Analogous to transmission of human T-cell leukemia virus type 1 (HTLV-
1) in vivo, an in vitro cell-to-cell infection model was established b
y coculturing MT-2 cells as virus donors and HUT78 cells as recipients
. At a donor:recipient ratio of 1:2, cell fusion occurred and a new ro
und of HTLV-1 genome replication was initiated in the cocultured cells
. Newly synthesized unintegrated viral DNA was detected by Southern bl
ot within 4-8 h and then increased between 8 and 48 h following cell m
ixing. The most dominant species of unintegrated viral DNA was 3.7 kb
in size which hybridized to a full-length HTLV-1 DNA probe but not to
a Kpnl viral DNA fragment that is absent from a defective proviral gen
ome that has been previously identified in MT-2 cells. Northern blot a
nalysis showed large amounts of viral RNA in the virus donor cells and
in the cocultured cells, with a 3.4-kb species being the most abundan
t. This 3.4-kb RNA gave a pattern identical to that of the 3.7-kb unin
tegrated viral DNA in hybridization studies using the two probes. It s
eems likely that the unspliced RNA transcript from the defective provi
ral genome in MT-2 cells was effectively reverse transcribed upon init
iation of cell-to-cell viral transmission to susceptible HUT78 cells.
Despite active de novo reverse transcription, however, viral RNA level
s remained unchanged following cell-to-cell transmission of HTLV-1 inf
ection and no viral antigen production could be attributed to the newl
y initiated round of viral genome replication. As an abortive infectio
n model this simple cell-to-cell infection system warrants more detail
ed study as it has the potential to provide reliable information regar
ding the early events in HTLV-I transmission and infection. (C) 1998 A
cademic Press.