Transmission of lymphocystis disease virus to cultured gilthead seabream,Sparus aurata L., larvae

The transmission of lymphocystis disease virus (LCDV) to gilthead seabream, Sparus aurata L., larvae was investigated using fertilized eggs from a farm with previous reports of lymphocystis disease. LCDV genome was detected by PCR‐hybridization in blood samples from 17.5% of the asymptomatic gilthead seabream broodstock analysed. Using the same methodology, eggs spawned from these animals were LCDV positive, as well as larvae hatched from them. The presence of infective viral particles was confirmed by cytopathic effects development on SAF‐1 cells. Whole‐mount in situ hybridization (ISH) and immunohistochemistry (IHC) showed the presence of LCDV in the epidermis of larvae hatched from LCDV‐positive eggs. When fertilized eggs were disinfected with iodine, no viral DNA was detected either in eggs (analysed by PCR‐hybridization) or in larvae (PCR‐hybridization and ISH). These results suggest the vertical transmission of LCDV, the virus being transmitted on the egg surface. Larvae hatched from disinfected eggs remain LCDV negative during the endotrophic phase, as showed by PCR‐hybridization, ISH and IHC. After feeding on LCDV‐positive rotifers, viral antigens were observed in the digestive tract, which suggests that viral entry could be achieved via the alimentary canal, and that rotifers can act as a vector in LCDV transmission to gilthead seabream larvae.     

Lymphocystis disease virus persists in the epidermal tissues of olive flounder, Paralichthys olivaceus (Temminch & Schlegel), at low temperatures

Olive flounder artificially infected with lymphocystis disease virus (LCDV) were reared at 10, 20 and 30 °C for 60 days, to compare LCD-incidence. In the fish reared at 20 °C, lymphocystis cells appeared on the skin and fins at 35 days post-challenge, and the cumulative LCD-incidence was 80% at 60 days. High levels of LCDV, with a mean polymerase chain reaction (PCR) titre of 10⁶ PCR-U mg⁻¹ tissue, were detected in the fins and skin of LCD-affected fish at 20 °C, but were not detected in the spleen, kidney, brain and intestinal tissues of these fish. No LCD clinical signs were observed in the fish reared at 10 °C and 30 °C; however, a low level of LCDV (10³ PCR-U mg⁻¹ tissue) was detected in the fins and skin of these fish. By increasing the rearing temperature from 10 to 20 °C, lymphocystis clusters appeared on the skin and fins of the fish with no previous LCD clinical signs within 33 days after the temperature change. It was shown that permissive cells for LCDV infection exist in the epidermis of olive flounder. At low temperatures, small amounts of LCDV were able to persist over a period extended for a further 45 days in the fish epidermis, even though the fish showed no LCD clinical signs. The optimum growth temperature of LCDV is near 20 °C.     

Phylogenetic analysis and molecular methods for the detection of lymphocystis disease virus from yellow perch, Perca flavescens (Mitchell)

Lymphocystis disease is a prevalent, non-fatal disease that affects many teleost fish and is caused by the DNA virus lymphocystis disease virus (LCDV). Lymphocystis-like lesions have been observed in yellow perch, Perca flavescens (Mitchell), in lakes in northern Alberta, Canada. In an effort to confirm the identity of the virus causing these lesions, DNA was extracted from these lesions and PCR with genotype generic LCDV primers specific to the major capsid protein (MCP) gene was performed. A 1357-base pair nucleotide sequence corresponding to a peptide length of 452 amino acids of the MCP gene was sequenced, confirming the lesions as being lymphocystis disease lesions. Phylogenetic analysis of the generated amino acid sequence revealed the perch LCDV isolate to be a distinct and novel genotype. From the obtained sequence, a real-time PCR identification method was developed using fluorgenic LUX primers. The identification method was used to detect the presence/absence of LCDV in yellow perch from two lakes, one where lymphocystis disease was observed to occur and the other where the disease had not been observed. All samples of fin, spleen and liver tested negative for LCDV in the lake where lymphocystis disease had not been observed. The second lake had a 2.6% incidence of LCD, and virus was detected in tissue samples from all individuals tested regardless of whether they were expressing the disease or not. However, estimated viral copy number in spleen and liver of symptomatic perch was four orders of magnitude higher than that in asymptomatic perch.     

