Susceptibility testing of fish cell lines for virus isolation

Abstract

Passage of cell cultures may adversely influence cell susceptibility to virus infection through selection of cell clones that thrive in vitro but may not necessarily display high sensitivity to virus infection. Susceptibility to a given virus can therefore vary not only between cell lines and laboratories, but also between lineages of the same cell line. To minimise the occurrence of false negatives in a cell culture based surveillance system, we have investigated methods, to select cell lineages that are relatively superior in their susceptibility to a panel of virus isolates. The procedures compare susceptibility between cell lines and between lineages within a laboratory and between laboratories (Inter-laboratory Proficiency Test). The objective being that the most sensitive cell line and lineages are routinely selected for diagnostic purposes. In comparing cell lines, we simulated “non-cell-culture-adapted” virus by propagating the virus in heterologous cell lines to the one tested. A stock of test virus was produced and stored at −80 °C and tests were conducted biannually. This procedure becomes complicated when several cell lines are in use and does not account for variation among lineages. In comparing cell lineages, we increased the number of isolates of each virus, propagated stocks in a given cell line and tested all lineages of that line in use in the laboratory. Testing of relative cell line susceptibility between laboratories is carried out annually via the Inter-laboratory Proficiency Test (Ariel et al., in press), which is organised by the European Community Reference Laboratory for Fish Diseases (CRL) in Denmark. In the year 2000, infected organ material rather than cell-culture-adapted virus was included in the test, to approach a realistic assessment of the variability in cell sensitivity for surveillance purposes within a cell line and between laboratories. In terms of economic and practical considerations as well as attempting to approach a realistic test system, we suggest the optimal procedure for susceptibility testing of fish cell lines for virus isolation to be a combination of biannual tests within the laboratory to compare cell lineages combined with the Inter-laboratory Proficiency Test.