Viral determinants of polarized assembly for the murine leukemia virus

SERINC5 Can Enhance Proinflammatory Cytokine Production by Primary Human Myeloid Cells in Response to Challenge with HIV-1 Particles

HIV-1 has to overcome physical barriers posed by host cell restriction factors (RFs) for efficient replication. Some RFs, including Trim5α and tetherin, trigger antiviral signaling in addition to directly impairing HIV replication. SERINC5 (S5) is an RF that is incorporated into HIV-1 particles to potently impair their infectivity and is efficiently antagonized by the viral pathogenesis factor Nef. Since effects of S5 on HIV-1 infectivity were mostly studied in reporter cell lines, we analyzed the effects of S5 during infection of primary HIV-1 target cells. In activated CD4⁺ T lymphocytes, virion incorporation of S5 only moderately impaired virion infectivity and was not associated with altered innate immune recognition. In contrast, in monocyte-derived macrophages, S5 virion incorporation potentiated the production of proinflammatory cytokines with very potent but donor-dependent effects on virion infectivity. Nef counteracted effects of S5 on both cytokine production and virion infectivity. Similar S5-induced cytokine production was observed in immature monocyte-derived dendritic cells. Notably, S5-mediated enhancement of cytokine production was not linked to the efficacy of productive infection and could be overcome by using vesicular stomatitis virus glycoprotein (VSV-G) but not infectivity restriction-insensitive HIV-1 Env for cell entry. Moreover, inhibiting entry of S5-negative HIV-1 ΔNef particles increased proinflammatory cytokine production comparably to virion incorporation of S5. Together, these results describe the sensitization of noninfectious HIV-1 particles to proinflammatory cytokine production by myeloid target cells as an additional and Nef-sensitive activity of S5. Moreover, the study reveals important cell-type and donor-dependent differences in the sensitivity of HIV target cells for antiviral effects of S5.IMPORTANCE SERINC5 (S5) is a host cell restriction factor (RF) that impairs the infectivity of HIV-1 particles in target cell lines. To assess the potential physiological relevance of this restriction, we assessed the effects of S5 on HIV-1 infection of relevant primary human target cells. We found that effects of S5 on infection of CD4⁺ T lymphocytes were negligible. In myeloid target cells, however, virion incorporation of S5 potently suppressed infectivity and promoted innate immune recognition of HIV-1 particles characterized by proinflammatory cytokine production. Both effects were not observed in cells of all donors analyzed, were exerted independently of one another, and were counteracted by the HIV-1 pathogenesis factor Nef. These results identify the sensitization of HIV-1 particles for innate immune recognition by myeloid target cells as a novel activity of S5 and emphasize the need to study RF function in the context of primary target cells and taking donor variabilities into account.

Antibody-induced internalisation of retroviral envelope glycoproteins is a signal initiation event

As obligate parasites, viruses highjack, modify and repurpose the cellular machinery for their own replication. Viral proteins have, therefore, evolved biological functions, such as signalling potential, that alter host cell physiology in ways that are still incompletely understood. Retroviral envelope glycoproteins interact with several host proteins, extracellularly with their cellular receptor and anti-envelope antibodies, and intracellularly with proteins of the cytoskeleton or sorting, endocytosis and recirculation pathways. Here, we examined the impact of endogenous retroviral envelope glycoprotein expression and interaction with host proteins, particularly antibodies, on the cell, independently of retroviral infection. We found that in the commonly used C57BL/6 substrains of mice, where murine leukaemia virus (MLV) envelope glycoproteins are expressed by several endogenous MLV proviruses, the highest expressed MLV envelope glycoprotein is under the control of an immune-responsive cellular promoter, thus linking MLV envelope glycoprotein expression with immune activation. We further showed that antibody ligation induces extensive internalisation from the plasma membrane into endocytic compartments of MLV envelope glycoproteins, which are not normally subject to constitutive endocytosis. Importantly, antibody binding and internalisation of MLV envelope glycoproteins initiates signalling cascades in envelope-expressing murine lymphocytic cell lines, leading to cellular activation. Similar effects were observed by MLV envelope glycoprotein ligation by its cellular receptor mCAT-1, and by overexpression in human lymphocytic cells, where it required an intact tyrosine-based YXXΦ motif in the envelope glycoprotein cytoplasmic tail. Together, these results suggest that signalling potential is a general property of retroviral envelope glycoproteins and, therefore, a target for intervention.

