Structural Basis for the Antiviral Activity of BST-2/Tetherin and Its Viral Antagonism

Multi-functional BST2/tetherin against HIV-1, other viruses and LINE-1

Bone marrow stromal cell antigen 2 (BST2), also known as CD317, HM1.24, or tetherin, is a type II transmembrane glycoprotein. Its expression is induced by IFN-I, and it initiates host immune responses by directly trapping enveloped HIV-1 particles onto the cell surface. This antagonistic mechanism toward the virus is attributable to the unique structure of BST2. In addition to its antiviral activity, BST2 restricts retrotransposon LINE-1 through a distinct mechanism. As counteractive measures, different viruses use a variety of proteins to neutralize the function or even stability of BST2. Interestingly, BST2 seems to have both a positive and a negative influence on immunomodulation and virus propagation. Here, we review the relationship between the structural and functional bases of BST2 in anti-HIV-1 and suppressing retrotransposon LINE-1 activation and focus on its dual features in immunomodulation and regulating virus propagation.

HIV-1 restriction by SERINC5

Serine incorporator 5 (SERINC5 or SER5) is a multipass transmembrane protein with ill-defined cellular activities. SER5 was recently described as a human immunodeficiency virus 1 (HIV-1) restriction factor capable of inhibiting HIV-1 that does not express its accessory protein Nef (Δ Nef). SER5 incorporated into the viral membrane impairs the entry of HIV-1 by disrupting the fusion between the viral and the plasma membrane after envelope receptor interaction induced the first steps of the fusion process. The mechanisms of how SER5 prevents membrane fusion are not fully understood and viral envelope proteins were identified that escape the SER5-mediated restriction. Primate lentiviruses, such as HIV-1 and simian immunodeficiency viruses (SIVs), use their accessory protein Nef to downregulate SER5 from the plasma membrane by inducing an endocytic pathway. In addition to being directly antiviral, recent data suggest that SER5 is an important adapter protein in innate signaling pathways leading to the induction of inflammatory cytokines. This review discusses the current knowledge about HIV-1 restriction by SER5.

MARCH8: the tie that binds to viruses

Membrane-associated RING-CH (MARCH) family member proteins are RING-finger E3 ubiquitin ligases that are known to downregulate cellular transmembrane proteins. MARCH8 is a novel antiviral factor that inhibits HIV-1 envelope glycoprotein and vesicular stomatitis virus G by downregulating these envelope glycoproteins from the cell surface, resulting in their reduced incorporation into virions. More recently, we have found that MARCH8 reduces viral infectivity via two different mechanisms. Additionally, several groups have reported further antiviral or virus-supportive functions of the MARCH8 protein and its other cellular mechanisms. In this review, we summarize the current knowledge about the molecular mechanisms by which MARCH8 can regulate cellular homeostasis and inhibit and occasionally support enveloped virus infection.

Deletion of BST2 Cytoplasmic and Transmembrane N-Terminal Domains Results in SARS-CoV, SARS-CoV-2, and Influenza Virus Production Suppression in a Vero Cell Line

SARS-CoV-2, which emerged in Wuhan (China), has become a great worldwide problem in 2020 and has led to more than 1,000,000 deaths worldwide. Many laboratories are searching for ways to fight this pandemic. We studied the action of the cellular antiviral protein tetherin, which is encoded by the BST2 gene. We deleted the transmembrane domain-encoding part of the gene in the Vero cell line. The transmembrane domain is a target for virus-antagonizing proteins. We showed a decrease in SARS-CoV-2 in cells with deleted transmembrane BST2 domains compared to the initial Vero cell line. Similar results were obtained for SARS-CoV and avian influenza virus. This finding may help the development of antiviral therapies competitively targeting the transmembrane domain of tetherin with viral-antagonizing proteins.

Beyond Tethering the Viral Particles: Immunomodulatory Functions of Tetherin (BST-2)

Host response to viral infection is a highly regulated process involving engagement of various host factors, cytokines, chemokines, and stimulatory signals that pave the way for an antiviral immune response. The response is manifested in terms of viral sequestration, phagocytosis, and inhibition of genome replication, and, finally, if required, lymphocyte-mediated clearance of virally infected cells. During this process, cross-talk between viral and host factors can shape disease outcomes and immunopathology. Bone marrow stromal antigen 2 (BST-2), also know as tetherin, is induced by type I interferon produced in response to viral infections, as well as in certain cancers. BST-2 has been shown to be a host restriction factor of virus multiplication through its ability to physically tether budding virions and restrict viral spread. However, BST-2 has other roles in the host antiviral response. This review focuses on the diverse functions of BST-2 and its downstream signaling pathways in regulating host immune responses.

