Peripheral B Lymphocyte Serves as A Reservoir for The Persistently Covert Infection of Mandarinfish Siniperca chuatsi Ranavirus

Genus Ranavirus in family Iridoviridae is composed of large members with various genomic sizes and viral gene contents, infecting a variety of ectothermic vertebrates including reptile, amphibians and bony fish worldwide. Mandarinfish ranavirus (MRV) is a very distinctive member among genus Ranavirus. Persistently convert infection of MRV were previously observed in natural outbreak of MRV, but the underlying mechanism remains unclear. We here evidenced that mandarinfish peripheral B lymphocytes are implemented as viral reservoirs to maintain persistent and covert infection. When mandarinfish were infected with sublethal dosage of MRV under nonpermissive temperature (19 {o}C) and permissive temperature (26 {o}C), respectively, all fish in 19 {o}C group survived and entered persistent phase of infection characterized with very low viral load in white blood cell, whereas partial fish died of MRV infection in 26 {o}C group, and the survivals then initiated persistent status. Gradually raising temperature, vaccination and dexamethasone treatment can reactivate the quiescent MRV to replicate and result in partial mortality. The viral reservoir investigates showed that IgM+-labelled B lymphocytes but not CD3{Delta}+-labelled T lymphocytes and MRC-1+-labelled macrophages are target cells for the persistent infection of MRV. Moreover, the quiescent MRV could not be reactivated by heat-killed Escherichia coli, indicating a very different reactivation mechanism from that of other known rannaviral member. Collectively, we are the first to confirm the presence of B cell-dependent persistent and covert infection of ranavirus, and provide a new clue for better understanding the complex infection mechanism of vertebrate iridovirus, especially regarding ranavirus. IMPORTANCEViruses known as HIV, HBV and EBV etc. evade host immune clearance by establishing long-term even lifelong persistent or latent infection. In vertebrate iridovirus, FV3, the type species of genus Ranavirus was evidenced to establish persistent infection by using Xenopus peritoneal macrophages as reservoirs. MRV is a very distinctive ranavirus from FV3 with very different genomic content and host species. We here uncovered MRV establishes persistent and covert infection by using peripheral B lymphocytes as virus reservoirs. During persistent infection, very low copies of quiescent MRV were harbored in peripheral B lymphocytes. Water temperature stress, vaccination stimulation, and dexamethasone treatment can reactivate quiescent MRV to replicate in abundance via a non-TLR5-mediated manner, and results in recurrence of MRV disease. Our finding suggests the diversity and complexity of the pathogenic mechanisms among ranaviruses, and also has important scientific significance for in-depth understanding of the infection and immunity interaction of vertebrate iridoviruses.