Serological techniques for detection of lymphocystis virus in fish

Three serological techniques (indirect immunofluorescence test, flow cytometry, and indirect dot–blot immunoenzymatic assay) have been evaluated for the detection of lymphocystis viral antigens using a gilt-head seabream cell line, SAF-1, and fish leukocytes. Six lymphocystis disease virus (LCDV) isolates from gilt-head seabream, and one reference strain (ATCC VR 342), were tested. Detection of viral LCDV antigens in SAF-1 cells and fish leukocytes by indirect immunofluorescence test occurs at similar periods (5–7 d post-inoculation), and viral antigens were detected as cytoplasmic inclusions located at the periphery of inoculated cells. The percentages of cells with LCDV antigens obtained by flow cytometry were very low, ranging between 0.9% at 5 d post-inoculation and 19.7% at 10 d post-inoculation. The optimal concentration of viral stocks detected by indirect dot–blot immunoenzymatic assay was 0.5 μg ml^–1, when purified viral stocks were used as antigens. Inoculated and uninoculated SAF-1 cells could not be distinguished using LCDV antiserum binding. On the basis of these results, indirect immunofluorescence and flow cytometry tests appear to be the best serological methods to detect LCDV antigens in both SAF-1 cells and fish leukocytes.     

Identification and characterization of a novel lymphocystis disease virus isolate from cultured grouper in China

Grouper Epinephelus spp. is one of the most important mariculture fish species in China and South-East Asian countries. The emerging viral diseases, evoked by iridovirus which belongs to genus Megalocytivirus and Ranavirus, have been well characterized in recent years. To date, few data on lymphocystis disease in grouper which caused by lymphocystis disease virus (LCDV) were described. Here, a novel LCDV isolate was identified and characterized. Based on the sequence of LCDV major capsid protein (MCP) and DNA polymerase gene, we found that the causative agents from different species of diseased groupers were the same one and herein were uniformly defined as grouper LCDV (GLCDV). Furthermore, H&E staining revealed that the nodules on the skin were composed of giant cells that contained inclusion bodies in the cytoplasm. Numerous virus particles with >210 nm in diameter and with hexagonal profiles were observed in the cytoplasm. In addition, phylogenetic analysis based on four iridovirus core genes, MCP, DNA polymerase, myristoylated membrane protein (MMP) and ribonucleotide reductase (RNR), consistently showed that GLCDV was mostly related to LCDV-C, followed by LCDV-1. Taken together, our data firstly provided the molecular evidence that GLCDV was a novel emerging iridovirus pathogen in grouper culture.     

Induction of apoptosis in a flounder gill cell line by lymphocystis disease virus infection

Lymphocystis disease virus (LCDV), a large icosahedral DNA virus classified to the iridovirus family, is the causative agent of lymphocystis, a disease which occurs in marine and freshwater fish species and is characterized by formation of papilloma-like lesions on the surface of the skin. In vitro, LCDV infection causes flounder gill cells, an adherent cell line, to exhibit an obvious cytopathic effect (CPE). In order to test whether apoptosis is responsible for the observed CPE, cells infected with LCDV at a multiplicity of infection (m.o.i.) of 5 PFU per cell were examined at various time intervals for the appearance of apoptotic signs. Nuclear fragmentation, DNA laddering and caspase activation were observed in the infected cells at the time (i.e. 10 days post-infection) when an intensive CPE was observed. These findings demonstrate that LCDV is capable of inducing apoptosis in vitro, which is different from the result of LCDV infection in vivo, and consequently suggest an intricate LCDV-host interaction.     

