the role of B-lymphocytes in LCDV dissemination and pathogenesis in teleost fish.

Lymphocystis Disease Virus (Iridoviridae) Enters Flounder (Paralichthys olivaceus) Gill Cells via a Caveolae-Mediated Endocytosis Mechanism Facilitated by Viral Receptors

In previous research, voltage-dependent anion channel protein 2 (VDAC2) and the receptor of activated protein C kinase 1 (RACK1) in flounder (Paralichthys olivaceus) were confirmed as functional receptors for lymphocystis disease virus (LCDV) entry; however, the underlying mechanism of VDAC2- and RACK1-mediated LCDV entry remains unclear. In this study, we elucidated the endocytosis pathway of LCDV entry into flounder gill (FG) cells by treatment with specific inhibitory agents, siRNAs, and co-localization analysis. LCDV entry was significantly inhibited by the disruption of caveolae-mediated endocytosis, dynamin, and microtubules, and the knockdown of caveoline-1 and dynamin expression, but was not inhibited by the disruption of clathrin-mediated endocytosis, micropinocytosis, or low-pH conditions. The disruption of caveolae-mediated and clathrin-mediated endocytosis was verified by the internalization of cholera toxin subunit B (CTB) and transferrin, respectively. Confocal immunofluorescence assay demonstrated that LCDV was co-localized with VDAC2 and RACK1, CTB was co-localized with VDAC2 and RACK1 and partially with LCDV, but transferrin was not co-localized with LCDV, VDAC2, or RACK1, indicating that LCDV utilized the same pathway as CTB, i.e., caveolae-mediated endocytosis. This was different from the pathway of transferrin, which used clathrin-mediated endocytosis. Furthermore, caveolin-1 was co-localized with LCDV, VDAC2, and RACK1, suggesting that caveolin-1 was involved in LCDV entry. These results revealed for the first time that LCDV entered into FG cells via caveolae-mediated endocytosis facilitated by VDAC2 and RACK1 receptors, relying on dynamin and microtubules in a pH-independent manner, which provided new insight into the molecular mechanisms of LCDV entry and potential for the development of antiviral agents, expanding our understanding of iridovirus infection.

Voltage-Dependent Anion Channel Protein 2 (VDAC2) and Receptor of Activated Protein C Kinase 1 (RACK1) Act as Functional Receptors for Lymphocystis Disease Virus Infection

In previous research, a 27.8-kDa protein in flounder Paralichthys olivaceus gill (FG) cells was identified as a putative cellular receptor (27.8R), which mediated lymphocystis disease virus (LCDV) infection via interaction with a 32-kDa viral attachment protein (VAP) of LCDV, and monoclonal antibodies (MAbs) against 27.8R and 32-kDa VAP were developed. In this study, the 27.8R was identified as voltage-dependent anion channel protein 2 (VDAC2) and receptor of activated protein C kinase 1 (RACK1) of flounder. Recombinant VDAC2 (rVDAC2) and RACK1 (rRACK1) were obtained by prokaryotic expression, and rabbit anti-VDAC2/RACK1 polyclonal antibodies were prepared. The rVDAC2, rRACK1, and 27.8-kDa proteins in FG cells were recognized by anti-27.8R MAbs and anti-VDAC2/RACK1 polyclonal antibodies simultaneously. Preincubation of FG cells with anti-VDAC2/RACK1 polyclonal antibodies significantly decreased the percentages of LCDV-infected cells and LCDV copy numbers, blocked virus infection, and delayed the development of cytopathic effect. The mRNA expressions of VDAC2 and RACK1 in FG cells were upregulated to maximum levels 12 h and 48 h after LCDV infection, respectively. VDAC2/RACK1 knockdown through short interfering RNA (siRNA) significantly reduced VDAC2/RACK1 expression and LCDV copy numbers in FG cells compared with negative controls, while VDAC2/RACK1 expression on LCDV-nonpermissive epithelial papillosum cells (EPCs) conferred susceptibility to LCDV infection, indicating the VDAC2 and RACK1 were sufficient to allow LCDV entry and infection. All these results collectively showed that VDAC2 and RACK1 function as receptors for LCDV entry and infection.IMPORTANCE Lymphocystis disease virus (LCDV) is the causative agent of lymphocystis disease in fish, which has caused huge economic losses to the aquaculture industry worldwide, but the molecular mechanism underlying the LCDV-host interaction remains unclear. Here, the 27.8-kDa putative cellular receptor for LCDV was identified as voltage-dependent anion channel protein 2 (VDAC2) and receptor of activated protein C kinase 1 (RACK1), and our results revealed that VDAC2 and RACK1 expression was sufficient to allow LCDV entry and that they are functional receptors that initiate LCDV infection for the first time, which leads to a better understanding of the molecular mechanism underlying LCDV infection and virus-host interactions.

Development and Characterization of Monoclonal Antibodies to the 32 kDa Viral Attachment Protein of Lymphocystis Disease Virus and Their Neutralizing Ability in Vitro

In previous research, a 32 kDa protein in lymphocystis disease virus (LCDV) was identified as viral attachment protein (VAP) that specifically interacted with the 27.8 kDa cellular receptor from flounder Paralichthys olivaceus gill (FG) cells, and the recombinant VAP (rVAP) was expressed in Escherichia coli strain BL21 (DE3). In this study, monoclonal antibodies (MAbs) against 32 kDa VAP are produced by immunization of BALB/c mice with the rVAP. Seven hybridoma secreting MAbs were screened by enzyme-linked immunosorbent assay, five of which designated as 1C6, 1C8, 3B5, 3D11 and 3H10 are cloned by the limiting dilution method, depending on the strongly positive results of ELISA. Western blotting analysis shows that the five MAbs can specifically react with the 32 kDa protein of LCDV and the purified 50 kDa rVAP, and the subtype of the MAbs is identified as IgG. Immunofluorescence results demonstrate that the specific fluorescence signals for LCDV appear in the cytoplasm of FG cells at 24 h post LCDV infection. Neutralization assay results indicate that pre-incubations of LCDV with the five MAbs can significantly decrease the LCDV copy numbers and delay the development of the cytopathic effect in FG cells, revealing that the five MAbs can neutralize the LCDV particles and block viral infection in vitro. The neutralizing MAbs against 32 kDa VAP would be useful for the study on the LCDV⁻host interaction and might be promising inhibitors of LCDV infection in fish.