In Silico Insights into HIV-1 Vpu-Tetherin Interactions and Its Mutational Counterparts

Tetherin, an interferon-induced host protein encoded by the bone marrow stromal antigen 2 (BST2/CD317/HM1.24) gene, is involved in obstructing the release of many retroviruses and other enveloped viruses by cross-linking the budding virus particles to the cell surface. This activity is antagonized in the case of human immunodeficiency virus (HIV)-1 wherein its accessory protein Viral Protein U (Vpu) interacts with tetherin, causing its downregulation from the cell surface. Vpu and tetherin connect through their transmembrane (TM) domains, culminating into events leading to tetherin degradation by recruitment of β-TrCP2. However, mutations in the TM domains of both proteins are reported to act as a resistance mechanism to Vpu countermeasure impacting tetherin's sensitivity towards Vpu but retaining its antiviral activity. Our study illustrates the binding aspects of blood-derived, brain-derived, and consensus HIV-1 Vpu with tetherin through protein-protein docking. The analysis of the bound complexes confirms the blood-derived Vpu-tetherin complex to have the best binding affinity as compared to other two. The mutations in tetherin and Vpu are devised computationally and are subjected to protein-protein interactions. The complexes are tested for their binding affinities, residue connections, hydrophobic forces, and, finally, the effect of mutation on their interactions. The single point mutations in tetherin at positions L23Y, L24T, and P40T, and triple mutations at {L22S, F44Y, L37I} and {L23T, L37T, T45I}, while single point mutations in Vpu at positions A19H and W23Y and triplet of mutations at {V10K, A11L, A19T}, {V14T, I18T, I26S}, and {A11T, V14L, A15T} have revealed no polar contacts with minimal hydrophobic interactions between Vpu and tetherin, resulting in reduced binding affinity. Additionally, we have explored the aggregation potential of tetherin and its association with the brain-derived Vpu protein. This work is a possible step toward an understanding of Vpu-tetherin interactions.

CRISPR-mediated activation of endogenous BST-2/tetherin expression inhibits wild-type HIV-1 production

The CRISPR technology not only can knock out target genes by using the RNA-guided Cas9 nuclease but also can activate their expression when a nuclease-deficient Cas9 (dCas9) is employed. Using the latter function, we here show the effect of the CRISPR-mediated pinpoint activation of endogenous expression of BST-2 (also known as tetherin), a virus restriction factor with a broad antiviral spectrum. Single-guide RNA (sgRNA) sequences targeting the BST-2 promoter were selected by promoter assays. Potential sgRNAs and dCas9 fused to the VP64 transactivation domain, along with an accessory transcriptional activator complex, were introduced into cells by lentiviral transduction. Increased expression of BST-2 mRNA in transduced cells was confirmed by real-time RT-PCR. Cells in which BST-2 expression was highly enhanced showed the effective inhibition of HIV-1 production and replication even in the presence of the viral antagonist Vpu against BST-2. These findings confirm that the physiological stoichiometry between host restriction factors and viral antagonists may determine the outcome of the battle with viruses.

BST-2 controls T cell proliferation and exhaustion by shaping the early distribution of a persistent viral infection

The interferon inducible protein, BST-2 (or, tetherin), plays an important role in the innate antiviral defense system by inhibiting the release of many enveloped viruses. Consequently, viruses have evolved strategies to counteract the anti-viral activity of this protein. While the mechanisms by which BST-2 prevents viral dissemination have been defined, less is known about how this protein shapes the early viral distribution and immunological defense against pathogens during the establishment of persistence. Using the lymphocytic choriomeningitis virus (LCMV) model of infection, we sought insights into how the in vitro antiviral activity of this protein compared to the immunological defense mounted in vivo. We observed that BST-2 modestly reduced production of virion particles from cultured cells, which was associated with the ability of BST-2 to interfere with the virus budding process mediated by the LCMV Z protein. Moreover, LCMV does not encode a BST-2 antagonist, and viral propagation was not significantly restricted in cells that constitutively expressed BST-2. In contrast to this very modest effect in cultured cells, BST-2 played a crucial role in controlling LCMV in vivo. In BST-2 deficient mice, a persistent strain of LCMV was no longer confined to the splenic marginal zone at early times post-infection, which resulted in an altered distribution of LCMV-specific T cells, reduced T cell proliferation / function, delayed viral control in the serum, and persistence in the brain. These data demonstrate that BST-2 is important in shaping the anatomical distribution and adaptive immune response against a persistent viral infection in vivo.

