CsA treatment suppresses HTLV-1 antibody responses in a dose-dependent manner. We used a HTLV-1 ELISA to determine the level of HTLV-1 antibodies in the plasma of rabbits after infection. (A) The 20 mg/kg CsA–treated rabbits displayed statistically significant (P < .001) decreases 6 and 10 weeks after infection. All CsA-treated rabbits displayed decreases in HTLV-1 antibodies at 4, 6, and 10 weeks after infection compared with control rabbits. (B) The 20 mg/kg CsA–delayed treatment rabbits had statistically significant decreases in HTLV-1 antibody titers 4, 6, and 10 weeks after infection compared with control rabbits (P < .001). (C) CsA treatment leads to a decreased antibody response to HTLV-1 Env SU. Western blot analysis was used to determine whether CsA treatment leads to a decrease in HTLV-1 antibodies directed against Env SU, transmembrane, and p19. (D) Densitometry was used to numerate the pixels in the bands. Rabbits treated with 20 mg/kg CsA before and after infection both had significant decreases in antibodies directed against HTLV-1 Env SU 10 weeks after infection.