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 covert 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 ºC) and permissive temperature (26 ºC), respectively, all fish in 19 ºC 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 ºC 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 CD3Δ+-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.