The outcome of viral infection depends on the balance between host immunity and the ability of the virus to avoid, evade or subvert it. The envelope glycoproteins of diverse viruses, including retroviruses, are displayed on the surface of virions and of infected cells and thus constitute the major target of the host antibody response. Antibody responses are elicited not only against infectious viruses we acquire during our life-history, but also against the numerous retroviral envelopes encoded by our genome and acquired during our species’ life-history. In turn, viruses have evolved ways to reduce exposure of their envelope glycoproteins to the host immune system, including constitutive endocytosis or antibody-induced internalisation. Using murine leukaemia viruses as models of infectious and endogenous retroviruses, we show that antibody binding to retroviral envelopes induces extensive internalisation of the envelope-antibody complex and initiates signalling cascades, ultimately leading to transcriptional activation of envelope glycoprotein-expressing lymphocytes. We further show that expression of endogenous retroviral envelopes is coupled to physiological lymphocyte activation, integrating them with the immune response. These findings reveal an unexpected layer of interaction between the host antibody response and retroviral envelope glycoproteins, which could be considered immune receptors.

Murine Leukemia Virus Exploits Innate Sensing by Toll-Like Receptor 7 in B-1 Cells To Establish Infection and Locally Spread in Mice

Lymph-borne Friend murine leukemia virus (FrMLV) exploits the sentinel macrophages in the draining popliteal lymph node (pLN) to infect highly permissive innate-like B-1 cells and establish infection in mice. The reason for FrMLV sensitivity of B-1 cells and their impact on viral spread is unknown. Here we demonstrate that Toll-like receptor 7 (TLR7) sensing and type I interferon (IFN-I) signaling in B-1 cells contribute to FrMLV susceptibility. FrMLV infection in B-1 cell-deficient mice (bumble; IκBNS dysfunctional) was significantly lower than that in the wild-type mice and was rescued by adoptive transfer of wild-type B-1 cells. This rescue of FrMLV infection in bumble mice was dependent on intact TLR7 sensing and IFN-I signaling within B-1 cells. Analyses of infected cell types revealed that the reduced infection in bumble mice was due predominantly to compromised virus spread to the B-2 cell population. Our data reveal how FrMLV exploits innate immune sensing and activation in the B-1 cell population for infection and subsequent spread to other lymphocytes.IMPORTANCE Viruses establish infection in hosts by targeting highly permissive cell types. The retrovirus Friend murine leukemia virus (FrMLV) infects a subtype of B cells called B-1 cells that permit robust virus replication. The reason for their susceptibility had remained unknown. We found that innate sensing of incoming virus and the ensuing type I interferon response within B-1 cells are responsible for their observed susceptibility. Our data provide insights into how retroviruses coevolved with the host to co-opt innate immune sensing pathways designed to fight virus infections for establishing infection. Understanding early events in viral spread can inform antiviral intervention strategies that prevent the colonization of a host.

Meeting Review: 2018 International Workshop on Structure and Function of the Lentiviral gp41 Cytoplasmic Tail

Recent developments in defining the role of the lentiviral envelope glycoprotein (Env) cytoplasmic tail (CT) in Env trafficking and incorporation into virus particles have advanced our understanding of viral replication and transmission. To stimulate additional progress in this field, the two-day International Workshop on Structure and Function of the Lentiviral gp41 Cytoplasmic Tail, co-organized by Eric Freed and James Hoxie, was held at the National Cancer Institute in Frederick, MD (26⁻27 April 2018). The meeting served to bring together experts focused on the role of gp41 in HIV replication and to discuss the emerging mechanisms of CT-dependent trafficking, Env conformation and structure, host protein interaction, incorporation, and viral transmission. The conference was organized around the following three main hot topics in gp41 research: the role of host factors in CT-dependent Env incorporation, Env structure, and CT-mediated trafficking and transmission. This review highlights important topics and the advances in gp41 research that were discussed during the conference.

