Background:  We, and others, recently indentified Nef as the viral gene product of SIV that counteracts restriction by rhesus macaque tetherin (BST2 or CD317), an interferon-inducible host-cell factor that inhibits the detachment of virions from infected cells. A role for Nef in opposing restriction by tetherin may help to explain the attenuated phenotype of nef-deleted SIV. However, since Nef is a polyfunctional protein, the loss of other activities may also contribute to attenuation. Macaques infected with nef-deleted strains of SIV typically control virus replication and do not develop AIDS. Nevertheless, under certain circumstances, nef-deleted viruses can regain the ability to replicate to high levels and cause disease in infected animals.

Methods:  To test the ability of a nef-deleted pathogenic strain of SIV to overcome restriction by tetherin, we co-transfected 293T cells with proviral DNA mutants and expression constructs for either human or rhesus tetherin. Virus release was measured by p27 antigen-capture ELISA and corroborated by Western blot analysis of cell lysates and culture supernatants.

Results:  We found that a pathogenic derivative of a nef-deleted strain of SIV that had acquired 14 amino acid changes in the cytoplasmic tail of gp41 and 8 amino acid changes in the N-terminus of Nef (upstream of the deletion), regained resistance to tetherin. The ability to antagonize rhesus macaque tetherin was dependent on amino acid changes in the cytoplasmic tail of gp41. Similar to SIV Nef, this activity was species-specific, since these changes conferred resistance to rhesus tetherin, but not to human tetherin.

Conclusions:  The selection of genetic changes in a pathogenic virus derived from a nef-deleted strain of SIV that restore resistance to tetherin demonstrates that the ability to antagonize tetherin is important for lentiviral pathogenesis. These observations are also consistent with a role for the HIV-2 envelope glycoprotein in counteracting restriction by human tetherin.