Mechanisms of antagonism of BST2 isoforms and implications for HIV pathogenesis - PROJECT SUMMARY The ability of HIV to circumvent host immune responses is key to its transmission and persistence. To do this, the virus encodes antagonists to cellular defenses, many of which are interferon-stimulated genes (ISGs) that promote an antiviral state in host cells. One such ISG with broad and potent antiviral activity is BST2/tetherin, which entraps budding virions on the cell surface and induces activation of NFκB signaling, among other antiviral effects. There are two isoforms of BST-2 – long and short – which vary in their function and sensitivity to viral mechanisms of counteraction. HIV-1 and HIV-2 encode different strategies to antagonize BST2, reflecting their distinct evolutionary histories: HIV-1 Group M and N encode Vpu, some HIV-1 Group O Nefs have anti-BST2 activity, and HIV-2 antagonizes BST2 through its Env. Of these, HIV-1 Group M Vpu is the best studied and most potent BST2 antagonist, employing multiple strategies for its counteraction including removal of BST2 from the cell surface, BST2 degradation, and displacement of BST2 from virion-assembly sites within the plane of the plasma membrane. The short form of BST2 lacks a YxYxxL clathrin adaptor binding sequence and an STS sequence that is putatively ubiquitinated in response to Vpu and so is refractory to Vpu-induced removal from the cell surface and degradation. However, Vpu can still displace short BST2 from sites of viral assembly. Conserved sequences near the C-terminus of Vpu have been identified which mediate this displacement-effect, although the mechanism remains undefined. This proposal seeks to determine the mechanisms of antagonism of the short isoform of BST2 by HIV-1 and HIV-2 and explore the consequences of these effects for viral infection and the immune response. SA1 will probe the mechanisms of the displacement effect, which is likely to involve clathrin based on preliminary mutant rescue studies. SA2 will evaluate the activity of different HIV-1 Nefs and HIV-2 Env against both isoforms of BST2 as well as their ability to mediate the displacement-effect. SA3 will build on preliminary observations that BST2 isoform expression ratios are influenced by interferon treatment and HIV infection to define the determinants and consequences of this regulation, including antibody-dependent cellular cytotoxicity (ADCC). The results of these studies will provide a more complete understanding of the cellular regulation and antagonism of BST2, informing therapeutic approaches aimed at HIV eradication and mitigation of immune dysregulation in chronic infection.
