Blood dendritic cells (DCs) efficiently capture and transmit HIV to CD4+ T lymphocytes, the main reservoir of virus infection in the body. DCs serve as immune system sentinels and patrol mucosal surfaces for invading pathogens to provide early warning to the adaptive immune system. Mucosal tissue exposure to HIV is the most frequent cause of HIV transmission worldwide. Because DCs are concentrated at these tissues, it is hypothesized that they are co-opted by HIV during mucosal transmission to gain access to the lymphatic system and seed infection of the CD4+ T cell compartment. DC-SIGN is a high-affinity HIV Env receptor expressed by myeloid-lineage DCs. DCs treated with antibodies to DC-SIGN bind less HIV and are less effective in stimulating infection of CD4+ T cells. In addition, ectopic expression of DC-SIGN in transformed B cells enables efficient HIV capture and transmission tosusceptible target cells. Cell biological analysis has revealed intracellular trafficking of HIV after capture by DCs, and concentration of HIV at synaptic junctions after DC contact with CD4+ target cells. It has been postulated that examination of DC-SIGN-mediated HIV transmission by transformed cell lines could aid in the characterization of these processes. Transformed cells would allow more extensive biochemical characterization and genetic manipulation of the cellular sorting machinery, cell signaling processes, and cell-cell adhesion molecules required for synaptic transfer of HIV. We have shown that the use of DC-SIGN transfectants has two significant shortcomings. First, DC-SIGN transmission of HIV by transformed cell lines is not analogous to HIV transmission by DCs. Second, DCs possess redundant mechanisms through which HIV can be captured for transmission, and DC-SIGN-dependent transmission is disfavored during the maturation of DCs. We have found that GFP-labeled virus-like particles (VLPs) can be used to study the requirements of DC-SIGN- or DC-mediated transmission of HIV. We are now using VLPs to investigate DC-SIGN-independent interactions with DCs and to screen for small-molecule inhibitors of the HIV Env interaction with DC-SIGN. These studies will increase our understanding of how DCs can disseminate HIV infection in lymphoid tissues.
