General analysis of receptor-mediated viral attachment to cell surfaces

Inter-individual Variation in Receptor Expression Influences MERS-CoV Infection and Immune Responses in Airway Epithelia

Middle East respiratory syndrome coronavirus (MERS-CoV) causes respiratory infection in humans, with symptom severity that ranges from asymptomatic to severe pneumonia. Known risk factors for severe MERS include male sex, older age, and the presence of various comorbidities. MERS-CoV gains entry into cells by binding its receptor, dipeptidyl peptidase 4 (DPP4), on the surface of airway epithelia. We hypothesized that expression of this receptor might be an additional determinant of outcomes in different individuals during MERS-CoV infection. To learn more about the role of DPP4 in facilitating MERS-CoV infection and spread, we used ELISA and immunofluorescent staining to characterize DPP4 expression in well-differentiated primary human airway epithelia (HAE). We noted wide inter-individual variation in DPP4 abundance, varying by as much as 1000-fold between HAE donors. This variability appears to influence multiple aspects of MERS-CoV infection and pathogenesis, with greater DPP4 abundance correlating with early, robust virus replication and increased cell sloughing. We also observed increased induction of interferon and some interferon-stimulated genes in response to MERS-CoV infection in epithelia with the greatest DPP4 abundance. Overall, our results indicate that inter-individual differences in DPP4 abundance are one host factor contributing to MERS-CoV replication and host defense responses, and highlight how HAE may serve as a useful model for identifying risk factors associated with heightened susceptibility to serious respiratory pathogens.

Making sense of virus size and the tradeoffs shaping viral fitness

Viruses span an impressive size range, with genome length varying a thousandfold and virion volume nearly a millionfold. For cellular organisms the scaling of traits with size is a pervasive influence on ecological processes, but whether size plays a central role in viral ecology is unknown. Here, we focus on viruses of aquatic unicellular organisms, which exhibit the greatest known range of virus size. We outline hypotheses within a quantitative framework, and analyse data where available, to consider how size affects the primary components of viral fitness. We argue that larger viruses have fewer offspring per infection and slower contact rates with host cells, but a larger genome tends to increase infection efficiency, broaden host range, and potentially increase attachment success and decrease decay rate. These countervailing selective pressures may explain why a breadth of sizes exist and even coexist when infecting the same host populations. Oligotrophic ecosystems may be enriched in "giant" viruses, because environments with resource-limited phagotrophs at low concentrations may select for broader host range, better control of host metabolism, lower decay rate and a physical size that mimics bacterial prey. Finally, we describe where further research is needed to understand the ecology and evolution of viral size diversity.

The viral protein corona directs viral pathogenesis and amyloid aggregation

Artificial nanoparticles accumulate a protein corona layer in biological fluids, which significantly influences their bioactivity. As nanosized obligate intracellular parasites, viruses share many biophysical properties with artificial nanoparticles in extracellular environments and here we show that respiratory syncytial virus (RSV) and herpes simplex virus type 1 (HSV-1) accumulate a rich and distinctive protein corona in different biological fluids. Moreover, we show that corona pre-coating differentially affects viral infectivity and immune cell activation. In addition, we demonstrate that viruses bind amyloidogenic peptides in their corona and catalyze amyloid formation via surface-assisted heterogeneous nucleation. Importantly, we show that HSV-1 catalyzes the aggregation of the amyloid β-peptide (Aβ42), a major constituent of amyloid plaques in Alzheimer's disease, in vitro and in animal models. Our results highlight the viral protein corona as an acquired structural layer that is critical for viral-host interactions and illustrate a mechanistic convergence between viral and amyloid pathologies.

Identification of insect genes involved in baculovirus AcMNPV entry into insect cells

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a model enveloped DNA virus that infects and replicates in lepidopteran insect cells, and can efficiently enter a wide variety of non-host cells. Budded virions of AcMNPV enter cells by endocytosis and traffic to the nucleus where the virus initiates gene expression and genome replication. While trafficking of nucleocapsids by actin propulsion has been studied in detail, other important components of trafficking during entry remain poorly understood. We used a recombinant AcMNPV virus expressing an EGFP reporter in combination with an RNAi screen in Drosophila DL1 cells, to identify host proteins involved in AcMNPV entry. The RNAi screen targeted 86 genes involved in vesicular trafficking, including genes coding for VPS and ESCRT proteins, Rab GTPases, Exocyst proteins, and Clathrin adaptor proteins. We identified 24 genes required for efficient virus entry and reporter expression, and 4 genes that appear to restrict virus entry.

Baculovirus Entry and Egress from Insect Cells

Baculoviruses are large DNA viruses of insects that are highly pathogenic in many hosts. In the infection cycle, baculoviruses produce two types of virions. These virion phenotypes are physically and functionally distinct, and each serves a critical role in the biology of the virus. One phenotype, the occlusion-derived virus (ODV), is occluded within a crystallized protein that facilitates oral infection of the host. A large complex of at least nine ODV envelope proteins called per os infectivity factors are critically important for ODV infection of insect midgut epithelial cells. Viral egress from midgut cells is by budding to produce a second virus phenotype, the budded virus (BV). BV binds, enters, and replicates in most other tissues of the host insect. Cell recognition and entry by BV are mediated by a single major envelope glycoprotein: GP64 in some baculoviruses and F in others. Entry and egress by the two virion phenotypes occur by dramatically different mechanisms and reflect a life cycle in which ODV is specifically adapted for oral infection while BV mediates dissemination of the infection within the animal.

An experimental and computational framework for engineering multifunctional nanoparticles: designing selective anticancer therapies

A key challenge in the treatment of cancer with nanomedicine is to engineer and select nanoparticle formulations that lead to the desired selectivity between tumorigenic and non-tumorigenic cells. To this aim, novel designed nanomaterials, deep biochemical understanding of the mechanisms of interaction between nanomaterials and cells, and computational models are emerging as very useful tools to guide the design of efficient and selective nanotherapies. This works shows, using a combination of detailed experimental approaches and simulations, that the specific targeting of cancer cells in comparison to non-tumorigenic cells can be achieved through the custom design of multivalent nanoparticles. A theoretical model that provides simple yet quantitative predictions to tune the nanoparticles targeting and cytotoxic properties by their degree of functionalization is developed. As a case study, a system that included a targeting agent and a drug and is amenable to controlled experimental manipulation and theoretical analysis is used. This study shows how at defined functionalization levels multivalent nanoparticles can selectively kill tumor cells, while barely affecting non-tumorigenic cells. This work opens a way to the rational design of multifunctionalized nanoparticles with defined targeting and cytotoxic properties for practical applications.

