1
|
Abstract
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.
Collapse
|
2
|
Barbosa JAF, Sparapani S, Boulais J, Lodge R, Cohen ÉA. Human Immunodeficiency Virus Type 1 Vpr Mediates Degradation of APC1, a Scaffolding Component of the Anaphase-Promoting Complex/Cyclosome. J Virol 2021; 95:e0097120. [PMID: 34011540 PMCID: PMC8274603 DOI: 10.1128/jvi.00971-20] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2020] [Accepted: 05/05/2021] [Indexed: 11/20/2022] Open
Abstract
HIV-1 encodes several accessory proteins-Nef, Vif, Vpr, and Vpu-whose functions are to modulate the cellular environment to favor immune evasion and viral replication. While Vpr was shown to mediate a G2/M cell cycle arrest and provide a replicative advantage during infection of myeloid cells, the mechanisms underlying these functions remain unclear. In this study, we defined HIV-1 Vpr proximity interaction network using the BioID proximity labeling approach and identified 352 potential Vpr partners/targets, including several complexes, such as the cell cycle-regulatory anaphase-promoting complex/cyclosome (APC/C). Herein, we demonstrate that both the wild type and cell cycle-defective mutants of Vpr induce the degradation of APC1, an essential APC/C scaffolding protein, and show that this activity relies on the recruitment of DCAF1 by Vpr and the presence of a functional proteasome. Vpr forms a complex with APC1, and the APC/C coactivators Cdh1 and Cdc20 are associated with these complexes. Interestingly, we found that Vpr encoded by the prototypic HIV-1 NL4.3 does not interact efficiently with APC1 and is unable to mediate its degradation as a result of a N28S-G41N amino acid substitution. In contrast, we show that APC1 degradation is a conserved feature of several primary Vpr variants from transmitted/founder virus. Functionally, Vpr-mediated APC1 degradation did not impact the ability of the protein to induce a G2 cell cycle arrest during infection of CD4+ T cells or enhance HIV-1 replication in macrophages, suggesting that this conserved activity may be important for other aspects of HIV-1 pathogenesis. IMPORTANCE The function of the Vpr accessory protein during HIV-1 infection remains poorly defined. Several cellular targets of Vpr were previously identified, but their individual degradation does not fully explain the ability of Vpr to impair the cell cycle or promote HIV-1 replication in macrophages. Here, we used the unbiased proximity labeling approach, called BioID, to further define the Vpr proximity interaction network and identified several potentially new Vpr partners/targets. We validated our approach by focusing on a cell cycle master regulator, the APC/C complex, and demonstrated that Vpr mediated the degradation of a critical scaffolding component of APC/C called APC1. Furthermore, we showed that targeting of APC/C by Vpr did not impact the known activity of Vpr. Since degradation of APC1 is a conserved feature of several primary variants of Vpr, it is likely that the interplay between Vpr and APC/C governs other aspects of HIV-1 pathogenesis.
Collapse
Affiliation(s)
| | | | | | - Robert Lodge
- Montreal Clinical Research Institute, Montréal, Québec, Canada
| | - Éric A. Cohen
- Montreal Clinical Research Institute, Montréal, Québec, Canada
- Department of Microbiology, Infectiology and Immunology, Université de Montréal, Montréal, Québec, Canada
| |
Collapse
|
3
|
Xu X, Wang Y, Xue F, Guan E, Tian F, Xu J, Zhang H. BST2 Promotes Tumor Growth via Multiple Pathways in Hepatocellular Carcinoma. Cancer Invest 2020; 38:329-337. [PMID: 32427495 DOI: 10.1080/07357907.2020.1769125] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Bone marrow stromal antigen 2 (BST2) is a transmembrane glycoprotein and plays an essential role in innate immunity. Here we firstly found that BST2 expression was significantly elevated in hepatocellular carcinoma (HCC) tissues. High BST2 was closely related to the larger tumor size and more tumor number. Moreover, HCC patients with higher expression of BST2 had poorer overall survival and BST2 was identified as an independent unfavorable prognosis factor. Finally, we demonstrated that BST2 can promote proliferation capacity of tumor cells. In conclusion, HCC patients with higher BST2 expression were more predisposed to poorer clinical symptoms and unfavorable prognosis.