牙鲆淋巴囊肿病毒一抗原蛋白的确认及定位

以牙鲆淋巴囊肿病毒（LCDV）为抗原免疫Balb/c小鼠，而后将小鼠脾细胞与P3U1骨髓瘤细胞融合，以囊肿组织冰冻切片的免疫荧光染色筛选杂交瘤细胞，阳性结果显示特异性块状荧光信号集中在囊肿细胞的细胞质边缘部分，且多个荧光信号相连呈现链圈状，有限稀释 法克隆阳性杂交瘤细胞，三次克隆后获得4株稳定产生抗LCDV抗体的单克隆杂交瘤细胞株（1A8、1D7、2B6、2D11）。应用Western-blotting法分析单抗识别蛋白的分子量，结果显示，单抗1D7 和2B6均能特异性结合一条分子量116 kD病毒多肽；应用免疫电镜技术定位单抗识别的抗原决定簇，结果发现胶体金颗粒集中吸附在病毒粒子衣壳周围，且背景清洁，无散在的金颗粒或其他污染物。实验结果说明分子量约为116 kD的蛋白多肽为LCDV病毒衣壳蛋白，且具有线性抗原决定簇。     

病毒铺膜印迹技术分离牙鲆鳃细胞上的淋巴囊肿病毒结合蛋白

利用非变性电泳与病毒铺膜印迹技术(VOPBA)分离了牙鲆(Paralichthys olivaceus)鳃细胞(FG)上淋巴囊肿病毒结合蛋白,结果显示在FG细胞膜上有分子量为135 kD的蛋白与淋巴囊肿病毒特异结合;对该蛋白切胶回收后进行SDS-PAGE与双向电泳,发现135 kD蛋白由3个蛋白组成,分子量分别为58.3 kD、44.6 kD及37.6 kD;135 kD蛋白SDS-PAGE的VOPBA显示,仅出现37.6 kD的蛋白带,而58.3 kD、44.6 kD蛋白皆不与淋巴囊肿病毒结合。结果表明牙鲆FG细胞上135 kD蛋白是淋巴囊肿病毒的结合蛋白,其37.6 kD蛋白具有病毒结合活性。     

淋巴囊肿病毒结构蛋白及其抗原性分析

病鱼为威海水产养殖场感染淋巴囊肿病的牙鲆（Paralichthys olivaceus）,收集病鱼的囊肿组织,匀浆破碎,采用差速离心和蔗糖密度梯度离心方法,分离纯化淋巴囊肿病毒粒子.负染后,电镜观察证实获得的病毒纯度高,杂质极少,病毒粒子呈近似于圆形的多角形,结构完整.纯化的淋巴囊肿病毒粒子经SDS-PAGE,硝酸银染色后,电泳图谱清晰显示病毒结构蛋白带共有22条,且分子量主要集中在123～26 kD.应用Western blotting法分析病毒结构蛋白的抗原性,结果显示,分子量分别为123.55 kD、65.292 kD和54.438 kD的3条蛋白带发生了免疫反应,其中分子量为65.292 kD的蛋白带反应强度明显高于其他2条蛋白带.本研究旨为确定淋巴囊肿病毒主要衣壳蛋白提供基础依据.[中国水产科学,2006,13（3）：415-420]     

淋巴囊肿病毒27.8 ku受体蛋白在牙鲆组织中的分布

应用抗牙鲆淋巴囊肿病毒(lymphocystis disease virus,LCDV)受体蛋白(27.8 ku)的单克隆抗体(2G11和3D9)定位LCDV受体蛋白在牙鲆组织中的分布。通过对牙鲆外周血、白细胞、鳃、胃、肠、表皮、肝脏、头肾、体肾、脾、性腺、脑、心脏等进行LCDV受体蛋白的间接免疫荧光与免疫组织化学定位观察,发现在牙鲆外周血白细胞的细胞膜、鳃上皮细胞、表皮、胃黏膜上皮细胞顶端、肠上皮细胞、肝细胞、脾表层结缔组织细胞及头肾后端的肾小管上皮细胞内均有较强的阳性信号,表明这些部位分布有LCDV的27.8 ku受体蛋白,但在体肾、性腺、脑、心脏及外周血红细胞中未观察到阳性信号。推测LCDV通过与鳃、表皮及消化道上皮的受体结合进入牙鲆体内,通过与外周血白细胞上的受体结合侵染白细胞而进入血液循环,进而感染肝脏、脾脏、头肾等器官。     