BacMam Delivery of a Protective Gene to Reduce Renal Ischemia-Reperfusion Injury

Ischemia-reperfusion (I/R) injury remains the primary contributor to delayed graft function in kidney transplantation. The beneficial application of manganese superoxide dismutase (sod), delivered by a BacMam vector, against renal I/R injury has not been evaluated previously. Therefore, this study overexpressed sod-2 in proximal tubular epithelial (HK-2) cells and porcine kidney organs during simulated renal I/R injury. Incubation of HK-2 cells with antimycin A and 2-deoxyglucose resulted in a significant decrease in intracellular adenosine triphosphate (ATP) levels; following reperfusion, ATP levels significantly increased over time in cells overexpressing sod-2. In addition, lactate dehydrogenase (LDH) release declined over 72 h in BacMam-transduced injured cells. Ex vivo delivery of sod-2 significantly increased ATP levels in organs after 24 h of cold perfusion. In vitro and ex vivo results suggested that BacMam transduction successfully delivered sod-2, which reduced injury associated with I/R, by improving ATP cell content and decreasing LDH release with a subsequent increase in kidney tissue viability. These data provide further evidence for the potential application of BacMam as a gene delivery system for attenuating injury after cold preservation.

Linear biocompatible glyco-polyamidoamines as dual action mode virus infection inhibitors with potential as broad-spectrum microbicides for sexually transmitted diseases

The initial steps of viral infections are mediated by interactions between viral proteins and cellular receptors. Blocking the latter with high-affinity ligands may inhibit infection. DC-SIGN, a C-type lectin receptor expressed by immature dendritic cells and macrophages, mediates human immunodeficiency virus (HIV) infection by recognizing mannose clusters on the HIV-1 gp120 envelope glycoprotein. Mannosylated glycodendrimers act as HIV entry inhibitors thanks to their ability to block this receptor. Previously, an amphoteric, but prevailingly cationic polyamidoamine named AGMA1 proved effective as infection inhibitor for several heparan sulfate proteoglycan-dependent viruses, such as human papilloma virus HPV-16 and herpes simplex virus HSV-2. An amphoteric, but prevailingly anionic PAA named ISA23 proved inactive. It was speculated that the substitution of mannosylated units for a limited percentage of AGMA1 repeating units, while imparting anti-HIV activity, would preserve the fundamentals of its HPV-16 and HSV-2 infection inhibitory activity. In this work, four biocompatible linear PAAs carrying different amounts of mannosyl-triazolyl pendants, Man-ISA7, Man-ISA14, Man-AGMA6.5 and Man-AGMA14.5, were prepared by reaction of 2-(azidoethyl)-α-D-mannopyranoside and differently propargyl-substituted AGMA1 and ISA23. All mannosylated PAAs inhibited HIV infection. Both Man-AGMA6.5 and Man-AGMA14.5 maintained the HPV-16 and HSV-2 activity of the parent polymer, proving broad-spectrum, dual action mode virus infection inhibitors.

Bioengineered baculoviruses as new class of therapeutics using micro and nanotechnologies: principles, prospects and challenges

Designing a safe and efficient gene delivery system is required for success of gene therapy trials. Although a wide variety of viral, non-viral and polymeric nanoparticle based careers have been widely studied, the current gene delivery vehicles are limited by their suboptimal, non-specific therapeutic efficacy and acute immunological reactions, leading to unwanted side effects. Recently, there has been a growing interest in insect-cell-originated baculoviruses as gene delivery vehicles for diverse biomedical applications. Specifically, the emergence of diverse types of surface functionalized and bioengineered baculoviruses is posed to edge over currently available gene delivery vehicles. This is primarily because baculoviruses are comparatively non-pathogenic and non-toxic as they cannot replicate in mammalian cells and do not invoke any cytopathic effect. Moreover, emerging advanced studies in this direction have demonstrated that hybridizing the baculovirus surface with different kinds of bioactive therapeutic molecules, cell-specific targeting moieties, protective polymeric grafts and nanomaterials can significantly improve the preclinical efficacy of baculoviruses. This review presents a comprehensive overview of the recent advancements in the field of bioengineering and biotherapeutics to engineer baculovirus hybrids for tailored gene therapy, and articulates in detail the potential and challenges of these strategies for clinical realization. In addition, the article illustrates the rapid evolvement of microfluidic devices as a high throughput platform for optimizing baculovirus production and treatment conditions.

How sticky should a virus be? The impact of virus binding and release on transmission fitness using influenza as an example

Budding viruses face a trade-off: virions need to efficiently attach to and enter uninfected cells while newly generated virions need to efficiently detach from infected cells. The right balance between attachment and detachment-the right amount of stickiness-is needed for maximum fitness. Here, we design and analyse a mathematical model to study in detail the impact of attachment and detachment rates on virus fitness. We apply our model to influenza, where stickiness is determined by a balance of the haemagglutinin (HA) and neuraminidase (NA) proteins. We investigate how drugs, the adaptive immune response and vaccines impact influenza stickiness and fitness. Our model suggests that the location in the 'stickiness landscape' of the virus determines how well interventions such as drugs or vaccines are expected to work. We discuss why hypothetical NA enhancer drugs might occasionally perform better than the currently available NA inhibitors in reducing virus fitness. We show that an increased antibody or T-cell-mediated immune response leads to maximum fitness at higher stickiness. We further show that antibody-based vaccines targeting mainly HA or NA, which leads to a shift in stickiness, might reduce virus fitness above what can be achieved by the direct immunological action of the vaccine. Overall, our findings provide potentially useful conceptual insights for future vaccine and drug development and can be applied to other budding viruses beyond influenza.

