HIV-1 MN infection of U937 promonocytic cells upregulates MCP-1. (A) MCP-1 protein levels and RT activity were measured at the indicated days postinfection. (B) MCP-1 expression measured by RT-PCR in uninfected and HIV-infected U937 cell cultures from which chemokine secretion was determined (A). RNA induction levels, calculated as described in the legend to Fig 2, were 1.5 at day 10 and 4.7 at day 28 postinfection, respectively.