HIV-induced syncytia of a T cell line form single giant pseudopods and are motile

Virus-Induced Cell Fusion and Syncytia Formation

Most enveloped viruses encode viral fusion proteins to penetrate host cell by membrane fusion. Interestingly, many enveloped viruses can also use viral fusion proteins to induce cell-cell fusion, both in vitro and in vivo, leading to the formation of syncytia or multinucleated giant cells (MGCs). In addition, some non-enveloped viruses encode specialized viral proteins that induce cell-cell fusion to facilitate viral spread. Overall, viruses that can induce cell-cell fusion are nearly ubiquitous in mammals. Virus cell-to-cell spread by inducing cell-cell fusion may overcome entry and post-entry blocks in target cells and allow evasion of neutralizing antibodies. However, molecular mechanisms of virus-induced cell-cell fusion remain largely unknown. Here, I summarize the current understanding of virus-induced cell fusion and syncytia formation.

Mathematical Modeling of Virus-Mediated Syncytia Formation: Past Successes and Future Directions

Many viruses have the ability to cause cells to fuse into large multi-nucleated cells, known as syncytia. While the existence of syncytia has long been known and its importance in helping spread viral infection within a host has been understood, few mathematical models have incorporated syncytia formation or examined its role in viral dynamics. This review examines mathematical models that have incorporated virus-mediated cell fusion and the insights they have provided on how syncytia can change the time course of an infection. While the modeling efforts are limited, they show promise in helping us understand the consequences of syncytia formation if future modeling efforts can be coupled with appropriate experimental efforts to help validate the models.

Human immunodeficiency virus transmission-Mechanisms underlying the cell-to-cell spread of human immunodeficiency virus

Despite the success of combined antiretroviral therapy in controlling viral load and reducing the risk of human immunodeficiency virus (HIV) transmission, an estimated 1.5 million new infections occurred worldwide in 2021. These new infections are mainly the result of sexual intercourse and thus involve cells present on the genital mucosa, such as dendritic cells (DCs), macrophages (Mø) and CD4+ T lymphocytes. Understanding the mechanisms by which HIV interacts with these cells and how HIV exploits these interactions to establish infection in a new human host is critical to the development of strategies to prevent and control HIV transmission. In this review, we explore how HIV has evolved to manipulate some of the physiological roles of these cells, thereby gaining access to strategic cellular niches that are critical for the spread and pathogenesis of HIV infection. The interaction of HIV with DCs, Mø and CD4+ T lymphocytes, and the role of the intercellular transfer of viral particles through the establishment of the infectious or virological synapses, but also through membrane protrusions such as filopodia and tunnelling nanotubes (TNTs), and cell fusion or cell engulfment processes are presented and discussed.

Galectin-3 facilitates cell-to-cell HIV-1 transmission by altering the composition of membrane lipid rafts in CD4 T cells

Galectin-3 (GAL3) is a β-galactoside-binding lectin expressed in CD4 T cells infected with human immunodeficiency virus-1 (HIV-1). GAL3 promotes HIV-1 budding by associating with ALIX and Gag p6. GAL3 has been shown to localize in membrane lipid rafts in dendritic cells and positively regulate cell migration. HIV-1 spreads between T cells by forming supramolecular structures (virological synapses [VSs]), whose integrity depends on lipid rafts. Here, we addressed the potential role of GAL3 in cell-to-cell transmission of HIV-1 in CD4 T cells. GAL3 expressed in donor cells was more important for facilitating HIV-1 cell-to-cell transfer than GAL3 expressed in target cells. GAL3 was found to be co-transferred with Gag from HIV-1-positive donor to HIV-1-negative target T cells. HIV-1 infection induced translocation of GAL3 together with Gag to the cell-cell interfaces and colocalize with GM1, where GAL3 facilitated VS formation. GAL3 regulated the coordinated transfer of Gag and flotillin-1 into plasma membrane fractions. Finally, depletion of GAL3 reduced the cholesterol levels in membrane lipid rafts in CD4 T cells. These findings provide evidence that endogenous GAL3 stimulates lipid raft components and facilitates intercellular HIV-1 transfer among CD4 T cells, offering another pathway by which GAL3 regulates HIV-1 infection. These findings may inform the treatment of HIV-1 infection based on targeting GAL3 to modulate lipid rafts.

Agent-based computational modeling of glioblastoma predicts that stromal density is central to oncolytic virus efficacy

Oncolytic viruses (OVs) are emerging cancer immunotherapy. Despite notable successes in the treatment of some tumors, OV therapy for central nervous system cancers has failed to show efficacy. We used an ex vivo tumor model developed from human glioblastoma tissue to evaluate the infiltration of herpes simplex OV rQNestin (oHSV-1) into glioblastoma tumors. We next leveraged our data to develop a computational, model of glioblastoma dynamics that accounts for cellular interactions within the tumor. Using our computational model, we found that low stromal density was highly predictive of oHSV-1 therapeutic success, suggesting that the efficacy of oHSV-1 in glioblastoma may be determined by stromal-to-tumor cell regional density. We validated these findings in heterogenous patient samples from brain metastatic adenocarcinoma. Our integrated modeling strategy can be applied to suggest mechanisms of therapeutic responses for central nervous system cancers and to facilitate the successful translation of OVs into the clinic.

