The herpesvirus of channel catfish : a biological and biochemical study

Abstract

A biological and biochemical investigation of channel catfish virus (CCV) was carried out in order to examine this virus and its relationship to the herpesvirus group. The oncogenic potential of the virus was also studied for its ability to cause cell transformation in vitro. In addition, some inhibitors of herpesvirus replication were investigated for their effect on CCV growth. Electron microscopical examination of negatively stained purified CCV preparations showed the virion to consist of a central core (81 ± 1 nm), enclosed by a capsid coat (96 ± 1 nm), that is surrounded by an irregular membrane envelope (165 ± 5 nm). The nucleocapsid exhibited icosahedral symmetry with 5 hollow capsomeres per facet edge - this would give a total of 162 capsomeric units for the virion. The mass of CCV was determined to be 1.275 g/cm^3 by equilibrium ultracentrifugation in a CsCl density gradient. The virus multiplied well in the brown bullhead (BB) catfish 000 cell line at 27 C, but not at 15°C and 37°C. Under single-cycle growth conditions, CCV showed a 3 hr eclipse period followed by an exponential growth period of around 20 hr. Light microscopical examination of virus-infected BB cells showed formation of cell syncytia and giemsa-stained cells revealed small nuclear inclusions with margination of the nuclear chromatin. The virus was very host-specific in that it did not replicate in cells of other fish such as RTG-2 (rainbow trout gonad), CHSE #214 (chinook salmon embryo), and OMAKA (Hawaiian marine fish larvae). Virus replication was reduced by inhibitors of DNA synthesis (5-fluorodeoxYuridine, cytosine arabinoside, adenine arabinoside), antibiotics (actinomycin-D, cycloheximide), and a chemical agent shown to interfere with herpesvirus specific DNA polymerase (phosphonoacetic acid). CCV was shown to be a DNA-containing virion by its ready incorporation of 3H-thymidine into the viral genome and by its sensitivity to inhibitors of DNA synthesis. The molecular weight (M) of the viral w DNA was estimated to be 103-105 x 10^6 daltons by its relative migration rate compared to T4 DNA in 1% agarose gels. The base composition of the viral DNA was calculated to be 57% G + C from its buoyant density (1.716 g/cm^3) in CsCl. The viral nucleic acid was found to be biologically active by assays employing either the diethylaminoethyldextran or the calcium phosphate co-precipitation techniques. This infectivity of CCV DNA was destroyed by treatment with deoxyribonuclease I. Eighteen viral proteins ranging from 17,000 - 350,000 Mw were identified by electrophoresis of purified degraded virus in 10% polyacrylamide gels. Analysis revealed that CCV has two major proteins of 44,000 and 132,000 Mw. The immunological studies on the basis of neutralization and fluorescent antibody staining showed CCV to have no cross-reactivity with HSV-1, HSV-2, human CMV, or Lucke frog herpesvirus. Therefore no viral antigens are shared between CCV and these herpesviruses. In the studies investigating the oncogenic potential of CCV, no morphologically transformed cells with altered growth characteristics were observed after interaction of UV-inactivated CCV with the fish cell lines BB, RTG-2, OMAKA, and the mouse BALB/3T3 cell line. This study of CCV has shown it to be similar to the herpesvirus group in terms of morphology, genetic material, protein constituents, mass, and reaction to various chemical and physical agents. It can therefore be concluded that CCV is indeed a herpesvirus.