The SU 90-94 and exon 8 peptides block HTLV-1 entry into CD4+ T cells or DCs. (A) Effect of the peptides on HTLV-1 SU binding on CD4+ T cells. MOLT4 were incubated in medium alone or in medium containing 2, 20, or 50 μg/mL peptides before incubation with 200 ng/mL HTLV-1 or ASLV SU-rFc. The data show the specific binding (MFISU-rFc–MFISU-ASLV-rFc) normalized on cells incubated with no peptide (100%) and are from 1 representative experiment of 2 performed. (B) Effect of the peptides on HTLV-1 virus binding to or internalization into CD4+ T cells. (Top panel) CD4+ T cells were preincubated with 20 μg/mL peptides, then HTLV-1 virions were added 30 minutes later, and the amount of virus binding was determined as described in “BIAcore binding analysis,” except that 1 μg rather than 5 μg virus was used. (Bottom panel) CD4+ T cells were incubated with 50 μg/mL peptides for 30 minutes. The cells were diluted and incubated with HTLV-1 virions for 3 hours (final concentration, 10 μg/mL peptide), and the amount of virus internalization was determined. HTLV-1 binding and internalization are normalized to the control peptide (100%), and the data are mean ± SD of at least 2 independent experiments. (C) Effect of the peptides on HIV-1 infection of CD4+ T cells. CD4+ T cells were preincubated with 50 ng/mL peptides for 30 minutes, and cells were infected with either a low dose (30 ng/mL) or high dose (80 ng/mL) of HIV-1 virions for 2 hours at 37°C. HIV-1 production was assessed 3 days after infection by measuring the amount of released particles using the anti-CAp24 ELISA. (D) Effect of the peptides on HTLV-1 binding to and infection of MDDCs. (Top panel) Cells were incubated with 20 μg/mL peptides for 30 minutes, exposed to the HTLV-1 SU-rFc (black lines) or the control SU-ASLV-rFc (gray lines), and the level of binding determined. (Bottom panel) Cells were incubated with 20 μg/mL peptides for 30 minutes and infected by cell-free HTLV-1. The extent of infection was determined by flow cytometry at 48 hours after exposure to virus by intracellular staining for the HTLV-1 Tax protein. The black lines represent the staining with Tax-specific antibody; gray lines, the staining with an isotype control.