Collapse
Affiliation(s)
- Xiaoguang Xu
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Yu Wang
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Fangxi Xue
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Encui Guan
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Feng Tian
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Jian Xu
- Department of Gastroenterology, Linyi Central Hospital, Shandong, Linyi, China
| | - Hongjin Zhang
- Department of Endocrinology and Nephrology, The Third People's Hospital of Linyi, Shandong, Linyi, China
| |
Collapse
|
4
|
Bego MG, Miguet N, Laliberté A, Aschman N, Gerard F, Merakos AA, Weissenhorn W, Cohen ÉA. Activation of the ILT7 receptor and plasmacytoid dendritic cell responses are governed by structurally-distinct BST2 determinants. J Biol Chem 2019; 294:10503-10518. [PMID: 31118237 DOI: 10.1074/jbc.ra119.008481] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2019] [Revised: 04/25/2019] [Indexed: 12/31/2022] Open
Abstract
Type I interferons (IFN-I) are key innate immune effectors predominantly produced by activated plasmacytoid dendritic cells (pDCs). By modulating immune responses at their foundation, IFNs can widely reshape immunity to control infectious diseases and malignancies. Nevertheless, their biological activities can also be detrimental to surrounding healthy cells, as prolonged IFN-I signaling is associated with excessive inflammation and immune dysfunction. The interaction of the human pDC receptor immunoglobulin-like transcript 7 (ILT7) with its IFN-I-regulated ligand, bone marrow stromal cell antigen 2 (BST2) plays a key role in controlling the IFN-I amounts produced by pDCs in response to Toll-like receptor (TLR) activation. However, the structural determinants and molecular features of BST2 that govern ILT7 engagement and activation are largely undefined. Using two functional assays to measure BST2-stimulated ILT7 activation as well as biophysical studies, here we identified two structurally-distinct regions of the BST2 ectodomain that play divergent roles during ILT7 activation. We found that although the coiled-coil region contains a newly defined ILT7-binding surface, the N-terminal region appears to suppress ILT7 activation. We further show that a stable BST2 homodimer binds to ILT7, but post-binding events associated with the unique BST2 coiled-coil plasticity are required to trigger receptor signaling. Hence, BST2 with an unstable or a rigid coiled-coil fails to activate ILT7, whereas substitutions in its N-terminal region enhance activation. Importantly, the biological relevance of these newly defined domains of BST2 is underscored by the identification of substitutions having opposing potentials to activate ILT7 in pathological malignant conditions.
Collapse
Affiliation(s)
- Mariana G Bego
- From the Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada
| | - Nolwenn Miguet
- the University Grenoble Alpes, Institut de Biologie Structurale (IBS), CEA, CNRS, 38044 Grenoble, France, and
| | - Alexandre Laliberté
- From the Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada
| | - Nicolas Aschman
- the University Grenoble Alpes, Institut de Biologie Structurale (IBS), CEA, CNRS, 38044 Grenoble, France, and
| | - Francine Gerard
- the University Grenoble Alpes, Institut de Biologie Structurale (IBS), CEA, CNRS, 38044 Grenoble, France, and
| | - Angelique A Merakos
- From the Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada
| | - Winfried Weissenhorn
- the University Grenoble Alpes, Institut de Biologie Structurale (IBS), CEA, CNRS, 38044 Grenoble, France, and
| | - Éric A Cohen
