Up close and 3-dimensional: HIV caught in the act inside the gut

Pamela Bjorkman, from the Howard Hughes Medical Institute and the California Institute of Technology, USA, and colleagues used electron tomography for a high-resolution study of HIV virus in the guts of "humanized" mice, whose immune system is made up to a large degree of human cells. They infected these "BLT mice" (so-called because they have human bone marrow, thymus, and liver cells) with HIV virus and developed methods that allowed them to safely examine and visualize the three-dimensional architecture of infected parts of the gut.

They saw HIV-infected human immune cells, caught virus particles in the act of budding from such cells, and also found groups of free immature and mature viruses. For one infected host cell (turned HIV factory) the researchers counted 63 virus particles it had likely released. The actual number is almost certainly much higher, because the method can only visualize virus particles surrounding the host cell within a relatively small part of the tissue. Nevertheless, they discovered that groups of viruses that were farther from the host cell were more mature than those closer to it, which suggested that the host cell releases new virus in a series of "semi-synchronized" waves.

Among the samples, the researchers found some where viruses released from one infected cell seemed directly to attach to a neighboring host cell, presumably infecting it. In addition to such "virological synapses," they also observed free virus particles that appear to have covered some distance between their "mother" cell and the cell that would become their target to infect.

These images provide the first 3D ultrastructural details on HIV infection and virus production in a setting that closely resembles the gut of human patients. Some results confirm earlier findings from in vitro experiments -- cells grown and infected in a petri dish -- but others are seen for the first time and advance the understanding of how HIV infection spreads in real life.

"To me, an important finding is that the majority of the viral transmission events within tissue involved free virus rather than virological synapses," says Bjorkman. "The spread of viruses through synapses had been speculated to be a major route of transmission in tissue, but our results reveal large pools of free virions. This discovery provides hope that possible therapeutics, such as antibodies, would be able to access infecting viruses that are not hidden within a virological synapse."

Video: http://www.plos.org/wp-content/uploads/2013/05/PLOS_Pathogens_Bjorkman_Jan30_Video.mp4