温度对牙鲆皮肤黏液抗体产生的影响

在9℃、15℃、21℃和26℃4种不同水温下,用淋巴囊肿病毒(LCDV)灭活疫苗腹腔注射牙鲆,应用间接酶联免疫吸附试验(ELISA)研究了其皮肤黏液中特异性抗体水平的变化,以分析温度对牙鲆皮肤黏液抗体产生的影响.ELISA结果表明,9℃和15℃水温下牙鲆黏液OD值分别在注射LCDV后第9周和第7周达到峰值(9℃:OD=0.179; 15℃:OD=0.233); 21℃水温下OD值上升最快,5周达到峰值(0.316); 26℃水温下OD值较21℃无显著差异,也于5周达到峰值(0.295).采用硫酸铵分步盐析等技术粗提不同温度下OD值最高时的牙鲆皮肤黏液中的免疫球蛋白(Ig),SDS-PAGE检测发现,各温度组黏液蛋白中均含有72 kD和26 kD蛋白条带.Western blotting结果显示,抗牙鲆血清Ig重链的单克隆抗体只与黏液蛋白中72 kD条带发生反应,确定为牙鲆皮肤黏液Ig重链.综上结果表明,牙鲆在最适生活温度(21℃)下,抗体应答强度最大.牙鲆粗提黏液蛋白中Ig的初步确定,为探索牙鲆黏液免疫机制提供了材料.     

Establishment of a novel fin cell line from Brown-marbled grouper, Epinephelus fuscoguttatus (Forsskål), and evaluation of its viral susceptibility

To lay a solid foundation of in vitro investigations of fish viral diseases, cytotechnology and cytotoxicology, a novel fin cell line from brown-marbled grouper, Epinephelus fuscoguttatus , was established and its viral susceptibility was evaluated. The fin tissues, digested with hyaluronidase and collagenase II, were used to initiate primary culture at 24 °C by using 20% foetal bovine serum-Dulbecco's modified Eagle medium/F12 medium, which was further supplemented with carboxymethyl–chitooligosaccharide, basic fibroblast growth factor and insulin-like growth factor-I. The fibroblastic fin cells grew at a steady rate during subsequent subculture and had a population doubling time of 50.6 h at passage 60. The modal diploid chromosome number was 48. A brown-marbled grouper fin cell line (bmGF-1) has been established and subcultured to passage 75 by now. Viral susceptibilities revealed that typical cytopathic effects of bmGF-1 cells emerged after being infected by turbot reddish-body iridovirus (TRBIV) or lymphocystis disease virus (LCDV). However, a large number of TRBIV and LCDV particles were also found in infected bmGF-1 cells. All these indicate that the bmGF-1 cell line has good susceptibility to TRBIV and LCDV, which may serve as a valuable tool for studies of cell–virus interactions and have potential applications in fish virus propagation and vaccine development.     

Detection of antigenic proteins expressed by lymphocystis virus as vaccine candidates in olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

Although the major capsid proteins (MCPs) of lymphocystis disease virus (LCDV) have been characterized, little is known about the host-derived immune response to MCPs and other LCDV antigenic proteins. To identify antigenic proteins of LCDV that could be used as vaccine candidates in olive flounder, Paralichthys olivaceus, we analysed the viral proteins responsible for its virulence by applying immuno-proteomics. LCDV proteins were separated by one-dimensional gel electrophoresis, transferred to polyvinylidene difluoride membrane, and probed with homogeneous P. olivaceus antisera elicited by LCDV natural infection and vaccination with formalin-killed LCDV. Four immune-reactive proteins were obtained at 68-, 51-, 41- and 21 kDa using antisera collected from natural infection while two proteins at 51- and 21 kDa exhibited response to antisera from vaccinated fish, indicating that the latter two proteins have vaccine potential. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and nanoelectrospray MS/MS, the 51 and 21 kDa proteins were identified as MCP and an unknown protein, respectively.     