The Influence of Anti-Hiv-1 Specific IgY In Inhibiting HIV-1 Infection in Binding Phase with Syncytium Examination of CD4 Receptor Density Using the Flowcytometry Method

HIV/ AIDS infections have increased and spread very quickly in the world, including in Indonesia. The absence of an effective vaccine and the fact that antiretroviral drugs can only suppress the progression of infection but cannot eradicate it lead to the efforts to find materials containing immunoglobulins that can replace the immune system which greatly declines in HIV/ AIDS patients. The successful use of specific IgY in other studies opens up opportunities for the use of anti-HIV-1 specific IgY as passive immunotherapy. This type of research is true experimental research design with post-test only control group design. IgY was obtained from Lohmann Laying hens chicken eggs immunized with the inactivated HIV-1 virus. The concentration of IgY was determined using the Bradford method and then the characterization test was continued using the AGPT, ELISA, SDS-PAGE and Western blot tests which showed anti-HIV-1 specific IgY. The results of the test showed specific anti-HIV-1 IgY was effective in inhibiting the formation of syncytium in HIV-1 infection against CD4+ T lymphocytes in the binding phase (entry stage) in the treatment group p-value 0.000 (p <0.05). The results of CD4 receptor density tests using the Flowcytometry method showed that specific anti-HIV-1 IgY was effective in inhibiting HIV-1 infection against CD4+ T lymphocytes in the binding phase (entry stage) in the treatment group p-value 0.047 (p <0.05).HIV/ AIDS infections have increased and spread very quickly in the world, including in Indonesia. The absence of an effective vaccine and the fact that antiretroviral drugs can only suppress the progression of infection but cannot eradicate it lead to the efforts to find materials containing immunoglobulins that can replace the immune system which greatly declines in HIV/ AIDS patients. The successful use of specific IgY in other studies opens up opportunities for the use of anti-HIV-1 specific IgY as passive immunotherapy. This type of research is true experimental research design with post-test only control group design. IgY was obtained from Lohmann Laying hens chicken eggs immunized with the inactivated HIV-1 virus. The concentration of IgY was determined using the Bradford method and then the characterization test was continued using the AGPT, ELISA, SDS-PAGE and Western blot tests which showed anti-HIV-1 specific IgY. The results of the test showed specific anti-HIV-1 IgY was effective in inhibiting the formation of syncytium in HIV-1 infection against CD4+ T lymphocytes in the binding phase (entry stage) in the treatment group p-value 0.000 (p <0.05). The results of CD4 receptor density tests using the Flowcytometry method showed that specific anti-HIV-1 IgY was effective in inhibiting HIV-1 infection against CD4+ T lymphocytes in the binding phase (entry stage) in the treatment group p-value 0.047 (p <0.05).

Ebola Virus VP40 Modulates Cell Cycle and Biogenesis of Extracellular Vesicles

Background

Ebola virus (EBOV) mainly targets myeloid cells; however, extensive death of T cells is often observed in lethal infections. We have previously shown that EBOV VP40 in exosomes causes recipient immune cell death.

Methods

Using VP40-producing clones, we analyzed donor cell cycle, extracellular vesicle (EV) biogenesis, and recipient immune cell death. Transcription of cyclin D1 and nuclear localization of VP40 were examined via kinase and chromatin immunoprecipitation assays. Extracellular vesicle contents were characterized by mass spectrometry, cytokine array, and western blot. Biosafety level-4 facilities were used for wild-type Ebola virus infection studies.

Results

VP40 EVs induced apoptosis in recipient T cells and monocytes. VP40 clones were accelerated in growth due to cyclin D1 upregulation, and nuclear VP40 was found bound to the cyclin D1 promoter. Accelerated cell cycling was related to EV biogenesis, resulting in fewer but larger EVs. VP40 EV contents were enriched in ribonucleic acid-binding proteins and cytokines (interleukin-15, transforming growth factor-β1, and interferon-γ). Finally, EBOV-infected cell and animal EVs contained VP40, nucleoprotein, and glycoprotein.

Conclusions

Nuclear VP40 upregulates cyclin D1 levels, resulting in dysregulated cell cycle and EV biogenesis. Packaging of cytokines and EBOV proteins into EVs from infected cells may be responsible for the decimation of immune cells during EBOV pathogenesis.

Mechanisms for Cell-to-Cell Transmission of HIV-1

While HIV-1 infection of target cells with cell-free viral particles has been largely documented, intercellular transmission through direct cell-to-cell contact may be a predominant mode of propagation in host. To spread, HIV-1 infects cells of the immune system and takes advantage of their specific particularities and functions. Subversion of intercellular communication allows to improve HIV-1 replication through a multiplicity of intercellular structures and membrane protrusions, like tunneling nanotubes, filopodia, or lamellipodia-like structures involved in the formation of the virological synapse. Other features of immune cells, like the immunological synapse or the phagocytosis of infected cells are hijacked by HIV-1 and used as gateways to infect target cells. Finally, HIV-1 reuses its fusogenic capacity to provoke fusion between infected donor cells and target cells, and to form infected syncytia with high capacity of viral production and improved capacities of motility or survival. All these modes of cell-to-cell transfer are now considered as viral mechanisms to escape immune system and antiretroviral therapies, and could be involved in the establishment of persistent virus reservoirs in different host tissues.

Tailored environments to study motile cells and pathogens

Motile cells and pathogens migrate in complex environments and yet are mostly studied on simple 2D substrates. In order to mimic the diverse environments of motile cells, a set of assays including substrates of defined elasticity, microfluidics, micropatterns, organotypic cultures, and 3D gels have been developed. We briefly introduce these and then focus on the use of micropatterned pillar arrays, which help to bridge the gap between 2D and 3D. These structures are made from polydimethylsiloxane, a moldable plastic, and their use has revealed new insights into mechanoperception in Caenorhabditis elegans, gliding motility of Plasmodium, swimming of trypanosomes, and nuclear stability in cancer cells. These studies contributed to our understanding of how the environment influences the respective cell and inform on how the cells adapt to their natural surroundings on a cellular and molecular level.

