Efficient BST2 antagonism by Vpu is critical for early HIV-1 dissemination in humanized mice

Background: Vpu is a multifunctional accessory protein that enhances the release of HIV-1 by counteracting the entrapment of nascent virions on infected cell surface mediated by BST2/Tetherin. Vpu-mediated BST2 antagonism involves physical association with BST2 and subsequent mislocalization of the restriction factor to intracellular compartments followed by SCF(beta-TrCP) E3 ligase-dependent lysosomal degradation. Apart from BST2 antagonism, Vpu also induces down regulation of several immune molecules, including CD4 and SLAMF6/NTB-A, to evade host immune responses and promote viral dissemination. However, it should be noted that the multiple functions of Vpu have been studied in cell-based assays, and thus it remains unclear how Vpu influences the dynamic of HIV-1 infection in in vivo conditions. Results: Using a humanized mouse model of acute infection as well as CCR5-tropic HIV-1 that lack Vpu or encode WT Vpu or Vpu with mutations in the beta-TrCP binding domain, we provide evidence that Vpu-mediated BST2 antagonism plays a crucial role in establishing early plasma viremia and viral dissemination. Interestingly, we also find that efficient HIV-1 release and dissemination are directly related to functional strength of Vpu in antagonizing BST2. Thus, reduced antagonism of BST2 due to beta-TrCP binding domain mutations results in decreased plasma viremia and frequency of infected T cells, highlighting the importance of Vpu-mediated beta-TrCP-dependent BST-2 degradation for optimal initial viral propagation. Conclusions: Overall, our findings suggest that BST2 antagonism by Vpu is critical for efficient early viral expansion and dissemination during acute infection and as such is likely to confer HIV-1 increased transmission fitness.