Cell viability, HIV-1 p24 and HTLV-1 p19 levels of HTLV-1 co- and mono-infected cells. HIV-1 IIIB/HTLV-1 co-infected and HTLV-1 mono-infected MT-4 cells were cultured in the absence or presence of varying concentrations of IFN-α (0 - 104 IU/ml). (A) Cell viability was determined after 5 days of culture, based on reduction of MTT and expressed in optical density (OD). In parallel, (B) HIV-1 p24 protein and (C) HTLV-1 p19 protein concentrations (pg/ml), were determined in cell-free supernatant of HIV-1/HTLV-1 co-infected and HTLV-1 mono-infected MT-4 cells, respectively, after 48 hours of culture. In addition, (D) HTLV-1 positive MT-2 cells were treated for 48 hours with no treatment (NT) or a fixed concentration of 1000 IU/ml of IFN-α, followed by quantification of HTLV-1 p19 protein levels in cell-free supernatant. (E) HTLV-1 p19 protein levels were also determined in cell-free supernatant of PBMCs of HAM/TSP (n = 5) and ATLL (n = 6) patients, after 48-72 hours of culture in the absence or presence of IFN-α (1000 IU/ml). Parametric t-tests were used with p-values indicated by asterisks (* < 0.05, ** < 0.01 and *** < 0.001).