Background: Dendritic cells (DCs) are among the first cells that encounter HIV-1 when it penetrates the mucosa. DCs pulsed with low amounts of HIV-1 efficiently transmit it to primary CD4⁺ T-cells. Molecular insight to this process was obtained in the characterization of the dendritic cell-restricted molecule known as DC-SIGN (dendritic cell-specific-ICAM-3-grabbing nonintegrin), which is able to capture HIV-1 on DCs by direct binding to HIV-1 Env. Examination of DC-SIGN by others has suggested that the DC-SIGN cytoplasmic tail may contribute to virus transmission. To investigate the mechanism of virion transfer to T-cells, we have created a series of mutations in the DC-SIGN tail and stably expressed these constructs in human THP-1 cells.

Methods: Expression of DC-SIGN mutations was confirmed by FACS analyses, cell protein immunoblots, and ICAM-3 adsorption. Stably transfected THP-1 cells were used in HIV-1 capture and transmission to T-cells. FACS assays were used to define regions in the DC-SIGN cytoplasmic tail necessary for internalization. Protection of bound HIV-1 was tested using protease treatment and long term culture of HIV-1 pulsed THP-1 cells prior to co-culture with T-cells.

Results: THP-1 cells expressing DC-SIGN were tested for their ability to bind ICAM-3, internalize HIV-1 particles, and capture and transmit HIV-1 in trans. In addition, these cells were assayed for their ability to protect bound virions from extracellular proteases and/or maintain viral infectivity over several days in culture. Although wild type DC-SIGN could be efficiently internalized in the THP-1 cells when cross-linked, we found that sequences necessary for DC-SIGN internalization did not correspond to those required for virus transmission. In contrast to other DC studies, we found that THP-1/DC-SIGN cells were inefficient at protecting virion particles from extracellular proteases.

Conclusions: Our data demonstrate that DC-SIGN expressed on THP-1 cells captures and efficiently transmits HIV-1 to T-cells without internalization. Because THP-1 and other model cell lines are physiologically distinct from DCs, it is unclear whether DC-SIGN functions in an analogous manner on DCs. Given the presence of DCs in mucosal tissue and their ability to stimulate HIV-1 transmission to CD4⁺ T-cells, understanding the mechanism of DC-SIGN mediated virus transmission will provide insight into their role in HIV-1 pathogenesis and in designing novel antiviral therapies.