In research on methods for the detection and enumeration of the fastidious adenovirus types 40 and 41 (Ad40/41), the PLC/PRF/5 cell line derived from a primary human hepatocellular carcinoma proved substantially more sensitive than the Graham 293 (human embryonic kidney), A549 (human lung), KB (human oral epidermoid) and Chang conjunctival cells generally used for propagation of these viruses. In comparative tests based on the detection of a cytopathic effect in microtitre plate monolayers inoculated with tenfold dilutions of virus suspensions and quantitation by most probable number calculation, the PLC/PRF/5 cell line yielded a hundred times higher count for a laboratory strain of Ad41, and ten times higher counts for a laboratory strain of Ad40 and two Ad41 stool isolates, than any of the other cells. The PLC/PRF/5 cells also retained an optimal condition for longer and displayed cytopathic effects earlier and more clearly than the other cell lines. Efforts to develop plaque assays for Ad40/41 by using various modifications of established plaque techniques were successful only for the laboratory strain and the two stool isolates of Ad41 using PLC/PRF/5 cells. The reason for the exceptional susceptibility of PLC/PRF/5 cells to Ad40/41, and the ability of these cells to plaque Ad41 but not Ad40, cannot be explained. Quantitative PLC/PRF/5 cell culture assays on viral suspensions in which numbers of viral particles were counted by electron microscopy using a calibrated colloidal gold particle suspension as reference, indicated that the minimal infective dose of Ad40/41 for humans may be in the same range as that of many other enteric viruses.

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