Patterns of chemokine secretion by HIV-infected U1 cells in unstimulated conditions and in response to TNF-α and IL-6. U1 cells were cultured with medium alone, TNF-α (0.02 nmol/L), or IL-6 (0.24 nmol/L) for 72 hours; supernatants were harvested daily and stored at −80°C until tested. The data shown are from one representative experiment of four independently performed. Four CC chemokines (MCP-1, RANTES, MIP-1α, and MIP-1β) and one CXC chemokine (IL-8) were measured in the supernatants of unstimulated and stimulated cultures. Low (MCP-1, RANTES, and IL-8) to undetectable (MIP-1α and MIP-1β) levels of chemokines were present in supernatants of unstimulated U1 cells. Both IL-6 and TNF-α upregulated MCP-1 secretion; a modest enhancement of IL-8 production was also noted in U1 cells stimulated with TNF-α and IL-6.