- From the Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada, .,the Department of Microbiology, Infectiology and Immunology, Université de Montréal, Montreal, Quebec H3T 1J4, Canada
| |
Collapse
|
5
|
Font-Haro A, Janovec V, Hofman T, Machala L, Jilich D, Melkova Z, Weber J, Trejbalova K, Hirsch I. Expression of TIM-3 on Plasmacytoid Dendritic Cells as a Predictive Biomarker of Decline in HIV-1 RNA Level during ART. Viruses 2018; 10:v10040154. [PMID: 29597250 PMCID: PMC5923448 DOI: 10.3390/v10040154] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2018] [Revised: 03/24/2018] [Accepted: 03/26/2018] [Indexed: 12/12/2022] Open
Abstract
Depletion and functional impairment of circulating plasmacytoid dendritic cells (pDCs) are characteristic attributes of HIV-1-infection. The mechanism of dysfunction of pDCs is unclear. Here, we studied the development of phenotype of pDCs in a cohort of HIV-1-infected individuals monitored before the initiation and during a 9-month follow up with antiretroviral therapy (ART). Using polychromatic flow cytometry, we detected significantly higher pDC-surface expression of the HIV-1 receptor CD4, regulatory receptor BDCA-2, Fcγ receptor CD32, pDC dysfunction marker TIM-3, and the marker of killer pDC, TRAIL, in treatment-naïve HIV-1-infected individuals before initiation of ART when compared to healthy donors. After 9 months of ART, all of these markers approached but did not reach the expression levels observed in healthy donors. We found that the rate of decline in HIV-1 RNA level over the first 3 months of ART negatively correlated with the expression of TIM-3 on pDCs. We conclude that immunogenic phenotype of pDCs is not significantly restored after sustained suppression of HIV-1 RNA level in ART-treated patients and that the level of the TIM-3 expressed on pDCs in treatment naïve patients could be a predictive marker of the rate of decline in the HIV-1 RNA level during ART.
Collapse
Affiliation(s)
- Albert Font-Haro
- Institute of Molecular Genetics of the Czech Academy of Sciences, 14220 Prague, Czech Republic.
- Department of Genetics and Microbiology, Charles University, Faculty of Sciences, BIOCEV, 25242 Vestec, Czech Republic.
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, IOCB & Gilead Research Center, 16610 Prague, Czech Republic.
| | - Vaclav Janovec
- Institute of Molecular Genetics of the Czech Academy of Sciences, 14220 Prague, Czech Republic.
- Department of Genetics and Microbiology, Charles University, Faculty of Sciences, BIOCEV, 25242 Vestec, Czech Republic.
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, IOCB & Gilead Research Center, 16610 Prague, Czech Republic.
| | - Tomas Hofman
- Department of Genetics and Microbiology, Charles University, Faculty of Sciences, BIOCEV, 25242 Vestec, Czech Republic.
| | - Ladislav Machala
- The Third Faculty of Medicine, Charles University and Hospital Na Bulovce, 18081 Prague, Czech Republic.
| | - David Jilich
- The First Faculty of Medicine, Charles University and Hospital Na Bulovce, 18081 Prague, Czech Republic.
| | - Zora Melkova
- Department of Immunology and Microbiology, Charles University, The First Faculty of Medicine, BIOCEV, 25242 Vestec, Czech Republic.
| | - Jan Weber
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, IOCB & Gilead Research Center, 16610 Prague, Czech Republic.
| | - Katerina Trejbalova
- Institute of Molecular Genetics of the Czech Academy of Sciences, 14220 Prague, Czech Republic.
| | - Ivan Hirsch
- Institute of Molecular Genetics of the Czech Academy of Sciences, 14220 Prague, Czech Republic.
- Department of Genetics and Microbiology, Charles University, Faculty of Sciences, BIOCEV, 25242 Vestec, Czech Republic.