Superinfection exclusion in alphabaculovirus infections is concomitant with actin reorganization

Superinfection exclusion is the ability of an established virus to interfere with a second virus infection. This effect was studied in vitro during lepidopteran-specific nucleopolyhedrovirus (genus Alphabaculovirus, family Baculoviridae) infection. Homologous interference was detected in Sf9 cells sequentially infected with two genotypes of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), each one expressing a different fluorescent protein. This was a progressive process in which a sharp decrease in the signs of infection caused by the second virus was observed, affecting not only the number of coinfected cells observed, but also the level of protein expression due to the second virus infection. Superinfection exclusion was concurrent with reorganization of cytoplasmic actin to F-actin in the nucleus, followed by budded virus production (16 to 20 h postinfection). Disruption of actin filaments by cell treatment with cytochalasin D resulted in a successful second infection. Protection against heterologous nucleopolyhedrovirus infection was also demonstrated, as productive infection of Sf9 cells by Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) was inhibited by prior infection with AcMNPV, and vice versa. Finally, coinfected cells were observed following inoculation with mixtures of these two phylogenetically distant nucleopolyhedroviruses--AcMNPV and SfMNPV--but at a frequency lower than predicted, suggesting interspecific virus interference during infection or replication. The temporal window of infection is likely necessary to maintain genotypic diversity that favors virus survival but also permits dual infection by heterospecific alphabaculoviruses.

IMPORTANCE

Infection of a cell by more than one virus particle implies sharing of cell resources. We show that multiple infection, by closely related or distantly related baculoviruses, is possible only during a brief window of time that allows additional virus particles to enter an infected cell over a period of ca. 16 h but then blocks multiple infections as newly generated virus particles begin to leave the infected cell. This temporal window has two important consequences. First, it allows multiple genotypes to almost simultaneously infect cells within the host, thus generating genetically diverse virus particles for transmission. Second, it provides a mechanism by which different viruses replicating in the same cell nucleus can exchange genetic material, so that the progeny viruses may be a mosaic of genes from each of the parental viruses. This opens a completely new avenue of research into the evolution of these insect pathogens.

Receptor concentration and diffusivity control multivalent binding of Sv40 to membrane bilayers

Incoming Simian Virus 40 particles bind to their cellular receptor, the glycolipid GM1, in the plasma membrane and thereby induce membrane deformation beneath the virion leading to endocytosis and infection. Efficient membrane deformation depends on receptor lipid structure and the organization of binding sites on the internalizing particle. To determine the role of receptor diffusion, concentration and the number of receptors required for stable binding in this interaction, we analyze the binding of SV40 to GM1 in supported membrane bilayers by computational modeling based on experimental data. We measure the diffusion rates of SV40 virions in solution by fluorescence correlation spectroscopy and of the receptor in bilayers by single molecule tracking. Quartz-crystal microbalance with dissipation (QCM-D) is used to measure binding of SV40 virus-like particles to bilayers containing the viral receptor GM1. We develop a phenomenological stochastic dynamics model calibrated against this data, and use it to investigate the early events of virus attachment to lipid membranes. Our results indicate that SV40 requires at least 4 attached receptors to achieve stable binding. We moreover find that receptor diffusion is essential for the establishment of stable binding over the physiological range of receptor concentrations and that receptor concentration controls the mode of viral motion on the target membrane. Our results provide quantitative insight into the initial events of virus-host interaction at the nanoscopic level.

Fusion of mApple and Venus fluorescent proteins to the Sindbis virus E2 protein leads to different cell-binding properties

Fluorescent proteins (FPs) are widely used in real-time single virus particle studies to visualize, track and quantify the spatial and temporal parameters of viral pathways. However, potential functional differences between the wild type and the FP-tagged virus may specifically affect particular stages in the virus life-cycle. In this work, we genetically modified the E2 spike protein of Sindbis virus (SINV) with two FPs. We inserted mApple, a red FP, or Venus, a yellow FP, at the N-terminus of the E2 protein of SINV to make SINV-Apple and SINV-Venus. Our results indicate that SINV-Apple and SINV-Venus have similar levels of infectivity and are morphologically similar to SINV-wild-type by negative stain transmission electron microscopy. Both mutants are highly fluorescent and have excellent single-particle tracking properties. However, despite these similarities, when measuring cell entry at the single-particle level, we found that SINV-Apple and SINV-Venus are different in their interaction with the cell surface and FPs are not always interchangeable. We went on to determine that the FP changes the net surface charge on the virus particles, the folding of the spike proteins, and the conformation of the spikes on the virus particle surface, ultimately leading to different cell-binding properties between SINV-Apple and SINV-Venus. Our results are consistent with recent findings that FPs may alter the biological and cellular localization properties of bacterial proteins to which they are fused.

Application of atomic force microscopy for characteristics of single intermolecular interactions

Atomic force microscopy (AFM) can be used to make measurements in vacuum, air, and water. The method is able to gather information about intermolecular interaction forces at the level of single molecules. This review encompasses experimental and theoretical data on the characterization of ligand-receptor interactions by AFM. The advantage of AFM in comparison with other methods developed for the characterization of single molecular interactions is its ability to estimate not only rupture forces, but also thermodynamic and kinetic parameters of the rupture of a complex. The specific features of force spectroscopy applied to ligand-receptor interactions are examined in this review from the stage of the modification of the substrate and the cantilever up to the processing and interpretation of the data. We show the specificities of the statistical analysis of the array of data based on the results of AFM measurements, and we discuss transformation of data into thermodynamic and kinetic parameters (kinetic dissociation constant, Gibbs free energy, enthalpy, and entropy). Particular attention is paid to the study of polyvalent interactions, where the definition of the constants is hampered due to the complex stoichiometry of the reactions.