HIV-1 Nucleocapsid Mimics the Membrane Adaptor Syntenin PDZ to Gain Access to ESCRTs and Promote Virus Budding

HIV-1 recruits cellular endosomal sorting complexes required for transport (ESCRTs) to bud virions from the membrane. Disruption of the viral nucleocapsid (NC) domain integrity affects HIV-1 budding. However, the molecular mechanisms of NC's involvement in HIV budding remain unclear. We find that NC mimics the PDZ domains of syntenin, a membrane-binding adaptor involved in cell-to-cell contact/communication, to capture the Bro1 domain of ALIX, which is an ESCRTs recruiting cellular adaptor. NC binds membranes via basic residues in either the distal or proximal zinc fingers, and NC-membrane binding is essential for Bro1 capture and HIV-1 budding. Removal of RNA enhances NC membrane binding, suggesting a dynamic competition between membrane lipids and RNA for the same binding sites in NC. Remarkably, syntenin PDZ can substitute for NC function in HIV-1 budding. Thus, NC mimics syntenin PDZs to function as a membrane-binding adaptor critical for HIV-1 budding at specific microdomains of the membrane.

HIV-1 Gag, Envelope, and Extracellular Determinants Cooperate To Regulate the Stability and Turnover of Virological Synapses

Retroviruses spread more efficiently when infected and uninfected cells form tight, physical interfaces known as virological synapses (VSs). VS formation is initiated by adhesive interactions between viral Envelope (Env) glycoproteins on the infected cell and CD4 receptor molecules on the uninfected cell. How high-avidity Env-CD4 linkages are resolved over time is unknown. We describe here a tractable two-color, long-term (>24 h) live cell imaging strategy to study VS turnover in the context of a large cell population, quantitatively. We show that Env's conserved cytoplasmic tail (CT) can potently signal the recruitment of Gag capsid proteins to the VS, a process also dependent on residues within Gag's N-terminal matrix (MA) domain. Additionally, we demonstrate that Env's CT and Gag's MA domain both regulate the duration of interactions between viral donor and target cells, as well as the stability of this interaction over time (i.e., its capacity to resolve or form a syncytium). Finally, we report the unexpected finding that modulating extracellular fluid viscosity markedly impacts target T cell trafficking and thus affects the duration, stability, and turnover of virus-induced cell-cell contacts. Combined, these results suggest a stepwise model for viral cell-to-cell transmission wherein (i) Env-receptor interactions anchor target cells to infected cells, (ii) Env signals Gag's recruitment to the cell-cell contact dependent on an intact Env CT and Gag MA, and (iii) Env CT and Gag MA, in conjunction with extracellular forces, combine to regulate VS stability and infectious outcomes.

IMPORTANCE

HIV-1 spreads efficiently at physical, cell-cell interfaces known as virological synapses (VSs). The VS provides for spatiotemporal coupling of virus assembly and entry into new host cells and may transmit signals relevant to pathogenesis. Disrupting this mode of transmission may be critical to the goal of abolishing viral persistence in infected individuals. We describe here a long-term live cell imaging strategy for studying virus-induced effects on cell behavior in the context of a large cell population. We demonstrate cooperative roles for viral Gag capsid proteins and Envelope glycoproteins in regulating VS formation and turnover. We also show that modulating fluid viscosity markedly affects T cell trafficking and VS stability. Thus, extracellular factors also play an important role in modulating the nature of infectious cell-cell interactions. In sum, our study provides new tools and insights relevant to exposing vulnerabilities in how HIV-1 and other viruses spread infection among cells, tissues, and people.

Retroviruses use CD169-mediated trans-infection of permissive lymphocytes to establish infection

Dendritic cells can capture and transfer retroviruses in vitro across synaptic cell-cell contacts to uninfected cells, a process called trans-infection. Whether trans-infection contributes to retroviral spread in vivo remains unknown. Here, we visualize how retroviruses disseminate in secondary lymphoid tissues of living mice. We demonstrate that murine leukemia virus (MLV) and human immunodeficiency virus (HIV) are first captured by sinus-lining macrophages. CD169/Siglec-1, an I-type lectin that recognizes gangliosides, captures the virus. MLV-laden macrophages then form long-lived synaptic contacts to trans-infect B-1 cells. Infected B-1 cells subsequently migrate into the lymph node to spread the infection through virological synapses. Robust infection in lymph nodes and spleen requires CD169, suggesting that a combination of fluid-based movement followed by CD169-dependent trans-infection can contribute to viral spread.