Isolation of lymphocystis disease virus from sole, Solea senegalensis Kaup, and blackspot sea bream, Pagellus bogaraveo (Brunnich)

Two viruses were isolated from cultured sole, Solea senegalensis, and wild blackspot sea bream, Pagellus bogaraveo, and preliminarily characterized as lymphocystis disease viruses (LCDVs). Viral isolates were characterized by morphological, biochemical and biophysical properties. In addition, the susceptibility of four fish cell lines was also tested. LCDV isolates developed cytopathic effects on the SAF-1 cell line at 5 and 6 days post-infection and reached titres of 10(6) TCID50 mL(-1). The antigenic and structural protein analysis of the two new LCDV isolates showed identical profiles to that obtained for LCDV strain Leetown NFH (ATCC VR-342), used as a reference viral strain, and for an LCDV isolate collected from gilt-head sea bream, Sparus aurata, cultured in southern Spain. Molecular confirmation was performed by polymerase chain reaction. Specific primers for LCDV produced a 270-bp DNA fragment, the expected size for LCDV.     

4种海水鱼淋巴囊肿组织的病理特征比较

取山东、河北、浙江等地感染淋巴囊肿病的牙鲆(Paralichthys olivaceus)、许氏平鮋(Sebastes schlegeli)、鲈鱼(Lateo-labrax japonicus)及纹腹叉鼻(Arothron hispidus),利用光镜和电镜技术及组织化学方法,对患病鱼淋巴囊肿组织的病理特征进行观察比较。结果发现,来自不同地区同一种鱼的淋巴囊肿组织的病理特征无明显差异,不同种鱼的淋巴囊肿细胞具有共同的特征:细胞膨大,细胞核不规则,细胞质内有嗜碱性的、呈Feulgen和Mann氏反应阳性的包涵体,囊肿细胞的细胞膜外有呈PAS反应阳性的均质囊壁,细胞质内病毒颗粒的大小200～220 nm,核周池内有高电子密度物质等。不同种鱼囊肿组织细胞的大小、细胞核的不规则程度、细胞质内包涵体的形态、细胞质内病毒粒子的分布状态,以及囊肿物的外观等有差异。虽然不同种鱼之间存在差异,囊肿组织共同的病理学特征仍可作为疾病诊断的可靠依据。     

A Novel Cell Line from Liver of Stone Flounder,Kareius bicoloratus,and Its Susceptibility to Different Iridoviruses

A novel cell line (SFL) was established from the liver of stone flounder, Kareius bicoloratus, and its susceptibility to different iridoviruses was evaluated. The SFL cells grew well in Dulbecco's Modified Eagle Medium supplemented with fetal bovine serum, basic fibroblast growth factor, and insulin‐like growth factor‐II, and have been subcultured over 82 passages. The optimal growth temperature was 22 C. The SFL cells were fibroblastic in morphology and grew at a steady rate, with a population doubling time of 38.8 h. Karyotype analysis showed that SFL cells exhibited chromosomal aneuploidy with a modal chromosome number of 48. The susceptibility evaluation of SFL cells revealed that cytopathic effects (CPE) appeared after infection by different iridoviruses, lymphocystis disease virus (LCDV) and turbot reddish body iridovirus (TRBIV). In addition, a large number of TRBIV and LCDV particles were observed in the infected SFL cells by electron microscope examination. It is suggested that the SFL cell line could be used as a valuable tool for isolation and propagation of different iridoviruses.     

军曹鱼淋巴囊肿的病理学研究

应用病理组织学和电镜方法,对患疑似淋巴囊肿病的军曹鱼(Rachycentron canadum)的各器官进行观察.结果表明,患病军曹鱼的皮肤囊肿组织由一些淋巴囊肿细胞集合体组成,这些囊肿细胞排列紧密,直径为10～150 μm,细胞呈圆形、锥形不规则状;细胞外有一层厚的囊膜;细胞质内散布有大量的嗜碱性包涵体,且多数集中在细胞的边缘部分;电镜观察到囊肿细胞质中有大量二十面体的病毒粒子,病毒颗粒直径220 nm.据此确认该病为病毒性淋巴囊肿病.其他器官主要组织病理学变化有:在心脏、肝脏、脾脏和头肾中也存在囊肿细胞,心肌纤维水肿;肾间质淋巴细胞增生,巨噬中心出现,肾小管上皮细胞变性和坏死;脾淋巴细胞增生,脾髓质出血;肝脂肪变性;鳃上皮肿胀.根据观察结果可以认为,该病毒不仅损伤鱼的皮肤,致使病鱼外观异样而严重影响其商品价值,而且对鱼的内脏和免疫器官也造成严重的致命损伤.     