Enhanced production of enveloped viruses in BST-2-deficient cell lines

Despite all the advantages that cell-cultured influenza vaccines have over egg-based influenza vaccines, the inferior productivity of cell-culture systems is a major drawback that must be addressed. BST-2 (tetherin) is a host restriction factor which inhibits budding-out of various enveloped viruses from infected host cells. We developed BST-2-deficient MDCK and Vero cell lines to increase influenza virus release in cell culture. BST-2 gene knock-out resulted in increased release of viral particles into the culture medium, by at least 2-fold and up to 50-fold compared to release from wild-type counterpart cells depending on cell line and virus type. The effect was not influenza virus/MDCK/Vero-specific, but was also present in a broad range of host cells and virus families; we observed similar results in murine, human, canine, and monkey cell lines with viruses including MHV-68 (Herpesviridae), influenza A virus (Orthomyxoviridae), porcine epidemic diarrhea virus (Coronaviridae), and vaccinia virus (Poxviridae). Our results suggest that the elimination of BST-2 expression in virus-producing cell lines can enhance the production of viral vaccines. Biotechnol. Bioeng.2017;114: 2289-2297. © 2017 Wiley Periodicals, Inc.

Single Amino Acid Substitution N659D in HIV-2 Envelope Glycoprotein (Env) Impairs Viral Release and Hampers BST-2 Antagonism

BST-2 or tetherin is a host cell restriction factor that prevents the budding of enveloped viruses at the cell surface, thus impairing the viral spread. Several countermeasures to evade this antiviral factor have been positively selected in retroviruses: the human immunodeficiency virus type 2 (HIV-2) relies on the envelope glycoprotein (Env) to overcome BST-2 restriction. The Env gp36 ectodomain seems involved in this anti-tetherin activity, however residues and regions interacting with BST-2 are not clearly defined. Among 32 HIV-2 ROD Env mutants tested, we demonstrated that the asparagine residue at position 659 located in the gp36 ectodomain is mandatory to exert the anti-tetherin function. Viral release assays in cell lines expressing BST-2 showed a loss of viral release ability for the HIV-2 N659D mutant virus compared to the HIV-2 wild type virus. In bst-2 inactivated H9 cells, those differences were lost. Subtilisin treatment of infected cells demonstrated that the N659D mutant was more tethered at the cell surface. Förster resonance energy transfer (FRET) experiments confirmed a direct molecular link between Env and BST-2 and highlighted an inability of the mutant to bind BST-2. We also tested a virus presenting a truncation of 109 amino acids at the C-terminal part of Env, a cytoplasmic tail partial deletion that is spontaneously selected in vitro. Interestingly, viral release assays and FRET experiments indicated that a full Env cytoplasmic tail was essential in BST-2 antagonism. In HIV-2 infected cells, an efficient Env-mediated antagonism of BST-2 is operated through an intermolecular link involving the asparagine 659 residue as well as the C-terminal part of the cytoplasmic tail.

Remodeling of the Host Cell Plasma Membrane by HIV-1 Nef and Vpu: A Strategy to Ensure Viral Fitness and Persistence

The plasma membrane protects the cell from its surroundings and regulates cellular communication, homing, and metabolism. Not surprisingly, the composition of this membrane is highly controlled through the vesicular trafficking of proteins to and from the cell surface. As intracellular pathogens, most viruses exploit the host plasma membrane to promote viral replication while avoiding immune detection. This is particularly true for the enveloped human immunodeficiency virus (HIV), which assembles and obtains its lipid shell directly at the plasma membrane. HIV-1 encodes two proteins, negative factor (Nef) and viral protein U (Vpu), which function primarily by altering the quantity and localization of cell surface molecules to increase virus fitness despite host antiviral immune responses. These proteins are expressed at different stages in the HIV-1 life cycle and employ a variety of mechanisms to target both unique and redundant surface proteins, including the viral receptor CD4, host restriction factors, immunoreceptors, homing molecules, tetraspanins and membrane transporters. In this review, we discuss recent progress in the study of the Nef and Vpu targeting of host membrane proteins with an emphasis on how remodeling of the cell membrane allows HIV-1 to avoid host antiviral immune responses leading to the establishment of systemic and persistent infection.