Mouse Siglec-1 Mediates trans-Infection of Surface-bound Murine Leukemia Virus in a Sialic Acid N-Acyl Side Chain-dependent Manner

Siglec-1 (sialoadhesin, CD169) is a surface receptor on human cells that mediates trans-enhancement of HIV-1 infection through recognition of sialic acid moieties in virus membrane gangliosides. Here, we demonstrate that mouse Siglec-1, expressed on the surface of primary macrophages in an interferon-α-responsive manner, captures murine leukemia virus (MLV) particles and mediates their transfer to proliferating lymphocytes. The MLV infection of primary B-cells was markedly more efficient than that of primary T-cells. The major structural protein of MLV particles, Gag, frequently co-localized with Siglec-1, and trans-infection, primarily of surface-bound MLV particles, efficiently occurred. To explore the role of sialic acid for MLV trans-infection at a submolecular level, we analyzed the potential of six sialic acid precursor analogs to modulate the sialylated ganglioside-dependent interaction of MLV particles with Siglec-1. Biosynthetically engineered sialic acids were detected in both the glycolipid and glycoprotein fractions of MLV producer cells. MLV released from cells carrying N-acyl-modified sialic acids displayed strikingly different capacities for Siglec-1-mediated capture and trans-infection; N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl side chain modifications resulted in up to 92 and 80% reduction of virus particle capture and trans-infection, respectively, whereas N-propanoyl or N-cyclopropylcarbamyl side chains had no effect. In agreement with these functional analyses, molecular modeling indicated reduced binding affinities for non-functional N-acyl modifications. Thus, Siglec-1 is a key receptor for macrophage/lymphocyte trans-infection of surface-bound virions, and the N-acyl side chain of sialic acid is a critical determinant for the Siglec-1/MLV interaction.

Murine leukemia virus Gag localizes to the uropod of migrating primary lymphocytes

B and CD4(+) T lymphocytes are natural targets of murine leukemia virus (MLV). Migrating lymphocytes adopt a polarized morphology with a trailing edge designated the uropod. Here, we demonstrate that MLV Gag localizes to the uropod in polarized B cells and CD4(+) T cells. The uropod localization of MLV Gag was dependent on plasma membrane (PM) association and multimerization of Gag but independent of the viral glycoprotein Env. Basic residues in MA that are required for MLV Gag recruitment to virological synapses between HEK293 and XC cells were dispensable for uropod localization in migrating B cells. Ultrastructural studies indicated that both wild-type and basic-residue mutant Gag localized to the outer surface of the PM at the uropod. Late-domain mutant virus particles were seen at the uropod in form of budding-arrested intermediates. Finally, uropods mediated contact between MLV-infected B cells and uninfected T cells to form virological synapses. Our results suggest that MLV, not unlike HIV, accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

IMPORTANCE

Viruses have evolved mechanisms to coordinate their assembly and budding with cell polarity to facilitate their spreading. In this study, we demonstrated that the viral determinants for MLV Gag to localize to the uropod in polarized B cells are distinct from the requirements to localize to virological synapses in transformed cell lines. Basic residues in MA that are required for the Gag localization to virological synapses between HEK293 and XC cells are dispensable for Gag localization to the uropod in primary B cells. Rather, plasma membrane association and capsid-driven multimerization of Gag are sufficient to drive MLV Gag to the uropod. MLV-laden uropods also mediate contacts between MLV-infected B cells and uninfected T cells to form virological synapses. Our results indicate that MLV accumulates at the uropod of primary lymphocytes to facilitate viral spreading through the formation of uropod-mediated cell-cell contacts.

HIV-1 Nef promotes the localization of Gag to the cell membrane and facilitates viral cell-to-cell transfer

Background

Newly synthesized HIV-1 particles assemble at the plasma membrane of infected cells, before being released as free virions or being transferred through direct cell-to-cell contacts to neighboring cells. Localization of HIV-1 Gag precursor at the cell membrane is necessary and sufficient to trigger viral assembly, whereas the GagPol precursor is additionally required to generate a fully matured virion. HIV-1 Nef is an accessory protein that optimizes viral replication through partly defined mechanisms. Whether Nef modulates Gag and/or GagPol localization and assembly at the membrane and facilitates viral cell-to-cell transfer has not been extensively characterized so far.