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, IOCB & Gilead Research Center, 16610 Prague, Czech Republic.
| |
Collapse
|
6
|
Foster TL, Pickering S, Neil SJD. Inhibiting the Ins and Outs of HIV Replication: Cell-Intrinsic Antiretroviral Restrictions at the Plasma Membrane. Front Immunol 2018; 8:1853. [PMID: 29354117 PMCID: PMC5758531 DOI: 10.3389/fimmu.2017.01853] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2017] [Accepted: 12/07/2017] [Indexed: 01/01/2023] Open
Abstract
Like all viruses, human immunodeficiency viruses (HIVs) and their primate lentivirus relatives must enter cells in order to replicate and, once produced, new virions need to exit to spread to new targets. These processes require the virus to cross the plasma membrane of the cell twice: once via fusion mediated by the envelope glycoprotein to deliver the viral core into the cytosol; and secondly by ESCRT-mediated scission of budding virions during release. This physical barrier thus presents a perfect location for host antiviral restrictions that target enveloped viruses in general. In this review we will examine the current understanding of innate host antiviral defences that inhibit these essential replicative steps of primate lentiviruses associated with the plasma membrane, the mechanism by which these viruses have adapted to evade such defences, and the role that this virus/host battleground plays in the transmission and pathogenesis of HIV/AIDS.
Collapse
Affiliation(s)
- Toshana L Foster
- Department of Infectious Disease, School of Immunology and Microbial Sciences, King's College London, London, United Kingdom
| | - Suzanne Pickering
- Department of Infectious Disease, School of Immunology and Microbial Sciences, King's College London, London, United Kingdom
| | - Stuart J D Neil
- Department of Infectious Disease, School of Immunology and Microbial Sciences, King's College London, London, United Kingdom
| |
Collapse
|
7
|
HIV-1 Vpu Downmodulates ICAM-1 Expression, Resulting in Decreased Killing of Infected CD4 + T Cells by NK Cells. J Virol 2017; 91:JVI.02442-16. [PMID: 28148794 DOI: 10.1128/jvi.02442-16] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2016] [Accepted: 01/26/2017] [Indexed: 12/20/2022] Open
Abstract
HIV-1 Vpu is known to alter the expression of numerous cell surface molecules. Given the ever-increasing list of Vpu targets identified to date, we undertook a proteomic screen to discover novel cell membrane proteins modulated by this viral protein. Plasma membrane proteome isolates from Vpu-inducible T cells were subjected to stable isotope labeling of amino acids in cell culture (SILAC)-based mass spectrometry analysis, and putative targets were validated by infection of primary CD4+ T cells. We report here that while intercellular adhesion molecule 1 (ICAM-1) and ICAM-3 are upregulated by HIV-1 infection, expression of Vpu offsets this increase by downregulating these molecules from the cell surface. Specifically, we show that Vpu is sufficient to downregulate and deplete ICAM-1 in a manner requiring the Vpu transmembrane domain and a dual-serine (S52/S56) motif necessary for recruitment of the beta-transducin repeat-containing E3 ubiquitin protein ligase (β-TrCP) component of the Skp, Cullin, F-box (SCFβ-TrCP) E3 ubiquitin ligase. Vpu interacts with ICAM-1 to induce its proteasomal degradation. Interestingly, the E3 ubiquitin ligase component β-TrCP-1 is dispensable for ICAM-1 surface downregulation yet is necessary for ICAM-1 degradation. Functionally, Vpu-mediated ICAM-1 downregulation lowers packaging of this adhesion molecule into virions, resulting in decreased infectivity. Importantly, while Vpu-mediated downregulation of ICAM-3 has a limited effect on the conjugation of NK cells to HIV-1-infected CD4+ T cells, downregulation of ICAM-1 by Vpu results in a reduced ability of NK cells to bind and kill infected T cells. Vpu-mediated ICAM-1 downregulation may therefore represent an evolutionary compromise in viral fitness by impeding the formation of cell-to-cell contacts between immune cells and infected T cells at the cost of decreased virion infectivity.IMPORTANCE The major barrier to eradicating HIV-1 infection is the establishment of treatment-resistant reservoirs early in infection. Vpu-mediated ICAM-1 downregulation may contribute to the evasion of cell-mediated immunity during acute infection to promote viral dissemination and the development of viral reservoirs. By aiding the immune system to clear infection prior to the development of reservoirs, novel treatments designed to disrupt Vpu-mediated ICAM-1 downregulation may be beneficial during acute infection or as a prophylactic treatment.