BST-2: at the crossroads of viral pathogenesis and oncogenesis

BST-2 is a moonlight protein with several protective and deleterious functions. Regulation of virus restriction and tumor aggressiveness are the most studied aspects of BST-2 function and thus, the main focus of this perspective. Virus inhibition roles of BST-2 have therapeutic potential that, if properly harnessed, could result in near broad spectrum antiviral. However, the involvement of BST-2 in cancer calls for additional studies on BST-2 biology and re-evaluation of the overall role of BST-2 in host protection, as it appears that BST-2 has pleiotropic effects in the host. Here, we analyze the antiviral and protumor roles of BST-2. We also discuss potential therapeutic options for BST-2 against viral infection and cancer.

A small molecule compound IMB-LA inhibits HIV-1 infection by preventing viral Vpu from antagonizing the host restriction factor BST-2

Human BST-2 inhibits HIV-1 replication by tethering nascent virions to the cell surface. HIV-1 codes Vpu that counteracts BST-2 by down-regulating this restriction factor from the cell surface. This important function makes Vpu a potential therapeutic target. Yet, no agents have been reported to block Vpu from antagonizing BST-2. In this study, we report a small molecule compound IMB-LA that abrogates the function of Vpu and thereby strongly suppresses HIV-1 replication by sensitizing the virus to BST-2 restriction. Further studies revealed that IMB-LA specifically inhibits Vpu-mediated degradation of BST-2 and restores the expression of BST-2 at the cell surface. Although IMB-LA does not prevent Vpu from interacting with BST-2 or β-TrCP2-containing ubiquitin E3 ligase, sorting of BST-2 into lysosomes in Vpu-expressing cells is blocked by IMB-LA. Most importantly, HIV-1 release and infection is inhibited by IMB-LA only in BST-2-expressing cells. In summary, results herein demonstrated that IMB-LA could specifically inhibit the degradation of BST-2 induced by Vpu, and impair HIV-1 replication in a BST-2 dependent manner, suggesting the feasibility of utilizing small molecule compounds to disable the antagonist function of Vpu and thereby expose HIV-1 to the restriction by BST-2.

The VP40 protein of Marburg virus exhibits impaired budding and increased sensitivity to human tetherin following mouse adaptation

The Marburg virus VP40 protein is a viral matrix protein that spontaneously buds from cells. It also functions as an interferon (IFN) signaling antagonist by targeting Janus kinase 1 (JAK1). A previous study demonstrated that the VP40 protein of the Ravn strain of Marburg virus (Ravn virus [RAVV]) failed to block IFN signaling in mouse cells, whereas the mouse-adapted RAVV (maRAVV) VP40 acquired the ability to inhibit IFN responses in mouse cells. The increased IFN antagonist function of maRAVV VP40 mapped to residues 57 and 165, which were mutated during the mouse adaptation process. In the present study, we demonstrate that maRAVV VP40 lost the capacity to efficiently bud from human cell lines, despite the fact that both parental and maRAVV VP40s bud efficiently from mouse cell lines. The impaired budding in human cells corresponds with the appearance of protrusions on the surface of maRAVV VP40-expressing Huh7 cells and with an increased sensitivity of maRAVV VP40 to restriction by human tetherin but not mouse tetherin. However, transfer of the human tetherin cytoplasmic tail to mouse tetherin restored restriction of maRAVV VP40. Residues 57 and 165 were demonstrated to contribute to the failure of maRAVV VP40 to bud from human cells, and residue 57 was demonstrated to alter VP40 oligomerization, as assessed by coprecipitation assay, and to determine sensitivity to human tetherin. This suggests that RAVV VP40 acquired, during adaptation to mice, changes in its oligomerization potential that enhanced IFN antagonist function. However, this new capacity impaired RAVV VP40 budding from human cells.

IMPORTANCE

Filoviruses, which include Marburg viruses and Ebola viruses, are zoonotic pathogens that cause severe disease in humans and nonhuman primates but do not cause similar disease in wild-type laboratory strains of mice unless first adapted to these animals. Although mouse adaptation has been used as a method to develop small animal models of pathogenesis, the molecular determinants associated with filovirus mouse adaptation are poorly understood. Our study demonstrates how genetic changes that accrued during mouse adaptation of the Ravn strain of Marburg virus have impacted the budding function of the viral VP40 matrix protein. Strikingly, we find impairment of mouse-adapted VP40 budding function in human but not mouse cell lines, and we correlate the impairment with an increased sensitivity of VP40 to restriction by human but not mouse tetherin and with changes in VP40 oligomerization. These data suggest that there are functional costs associated with filovirus adaptation to new hosts and implicate tetherin as a filovirus host restriction factor.

