Induction of MCP-1 expression by IL-6 and IFN-γ in HIV-infected U1 cells. (A) Northern blot analysis of MCP-1 expression after 4 and 18 hours of culture with medium alone, IL-6 (0.24 nmol/L), or IFN-γ (1.5 nmol/L). The results of one representative experiment of three independently performed are shown. Intermediate (11 hours) and later (36 hours) time points were evaluated in the additional two experiments, showing that baseline and cytokine-induced MCP-1 message decreases after 36 hours of culture. Ethidium bromide staining of the 18S and 28S ribosomal RNA is shown as a control for RNA loading. (B) MCP-1 secretion in supernatants harvested after 4 and 18 hours of culture from the same cells of the experiment shown in (A). Low, but detectable levels of MCP-1 were already present in the supernatants 4 hours after stimulation.