Impact of tumor HER2/ERBB2 expression level on HER2-targeted liposomal doxorubicin-mediated drug delivery: multiple low-affinity interactions lead to a threshold effect

Numerous targeted nanotherapeutics have been described for potential treatment of solid tumors. Although attention has focused on antigen selection and molecular design of these systems, there has been comparatively little study of how cellular heterogeneity influences interaction of targeted nanoparticles with tumor cells. Antigens, such as HER2/ERBB2, are heterogeneously expressed across different indications, across patients, and within individual tumors. Furthermore, antigen expression in nontarget tissues necessitates optimization of the therapeutic window. Understanding the performance of a given nanoparticle under different regimens of antigen expression has the ability to inform patient selection and clinical development decisions. In this work, HER2-targeted liposomal doxorubicin was used as a model-targeted nanoparticle to quantitatively investigate the effect of HER2 expression levels on delivery of doxorubicin to the nucleus. We find quantitatively greater nuclear doxorubicin delivery with increasing HER2 expression, exhibiting a threshold effect at approximately 2 × 10(5) HER2 receptors/cell. Kinetic modeling indicated that the threshold effect arises from multiple low-affinity interactions between the targeted liposome and HER2. These results support previous data showing little or no uptake into human cardiomyocytes, which express levels of HER2 below the threshold. Finally, these results suggest that HER2-targeted liposomal doxorubicin may effectively target tumors that fall below traditional definitions of HER2-positive tumors, thereby expanding the potential population of patients that might benefit from this agent.

Baculovirus superinfection: a probable restriction factor on the surface display of proteins for library screening

In addition to the expression of recombinant proteins, baculoviruses have been developed as a platform for the display of complex eukaryotic proteins on the surface of virus particles or infected insect cells. Surface display has been used extensively for antigen presentation and targeted gene delivery but is also a candidate for the display of protein libraries for molecular screening. However, although baculovirus gene libraries can be efficiently expressed and displayed on the surface of insect cells, target gene selection is inefficient probably due to super-infection which gives rise to cells expressing more than one protein. In this report baculovirus superinfection of Sf9 cells has been investigated by the use of two recombinant multiple nucleopolyhedrovirus carrying green or red fluorescent proteins under the control of both early and late promoters (vAcBacGFP and vAcBacDsRed). The reporter gene expression was detected 8 hours after the infection of vAcBacGFP and cells in early and late phases of infection could be distinguished by the fluorescence intensity of the expressed protein. Simultaneous infection with vAcBacGFP and vAcBacDsRed viruses each at 0.5 MOI resulted in 80% of infected cells co-expressing the two fluorescent proteins at 48 hours post infection (hpi), and subsequent infection with the two viruses resulted in similar co-infection rate. Most Sf9 cells were re-infectable within the first several hours post infection, but the re-infection rate then decreased to a very low level by 16 hpi. Our data demonstrate that Sf9 cells were easily super-infectable during baculovirus infection, and super-infection could occur simultaneously at the time of the primary infection or subsequently during secondary infection by progeny viruses. The efficiency of super-infection may explain the difficulties of baculovirus display library screening but would benefit the production of complex proteins requiring co-expression of multiple polypeptides.

A model of the binding, entry, uncoating, and RNA synthesis of Semliki Forest virus in baby hamster kidney (BHK-21) cells

A quantitative understanding of viral trafficking would be useful in treating viral-mediated diseases, designing protocols for viral gene therapy, and optimizing heterologous protein production. In this article, a model for the trafficking of Semliki Forest virus and its RNA synthesis in baby hamster kidney (BHK-21) cells is presented. This model includes the various steps leading to infection such as attachment, endocytosis, and viral fusion in the endosome. The model estimates a mean fusion time of 4 to 6 min for the wild-type virus, and 38 min for Fus-1, an SFV mutant which requires a lower pH for fusion. These mean fusion times are consistent with the time-scale of endosomal acidification, suggesting viruses fuse almost instantaneously with the endosomal membrane as soon as the pH of the endosome drops below the pH threshold of the virus. Infection is most likely controlled at the level of viral uncoating, as shown by the close agreement between the efficiency of uncoating and the experimentally determined fraction of viruses that is infectious. The viral RNA synthesized per cell is best described by assuming that it depends on the number of uncoated viruses prior to the onset of replication according to a saturation-type expression. A Poisson distribution is used to determine the distribution of uncoated viruses among the cells. Because attachment is the rate-limiting step in the uncoating of the virus, increasing the attachment rate can lead to enhanced RNA synthesis and, hence, new virion production. Such an increase in the attachment rate may be obtained by lowering the medium pH or the addition of a polycation. (c) 1995 John Wiley & Sons, Inc.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

An overview of influenza A virus receptors

Influenza A viruses are spherical particles that attach to cells through bonds between hemagglutinin and specific cellular receptors. Numerous studies performed have recently revealed that Sialic acid (SA) is a crucial component of influenza A virus receptors. This brief review summarizes recent advances in our understanding of influenza A virus receptors. The introduction describes the classification of influenza A virus receptors and the review continues with a survey of the distribution of SA in different tissue and host. This is followed by research applications of influenza A virus receptors, and explanation of why receptor studies are so important on a world-wide scale.

Modeling the population dynamics of baculovirus-infected insect cells: Optimizing infection strategies for enhanced recombinant protein yields

The insect cell-baculovirus model presented here is capable of simulating cell population dynamics, extracellular virion densities, and heterologous product titers in reasonable agreement with experimental data for a wide rang of multiplicities of infection (MOI) and times of infection. The model accounts for the infection of a single cell by multiple virions and the consequences on the time course of infection. The probability of infection by more than one virion was approximated using the Poisson distribution, which proved to be a refinement over second-order kinetics. The model tracks initiation and duration of important events in the progression of infected cell development (virus replication, recombinant protein synthesis, and cell lysis) for subpopulations delineated by the time and extent of their initial infection. The model suggests infection strategies, weighing the importance of MOI and infection time. Maximum product titers result from infection in the early exponential growth phase with low MOI.

Correlation between the composition of multivalent antibody conjugates with colloidal gold nanoparticles and their affinity

Interactions between multivalent preparations of antibodies (conjugated with colloidal gold nanoparticles (GNP) as a carrier system) and a multivalent ligand were investigated. The aim of the present study was to reveal the relationship between the affinity of the conjugate and its composition (i.e., the valency). Surface plasmon resonance was applied to study the affinity and the kinetics of the interaction of multivalent conjugates and multivalent virus (on the example of the plum pox virus (PPV)). Three monoclonal antibodies against PPV were prepared. Five GNP preparations with an average particle size in the range from 5 to 60nm (according to electron microscopy measurements) were obtained. The series of preparations allowed us to synthesize GNP-antibody conjugates with different surface areas for immobilization of antibodies, and, consequently, conjugates with different valencies. It was shown that the affinity of the conjugates changes with size of colloidal carriers (i.e. with the valency of the conjugates). The affinity of the virus-antibody interaction (antibodies with affinities of 1.46.10(-8)M and 1.73.10(-8)M) is one to three orders of magnitude lower (depending on the valency of the conjugate) compared to that of the interactions of the virus with GNP conjugates (conjugates with the affinity varying from 1.69.10(-9) to 7.02.10(-12)M and from 2.39.10(-9) to 2.62.10(-11)M, respectively). An increase in the conjugate size leads to an increase in its affinity. The similar trends were observed for the potato virus X.