HIV-1-Induced Small T Cell Syncytia Can Transfer Virus Particles to Target Cells through Transient Contacts

HIV-1 Env mediates fusion of viral and target cell membranes, but it can also mediate fusion of infected (producer) and target cells, thus triggering the formation of multinucleated cells, so-called syncytia. Large, round, immobile syncytia are readily observable in cultures of HIV-1-infected T cells, but these fast growing “fusion sinks” are largely regarded as cell culture artifacts. In contrast, small HIV-1-induced syncytia were seen in the paracortex of peripheral lymph nodes and other secondary lymphoid tissue of HIV-1-positive individuals. Further, recent intravital imaging of lymph nodes in humanized mice early after their infection with HIV-1 demonstrated that a significant fraction of infected cells were highly mobile, small syncytia, suggesting that these entities contribute to virus dissemination. Here, we report that the formation of small, migratory syncytia, for which we provide further quantification in humanized mice, can be recapitulated in vitro if HIV-1-infected T cells are placed into 3D extracellular matrix (ECM) hydrogels rather than being kept in traditional suspension culture systems. Intriguingly, live-cell imaging in hydrogels revealed that these syncytia, similar to individual infected cells, can transiently interact with uninfected cells, leading to rapid virus transfer without cell-cell fusion. Infected cells were also observed to deposit large amounts of viral particles into the extracellular space. Altogether, these observations suggest the need to further evaluate the biological significance of small, T cell-based syncytia and to consider the possibility that these entities do indeed contribute to virus spread and pathogenesis.

Proteomics approach to understand reduced clearance of mycobacteria and high viral titers during HIV-mycobacteria co-infection

Environmental mycobacteria, highly prevalent in natural and artificial (including chlorinated municipal water) niches, are emerging as new threat to human health, especially to HIV-infected population. These seemingly harmless non-pathogenic mycobacteria, which are otherwise cleared, establish as opportunistic infections adding to HIV-associated complications. Although immune-evading strategies of pathogenic mycobacteria are known, the mechanisms underlying the early events by which opportunistic mycobacteria establish infection in macrophages and influencing HIV infection are unclear. Proteomics of phagosome-enriched fractions from Mycobacterium bovis Bacillus Calmette-Guérin (BCG) mono-infected and HIV-M. bovis BCG co-infected THP-1 cells by LC-MALDI-MS/MS revealed differential distribution of 260 proteins. Validation of the proteomics data showed that HIV co-infection helped the survival of non-pathogenic mycobacteria by obstructing phagosome maturation, promoting lipid biogenesis and increasing intracellular ATP equivalents. In turn, mycobacterial co-infection up-regulated purinergic receptors in macrophages that are known to support HIV entry, explaining increased viral titers during co-infection. The mutualism was reconfirmed using clinically relevant opportunistic mycobacteria, Mycobacterium avium, Mycobacterium kansasii and Mycobacterium phlei that exhibited increased survival during co-infection, together with increase in HIV titers. Additionally, the catalogued proteins in the study provide new leads that will significantly add to the understanding of the biology of opportunistic mycobacteria and HIV coalition.

Impact of viral factors on subcellular distribution and RNA export activity of HIV-1 rev in astrocytes 1321N1

CNS associated cells are permissive to HIV-1 infection, but poor in virus production due to attenuated Rev activity. The temporal and the spatial distribution of Rev in human astrocyte 1321N1 and glioblastoma GO-G-CCM were monitored for explaining the reduced Rev activity and low viral production during HIV-1 infection. Rev remained localized to the nuclei of these cells upon infection, attenuating its export activity, as manifested by low copy number of RRE-containing viral-mRNA in the cytoplasm of these cells. In contrast to infection, when Rev alone was transiently expressed, it localized in the cytoplasm of 1321N1. The localization changed to the nucleus when Rev was expressed in the presence of other viral proteins through pro-viral DNA pNL4-3. This study, for the first time, revealed the impact of other HIV-1 proteins apart from host factors in regulating the subcellular localization of Rev in astrocytes and hence the fate of HIV-1 infection in these cells.

CCR6 functions as a new coreceptor for limited primary human and simian immunodeficiency viruses

More than 12 chemokine receptors (CKRs) have been identified as coreceptors for the entry of human immunodeficiency virus type 1 (HIV-1), type 2 (HIV-2), and simian immunodeficiency viruses (SIVs) into target cells. The expression of CC chemokine receptor 6 (CCR6) on Th17 cells and regulatory T cells make the host cells vulnerable to HIV/SIV infection preferentially. However, only limited information is available concerning the specific role of CCR6 in HIV/SIV infection. We examined CCR6 as a coreceptor candidate in this study using NP-2 cell line-based in-vitro studies. Normally, CD4-transduced cell line, NP-2/CD4, is strictly resistant to all HIV/SIV infection. When CCR6 was transduced there, the resultant NP-2/CD4/CCR6 cells became susceptible to HIV-1HAN2, HIV-2MIR and SIVsmE660, indicating coreceptor roles of CCR6. Viral antigens in infected cells were detected by IFA and confirmed by detection of proviral DNA. Infection-induced syncytia in NP-2/CD4/CCR6 cells were detected by Giemsa staining. Amount of virus release through CCR6 has been detected by RT assay in spent culture medium. Sequence analysis of proviral DNA showed two common amino acid substitutions in the C2 envelope region of HIV-2MIR clones propagated through NP-2/CD4/CCR6 cells. Conversely, CCR6-origin SIVsmE660 clones resulted two amino acid changes in the V1 region and one change in the C2 region. The substitutions in the C2 region for HIV-2MIR and the V1 region of SIVsmE660 may confer selection advantage for CCR6-use. Together, the results describe CCR6 as an independent coreceptor for HIV and SIV in strain-specific manner. The alteration of CCR6 uses by viruses may influence the susceptibility of CD4+ CCR6+ T-cells and dendritic cell subsets in vivo and therefore, is important for viral pathogenesis in establishing latent infections, trafficking, and transmission. However, clinical relevance of CCR6 as coreceptor in HIV/SIV infections should be investigated further.

