Factors affecting the yield of virus in a cloned cell line of Trichoplusia ni infected with a nuclear polyhedrosis virus

The TN-368 tissue culture line of the cabbage looper, Trichoplusia ni, has been cloned. The doubling times of three clones at 27°C were 27.6 ± 3.4 hr, 21.9 ± 1.7 hr, and 27.4 ± 5.9 hr and that of the uncloned culture was 15.8 ± 1.5 hr. Growth of cells in all cultures was arrested after infection with a nuclear polyhedrosis virus of T. ni. There was little difference in the yield of polyhedra from cultures of uncloned or cloned cells infected at a multiplicity of infection (m.o.i) = 4. Yields of polyhedra were about the same when a m.o.i. was in the range of 0.01–4.0, but the yield tripled in the range m.o.i. = 20–30. At higher multiplicities, up to m.o.i. = 500 the yield of polyhedra progressively fell. It is concluded that the observed variation in numbers of polyhedra borne by individual cells in culture is not due to genetic variability among cells, nor can it be accounted for as a consequence of differing m.o.i. by virus. It is postulated that variation in polyhedra yield among cells in culture may be due to such factors as (1) strain differences in the virus, (2) the stage in the cell cycle at which a particular cell is present when infected.