Collapse
|
8
|
Mack K, Starz K, Sauter D, Langer S, Bibollet-Ruche F, Learn GH, Stürzel CM, Leoz M, Plantier JC, Geyer M, Hahn BH, Kirchhoff F. Efficient Vpu-Mediated Tetherin Antagonism by an HIV-1 Group O Strain. J Virol 2017; 91:e02177-16. [PMID: 28077643 PMCID: PMC5331793 DOI: 10.1128/jvi.02177-16] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2016] [Accepted: 12/22/2016] [Indexed: 11/20/2022] Open
Abstract
Simian immunodeficiency viruses (SIVs) use their Nef proteins to counteract the restriction factor tetherin. However, a deletion in human tetherin prevents antagonism by the Nef proteins of SIVcpz and SIVgor, which represent the ape precursors of human immunodeficiency virus type 1 (HIV-1). To promote virus release from infected cells, pandemic HIV-1 group M strains evolved Vpu as a tetherin antagonist, while the Nef protein of less widespread HIV-1 group O strains acquired the ability to target a region adjacent to this deletion. In this study, we identified an unusual HIV-1 group O strain (RBF206) that evolved Vpu as an effective antagonist of human tetherin. While both RBF206 Vpu and Nef exert anti-tetherin activity in transient-transfection assays, mainly Vpu promotes RBF206 release in infected CD4+ T cells. Although mutations distinct from the adaptive changes observed in group M Vpus (M-Vpus) were critical for the acquisition of its anti-tetherin activity, RBF206 O-Vpu potently suppresses NF-κB activation and reduces CD4 cell surface expression. Interestingly, RBF206 Vpu counteracts tetherin in a largely species-independent manner, degrading both the long and short isoforms of human tetherin. Downmodulation of CD4, but not counteraction of tetherin, by RBF206 Vpu was dependent on the cellular ubiquitin ligase machinery. Our data present the first example of an HIV-1 group O Vpu that efficiently antagonizes human tetherin and suggest that counteraction by O-Nefs may be suboptimal.IMPORTANCE Previous studies showed that HIV-1 groups M and O evolved two alternative strategies to counteract the human ortholog of the restriction factor tetherin. While HIV-1 group M switched from Nef to Vpu due to a deletion in the cytoplasmic domain of human tetherin, HIV-1 group O, which lacks Vpu-mediated anti-tetherin activity, acquired a Nef protein that is able to target a region adjacent to the deletion. Here we report an unusual exception, identifying a strain of HIV-1 group O (RBF206) whose Vpu protein evolved an effective antagonism of human tetherin. Interestingly, the adaptive changes in RBF206 Vpu are distinct from those found in M-Vpus and mediate efficient counteraction of both the long and short isoforms of this restriction factor. Our results further illustrate the enormous flexibility of HIV-1 in counteracting human defense mechanisms.
Collapse
Affiliation(s)
- Katharina Mack
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| | - Kathrin Starz
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| | - Daniel Sauter
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| | - Simon Langer
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| | | | - Gerald H Learn
- Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA
| | - Christina M Stürzel
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| | - Marie Leoz
- Laboratoire de Virologie, CHU Charles Nicolle, Rouen, France
- EA 2656 GRAM Université de Rouen, Rouen, France
| | - Jean-Christophe Plantier
- Laboratoire de Virologie, CHU Charles Nicolle, Rouen, France
- EA 2656 GRAM Université de Rouen, Rouen, France
- Laboratoire associé au Centre National de Référence du VIH, CHU Charles Nicolle, Rouen, France
| | - Matthias Geyer
- Department of Structural Immunology, Institute of Innate Immunity, University of Bonn, Bonn, Germany
| | - Beatrice H Hahn
- Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA
- Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
| | - Frank Kirchhoff
- Institute of Molecular Virology, Ulm University Medical Center, Ulm, Germany
| |
Collapse
|