Analysis of multiple cell reservoirs expressing unspliced HIV-1 gag-pol mRNA in patients on antiretroviral therapy

AIMS: Longitudinal percentage change of eight HIV-1 gag-pol mRNA cellular reservoirs from HIV-infected subjects on antiretroviral therapy was ascertained by simultaneous ultrasensitive subpopulation staining/hybridization in situ (SUSHI). MATERIALS #ENTITYSTARTX00026; METHODS: Serial peripheral blood mononuclear cells were taken from three subjects with treatment success, limited response and viral breakthrough plasma viral load (PVL) profiles. SUSHI was carried out on monocytes, macrophages, CD4(+) cells and naive, memory and activated T-cell reservoirs followed with broad light scatter flow cytometry. RESULTS: All gag-pol(+) reservoirs declined in the treatment success patient and similar to PVL. Only some gag-pol(+) reservoirs responded similarly to PVL for the limited treatment patient, and most gag-pol(+) reservoirs increased 16 weeks prior to PVL breakthrough in the viral breakthrough patient. CONCLUSION: SUSHI measures changes in a wide range of gag-pol(+) reservoirs in response to antiretroviral therapy.

Fusion-pore expansion during syncytium formation is restricted by an actin network

Cell-cell fusion in animal development and in pathophysiology involves expansion of nascent fusion pores formed by protein fusogens to yield an open lumen of cell-size diameter. Here we explored the enlargement of micron-scale pores in syncytium formation, which was initiated by a well-characterized fusogen baculovirus gp64. Radial expansion of a single or, more often, of multiple fusion pores proceeds without loss of membrane material in the tight contact zone. Pore growth requires cell metabolism and is accompanied by a local disassembly of the actin cortex under the pores. Effects of actin-modifying agents indicate that the actin cortex slows down pore expansion. We propose that the growth of the strongly bent fusion-pore rim is restricted by a dynamic resistance of the actin network and driven by membrane-bending proteins that are involved in the generation of highly curved intracellular membrane compartments.

Cytoskeleton reorganization in influenza hemagglutinin-initiated syncytium formation

Little is known about the mechanisms of cell-cell fusion in development and diseases and, especially, about fusion stages downstream of an opening of nascent fusion pore(s). Earlier works on different cell-cell fusion reactions have indicated that cytoskeleton plays important role in syncytium formation. However, due to complexity of these reactions and multifaceted contributions of cytoskeleton in cell physiology, it has remained unclear whether cytoskeleton directly drives fusion pore expansion or affects preceding fusion stages. Here we explore cellular reorganization associated with fusion pore expansion in syncytium formation using relatively simple experimental system. Fusion between murine embryonic fibroblasts NIH3T3-based cells is initiated on demand by well-characterized fusogen influenza virus hemagglutinin. We uncouple early fusion stages dependent on protein fusogens from subsequent fusion pore expansion stage and establish that the transition from local fusion to syncytium requires metabolic activity of living cells. Effective syncytium formation for cells with disorganized actin and microtubule cytoskeleton argues against hypothesis that cytoskeleton drives fusion expansion.

The possible contribution of HIV-1-induced syncytia to the generation of intersubtype recombinants in vitro

OBJECTIVE

To develop a method for single syncytia isolation and delineate the possible contribution of syncytia to intersubtype recombination.

DESIGN

We dually infected whole peripheral mononuclear blood cells with subtype A and D viruses and studied syncytia in vitro and developed a method to isolate individual syncytia to further study HIV variants/dual infections, viral isolation, proviral copies in single syncytia and possible intersubtype recombination in dual cultures containing syncytia using real time PCR.

METHODS

Cell culture-based single syncytia isolation, PCR and cloning to determine the nature of HIV variants and real-time PCR to determine proviral copies per individual syncytium and intersubtype recombination in dual cultures. Viral coculture from single syncytia and p24 antigen determination for assessing viral replication in vitro.

RESULTS

Our results show the feasibility that not only can single syncytia be successfully isolated, but the viruses from individual syncytia can also be grown in vitro. They also demonstrate the ability of single syncytia to bring diverse HIV-1 subtypes together along with the possible contribution to intersubtype recombination in vitro. Up to 40% of single syncytia harbored both input HIV-1 subtypes and single syncytium could harbor as many as 2000 proviral DNA copies, which exceeds the limit seen in a single cell.

CONCLUSION

These analyses are unique in experimentally confirming the previously held belief that single syncytia can harbor multiple HIV strains and that they can serve as a breeding ground for heterozygous virions and this may contribute toward viral diversity and intersubtype recombination.

