Human cells have an antiviral activity that inhibits the release of retrovirus particles (membrane-encapsulated viruses possessing an RNA genome and replicated via a DNA intermediate), and other enveloped virus particles.
Two years ago, Dr. Paul Bieniasz, Head of the Laboratory of Retrovirology and ADARC scientist, discovered that this inhibitory activity is antagonized by the human immunodeficiency virus (HIV) type-1 viral protein U (Vpu). He found that using their Vpu, normal HIV-1 particles are able to extricate themselves from the sticky cell-membrane surface. The HIV-1 Vpu was found to promote the release of diverse retroviruses from human cells.
Recently, Dr. Bieniasz and his colleagues identified the cellular “glue” that keeps viruses tied to a cell. This antiviral mechanism consists of protein-based tethers, termed ‘tetherins’, which cause retention of fully formed virions (virus particles that exist outside a host cell) on infected cell surfaces.
In order to identify the cause of cell surface stickiness, Dr. Bieniasz and his team analyzed gene expression and activity across all known human genes, and compared between cells that require Vpu for HIV-1 release and those that do not. Ultimately, they identified CD317, a membrane protein, as tetherin. The researchers demonstrated that CD317 expression correlated with a requirement for Vpu during release of HIV-1 and murine leukemia virus particles. When Vpu was absent but CD317 was expressed, large numbers of virus particles accumulated on the infected cell’s surface, but when CD317 was missing, even the Vpu-deficient viruses were able to extricate from the cell.
By discovering that the CD317 tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu, the scientists revealed an anti-virus defense mechanism used by our cells. Dr. Bieniasz’s team is now planning to focus on the scope of tetherin’s antiviral activity, and will try to determine whether there are variations that might confer additional immunity or sensitivity to HIV and other viruses. Virology researchers may also study the ways by which viruses evade the tethering mechanism.
Dr. Bieniasz notes that if drug researchers are able to interfere with the interaction between tetherin and Vpu, his newly discovered protein may even provide a potential therapeutic target. Inhibition of Vpu function and mobilization of tetherin’s antiviral activity may constitute a potential therapeutic strategy in fighting AIDS and other retroviral diseases.
Another discovery of human cells’ natural ability to defend themselves against HIV infection, previously covered by TFOT, involves the nuclear proteins XPB and XPD.
More information about the tetherin research is available on this Rockefeller University webpage.