Effect of exogenous Tat on MCP-1 secretion in U937 and U1 cell lines and in MDM. Both U937 and U1 cells (4 × 10⁵cells/mL) and 5-day old MDM of two donors were incubated with LPS (10 ng/mL), IL-6 (10 ng/mL), or Tat at the indicated concentrations. MCP-1 was measured in U937 and U1 culture supernatants after 48 hours and in MDM culture supernatants after 72 hours of incubation.