HIV-1 Tat represses transcription from the mannose receptor promoter

The mannose receptor is expressed on mature macrophages and immature dendritic cells, and functions to mediate phagocytosis of pathogens and capture of Ags for delivery to MHC class II-containing intracellular compartments. It has been previously reported that HIV-1-infected macrophages have reduced functions associated with the mannose receptor, including impaired Pneumocystis carinii phagocytosis and mannosylated albumin uptake. Several HIV-1-derived proteins including the Tat protein have been shown to transcriptionally repress host cell genes. The present study was undertaken to define the role of the HIV-1-derived protein Tat in HIV-mediated mannose receptor down-regulation. Cotransfection of the human macrophage cell line U937 with a Tat expression vector and a mannose receptor promoter-luciferase reporter construct resulted in down-regulation of mannose receptor promoter activity. This repression was targeted to the basal promoter. Expression of either one- or two-exon Tat resulted in decreased promoter activity. The addition of the transactivation response element (TAR) sequence enhanced the Tat-mediated repression. Down-regulation was also seen when transfected cells were treated with exogenously added Tat protein. These results are consistent with a mechanism whereby Tat reduces mannose receptor promoter activity by interfering with the host transcriptional initiation machinery, potentially resulting in decreased levels of surface mannose receptor available for Ag or pathogen capture.