Immune response of DNA vaccine against lymphocystis disease virus and expression analysis of immune‐related genes after vaccination

In this study, we found that an intramuscular injection of Japanese flounder (Paralichthys olivaceus, 60–80 g in weight and 15–20 mL in length) with 5 μg of a DNA vaccine (pEGFP‐N2‐LCDV‐cn‐MCP 0.6 kb, containing lymphocystis disease virus major capsid protein gene) induced a strong immune response. Subsequent real‐time polymerase chain reaction showed that the expression of immune‐related genes [e.g., major histocompatibility complex (MHC) class I α, MHC II α, T‐cell receptor (TCR), tumour necrosis factor (TNF), tumour necrosis factor receptor (TNFR), Mx, interleukin (IL)‐1β, CXC and IL‐8R] was significantly changed after DNA vaccination. The most remarkable alternation was the expression of MHC I α and MHC II α genes: MHC II α reached the maximum on day 8 in different tissues, and MHC I α on day 2 in the intestine and gills. The expression of TCR increased and reached a plateau in 2 days in the spleen, gills, kidney and liver after vaccination and then decreased after day 8. In contrast, the expression of TCR in the intestine increased and reached a plateau in 8 days. The expression of IL‐8R reached the maximum on day 2 in different tissues and then decreased on day 8. Mx increased in the gills, kidney, spleen and liver on days 2, 8, 2 and 2, but decreased in the intestine, gills, spleen and liver on days 2, 8, 8 and 8 respectively. The TNFR expression increased in the spleen, kidney and gills on days 2, 8 and 8, but decreased in intestine, liver and gills on days 2, 8 and 8 respectively. The expression of TNF, CXC and IL‐1β increased 2 and 8 days after the injection of DNA vaccine. However, the expression of TNF, CXC and IL‐1β altered on days 2 and 8 with different patterns in different tissues respectively. The fish responded to the DNA vaccine by yielding a specific immunoglobulin against lymphocystis disease virus (LCDV) as observed with indirect ELISA. The DNA vaccine induced a unique humoral response, suggesting that the DNA vaccine activated both cellular and humoral defences of the specific immune system of Japanese flounder.     

同步检测7种鱼类病毒的扩增子拯救多重PCR(Arm-PCR)方法的建立和应用

淋巴囊肿病毒(LCDV)、肿大细胞病毒属虹彩病毒(Mega)、赤点石斑鱼神经坏死病毒(RGNNV)、传染性造血器官坏死病毒(IHNV)、传染性胰脏坏死病毒(IPNV)、病毒性出血败血症病毒(VHSV)和传染性鲑鱼贫血症病毒(ISAV)是养殖鱼类主要的病毒性病原,危害巨大。为实现这7种病原的高通量、同步检测,本研究在分析这7种病毒基因序列的基础上,设计了9组扩增子拯救多重PCR(Arm-PCR)引物,并对扩增体系中的Taq酶、Mg2+、dNTP、Primer Mix浓度及退火温度等参数进行调整和优化,结合基因芯片检测技术,建立了同步检测7种鱼类病毒的Arm-PCR方法。优化后的Arm-PCR方法第一步PCR体系为：Taq酶(2.5 U/μl)1.0μl,10×PCR Buffer(含20 mmol/L的Mg2+)5μl,dNTP(各2.5 mmol/L)5μl,10×Primer Mix(各2μmol/L)9μl,模板1μl,ddH2O补足至50μl,退火温度为56℃。研究结果显示,该方法可以在1支反应管内对上述7种病毒的9个致病基因同步进行扩增和检测,检测灵敏度分别为101 copies/μl (RGNNV、VHSV、ISAV-NS、ISAV-MA)、102 copies/μl (LCDV、Mega、IHNV、IPNV)和103 copies/μl (大菱鲆红体病虹彩病毒,TRBIV)。该方法特异性强,与半滑舌鳎、石斑鱼、大菱鲆和牙鲆基因组DNA不产生交叉反应。本研究建立的可同步检测7种鱼类病毒的Arm-PCR方法具有高通量、高灵敏度、高准确性的优势,能有效提高工作效率,在鱼类病毒的筛查和流行病学调查领域有广泛的应用前景。