A mathematical model of the trafficking of acid-dependent enveloped viruses: application to the binding, uptake, and nuclear accumulation of baculovirus

A quantitative understanding of virus trafficking would be useful in treating viral-mediated diseases, developing protocols for viral gene therapy, designing infection regimens for viral expression systems, and optimizing vaccine and recombinant protein production. Here, we present a mathematical model of the attachment, internalization, endosomal fusion, lysosomal routing, and nuclear accumulation of baculovirus in SF21 insect cells. The model accounts for multivalent bond formation of the virus with cell surface receptors. The model mimics accurately the experimental trafficking dynamics of the virus at both low and high virion to cell ratios, and estimates a receptor number of 11,000 per cell. A significant amount of virus was degraded intracellularly. Independent of the virion to cell ratio, half of the internalized virus was degraded with the rest accumulating in the nucleus. The formalism used in the model may be generally useful for other acid-dependent enveloped viruses. A subset of the model has been used previously to describe the trafficking of Semliki Forest virus, an acid-dependent enveloped RNA virus.Two pathways have previously been implicated for the in vitro entry of the budded form of the baculovirus: adsorptive endocytosis and plasma membrane fusion. Experimental evidence is presented which strongly suggests that the physical number of viruses entering by plasma membrane fusion is not significant relative to receptor-mediated endocytosis.

Inducing single-cell suspension of BTI-TN5B1-4 insect cells: II. The effect of sulfated polyanions on baculovirus infection

Sulfated polyanions can be used to rapidly induce and maintain single-cell suspensions of BTI-TN5B1-4 insect cells, a cell line which clumps in suspension. Elimination of cell clumping results in a significant increase in volumetric yield of the baculovirus expression vector system. Sulfated polyanions, however, inhibited baculovirus infection of BTI-TN5B1-4. Data from binding studies and fusion assays suggest that the inhibition of infection was not due to the observed reduction in viral attachment rate but to inhibition of viral membrane fusion in the endosome.The three most effective polyanions for inducing single cells are dextran sulfate, pentosan sulfate, and polyvinyl sulfate. At concentrations required for single-cell formation, dextran sulfate and pentosan sulfate did not affect viral infection at multiplicities of infection greater than one plaque forming unit per cell. In contrast, polyvinyl sulfate blocked viral infection even at a high multiplicity of infection of 20 plaque-forming units per cell. To bypass this inhibition, polyvinyl sulfate can be removed by resuspending the cells in fresh medium before virus addition, and then added back to the cell suspension after a substantial amount of virus has been internalized. Alternatively, polyvinyl sulfate can be neutralized with a polycation before virus addition, and an equivalent amount of polyvinyl sulfate added back after most of the virus has been internalized. We present a simple mathematical model of the attachment and entry of baculovirus in BTI-TN5B1-4, which can be used to design appropriate infection regimens.

An arginine switch in the species B adenovirus knob determines high-affinity engagement of cellular receptor CD46

Adenoviruses (Ads) are icosahedral, nonenveloped viruses with a double-stranded DNA genome. The 51 known Ad serotypes exhibit profound variations in cell tropism and disease types. The number of observed Ad infections is steadily increasing, sometimes leading to fatal outcomes even in healthy individuals. Species B Ads can cause kidney infections, hemorrhagic cystitis, and severe respiratory infections, and most of them use the membrane cofactor protein CD46 as a cellular receptor. The crystal structure of the human Ad type 11 (Ad11) knob complexed with CD46 is known; however, the determinants of CD46 binding in related species B Ads remain unclear. We report here a structural and functional analysis of the Ad11 knob, as well as the Ad7 and Ad14 knobs, which are closely related in sequence to the Ad11 knob but have altered CD46-binding properties. The comparison of the structures of the three knobs, which we determined at very high resolution, provides a platform for understanding these differences and allows us to propose a mechanism for productive high-affinity engagement of CD46. At the center of this mechanism is an Ad knob arginine that needs to switch its orientation in order to engage CD46 with high affinity. Quantum chemical calculations showed that the CD46-binding affinity of Ad11 is significantly higher than that of Ad7. Thus, while Ad7 and Ad14 also bind CD46, the affinity and kinetics of these interactions suggest that these Ads are unlikely to use CD46 productively. The proposed mechanism is likely to determine the receptor usage of all CD46-binding Ads.

Micro and Nanopatterning for Bacteria- and Virus-Based Biosensing Applications

Current technologies capable of rapidly and accurately detecting the presence of infectious diseases and toxic compounds in the human body and the environment are inadequate and new, novel techniques are required to ensure the safety of the general population. To develop these technologies, researchers must broaden their scope of interest and investigate scientific areas that have yet to be fully explored. Lithography is a common name given to technologies designed to print materials onto smooth surfaces. More specifically, micropatterning encompasses the selective binding of materials to surfaces in organized microscale arrays. The selective micropatterning of bacteria and viruses is currently an exciting area of research in the field of biomedical engineering and can potentially offer attractive qualities to biosensing applications in terms of increased sensing accuracy and reliability. This chapter focuses on briefly introducing the reader to the fundamentals of bacterial and viral surface interactions and describing several different micropatterning techniques and their advantages and disadvantages in the field of biosensing. The application of these techniques in healthcare and environmental settings is also discussed.

