Opioids activate HIV replication in vitro. Latently-HIV-infected ACH-2 T lymphoblasts were cultured for 24 hours in the presence of different concentrations of morphine (A) or heroin (B). HIV replication was quantified by intracellular staining of HIV p24-antigen and flow cytometry. C, D: representative dot-plot analyses of ACH-2 cells left untreated (C) or treated with 4 mM heroin (D). The x-axis displays the forward scatter, the y-axis displays p24-expression. Pink rectangles represent the gates for p24-positive cells and the small numbers within indicate the proportion (%) of p24-positive cells from the total number of events. E: Heroin has no effects on HIV reactivation at concentrations found in plasma of IDUs (1-10 μM). F: Latently-HIV-infected ACH-2 T lymphoblasts were cultured for 24 hours in the presence of 5 mM heroin, together with different concentrations of naloxone (0 μM and 0.1μM – 1 mM). HIV replication was quantified by intracellular staining of HIV p24-antigen and flow cytometry. Data as mean ± S.E.M. from duplicates and linear regression. The slopes of both regressions are positive and significantly differ from = zero (p = 0.0008 for medium and p = 0.0152 for heroin).