P247 and p523: two in vivo-expressed megalocytivirus proteins that induce protective immunity and are essential to viral infection.

Jian Zhang,Bao Cun Zhang,Li Sun,Tzong-Yueh Chen

doi:10.1371/journal.pone.0121282

Abstract

Megalocytivirus is a DNA virus with a broad host range among teleost fish. Although the complete genome sequences of a number of megalocytivirus isolates have been reported, the functions of most of the genes of this virus are unknown. In this study, we selected two megalocytivirus immunogens, P247 and P523, which were expressed during host infection and, when in the form of DNA vaccines (pCN247 and pCN523 respectively), elicited strong protectivity against lethal megalocytivirus challenge in a turbot (Scophthalmus maximus) model. Compared to control fish, fish vaccinated with pCN247 and pCN523 exhibited drastically reduced viral loads in tissues and high levels of survival rates. Immune response analysis showed that pCN247 and pCN523 (i) induced production of specific serum antibodies, (ii) caused generation of cytotoxic immune cells and specific memory immune cells that responded to secondary antigen stimulation, and (iii) upregulated the expression of genes involved in innate and adaptive immunity. To examine the potential role of P247 and P523 in viral infection, the expression of P247 and P523 was knocked down by siRNA. Subsequent in vivo infection study showed that P247 and P523 knockdown significantly impaired viral replication. Furthermore, whole-genome transcriptome analysis revealed that P247 and P523 knockdown altered the expression profiles of 26 and 41 viral genes, respectively, putatively participating in diverse aspects of viral infection. Taken together, these results indicate that P247 and P523 induce protective immunity in teleost and play fundamental roles essential to viral replication. These observations provide the first evidence that suggests a likely link between the protectivity of viral immunogens and their biological significance in viral replication.

Highlights

Iridoviruses are a family of double-stranded DNA viruses ranging between 120–300 nm in diameter
The results showed that fish vaccinated with two of the vaccine plasmids, pCN247 and pCN523, exhibited high survival rates. pCN247 and pCN523 were constructed based on the ORF107 and ORF86, respectively, of megalocytivirus rock bream iridovirus (RBIV)-C1
We examined the vaccine potentials of a set of megalocytivirus genes as DNA vaccines based on the knowledge that DNA vaccine has the unique feature of stimulating both humoral and cellular immune responses, which are important in the control of viral diseases [39,40,41]

Summary

Introduction

Iridoviruses are a family of double-stranded DNA viruses ranging between 120–300 nm in diameter. The family has five genera named Iridovirus, Chloriridovirus, Lymphocystivirus, Ranavirus, and Megalocytivirus [1]. Of these genera, Megalocytivirus is relatively newly identified and ranked worldwide as an important pathogen to finfish [2,3]. In countries other than China, megalocytivirus-associated disease outbreaks have been documented in a large number of farmed fish including red sea bream (Pagrus major) [8,9,10], turbot [11], flounder (Paralichthys olivaceus) [12], sea perch (Lateolabrax japonicas) [13], and rock bream [14].

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Results

Conclusion