Heat shock protein inhibitors increase the efficacy of measles virotherapy

Oncolytic measles virus strains have activity against multiple tumor types and are currently in phase I clinical testing. Induction of the heat shock protein 70 (HSP70) constitutes one of the earliest changes in cellular gene expression following infection with RNA viruses including measles virus, and HSP70 upregulation induced by heat shock has been shown to result in increased measles virus cytotoxicity. HSP90 inhibitors such as geldanamycin (GA) or 17-allylaminogeldanamycin result in pharmacologic upregulation of HSP70 and they are currently in clinical testing as cancer therapeutics. We therefore investigated the hypothesis that heat shock protein inhibitors could augment the measles virus-induced cytopathic effect. We tested the combination of a measles virus derivative expressing soluble human carcinoembryonic antigen (MV-CEA) and GA in MDA-MB-231 (breast), SKOV3.IP (ovarian) and TE671 (rhabdomyosarcoma) cancer cell lines. Optimal synergy was accomplished when GA treatment was initiated 6-24 h following MV infection. Western immunoblotting confirmed HSP70 upregulation in combination-treated cells. Combination treatment resulted in statistically significant increase in syncytia formation as compared to MV-CEA infection alone. Clonogenic assays demonstrated significant decrease in tumor colony formation in MV-CEA/GA combination-treated cells. In addition there was increase in apoptosis by 4,6-diamidino-2-phenylindole staining. Western immunoblotting for caspase-9, caspase-8, caspase-3 and poly(ADP-ribose) polymerase (PARP) demonstrated increase in cleaved caspase-8 and PARP. The pan-caspase inhibitor Z-VAD-FMK and caspase-8 inhibitor Z-IETD-FMK, but not the caspase-9 inhibitor Z-IEHD-FMK, protected tumor cells from MV-CEA/GA-induced PARP activation, indicating that apoptosis in combination-treated cells occurs mainly via the extrinsic caspase pathway. Treatment of normal cells, such as normal human fibroblasts, however, with the MV-CEA/GA combination, did not result in cytopathic effect, indicating that GA did not alter the MV-CEA specificity for tumor cells. One-step viral growth curves, western immunoblotting for MV-N protein expression, QRT-PCR quantitation of MV-genome copy number and CEA levels showed comparable proliferation of MV-CEA in GA-treated vs -untreated tumor cells. Rho activation assays and western blot for total RhoA, a GTPase associated with the actin cytoskeleton, demonstrated decrease in RhoA activation in combination-treated cells, a change previously shown to be associated with increase in paramyxovirus-induced cell-cell fusion. The enhanced cytopathic effect resulting from measles virus/GA combination supports the translational potential of this approach in the treatment of cancer.

Real-time visualization of HIV-1 GAG trafficking in infected macrophages

HIV-1 particle production is driven by the Gag precursor protein Pr55(Gag). Despite significant progress in defining both the viral and cellular determinants of HIV-1 assembly and release, the trafficking pathway used by Gag to reach its site of assembly in the infected cell remains to be elucidated. The Gag trafficking itinerary in primary monocyte-derived macrophages is especially poorly understood. To define the site of assembly and characterize the Gag trafficking pathway in this physiologically relevant cell type, we have made use of the biarsenical-tetracysteine system. A small tetracysteine tag was introduced near the C-terminus of the matrix domain of Gag. The insertion of the tag at this position did not interfere with Gag trafficking, virus assembly or release, particle infectivity, or the kinetics of virus replication. By using this in vivo detection system to visualize Gag trafficking in living macrophages, Gag was observed to accumulate both at the plasma membrane and in an apparently internal compartment that bears markers characteristic of late endosomes or multivesicular bodies. Significantly, the internal Gag rapidly translocated to the junction between the infected macrophages and uninfected T cells following macrophage/T-cell synapse formation. These data indicate that a population of Gag in infected macrophages remains sequestered internally and is presented to uninfected target cells at a virological synapse.

Tetraspanins CD9 and CD81 modulate HIV-1-induced membrane fusion

Protein organization on the membrane of target cells may modulate HIV-1 transmission. Since the tetraspanin CD81 is associated to CD4, the receptor of HIV-1 envelope protein (Env; gp120/gp41), we have explored the possibility that this molecule may modulate the initial steps of HIV-1 infection. On the other hand, CD81 belongs to the tetraspanin family, which has been described as organizers of protein microdomains on the plasma membrane. Therefore, the role of CD81 and other related tetraspanin, CD9, on the cell-to-cell fusion process mediated by HIV-1 was studied. We found that anti-tetraspanin Abs enhanced the syncytia formation induced by HIV-1 envelope proteins and viral entry in human T lymphoblasts. In addition, anti-CD81 Abs triggered its clustering in patches, where CD4 and CXCR4 were included. Moreover, the knocking down of CD81 and CD9 expression resulted in an increase in syncytia formation and viral entry. Accordingly, overexpression of CD81 and CD9 rendered cells less susceptible to Env-mediated syncytia formation. These data indicate that CD9 and CD81 have an important role in membrane fusion induced by HIV-1 envelope.

Precise identification of a human immunodeficiency virus type 1 antigen processing mutant

Human immunodeficiency virus type 1 (HIV-1) evokes a strong immune response, but the virus persists. Polymorphisms within known antigenic sites result in loss of immune recognition and can be positively selected. Amino acid variation outside known HLA class I restricted epitopes can also enable immune escape by interfering with the processing of the optimal peptide antigen. However, the lack of precise rules dictating epitope generation and the enormous genetic diversity of HIV make prediction of processing mutants very difficult. Polymorphism E169D in HIV-1 reverse transcriptase (RT) is significantly associated with HLA-B*0702 in HIV-1-infected individuals. This polymorphism does not map within a known HLA-B*0702 epitope; instead, it is located five residues downstream of a HLA-B*0702-restricted epitope SPAIFQSSM (SM9). Here we investigate the association between E169D and HLA-B*0702 for immune escape via the SM9 epitope. We show that this single amino acid variation prevents the immune recognition of the flanked SM9 epitope by cytotoxic T cells through lack of generation of the epitope, which is a result of aberrant proteasomal cleavage. The E169D polymorphism also maps within and abrogates the recognition of an HLA-A*03-restricted RT epitope MR9. This study highlights the potential for using known statistical associations as indicators for viral escape but also the complexity involved in interpreting the immunological consequences of amino acid changes in HIV sequences.

