Construction and characterization of deltaretrovirus indicator cell lines

The deltaretroviruses, which include bovine leukemia virus (BLV) and human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2), replicate poorly in culture and the molecular details of their life cycles are limited. To facilitate the analysis of virus replication, mammalian cell lines were created with the long terminal repeats (LTRs) of each virus driving expression of the enhanced green fluorescent protein gene (egfp). The BLGFP, H1GFP and H2GFP cell lines detect virus infection by the expression of GFP via the transactivation of the LTR via the Tax protein of BLV, HTLV-1 or HTLV-2, respectively. GFP expression was measured by flow cytometry, yielding sensitive and rapid detection of virus infectivity. Interestingly, we observed that the Tax proteins of HTLV-1 and HTLV-2 could transactivate the BLV LTR at levels that were comparable to that of BLV Tax. In contrast, the BLV Tax showed low levels of transactivation in H1GFP and H2GFP cells. HTLV-1 and HTLV-2 Tax proteins efficiently transactivated both the HTLV-1 and HTLV-2 LTRs. Finally, spinoculation of BLV resulted in only a two-fold increase in viral titer.