HTLV-1-induced cell fusion is limited at two distinct steps in the fusion pathway after receptor binding

Human T-cell leukemia virus type 1 (HTLV-1) is notable among retroviruses f or its poor ability to infect permissive cells, particularly as cell free v irus. The virus is most efficiently transmitted between individuals by infe cted cells, where it is presumed that intracellular particles and viral RNA are transferred to target cells following fusion, Although the mandatory f irst step for HTLV-1 fusion is the binding of envelope SU (gp46) to the rec eptor, the events which follow this interaction and lead to fusion and infe ction have not been well characterized. To investigate these events, we stu died two HTLV-1 chronically infected cell lines with different abilities to fuse with K562 target cells. Although not inherently fusion incompetent, t he HTLV-1 envelope protein on MT2 cells was poorly able to undergo a change in membrane hydrophobicity required for fusion with the target cell membra ne after binding to the receptor. High level expression of a fusion-compete nt HTLV-1 envelope protein on MT2 cells had little effect on improving this suggesting that the defect was encoded by the parent cell. Visible syncyti a were seen after incubation of these cells with K562 target cells but comp lete fusion as measured by transfer of cellular contents into the recipient cell was not observed, In C91-PL cells, binding of SU to the receptor resu lted in a sustained hydrophobic change of envelope accompanied by a cytopat hic effect in mixed cell cultures and complete fusion, However, in C91-PL c ells, overexpression of envelope protein blocked the transfer of cell conte nts after receptor engagement and initiation of cytopathic membrane changes , indicating that post binding fusion events were blocked. These data sugge st that HTLV-1 fusion is a multistep process which is susceptible to inhibi tion at two separate stages of the fusion pathway post receptor binding. Th is, and the inefficient infection by cell-free virions, may explain the poo r infectivity of HTLV-1 in vivo and suggests strategies for preventative th erapy.