Interrelationship between tetherin-mediated restriction and its Vpu-mediated antagonism in HIV-1 cell-to-cell spread and the potential to develop Vpu as an antiviral target

This doctoral thesis comprises three parts. The first two parts (Chapter 2 and 3) define the role of the host cell restriction factor tetherin in restriction of HIV-1 cell-to-cell spread. These chapters also characterize the tetherin-inducible cell line Sup-T1 that was used in Chapter 4 to investigate whether the antiviral activity of protease inhibitors might partially be attributed to tetherin modulation. Tetherin is an intrinsic host cell restriction factor that inhibits virus release by linking the viral membrane of the budding virus to the cellular membrane. The antiviral activity of tetherin has been commonly attributed to its cell surface expression. In HIV-1 infections, the viral protein Vpu antagonizes the tetherin-mediated restriction of virus release and downmodulates tetherin from the cell surface. In Chapter 2, we show that tetherin, besides restricting virus release, also restricts direct viral cell-to-cell spread. We also provide evidence that Vpu poses a fitness cost to HIV-1 in regard to cell-to-cell spread in the absence of tetherin, but is necessary for efficient cell-to-cell spread in the presence of tetherin. Tetherin appears also to play a role in synapse formation. In Chapter 3, we characterized cell line specific differences of the tetherin-Vpu interrelation in regard to cell surface expression of tetherin and virus release. However, Vpu-mediated tetherin antagonism in regard to cell-to-cell spread was similar in all cell lines and seemed to be independent of the level of tetherin cell surface downmodulation. In Chapter 4 we assessed whether protease inhibitors, whose antiviral activity partially depends on modulation of transmembrane proteins, may also modulate tetherin cell surface expression and/or the Vpu-mediated downmodulation of cell surface tetherin. As such, modulation was not apparent; thus, the antiviral activity of PIs is unlikely to be influenced by tetherin modulation activity.