Background: DC-SIGN, an ICAM-3 receptor that is expressed on the surface of dendritic cells (DC), is the first known example of a virus attachment receptor that works most potently in trans. Through a high affinity interaction with primate lentiviral envelope glycoproteins, DC-SIGN captures virus and promotes the trans infection of CD4⁺ T-cells. Our studies focus on the mechanism by which DC-SIGN efficiently presents captured virus to cells expressing HIV-1 receptor molecules.

Methods: Fluorescence-activated cell sorting (FACS) was used to sort and characterize the DC-SIGN-expressing cell lines. DC-SIGN positive cell lines were further manipulated by the introduction of exogenous genes that would make them more or less similar to primary DC. HIV-1 binding to DC-SIGN expressing cell lines was measured via capsid protein ELISA. HIV-1 single round infectious viral vectors expressing marker genes were used in DC-SIGN transmission assays. DC-SIGN mediated transmission of HIV-1 vectors was performed in the presence and absence of agents blocking HIV-1 Env interactions with DC-SIGN.

Results: Although physiological interactions between DC-SIGN and ICAM-3 are not necessary for HIV-1 transmission, our experiments indicated that donor cell and target cell contact was required for efficient DC-SIGN transmission of HIV-1. This prompted us to examine whether cell context and the presence or absence of different cellular adhesion molecules could affect DC-SIGN-mediated virus transmission. Expression of DC-SIGN in cell lines derived from different species or tissue types results in cells with comparably increased binding but variable transmission of HIV-1 to target cells, suggesting that DC-SIGN-mediated HIV-1 transmission is dependent on the cell type in which DC-SIGN is expressed. We have identified a conserved set of antigen presenting cell (APC)-related surface factors present on immature DC and on cells that transmit HIV-1 efficiently via DC-SIGN. Using this information, we have silenced or restored expression of candidate factors in DC-SIGN positive cell lines with different HIV-1 transmission abilities, respectively, in an attempt to either suppress or rescue the viral transmission function, and thus identify previously unrecognized participants in the transmission process.

Conclusions: Our studies indicate that enhanced HIV-1 trans infection may require the collaboration of DC-SIGN with other APC-specific features.