A simple macroscopic model for the diffusion and adsorption kinetics of r-adenovirus

The diffusion of viruses toward cells is a limiting step of the infection process. To be modeled correctly, this step must be evaluated in combination with the adsorption of the virus to the cell surface, which is a rapid but reversible step. In this paper, the recombinant adenovirus (rAd) diffusion and its adsorption to 293S cells in suspension were both measured and modeled. First, equilibrium experiments permitted to determine the number of receptors on the surface of 293S (R(T) = 3,500 cell(-1)) and the association constant (K(A) = 1.9 x 10(11) M(-1)) for rAd on these cells based on a simple monovalent adsorption model. Non-specific binding of the virus to the cell surface was not found to be significant. Second, total virus particle degradation rates between 5.2 x 10(-3) and 4.0 x 10(-2) min(-1) were measured at 37 degrees C in culture medium, but no significant virus degradation was observed at 4 degrees C. Third, free viral particle disappearance rates from a mixed suspension of virus and cells were measured at different virus concentrations. Experimental data were compared to a phenomenological dynamic model comprising both the diffusion and the adsorption steps. The diffusion to adsorption ratio, a fitted parameter, confirmed that the contact process of a virus with a cell is indeed diffusion controlled. However, the characteristic diffusion time constants obtained, based on a reversible adsorption model, were eightfolds smaller than those reported in the literature, based on diffusion models that assume irreversible adsorption.

Structural basis for the recognition of blood group trisaccharides by norovirus

Noroviruses are one of the major causes of nonbacterial gastroenteritis epidemics in humans. Recent studies on norovirus receptors show that different noroviruses recognize different human histo-blood group antigens (HBGAs), and eight receptor binding patterns of noroviruses have been identified. The P domain of the norovirus capsids is directly involved in this recognition. To determine the precise locations and receptor binding modes of HBGA carbohydrates on the viral capsids, a recombinant P protein of a GII-4 strain norovirus, VA387, was cocrystallized with synthetic type A or B trisaccharides. Based on complex crystal structures observed at a 2.0-A resolution, we demonstrated that the receptor binding site lies at the outermost end of the P domain and forms an extensive hydrogen-bonding network with the saccharide ligand. The A and B trisaccharides display similar binding modes, and the common fucose ring plays a key role in this interaction. The extensive interface between the two protomers in a P dimer also plays a crucial role in the formation of the receptor binding interface.

Modeling rotavirus-like particles production in a baculovirus expression vector system: Infection kinetics, baculovirus DNA replication, mRNA synthesis and protein production

Rotavirus is the most common cause of severe diarrhoea in children worldwide, responsible for more than half a million deaths in children per year. Rotavirus-like particles (Rota VLPs) are excellent vaccine candidates against rotavirus infection, since they are non-infectious, highly immunogenic, amenable to large-scale production and safer to produce than those based on attenuated viruses. This work focuses on the analysis and modeling of the major events taking place inside Spodoptera frugiperda (Sf-9) cells infected by recombinant baculovirus that may be critical for the expression of rotavirus viral proteins (VPs). For model validation, experiments were performed adopting either a co-infection strategy, using three monocistronic recombinant baculovirus each one coding for viral proteins VP(2), VP(6) and VP(7), or single-infection strategies using a multigene baculovirus coding for the three proteins of interest. A characteristic viral DNA (vDNA) replication rate of 0.19+/-0.01 h(-1) was obtained irrespective of the monocistronic or multigene vector employed, and synthesis of progeny virus was found to be negligible in comparison to intracellular vDNA concentrations. The timeframe for vDNA, mRNA and VP synthesis tends to decrease with increasing multiplicity of infection (MOI) due to the metabolic burden effect. The protein synthesis rates could be ranked according to the gene size in the multigene experiments but not in the co-infection experiments. The model exhibits acceptable prediction power of the dynamics of intracellular vDNA replication, mRNA synthesis and VP production for the three proteins involved. This model is intended to be the basis for future Rota VLPs process optimisation and also a means to evaluating different baculovirus constructs for Rota VLPs production.

Expression of Coxsackie-Adenovirus receptor (CAR) in the developing mouse olfactory system

Interest in manipulating gene expression in olfactory sensory neurons (OSNs) has led to the use of adenoviruses (AdV) as gene delivery vectors. OSNs are the first order neurons in the olfactory system and the initial site of odor detection. They are highly susceptible to adenovirus infection although the mechanism is poorly understood. The Coxsackie-Adenovirus receptor (CAR) and members of the integrin family have been implicated in the process of AdV infection in various systems. Multiple serotypes of AdV efficiently bind to the CAR, leading to entry and infection of the host cell by a mechanism that can also involve integrins. Cell lines that do not express CAR are relatively resistant, but not completely immune to AdV infection, suggesting that other mechanisms participate in mediating AdV attachment and entry. Using in situ hybridization and western blot analyses, we show that OSNs and olfactory bulbs (OB) of mice express abundant CAR mRNA at embryonic and neonatal stages, with progressive diminution during postnatal development. By contrast to the olfactory epithelium (OE), CAR mRNA is still present in the adult mouse OB. Furthermore, despite a similar postnatal decline, CAR protein expression in the OE and OB of mice continues into adulthood. Our results suggest that the robust AdV infection observed in the postnatal olfactory system is mediated by CAR and that expression of even small amounts of CAR protein as seen in the adult rodent, permits efficient AdV infection and entry. CAR is an immunoglobulin domain-containing protein that bears homology to cell-adhesion molecules suggesting the possibility that it may participate in organization of the developing olfactory system.

A novel method for in vitro radiolabeling and testing enveloped viruses by phosphatidylethanolamineN-methyltransferase and host cell-specific binding

The present study developed a novel virus labeling and testing method, referred to as an envelope-labeled virus assay (ELVA), in which virus envelope is labeled in vitro by the action of phosphatidylethanolamine N-methyltransferase (PEMT) and tested through a host cell-specific binding. A recombinant strain (vGFPuv) of Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) and Spodoptera frugiperda (Sf-9) insect cells were used as a model of viruses and host cells, respectively. The labeling mixture, which contained PEMT, [methyl-3H]S-adenosylmethionine (SAM), and a trace amount of detergent Triton X-100, brought about little change in virus titer of vGFPuv on a 1-h incubation, but was so toxic to Sf-9 cells as to immediately cause cell death. After being incubated with vGFPuv, therefore, the labeling mixture was neutralized by adsorptive removal of PEMT and Triton X-100 before Sf-9 cells were contacted with the mixture to extract the virus. The Sf-9 cells were then washed with a phosphate buffered saline (PBS), and lipid extracts with a 1% SDS solution were subjected to a liquid scintillation analysis for the determination of labeling efficiency. As a result, a significant amount of radioactivity was determined in the extracts, demonstrating the validity of ELVA for labeling and testing enveloped viruses. The conditions for the PEMT reaction and cell-virus binding were examined, and the lower detection limit of AcMNPV by ELVA was found to lie in the order of 10(3) plaque forming unit (pfu) per milliliter. Since the labeling reaction and detection of virus are based on neither immunological nor genetic characteristics of virus, ELVA is also expected to be a convenient and comprehensive test of other enveloped viruses.