HIV-1-mediated syncytium formation promotes cell-to-cell transfer of Tax protein and HTLV-I gene expression

An important increase in luciferase activity was detected following co-culture of Jurkat T cells transiently transfected with an HTLV-I-LTR-driven reporter construct with HIV-1- and HTLV-I-infected cells. Production of infectious HTLV-I and expression of the HTLV-I envelope were not required for this HIV-1-dependent induction while it was severely hampered by anti-gp120 and anti-CD4 antibodies. The HTLV-I Tax protein and the TRE1 repeats were found to be necessary for the HIV-1-mediated enhancement of HTLV-I LTR activity in the co-culture assay. As these results suggested triple fusion events involving all three cell types and the intracellular transfer of Tax, we labelled each cell line with a distinct fluorescent probe. Through confocal microscopy, a number of resulting syncytia and cell clusters were indeed observed to be positive for all three probes. We are proposing a model in which HIV-1-mediated syncytium formation between HIV-1- and HTLV-I-infected cells and uninfected T cells forms a "bridge" or "tunnel" through which Tax from the HTLV-I-infected cells can diffuse and activate HTLV-I-LTR transcription.

Involvement of a small GTP binding protein in HIV-1 release

Background

There is evidence suggesting that actin binding to HIV-1 encoded proteins, or even actin dynamics themselves, might play a key role in virus budding and/or release from the infected cell. A crucial step in the reorganisation of the actin cytoskeleton is the engagement of various different GTP binding proteins. We have thus studied the involvement of GTP-binding proteins in the final steps of the HIV-1 viral replication cycle.

Results

Our results demonstrate that virus production is abolished when cellular GTP binding proteins involved in actin polymerisation are inhibited with specific toxins.

Conclusion

We propose a new HIV budding working model whereby Gag interactions with pre-existing endosomal cellular tracks as well as with a yet non identified element of the actin polymerisation pathway are required in order to allow HIV-1 to be released from the infected cell.

Actin cytoskeletal reorganizations and coreceptor-mediated activation of rac during human immunodeficiency virus-induced cell fusion

The membrane fusion events which initiate human immunodeficiency virus type 1 (HIV-1) infection and promote cytopathic syncytium formation in infected cells commence with the binding of the HIV envelope glycoprotein (Env) to CD4 and an appropriate coreceptor. Here, we show that HIV Env-coreceptor interactions activate Rac-1 GTPase and stimulate the actin filament network reorganizations that are requisite components of the cell fusion process. Disrupting actin filament dynamics with jasplakinolide or latrunculin A arrested fusion at a late step in the formation of Env-CD4-coreceptor complexes. Time-lapse confocal microscopy of living cells revealed vigorous activity of actin-based, target cell membrane extensions at the target cell-Env-expressing cell interface. The expression of dominant-negative forms of actin-regulating Rho-family GTPases established that HIV Env-mediated syncytium formation relies on Rac-1 but not on Cdc42 or Rho activation in target cells. Similar dependencies were found when cell fusion was induced by Env expressed on viral or cellular membranes. Additionally, Rac activity was specifically upregulated in a coreceptor-dependent manner in fusion reaction cell lysates. These results define a role for HIV Env-coreceptor interactions in activating the cellular factors essential for virus-cell and cell-cell fusion and provide evidence for the participation of pertussis toxin-insensitive signaling pathways in HIV-induced membrane fusion.

Role of cholesterol in human immunodeficiency virus type 1 envelope protein-mediated fusion with host cells

In this study we examined the effects of target membrane cholesterol depletion and cytoskeletal changes on human immunodeficiency virus type 1 (HIV-1) Env-mediated membrane fusion by dye redistribution assays. We found that treatment of peripheral blood lymphocytes (PBL) with methyl-beta-cyclodextrin (MbetaCD) or cytochalasin reduced their susceptibility to membrane fusion with cells expressing HIV-1 Env that utilize CXCR4 or CCR5. However, treatment of human osteosarcoma (HOS) cells expressing high levels of CD4 and coreceptors with these agents did not affect their susceptibility to HIV-1 Env-mediated membrane fusion. Removal of cholesterol inhibited stromal cell-derived factor-1alpha- and macrophage inflammatory protein 1beta-induced chemotaxis of both PBL and HOS cells expressing CD4 and coreceptors. The fusion activity as well as the chemotactic activity of PBL was recovered by adding back cholesterol to these cells. Confocal laser scanning microscopy analysis indicated that treatment of lymphocytes with MbetaCD reduced the colocalization of CD4 or of CXCR4 with actin presumably in microvilli. These findings indicate that, although cholesterol is not required for HIV-1 Env-mediated membrane fusion per se, its depletion from cells with relatively low coreceptor densities reduces the capacity of HIV-1 Env to engage coreceptor clusters required to trigger fusion. Furthermore, our results suggest that coreceptor clustering may occur in microvilli that are supported by actin polymerization.

Changes in the motility, morphology, and F-actin architecture of human dendritic cells in an in vitro model of dendritic cell development

An in vitro model has been developed for analyzing the two developmental phases of human dendritic cell (DC) migration. Employing the age of the culture and the addition of GM-CSF, IL-4, and serum to regulate cellular phenotype, and glass coated with acid-precipitated human plasma proteins to facilitate persistent DC translocation, the model produces three sequential in vitro phenotypes with the following suggested in vivo counterparts: (1) DCs recently isolated from blood, which are highly polar and motile, and reflect the behavior of "undifferentiated" DCs that must extravasate from the blood stream and migrate into peripheral tissue; (2) large, nonmotile, stellate DCs, which reflect the highly "differentiated" signature phenotype of DCs in peripheral tissue, whose function is to capture foreign antigens; and (3) the large, motile "dedifferentiated" DCs, which reflect the behavior of "veiled cells" that have captured an antigen, retracted dendritic processes, migrated out of peripheral tissue, and are in the process of transporting a captured antigen to a proximal draining lymph node for presentation to T cells. Computer-assisted motion analysis of the three sequential phenotypes and fluorescent staining of F-actin reveal three unique behavioral states and unique cellular architecture consistent with inferred in vivo function. This in vitro model should serve as a starting point for elucidating the cues and molecular mechanisms involved in the regulation of DC differentiation and motility.