Results

We report that Nef increases the total amount of Gag proteins present in infected cells, and promotes Gag localization at the cell membrane. Moreover, the processing of p55 into p24 is improved in the presence of Nef. We also examined the effect of Nef during HIV-1 cell-to-cell transfer. We show that without Nef, viral transfer through direct contacts between infected cells and target cells is impaired. With a nef-deleted virus, the number of HIV-1 positive target cells after a short 2h co-culture is reduced, and viral material transferred to uninfected cells is less matured. At later time points, this defect is associated with a reduction in the productive infection of new target cells.

Conclusions

Our results highlight a previously unappreciated role of Nef during the viral replication cycle. Nef promotes HIV-1 Gag membrane localization and processing, and facilitates viral cell-to-cell transfer.

In vivo imaging of virological synapses

Retroviruses such as the human immunodeficiency virus, human T-cell lymphotropic virus and murine leukaemia virus are believed to spread via sites of cell-cell contact designated virological synapses. Support for this model is based on in vitro evidence in which infected cells are observed to specifically establish long-lived cell-cell contact with uninfected cells. Whether virological synapses exist in vivo is unknown. Here we apply intravital microscopy to identify a subpopulation of B cells infected with the Friend murine leukaemia virus that form virological synapses with uninfected leucocytes in the lymph node of living mice. In vivo virological synapses are, like their in vitro counterpart, dependent on the expression of the viral envelope glycoprotein and are characterized by a prolonged polarization of viral capsid to the cell-cell interface. Our results validate the concept of virological synapses and introduce intravital imaging as a tool to visualize retroviral spreading directly in living mice.

Basic residues in the matrix domain and multimerization target murine leukemia virus Gag to the virological synapse

Murine leukemia virus (MLV) can efficiently spread in tissue cultures by polarizing assembly to virological synapses. The viral envelope glycoprotein (Env) establishes cell-cell contacts and subsequently recruits Gag by a process that depends on its cytoplasmic tail. MLV Gag is recruited to virological synapses through the matrix domain (MA) (J. Jin, F. Li, and W. Mothes, J. Virol. 85:7672-7682, 2011). However, how MA targets Gag to sites of cell-cell contact remains unknown. Here we report that basic residues within MA are critical for directing MLV Gag to virological synapses. Alternative membrane targeting domains (MTDs) containing multiple basic residues can efficiently substitute MA to direct polarized assembly. Similarly, mutations in the polybasic cluster of MA that disrupt Gag polarization can be rescued by N-terminal addition of MTDs containing basic residues. MTDs containing basic residues alone fail to be targeted to the virological synapse. Systematic deletion experiments reveal that domains within Gag known to mediate Gag multimerization are also required. Thus, our data predict the existence of a specific "acidic" interface at virological synapses that mediates the recruitment of MLV Gag via the basic cluster of MA and Gag multimerization.

Cell-to-cell transmission can overcome multiple donor and target cell barriers imposed on cell-free HIV

Virus transmission can occur either by a cell-free mode through the extracellular space or by cell-to-cell transmission involving direct cell-to-cell contact. The factors that determine whether a virus spreads by either pathway are poorly understood. Here, we assessed the relative contribution of cell-free and cell-to-cell transmission to the spreading of the human immunodeficiency virus (HIV). We demonstrate that HIV can spread by a cell-free pathway if all the steps of the viral replication cycle are efficiently supported in highly permissive cells. However, when the cell-free path was systematically hindered at various steps, HIV transmission became contact-dependent. Cell-to-cell transmission overcame barriers introduced in the donor cell at the level of gene expression and surface retention by the restriction factor tetherin. Moreover, neutralizing antibodies that efficiently inhibit cell-free HIV were less effective against cell-to-cell transmitted virus. HIV cell-to-cell transmission also efficiently infected target T cells that were relatively poorly susceptible to cell-free HIV. Importantly, we demonstrate that the donor and target cell types influence critically the extent by which cell-to-cell transmission can overcome each barrier. Mechanistically, cell-to-cell transmission promoted HIV spread to more cells and infected target cells with a higher proviral content than observed for cell-free virus. Our data demonstrate that the frequently observed contact-dependent spread of HIV is the result of specific features in donor and target cell types, thus offering an explanation for conflicting reports on the extent of cell-to-cell transmission of HIV.

