HIV-1 Vpu Promotes Phagocytosis of Infected CD4+ T Cells by Macrophages through Downregulation of CD47.

Human immunodeficiency virus (HIV) remodels the cell surface of infected cells to facilitate viral dissemination and promote immune evasion. The membrane-associated viral protein U (Vpu) accessory protein encoded by HIV-1 plays a key role in this process by altering cell surface levels of multiple host proteins. Using an unbiased quantitative plasma membrane profiling approach, we previously identified CD47 as a putative host target downregulated by Vpu. CD47 is a ubiquitously expressed cell surface protein that interacts with the myeloid cell inhibitory receptor signal regulatory protein-alpha (SIRPα) to deliver a "don't-eat-me" signal, thus protecting cells from phagocytosis. In this study, we investigate whether CD47 modulation by HIV-1 Vpu might promote the susceptibility of macrophages to viral infection via phagocytosis of infected CD4+ T cells. Indeed, we find that Vpu downregulates CD47 expression on infected CD4+ T cells, leading to enhanced capture and phagocytosis by macrophages. We further provide evidence that this Vpu-dependent process allows a C-C chemokine receptor type 5 (CCR5)-tropic transmitted/founder (T/F) virus, which otherwise poorly infects macrophages in its cell-free form, to efficiently infect macrophages. Importantly, we show that HIV-1-infected cells expressing a Vpu-resistant CD47 mutant are less prone to infecting macrophages through phagocytosis. Mechanistically, Vpu forms a physical complex with CD47 through its transmembrane domain and targets the latter for lysosomal degradation. These results reveal a novel role of Vpu in modulating macrophage infection, which has important implications for HIV-1 transmission in early stages of infection and the establishment of viral reservoir. IMPORTANCE Macrophages play critical roles in human immunodeficiency virus (HIV) transmission, viral spread early in infection, and as a reservoir of virus. Selective capture and engulfment of HIV-1-infected T cells was shown to drive efficient macrophage infection, suggesting that this mechanism represents an important mode of infection notably for weakly macrophage-tropic T/F viruses. In this study, we provide insight into the signals that regulate this process. We show that the HIV-1 accessory protein viral protein U (Vpu) downregulates cell surface levels of CD47, a host protein that interacts with the inhibitory receptor signal regulatory protein-alpha (SIRPα), to deliver a "don't-eat-me" signal to macrophages. This allows for enhanced capture and phagocytosis of infected T cells by macrophages, ultimately leading to their productive infection even with transmitted/founder (T/F) virus. These findings provide new insights into the mechanisms governing the intercellular transmission of HIV-1 to macrophages with implications for the establishment of the macrophage reservoir and early HIV-1 dissemination in vivo.

Differential Control of BST2 Restriction and Plasmacytoid Dendritic Cell Antiviral Response by Antagonists Encoded by HIV-1 Group M and O Strains.

BST2/tetherin is a type I interferon (IFN-I)-stimulated host factor that restricts the release of HIV-1 by entrapping budding virions at the cell surface. This membrane-associated protein can also engage and activate the plasmacytoid dendritic cell (pDC)-specific immunoglobulin-like transcript 7 (ILT7) inhibitory receptor to downregulate the IFN-I response by pDCs. Pandemic HIV-1 group M uses Vpu (M-Vpu) to counteract the two BST2 isoforms (long and short) that are expressed in human cells. M-Vpu efficiently downregulates surface long BST2, while it displaces short BST2 molecules away from viral assembly sites. We recently found that this attribute is used by M-Vpu to activate the BST2/ILT7-dependent negative-feedback pathway and to suppress pDC IFN-I responses during sensing of infected cells. However, whether this property is conserved in endemic HIV-1 group O, which has evolved Nef (O-Nef) to counteract specifically the long BST2 isoform, remains unknown. In the present study, we validated that O-Nefs have the capacity to downregulate surface BST2 and enhance HIV-1 particle release although less efficiently than M-Vpu. In contrast to M-Vpu, O-Nef did not efficiently enhance viral spread in T cell culture or displace short BST2 from viral assembly sites to prevent its occlusion by tethered HIV-1 particles. Consequently, O-Nef impairs the ability of BST2 to activate negative ILT7 signaling to suppress the IFN-I response by pDC-containing peripheral blood mononuclear cells (PBMCs) during sensing of infected cells. These distinctive features of BST2 counteraction by O-Nefs may in part explain the limited spread of HIV-1 group O in the human population. IMPORTANCE: The geographical distributions and prevalences of different HIV-1 groups show large variations. Understanding drivers of distinctive viral spread may aid in the development of therapeutic strategies for controlling the spread of HIV-1 pandemic strains. The differential spread of HIV-1 groups appears to be linked to their capacities to antagonize the long and short isoforms of the BST2 restriction factor. We found that the endemic HIV-1 group O-encoded BST2 antagonist Nef is unable to counteract the restriction mediated by short BST2, a condition that impairs its ability to activate ILT7 and suppress pDC antiviral responses. This is in contrast to the pandemic HIV-1 group M-specified BST2 countermeasure Vpu, which displays a diverse array of mechanisms to counteract short and long BST2 isoforms, an attribute that allows the effective control of pDC antiviral responses. These findings may help explain the limited spread of HIV-1 group O as well as the continued predominance of HIV-1 group M throughout the world.