Quantitative analysis of Caenorhabditis elegans sperm motility and how it is affected by mutants spe11 and unc54

The sperm of Caenorhabditis elegans translocate in a fashion similar to sperm of Ascaris suum even though their pseudopods are longer, more plastic in shape, and form multiple expansions zones around their perimeter. Mutants in spe-11 form primary spermatocytes with a defective perinuclear region, but the resulting spermatozoa can still crawl and fertilize eggs. However, the resultant zygotes die due to the absence of sperm-supplied spe-11. Computer-assisted analysis of translocating spe-11 sperm reveals a novel defect in the dynamic morphology of their pseudopods. A similar analysis of the C. elegans mutant unc-54, which lacks the most abundant isoform of myosin II, reveals no defect in sperm motility, as expected, since C. elegans sperm have substituted the protein MSP for actin in the process of pseudopod expansion. These results reveal an unexpected defect in the dynamic morphology of pseudopods of spe-11 sperm. This defect, however, does not significantly affect crawling velocity, and it demonstrates how computer-assisted motion analysis systems can reveal subtle behavioral phenotypes in C. elegans mutant spermatozoa.

A RhoA-derived peptide inhibits syncytium formation induced by respiratory syncytial virus and parainfluenza virus type 3

The fusion glycoproteins of human respiratory syncytial virus (RSV) and human parainfluenza virus type-3 (PIV-3) mediate virus entry and syncytium formation. Interaction between the fusion protein of RSV and RhoA, a small GTPase, facilitates virus-induced syncytium formation. We show here a RhoA-derived peptide inhibits RSV and syncytium formation induced by RSV and PIV-3, both in vitro by inhibition of cell-to-cell fusion and in vivo by reduction of peak titer by 2 log10 in RSV-infected mice. These findings indicate that the interaction between these two paramyxovirus fusion proteins and RhoA is an important target for new antiviral strategies.

HIV-induced T-cell syncytia release a two component T-helper cell chemoattractant composed of Nef and Tat

Using a newly developed gradient chamber to provide independent measurements of chemokinesis (stimulated motility) and chemotaxis (stimulated motility up a concentration gradient) of individual T-helper cells, it was recently demonstrated that HIV-induced T-cell syncytia release two distinct chemotactic activities that are separable by their rates of diffusion. The molecular masses of the two chemoattractant activities were estimated to be 30 and 120 kDa. The higher molecular mass activity was demonstrated to be the viral glycoprotein gp120. In an attempt to identify the lower molecular mass activity, chemotaxis and chemokinesis of T-helper cells were analyzed in individual concentration gradients of the virally encoded proteins Rev, p24, Tat and Nef. None functioned alone as a chemoattractant, but both Tat and Nef alone functioned as chemokinetic stimulants. When Tat and Nef were used together to generate parallel gradients, they stimulated chemotaxis. Antibody to either Tat or Nef neutralized the lower molecular mass chemotactic activity released by syncytia. The addition of antibody to the CD4 receptor or the addition of soluble CD4 inhibited high molecular mass chemotactic activity but not the low molecular mass chemotactic activity in HIV-induced syncytium-conditioned medium, demonstrating that the former but not the latter activity is mediated through the CD4 receptor. These results identify the combination of Nef and Tat as the lower molecular mass T cell chemoattractant released by HIV-induced syncytia, and provide the first evidence suggesting that parallel concentration gradients of two proteins are necessary for chemotaxis.

Computer-assisted three-dimensional reconstruction and motion analysis of living, crawling cells

A computer-assisted three-dimensional dynamic image analysis system (3D-DIAS) has been developed for reconstructing and motion analyzing living, crawling cells. The system simultaneously reconstructs the cell surface, the nucleus and pseudopodia, both expanding and retracting. Although this system has been developed for single cell analysis, it can be used for the dynamic reconstruction and motion analysis of cells in early embryos, the human heart and any other cell, organ or object changing shape over time. Ongoing development of a dynamic analysis system with a confocal front-end, a high speed reconstruction system, a near-real time system and a virtual reality system are described.

T cell syncytia induced by HIV release. T cell chemoattractants: demonstration with a newly developed single cell chemotaxis chamber

A chemotaxis chamber has been developed to analyze both the velocity and the directionality of individual T cells in gradients of high molecular mass molecules over long periods of time. Employing this chamber, it is demonstrated that syncytia induced by HIV in SUP-T1 cell cultures release two T cell chemoattractants with approximate molecular masses of 30 and 120 kDa. Neither uninfected single cells nor polyethylene glycol-induced syncytia release detectable chemoattractant, suggesting that these chemoattractants are linked to HIV infection. Soluble gp120 functions as a T cell chemoattractant and the addition of anti-gp120 antibody to syncytium-conditioned medium blocks the high molecular mass chemoattractant activity but not the low molecular mass activity. The addition of anti-CD4 antibody to syncytium-conditioned medium also blocks the high molecular mass chemoattractant activity but not the low molecular mass activity. These results demonstrate that HIV-induced T cell syncytia release a low and a high molecular mass T cell chemoattractant, and suggest that the high molecular mass factor is gp120 and that it functions through the CD4 receptor.

