3-D Structure of Dendritic Cell-T Cell Virological Synapses Revealed
HIV can either be transmitted as free virus or be transferred from infected to non-infected cells through so-called virological synapses. How often HIV transmission in vivo results from cell-cell viral transfer is unknown, but it has been shown that this mode of transmission is likely more efficient than cell-free transmission. Previous studies suggest that HIV infects CD4+ T cells about 1,000 times more efficiently in the presence of dendritic cells (DCs), presumably because in this case HIV is transmitted through a virological synapse between the DC and the CD4+ T cell.
Now, a study using electron microscopy has shown the three-dimensional (3-D) structure of DC-T cell synapses in unprecedented detail, providing some clues as to what makes this type of transmission so efficient (1). The study, led by Sriram Subramaniam, chief of the biophysics section in the laboratory of cell biology at the National Cancer Institute (NCI) in Bethesda, also implies that it may be harder for molecules such as antibodies to reach HIV particles in a DC-T cell synapse.
The researchers cultured human DCs with HIV for one to two hours, then added CD4+ T cells from the same human donor for another two hours before fixing the cells for microscopic analysis. The study is the first that uses a method called ion abrasion scanning electron microscopy (IA-SEM) to view cell-to-cell contacts, Subramaniam says. IA-SEM uses an ion beam that takes slices off the surface of a biological specimen, such as a cell, and captures an image of the surface after each slicing. The resulting images are then reassembled into a 3-D image.
In this study, researchers used light microscopy to show that the HIV particles are at the interface between the DC and the T cell. Closer inspection with IA-SEM and electron tomography revealed that the HIV particles are enclosed in spaces inside DCs that are connected to the surface by channels.
IA-SEM also showed that the DCs had large veil-like extensions on their surface (see cover image, right margin), which catch HIV particles from the surrounding medium and enclose them in spaces that remain connected with the surface, says Subramaniam, who last year used IA-SEM to show similar structures in HIV-infected macrophages (see The Beauty Behind the Beasts, IAVI Report, Nov.-Dec. 2009).
In this electron tomographic image, a CD4+ T cell (yellow) appears to use filopodia to pick up HIV particles (red) that are buried inside channels in a dendritic cell (brown). Image courtesy of Donald Bliss and Sriram Subramaniam, NCI.
For the first time, Subramaniam says, the study also found that these membrane extensions from DCs sometimes enclose entire T cells, shielding off the virological synapse. This, together with the fact that the HIV particles are found in spaces deep inside the DCs, suggests that HIV particles in DC-T synapses may be harder for molecules like antibodies to reach than free virus, Subramaniam says. But this may not be true for all types of virological synapses. A recent study showed that HIV particles transmitted between CD4+ T cells are not harder to reach than cell-free HIV (see Research Briefs, IAVI Report, Mar.-Apr. 2010).
The researchers also found that the CD4+ T cells appear to use filopodia to pick up the HIV particles by reaching into the channels and spaces in the DC where the HIV particles are found (see Figure, left). In addition, when they added antibody to the CD4 co-receptor, the main receptor HIV uses to infect CD4+ T cells, they were surprised to find that HIV remained stuck to the DCs. “I thought the viruses would just fall off, but it looks as though they are anchored to the surface of the dendritic cell,” Subramaniam says. This suggests that the CD4+ T cells actively pluck the virus off the DCs, ensuring that no virus is just wasted without infecting its target cell, which may in part explain why HIV is transmitted so efficiently via DC-T cell synapses. “The virus is not transferred under conditions when it knows it’s not going to infect the T cell,” he adds.
Once the HIV particles are picked up by the filopodia, they need to make their way toward the cell bodies of the T cells to eventually enter and infect them. How they do that is unclear, but Subramaniam says that a possible mechanism is a process called virus surfing, by which viruses move along the filopodia of target cells. Walther Mothes of Yale University, who discovered virus surfing several years ago, calls the findings “exciting evidence that raises the possibility that the virus surfing also contributes to the transmission of HIV in lymphocytes.” Mothes previously only showed virus surfing of HIV particles between cultured fibroblasts.
The overarching question that remains is just how often HIV transmission of CD4+ T cells in vivo occurs because of DC-T cell transmission. Subramaniam therefore wants to take an electron-microscopic look at HIV transmission in cultured tissues, which has not yet been done. “This is where we are headed,” Subramaniam says. —Andreas von Bubnoff