Viral miniproteins

Many viruses encode short transmembrane proteins that play vital roles in virus replication or virulence. Because many of these proteins are less than 50 amino acids long and not homologous to cellular proteins, their open reading frames were often overlooked during the initial annotation of viral genomes. Some of these proteins oligomerize in membranes and form ion channels. Other miniproteins bind to cellular transmembrane proteins and modulate their activity, whereas still others have an unknown mechanism of action. Based on the underlying principles of transmembrane miniprotein structure, it is possible to build artificial small transmembrane proteins that modulate a variety of biological processes. These findings suggest that short transmembrane proteins provide a versatile mechanism to regulate a wide range of cellular activities, and we speculate that cells also express many similar proteins that have not yet been discovered.

Identification of novel key amino acids at the interface of the transmembrane domains of human BST-2 and HIV-1 Vpu

Background

BST-2 (bone marrow stromal cell antigen 2) is an interferon-inducible protein that inhibits virus release by tethering viral particles to the cell surface. This antiviral activity of BST-2 is antagonized by HIV-1 accessory protein Vpu. Vpu physically interacts with BST-2 through their mutual transmembrane (TM) domains. In this study, we utilized the BRET assay and molecular dynamics (MD) simulation method to further characterize the interaction of BST-2 and Vpu.

Results

Amino acids I34, L37, P40 and L41 in the TM domain of BST-2, and L11, A18 and W22 in the TM domain of Vpu were identified to be critical for the interaction between BST-2 and Vpu. The residues P40 in the TM domain of BST-2 and L11 in the TM domain of Vpu were shown, for the first time, to be important for their interaction. Furthermore, triple-amino-acid substitutions, 14–16 (AII to VAA) and 26–28 (IIE to AAA) in Vpu TM, not the single-residue mutation, profoundly disrupted BST-2/Vpu interaction. The results of MD simulation revealed significant conformational changes of the BST-2/Vpu complex as a result of mutating P40 of BST-2 and L11, 14–16 (AII to VAA) and 26–28 (IIE to AAA) of Vpu. In addition, disrupting the interaction between BST-2 and Vpu rendered BST-2 resistant to Vpu antagonization.

Conclusions

Through use of the BRET assay, we identified novel key residues P40 in the TM domain of BST-2 and L11 in the TM domain of Vpu that are important for their interaction. These results add new insights into the molecular mechanism behind BST-2 antagonization by HIV-1 Vpu.

Molecular mechanism of arenavirus assembly and budding

Arenaviruses have a bisegmented negative-strand RNA genome, which encodes four viral proteins: GP and NP by the S segment and L and Z by the L segment. These four viral proteins possess multiple functions in infection, replication and release of progeny viruses from infected cells. The small RING finger protein, Z protein is a matrix protein that plays a central role in viral assembly and budding. Although all arenaviruses encode Z protein, amino acid sequence alignment showed a huge variety among the species, especially at the C-terminus where the L-domain is located. Recent publications have demonstrated the interactions between viral protein and viral protein, and viral protein and host cellular protein, which facilitate transportation and assembly of viral components to sites of virus egress. This review presents a summary of current knowledge regarding arenavirus assembly and budding, in comparison with other enveloped viruses. We also refer to the restriction of arenavirus production by the antiviral cellular factor, Tetherin/BST-2.

SAMHD1-Dependent and -Independent Functions of HIV-2/SIV Vpx Protein

Both human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) encode a unique set of accessory proteins that enhance viral replication in the host. Two similar accessory proteins, Vpx and Vpr, are encoded by HIV-2. In contrast, HIV-1 encodes Vpr but not Vpx. Recent studies have indicated that Vpx counteracts a particular host restriction factor, thereby facilitating reverse transcription in myeloid cells such as monocyte-derived macrophages and monocyte-derived dendritic cells. This mechanism of counteraction is similar to that of the accessory proteins Vif and Vpu which antagonize other host factors. In 2011, the protein SAMHD1 was identified as the restriction factor counteracted by Vpx. Studies have since revealed that SAMHD1 degrades deoxynucleoside triphosphates (dNTPs), which are components of viral genomic cDNA, in order to deprive viruses of dNTPs. Although interactions between SAMHD1 and Vpx continue to be a major research focus, Vpx has also been shown to have an apparent ability to enhance nuclear import of the viral genome in T lymphocytes. This review summarizes the current knowledge regarding SAMHD1-dependent and -independent functions of Vpx, and discusses possible reasons why HIV-2 encodes both Vpx and Vpr, unlike HIV-1.

