Comparative susceptibilities of insect cell lines to infection by the occlusion-body derived phenotype of baculoviruses

Twelve insect cell lines from six species were tested for susceptibility to baculovirus infection by occlusion-derived virus (ODV) phenotype through the use of a typical endpoint assay procedure. ODV from three nucleopolyhedroviruses were prepared by alkali treatment (sodium carbonate) of occlusion bodies (OBs) and the virus preparations were titered on various cell lines. More than a four-log difference was realized for each of theses viruses between the various cell lines. The TN368 line from Trichoplusia ni was only marginally susceptible to ODV from each virus, showing only 3-6 infectious units (IU) per million OBs while the gypsy moth line, LdEp was most susceptible, realizing more than 100,000 IU/million OBs. The other lines tested showed various levels of susceptibility between these two extremes and also varied between the three viruses tested. In additional tests, the ODV were treated with trypsin prior to application to the cells. With most cell lines, this treatment increased the infectivity of each virus by 2-10-fold. Exceptions to this trend included the gypsy moth LdEp line, on which the trypsinized ODV from two of the viruses were slightly less infectious than each virus without trypsin, and the TN-368 line, on which the trypsinized ODV was 5,000-75,000 times more infectious. The variable results of trypsinized virus on the different lines are probably due to the levels of endogenous protease activity in the various lines, but the mode of action of the trypsin has not been elucidated. Ultimately, the variable response of cell lines to ODV of different viruses, and the variable effects of trypsin on the ODV may lead to an improved understanding of the infection process of this virus phenotype as well as factors relating to baculovirus host range.