Baculovirus vectors for gene therapy

Since the discovery that baculoviruses can efficiently transduce mammalian cells, baculoviruses have been extensively studied as potential vectors for both in vitro and in vivo gene therapy. This chapter reviews the history of this research area, cells permissive to baculovirus transduction, factors influencing transduction and transgene expression, efforts to improve transduction, mechanisms of virus entry and intracellular trafficking, applications for in vivo and ex vivo gene therapy, as well as advantages, limitations, and safety issues concerning use of baculoviruses as gene therapy vectors. Recent progress and efforts directed toward overcoming existing bottlenecks are emphasized.

The Function of Envelope Protein P74 from Autographa californica Multiple Nucleopolyhedrovirus in Primary Infection to Host

This research investigated the function of envelope protein P74 of Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) in primary infection to host. A p74-inactivation recombinant baculovirus, rAc-gfp(Delta) p74, was constructed by inserting gfp driven by AcMNPV polyhedrin promoter into the p74 locus of AcMNPV genome. Bioassays showed that the P74-null occlusion bodies (OBs) failed to infect its natural host larvae, Spodoptera exigua, per os, while the p74-null budded virus (BVs) could infect host larvae by injection. However, its inability for oral infectivity was rescued by a mixed infection with wild-type OBs or with the purified P74 protein expressed in Spodoptera frugiperda Sf-9 cells, and the P74 protein rescue was in a dosage-dependent manner. The 50% lethal dosage (LD50) value of a P74 overexpression recombinant virus, rAc-p74(++)-polh+, which contained two copies of p74 gene, was not significantly different from that of wild-type virus. One-step growth curve assays of viruses suggested that BV production from cells infected with p74-null virus was similar to that from cells infected with wild-type virus or the P74 overexpression virus. ELISA analysis indicated that P74 protein could bind its host brush border membrane vesicles (BBMV) efficiently with saturation, but it could only bind its sensitive midgut BBMV specifically. In vitro pull-down assay showed that a protein of approximately 35 kDa in the BBMV was involved in the specific binding. These results demonstrated that the P74 protein is essential for oral infectivity of occlusion-derived virus (ODV) and plays a role in midgut attachment and fusion.

Structured model of influenza virus replication in MDCK cells

Intracellular events that take place during influenza virus replication in animal cells are well understood qualitatively. However, to better understand the complex interaction of the virus with its host cell and to quantitatively analyze the use of cellular resources for virion formation or the overall dynamic for the entire infection cycle, a mathematical model for influenza virus replication has to be formulated. Here, we present a structured model for the single-cell reproductive cycle of influenza A virus in animal cells that accounts for the individual steps of the process such as attachment, internalization, genome replication and translation, and progeny virion assembly. The model describes an average cell surrounded by a small quantity of medium and infected by a low number of virus particles. The model allows estimation of the cellular resources consumed by virus replication. Simulation results show that the number of cellular surface receptors and endosomes, as well as other resources, such as the number of free nucleotides or amino acids, is not significantly influenced by influenza virus propagation. A factor that limits the growth rate of progeny viruses and their release is the total amount of matrix proteins (M1) in the nucleus while other newly synthesized viral proteins (e.g., nucleoprotein NP) and viral RNAs accumulate. During budding, synthesis of vRNPs (viral ribonucleoprotein complexes) represents another limiting factor. Based on this model it is also possible to analyze effects of parameter changes on the dynamics of virus replication, to identify possible targets for molecular engineering, or to develop strategies for improving yields in vaccine production processes. Furthermore, a better insight into the interactions of viruses and host cells might help to improve our understanding of virus-related diseases and to develop therapies.

Brownian adhesive dynamics (BRAD) for simulating the receptor-mediated binding of viruses

Current viral docking models have relied upon the assumption that bond formation and breakage are independent of viral and docking surface geometry, as well as the forces exerted on the bonds. This assumption, known as the equivalent site hypothesis (ESH), is examined in detail using a newly developed simulation technique-Brownian adhesive dynamics (BRAD). The simulation couples the thermal motion of viral particles with adhesive dynamics models to characterize the effect of bonding on viral motion. We use the binding of HIV-like particles to CD4 expressing cells as a model system to illustrate the utility of BRAD. Comparison of the transition rates between bound states predicted by ESH and the rates resulting from BRAD simulations show dramatic differences; at values of the equilibrium crosslinking constant, K(x)R(T), where ESH suggests all virus adhesion proteins will be bound (K(x)R(T) = 10(6)), BRAD predicts not all virus adhesion proteins will be bound. At values of the equilibrium crosslinking constant used in typical ESH calculations of virus docking (K(x)R(T) = 1) we find BRAD simulations predict no binding. The mean bond density from BRAD models is often much lower than that predicted by ESH for equivalent parameter values. BRAD suggests that the viruses are much less well bound than ESH predicts. The differences suggest that binding models for viruses need to be reexamined closely. BRAD is a simulation technique that will be useful for quantifying the receptor-mediated binding of a wide variety of viruses to cells.