Morphometric description of the wandering behavior in Drosophila larvae: aberrant locomotion in Na+ and K+ channel mutants revealed by computer-assisted motion analysis

Wandering is a simple behavior in Drosophila larvae prior to metamorphosis. Using the Dynamic Image Analysis System (DIAS) initially developed for analyzing amoeboic movements of single cells, we have analyzed videotaped behaviors of Drosophila larvae at the wandering stage. Previous studies show that mutations in the Na+ channel gene paralytic (para) cause paralysis at 29 degrees C, and mutations in the K+ channel beta subunit gene Hyperkinetic (Hk) lead to leg-shaking under ether anesthesia. The application of DIAS revealed quantifiable abnormalities in the larval locomotion of both ion channel mutants even under "permissive" conditions. Analysis of centroid movement indicates that, compared to wild type, both Hk and para larvae crawled at a slower average speed, but a similar peak instantaneous speed during a contraction cycle. Nevertheless, contraction in the body length was greater in mutants, implying a lower efficiency in conversion of muscular contraction to distance translocation. In addition, each mutant produced a characteristic crawling pattern distinct from the wild-type control. The larval crawling pattern was determined by periods of linear locomotion interposed by non-locomotive, "searching and decision-making" episodes, after which the crawling was resumed in a new direction. Our results demonstrate that mutations in single ion channel subunits resulted in stereotypic modifications in locomotion control and crawling patterns, and that DIAS is a powerful tool in revealing subtle differences in animal behavior and quantifying mutational effects on the interplay of discrete behavioral components.

Identification of differentially expressed mRNA species during HIV infection by RNA arbitrarily primed PCR

BACKGROUND

A number of strategies, such as subtractive cDNA libraries and high through-put sequencing, have been devised to assess differential gene expression. Most of these approaches, however, are cumbersome and/or require tremendous technological power. In this paper, we describe a method, RNA fingerprinting using arbitrarily primed polymerase chain reaction (RAP-PCR), that is rapid, less cumbersome and can differentiate low levels of mRNA expression.

OBJECTIVES

To identify genes that are differentially expressed following human immunodeficiency virus type 1 (HIV-1) infection in different cell types by RAP-PCR.

STUDY DESIGN

RNA was extracted from both HIV-1-infected and uninfected HUT78 cells and peripheral blood mononuclear cells (PBMCs), reverse transcribed, and RAP-PCR amplified using numerous primer sets.

RESULTS

Three genes, gamma-actin, the HIV-1 nef and an unknown sequence, were identified as being differentially expressed in HUT78 cells. The level of gamma-actin mRNA expression is increased after HIV infection and, as expected, the nef gene was solely expressed in HIV-infected cells. In contrast, the unknown mRNA is down-regulated by HIV. Northern blot analysis and/or specific PCR confirmed the differential expression of these three genes. RNA fingerprinting using phytohemagglutinin (PHA)-activated PBMCs infected by HIV in vitro, revealed that gamma-actin is still up-regulated by HIV, whereas the unknown product no longer shows down-regulation.

CONCLUSIONS

These results illustrate the usefulness of the RAP-PCR method for isolating and identifying differentially expressed genes during HIV-1 infection of primary lymphocytes.

Phagocytosis of individual CD4+ T cells by HIV-induced T cell syncytia

Transmission electron microscopic analysis of HIV-induced syncytia of the CD4+ SupT1 cell line has revealed profiles of whole T cells in the syncytium cytoplasm. Serial sections demonstrate that these T cells are completely enveloped by a second membrane in the syncytium cytoplasm and represent phagosomes. Pycnosis of engulfed T cell nuclei, vacuolation of the cytoplasm of engulfed T cells, and the association of engulfed T cells with dense vesicular clusters in the syncytium cytoplasm support the conclusion that they represent phagosomes. In addition, transmission electron micrographs of the syncytium surface reveal giant pseudopodial extensions wrapping around T cells, in a fashion similar to bacterial and yeast phagocytosis by professional phagocytes. These results suggest that phagocytosis is a characteristic acquired during HIV-induced syncytium formation, and that it may represent an avenue of T cell death in addition to fusion in HIV-infected SupT1 cell cultures.

In Ascaris sperm pseudopods, MSP fibers move proximally at a constant rate regardless of the forward rate of cellular translocation

Computer-assisted methods have been employed to obtain a high resolution description of pseudopod expansion, cellular translocation, and the subcellular dynamics of MSP fiber complexes in the motile sperm of the nematode Ascaris suum. Although Ascaris sperm translocating in a straight line or along a curved path do not retract their pseudopod or significantly alter pseudopod shape, they move in a cyclic fashion, with an average period between velocity peaks of 0.35 +/- 0.05 min, which is independent of the forward velocity of sperm translocation. Expansion is confined to a central zone at the distal edge of the pseudopod for sperm translocating in a straight line and to a left-handed or right-handed lateral zone in the direction of turning, for sperm translocating along a curved path. For cells translocating in a straight line, the branch points and kinks of MSP fiber complexes move in a retrograde direction in relation to the substratum at an average velocity of 11 microns per min which is independent of the forward velocity of sperm translocation. The distal (anterior) end of a fiber complex, however, moves distally at the speed of sperm translocation when it emanates from the expansion zone, but when it is displaced to a nonexpanding surface of the pseudopod, it stops moving distally. When a cell is anchored to the substratum and is, therefore, nonmotile, the velocity of fiber complexes moving in a retrograde direction doubles. The unique aspects of pseudopod and MSP fiber complex dynamics in Ascaris are compared to the dynamics of pseudopod formation and actin filament dynamics in traditional actin-based amoeboid cells, and the treadmill model for MSP polymerization is reassessed in light of the discovery that fiber complex branch points move proximally (posteriorly) at a fixed rate.