Species tropism of HIV-1 modulated by viral accessory proteins

Human immunodeficiency virus type 1 (HIV-1) is tropic and pathogenic only for humans, and does not replicate in macaque monkeys routinely used for experimental infections. This specially narrow host range (species tropism) has impeded much the progress of HIV-1/acquired immunodeficiency syndrome (AIDS) basic research. Extensive studies on the underlying mechanism have revealed that Vif, one of viral accessory proteins, is critical for the HIV-1 species tropism in addition to Gag-capsid protein. Another auxiliary protein Vpu also has been demonstrated to affect this HIV-1 property. In this review, we focus on functional interactions of these HIV-1 proteins and species specific-restriction factors. In addition, we describe an evolutional viewpoint that is relevant to the species tropism of HIV-1 controlled by the accessory proteins.

HIV-1, interferon and the interferon regulatory factor system: an interplay between induction, antiviral responses and viral evasion

Thirty years after the first isolation of the etiological agent of AIDS, the virus HIV-1 is still a major threat worldwide with millions of individuals currently infected. Although current combination therapies allow viral replication to be controlled, HIV-1 is not eradicated and persists in drug- and immune system-insensitive reservoirs and a cure is still lacking. Pathogens such as HIV-1 that cause chronic infections are able to adapt to the host in a manner that ensures long term residence and survival, via the evolution of numerous mechanisms that evade various aspects of the innate and adaptive immune response. One such mechanism is targeted to members of the interferon (IFN) regulatory factor (IRF) family of proteins. These transcription factors regulate a variety of biological processes including interferon induction, immune cell activation and downstream pattern recognition receptors (PRRs). HIV-1 renders IRFs harmless and hijacks them to its own advantage in order to facilitate its replication and evasion of immune responses. Type I interferon (IFN), the canonical antiviral innate response, can be induced in both acute and chronic HIV-1 infection in vivo, but in the majority of individuals this initial response is not protective and can contribute to disease progression. Type I IFN expression is largely inhibited in T cells and macrophages in order to successfully establish productive infection, whereas sustained IFN production by plasmacytoid dendritic cells is considered an important source of chronic immune activation, a hallmark to AIDS progression.

Vpu and BST2: Still Not There Yet?

Extensive investigations have identified two cellular proteins in humans that potently inhibit HIV type 1 (HIV-1) replication and are widely accepted as "restriction factors." APOBEC3G was identified as a restriction factor that diminishes HIV-1 replication by inducing G-to-A hypermutation in the viral genome, while BST2 has been identified as another restriction factor that impairs the release of nascent virions by tethering them on the surface of infected cells. To counter these restriction factors, HIV-1 has equipped itself with its own weapons: viral infectivity factor (Vif) degrades APOBEC3G, while viral protein U (Vpu) antagonizes BST2. These findings have allowed us to further our understanding of virus-host interaction, namely, the interplay between viral factors versus host restriction factors. In the first case, the interplay between APOBEC3G and Vif is clear: vif-deficient HIV-1 is incapable of replicating in APOBEC3G-expressing cells. This insight directly indicates that APOBEC3G is a bona fide restriction factor and has intrinsic immunity against HIV-1, and that Vif is a prerequisite for HIV-1 infection. In other words, the relationship between Vif and APOBEC3G has already "matured," and Vif has highly evolved to overcome APOBEC3G. On the other hand, although BST2 drastically impairs the release of vpu-deficient HIV-1 virions, it is puzzling that vpu-deficient HIV-1 is still able to replicate in BST2-expressing cells. These insights imply that BST2-mediated anti-HIV-1 activity is vulnerable, and that Vpu is dispensable for HIV-1 infection. If so, why has Vpu acquired the counteracting potential against BST2? Was it necessary or important for HIV-1? Or is the relationship between Vpu and BST2 still "immature"? In this review, we particularly focus on the interplay between Vpu and BST2. We discuss the possibility that Vpu has evolved as a potent antagonist against BST2, and finally, propose a hypothesis that Vpu has evolved as a promoter of human-to-human HIV-1 transmission. Since the first report of acquired immunodeficiency syndrome patients in 1981, HIV-1 has spread explosively worldwide and is currently a pandemic. This review proposes a concept suggesting that the current HIV-1 pandemic may be partly attributed by Vpu.