C/EBP activators are required for HIV-1 replication and proviral induction in monocytic cell lines. 
Previous work has shown that C/EBP sites and C/EBP transcriptional activators are necessary for HIV-1 LTR activity in monocytes/macrophages. We have investigated the role that C/EBP proteins play in induction and replication of HIV-1. Ectopic expression of the dominant negative C/EBP protein LIP inhibited HIV-1 mRNA and virus production in activated U1 cells, demonstrating that C/EBP proteins are required for provirus induction. U1 lines overexpressing C/EBP activator NF-IL-6 produced more viral mRNA and virus particles following cellular activation than control lines, demonstrating that C/EBP proteins are limiting for virus transcription. HIV-1 harboring mutations within two C/EBP sites were crippled in their ability to replicate in U937 promonocytic cells, indicating that these sites are required for replication. These data identify C/EBP proteins as regulators of HIV-1 expression in monocytes/macrophages.