Mode of transmission of human T-cell leukemia virus type I (HTLV I) in a human promyelocytic leukemia HL60 cell

Abstract
HTLV‐I propagated in IMR90 human diploid fibroblasts was transmitted to human myeloid leukemia HL60 cells at a low efficiency. After co‐cultivation for 3 months, the viral genome was detected in 14/48 HL60 cell clones. Among the 14 HTLV‐I‐infected clones, 8 contained subgenomic fragments alone or in addition to the complete HTLV‐I genome. The frequency of deleted proviruses (9/24 total proviruses) was unexpectedly high. Hirt's supernatant of some of the clones harboring complete HTLV‐I genome(s) in the chromosome contained both linear and circular HTLV‐I proviral DNAs. The circular DNAs were composed of one LTR and 2 LTR closed circular proviruses. These clones produced infectious HTLV‐I constitutively, which was proved by transmission of the viral genome into fresh IMR90 cells by co‐cultivation. However, in these clones, re‐integration of extrachromosomal provirus into their own chromosomes was not observed.