Cellular entry of HIV is dependent on interactions between the viral envelope glycoprotein, host CD4 molecules and chemokine receptors (CCR5 and CXCR4). Much of the work on mechanisms of HIV entry has been performed in vitro, and additonal factors that could be important for infections in vivo remain poorly characterized. Recent work on HIV infection has focused on the role of the dendritic cell (DC)-specific lectin DC-SIGN (dendritic-cell-specific intercellular adhesion molecule (ICAM)-3-grabbing non-integrin). Two papers in Immunity now show that the HIV-1 protein Nef upregulates DC-SIGN expression on DCs and that binding of DC-SIGN mediates internalization of virus particles into a non-lysosomal compartment, which enhances infectivity.

DC-SIGN was previously shown to bind to glycoprotein 120 (gp120) of HIV. DCs that express DC-SIGN can retain virions in an infectious state for several days and augment infection of T cells, although HIV is known to replicate inefficiently in DCs. It has been proposed that DC-mediated transport of virus from mucosal sites to secondary lymphoid tissues is important for the efficient transfer of virus to T cells, but whether the virus is actually internalized in DCs remains unclear.

The study from Olivier Schwartz' group examined the role of HIV-1 Nef in viral infectivity. DC-SIGN was found to be upregulated at the surface of infected, but not uninfected, cells. Upregulation was dependent on Nef, given that DCs infected with Nef-deficient virus showed DC-SIGN staining patterns similar to uninfected cells. Two putative internalization signals — a tyrosine-based motif and a dileucine motif — are located in the cytoplasmic tail of DC-SIGN. The role of these motifs was examined using DC-SIGN-negative HeLa cells transfected with plasmids encoding DC-SIGN or Nef, or both. HeLa cells expressing wild-type DC-SIGN had very little surface expression but a high rate of endocytosis. By contrast, dileucine mutants of DC-SIGN showed high levels of surface expression with little or no internalization. Co-transfection with wild-type DC-SIGN and Nef resulted in stable expression of DC-SIGN at the plasma membrane. These results show that Nef upregulates DC-SIGN surface expression by preventing internalization, which enhances the ability of DCs to form clusters with lymphocytes and facilitates viral transmission.

The study from Dan Littman's group focused on the role of DC-SIGN in internalization and infectivity of HIV-1. DC-SIGN was shown to mediate the internalization of gp120 in a monocyte cell line transduced with a DC-SIGN-encoding virus. Fluorescence confocal microscopy showed that whole viral particles could be internalized by human DCs. After 45 minutes, most of the viral particles were localized in a non-lysosomal compartment, and these could be transmitted to a second target cell. Low pH in this compartment was important for maintenance of infectivity, given that neutralization of pH abolished the ability of DC-SIGN-positive cells to enhance infection of T cells.

Taken together, these results suggest that HIV has evolved to exploit the function of DCs and enhance viral transmissibility and infectivity. Although several questions — such as how HIV returns to the cell surface and what regulates this process — remain to be answered, this new work has implications for our understanding of HIV pathogenesis.