Viruses bridge gap to healthy cells

In a striking demonstration of how retroviruses exploit cellular machinery to infect healthy cells, researchers have caught the viruses crawling along long, thin cytoplasmic filaments from infected to target cells. Using high-resolution live imaging, the researchers provide evidence of this previously unknown mode of viral transmission in this week's Nature Cell Biology.

"It almost looks like the virus is walking from one cell to another," said Dimiter Dimitrov of the National Cancer Institute in Bethesda, Md., who was not involved in the study. "The study is a step further in our understanding of the mechanics of cell to cell transmission."

The team, led by Walther Mothes of Yale University in New Haven, Conn., worked with different co-cultures of healthy cells and cells infected with murine leukemia virus (MLV). The researchers observed that viral particles that moved from infected to target cells traveled mostly along filopodia -- thin, elongated actin-based filaments that normally help cells migrate. In the co-culture, these structures resembled the long-lived filopodia associated with cell-cell communication in fruit flies, called cytonemes. "Our work demonstrates for the first time that cytonemes can transport ligands -- viruses in our case -- from cell to cell," Mothes said.

In order to see the viruses in action, the researchers tagged a capsid and viral envelope protein in infected cells, and tagged the MLV receptor in uninfected cells. They co-cultured the cells in a thin layer on a cover slip, and imaged them at regular intervals using a fluorescence microscope. The resulting movies track viral particles as they bud out of infected cells, move along a cytoneme, and enter their target cells -- a journey that typically takes less than 20 minutes. "The movies are striking," Mothes told The Scientist. "They give us a first visual idea of why viruses are up to 1000 times more infectious in cultures."

Cytonemes form only between infected and healthy cells, the study shows. Using its endocytic force, an infected cell sucks in a target cell's filopodia until they form stable bridges between the two cells, up to 30 microns long. The researchers found that viruses move against the direction of cytoneme extension, using actin-driven flow to reach the target. "The actin-myosin II machinery is one of the strongest engines the cell has," Mothes said. "It is really smart of the virus to engage this machinery to hop from one cell to the other."

Mothes and his colleagues also found that both the viral envelope protein Env and the viral receptor accumulate at the ends of the cytonemes, suggesting that the two proteins interact to anchor the structures. Indeed, when the researchers added neutralizing antibodies against the extra-cellular domain of Env, the bridges collapsed. Nor could the bridges form in the absence of wild-type Env in the infected cells.

"This is an intriguing and novel way in which viruses appear to be transported," Sriram Subramaniam of the National Cancer Institute, who was not involved in the study, told The Scientist. "It will be useful to know how efficient this process is relative to other modes of transmission."

Recent research has pinpointed another mode of transmission, in which the virus uses the so-called infectious synapse formed at the boundary between a virus-carrying dendritic cell and a healthy T cell. As David McDonald, now at the Case Western Reserve University in Cleveland, showed in 2003, HIV and other retroviruses exploit this immune mechanism to spread to uninfected T cells. "If cells have large contact areas, such as infectious synapses, the virus will likely pass through those," McDonald told The Scientist. "But in other settings, such as macrophages in a tissue, these filopodial bridges may be important for transmission."

Chandra Shekhar
mail@the-scientist.com

Links within this article

N.M.Sherer, et al., "Retroviruses can establish filopodial bridges for efficient cell-to-cell transmission," Nat Cell Biol, Feb 11, 2007.
http://www.nature.com/ncb/journal/vaop/ncurrent/abs/ncb1544.html

Dimiter Dimitrov
http://www-lecb.ncifcrf.gov/_{dimitrov/dimitrov.html}

Walther Mothes
http://www.med.yale.edu/micropath/fac_mothes.html

C Choi, "Nanotubes link immune cells," The Scientist, September 20, 2005.
http://www.the-scientist.com/article/display/22774

Sriram Subramaniam
http://ccr.nci.nih.gov/Staff/Staff.asp?profileid=5614

David McDonald
http://www.case.edu/med/microbio/mcdonald.htm

D. McDonald, et al., "Recruitment of HIV and its receptors to dendritic cell-T cell junctions," Science, 300:1295-7, May 2003
http://www.the-scientist.com/pubmed/12730499

T Toma, "MDDC involvement in HIV infection," The Scientist, May 2, 2003.
http://www.the-scientist.com/article/display/21299