From Nature Cell Biology magazine:
Walther Mothes of Yale University in New Haven, Connecticut, and his colleagues fluorescently labelled three retroviruses, including HIV, and tracked their movement between cells. Compressing a video of the process into a single picture reveals the tracks of viral particles (which appear green in the image) crossing cytoplasmic bridges. Interactions between a viral envelope protein and proteins on the uninfected cell surface seem to stabilize the bridges.-Wes
The experiments were performed in cell culture, but if the findings hold in vivo they could suggest new therapies to limit retroviral spread.