Cell-to-cell transmission of viruses

The life cycle of most viruses involves the release of particles into the extracellular space. Consequently, the study of virus egress as well as virus entry has focused almost exclusively on the biology of cell-free virus. However, cell-free virus spread is often very inefficient. Specific barriers, either located in the donor cell or in the target cell, prevent efficient spread by the cell-free mode. In contrast, viral spread by direct cell-cell contact is largely unaffected by most of these barriers resulting in preferential spread by cell-to-cell transmission. Virus cell-to-cell transmission allows an efficient coordination of several steps of the viral life cycle. It often involves complex inter-cellular adhesion, cellular polarity and intra-cellular trafficking. Because virus cell-to-cell transmission can involve transmission through zones of tight cell-cell contact that are resistant to neutralizing antibodies and reach a high local particle concentration, cell-to-cell transmission can contribute to the pathogenesis of viral infections.

Multiple Gag domains contribute to selective recruitment of murine leukemia virus (MLV) Env to MLV virions

Retroviruses, like all enveloped viruses, must incorporate viral glycoproteins to form infectious particles. Interactions between the glycoprotein cytoplasmic tail and the matrix domain of Gag are thought to direct recruitment of glycoproteins to native virions for many retroviruses. However, retroviruses can also incorporate glycoproteins from other viruses to form infectious virions known as pseudotyped particles. The glycoprotein murine leukemia virus (MLV) Env can readily form pseudotyped particles with many retroviruses, suggesting a generic mechanism for recruitment. Here, we sought to identify which components of Gag, particularly the matrix domain, contribute to recruitment of MLV Env into retroviral particles. Unexpectedly, we discovered that the matrix domain of HIV-1 Gag is dispensable for generic recruitment, since it could be replaced with a nonviral membrane-binding domain without blocking active incorporation of MLV Env into HIV virions. However, MLV Env preferentially assembles with MLV virions. When MLV and HIV particles are produced from the same cell, MLV Env is packaged almost exclusively by MLV particles, thus preventing incorporation into HIV particles. Surprisingly, the matrix domain of MLV Gag is not required for this selectivity, since MLV Gag containing the matrix domain from HIV is still able to outcompete HIV particles for MLV Env. Although MLV Gag is sufficient for selective incorporation to occur, no single Gag domain dictates the selectivity. Our findings indicate that Env recruitment is more complex than previously believed and that Gag assembly/budding sites have fundamental properties that affect glycoprotein incorporation.

Retroviral env glycoprotein trafficking and incorporation into virions

Together with the Gag protein, the Env glycoprotein is a major retroviral structural protein and is essential for forming infectious virus particles. Env is synthesized, processed, and transported to certain microdomains at the plasma membrane and takes advantage of the same host machinery for its trafficking as that used by cellular glycoproteins. Incorporation of Env into progeny virions is probably mediated by the interaction between Env and Gag, in some cases with the additional involvement of certain host factors. Although several general models have been proposed to explain the incorporation of retroviral Env glycoproteins into virions, the actual mechanism for this process is still unclear, partly because structural data on the Env protein cytoplasmic tail is lacking. This paper presents the current understanding of the synthesis, trafficking, and virion incorporation of retroviral Env proteins.

Nucleic Acid, Antibody, and Virus Culture Methods to Detect Xenotropic MLV-Related Virus in Human Blood Samples

The MLV-related retrovirus, XMRV, was recently identified and reported to be associated with both prostate cancer and chronic fatigue syndrome. At the National Cancer Institute-Frederick, MD (NCI-Frederick), we developed highly sensitive methods to detect XMRV nucleic acids, antibodies, and replication competent virus. Analysis of XMRV-spiked samples and/or specimens from two pigtail macaques experimentally inoculated with 22Rv1 cell-derived XMRV confirmed the ability of the assays used to detect XMRV RNA and DNA, and culture isolatable virus when present, along with XMRV reactive antibody responses. Using these assays, we did not detect evidence of XMRV in blood samples (N = 134) or prostate specimens (N = 19) from two independent cohorts of patients with prostate cancer. Previous studies detected XMRV in prostate tissues. In the present study, we primarily investigated the levels of XMRV in blood plasma samples collected from patients with prostate cancer. These results demonstrate that while XMRV-related assays developed at the NCI-Frederick can readily measure XMRV nucleic acids, antibodies, and replication competent virus, no evidence of XMRV was found in the blood of patients with prostate cancer.

The direct passage of animal viruses between cells

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.