The effect of cellular receptor diffusion on receptor-mediated viral binding using Brownian adhesive dynamics (BRAD) simulations

Brownian adhesive dynamics (BRAD) is a new method for simulating the attachment of viruses to cell surfaces. In BRAD, the motion of the virus is subject to stochastic bond formation and breakage, and thermal motion owing to collisions from the solvent. In the model, the virus is approximated as a rigid sphere and the cell surface is approximated as a rigid plane coated with receptors. In this article, we extend BRAD to allow for the mobility of receptors in the plane of the membrane, both before and after they are ligated by viral attachment proteins. Allowing the proteins to move within the membrane produced several differences in behavior from when the receptors are immobilized. First, the mean steady-state bond number is unaffected by changes in cellular receptor density because proteins are now free to diffuse into the contact area, and the extent of binding is dictated by the availability of viral attachment proteins. Second, the time required to reach steady-state binding increases as both the cellular receptor number decreases and the receptor mobility decreases. This is because receptor diffusion is a slower process than the binding kinetics of the proteins. Decreasing the rate of protein binding was found to decrease the fraction of viruses bound to steady state, but not the extent of binding for those viruses that were bound. Increasing the binding rate increased the fraction of viruses bound, until no further viruses could bind. Alterations in receptor binding kinetics had no discernable effect on the mean steady-state bond number between virus and cell, because interactions were of sufficiently high affinity that all available receptor-viral attachment proteins were destined to bind at steady state.

Charged polymers modulate retrovirus transduction via membrane charge neutralization and virus aggregation

The specific mechanisms of charged polymer modulation of retrovirus transduction were analyzed by characterizing their effects on virus transport and adsorption. From a standard colloidal perspective two mechanisms, charge shielding and virus aggregation, can potentially account for the experimentally observed changes in adsorption behavior and biophysical parameters due to charged polymers. Experimental testing revealed that both mechanisms could be at work depending on the characteristics of the cationic polymer. All cationic polymers enhanced adsorption and transduction via charge shielding; however, only polymers greater than 15 kDa in size were capable of enhancing these processes via the virus aggregation mechanism, explaining the higher efficiency enhancement of the high molecular weight molecules. The role of anionic polymers was also characterized and they were found to inhibit transduction via sequestration of cationic polymers, thereby preventing charge shielding and virus aggregation. Taken together, these findings suggest the basis for a revised physical model of virus transport that incorporates electrostatic interactions through both virus-cell repulsive and attractive interactions, as well as the aggregation state of the virus.

Evidence that binding of cucumber necrosis virus to vector zoospores involves recognition of oligosaccharides

Despite the importance of vectors in natural dissemination of plant viruses, relatively little is known about the molecular features of viruses and vectors that permit their interaction in nature. Cucumber necrosis virus (CNV) is a small spherical virus whose transmission in nature is facilitated by zoospores of the fungus Olpidium bornovanus. Previous studies have shown that specific regions of the CNV capsid are involved in transmission and that transmission defects in several CNV transmission mutants are due to inefficient attachment of virions to the zoospore surface. In this study, we have undertaken to determine if zoospores contain specific receptors for CNV. We show that in vitro binding of CNV to zoospores is saturable and that vector zoospores bind CNV more efficiently than nonvector zoospores. Further studies show that treatment of zoospores with periodate and trypsin reduces CNV binding, suggesting the involvement of glycoproteins in zoospore attachment. In virus overlay assays, CNV binds to several proteins, whereas CNV transmission mutants either fail to bind or bind at significantly reduced levels. The possible involvement of specific sugars in attachment was investigated by incubating CNV with zoospores in the presence of various sugars. Two mannose derivatives (methyl alpha-D-mannopyranoside and D-mannosamine), as well as three mannose-containing oligosaccharides (mannotriose, alpha3,alpha6-mannopentaose, and yeast mannan) and L-(-)-fucose, all inhibited CNV binding at relatively low concentrations. Taken together, our studies suggest that binding of CNV to zoospores is mediated by specific mannose and/or fucose-containing oligosaccharides. This is the first time sugars have been implicated in transmission of a plant virus.

Production of core and virus-like particles with baculovirus infected insect cells

In this paper the fundamental aspects of process development for the production of core and virus-like particles with baculovirus infected insect cells are reviewed. The issues addressed include: particle formation and monomer composition, chemical and physical conditions for optimal cell growth, baculovirus replication and product expression, multiplicity of infection strategy, and scale-up of the process. Study of the differences in the metabolic requirements of infected and non-infected cells is necessary for high cell density processes. In the bioreactor, the specific oxygen uptake rate (OURsp) plays a central role in process scale-up, leading to the specification of the bioreactor operational parameters. Shear stress can also be an important variable for bioreactor operation due to its influence on cell growth and product expression. The determination of the critical variables in process development is discussed, showing the relevance of the mathematical models that have been developed for the insect cells/baculovirus system in process implementation and control.

Artificial in vivo antigen presentation by the APCs and subsequent T-cell activation: a feasibility analysis

Inappropriate antigen presentation by the antigen-presenting cells (APCs) is a cause of various diseases. One of the ways to combat these diseases is to immobilize the APCs near the infected tissue or a tissue which is susceptible to an antigen. The antigen is presented by the APCs present in the immobilized form on an implant and these upon binding to T(H)-cells result in triggering of a cascade of events as part of the natural immune response leading to the destruction of the antigen. This system has been modeled as a dialysis bag containing immobilized receptors inside the bag and the ligand diffusing out of the bag. The simulations show that by using the implant, the concentration of the ligand that has diffused into the tissue matrix can be substantially reduced and by suitably choosing the coupler size, the T(H)-cells can also effectively be activated.

Nucleic acid biotechnology

Driven by advances in the acquisition of genetic sequence information and the ability to manipulate small quantities of nucleic acid, a number of technologies are emerging that exploit nucleic acids for research, diagnostic, and therapeutic utility. In this review, we cover three technologies based on nucleic acids--DNA microarrays, antisense technology, and gene therapy--that are especially promising and may make a substantial impact in the laboratory and in the clinic during the coming years. For each of these areas, an overview of the current status and applications is provided, followed by a discussion of critical issues and challenges to be faced for further advancement of the technology; an emphasis is placed on quantitative and engineering aspects.

T-cell activation by soluble MHC oligomers can be described by a two-parameter binding model

T-cell activation is essential for initiation and control of immune system function. T cells are activated by interaction of cell-surface antigen receptors with major histocompatibility complex (MHC) proteins on the surface of other cells. Studies using soluble oligomers of MHC-peptide complexes and other types of receptor cross-linking agents have supported an activation mechanism that involves T cell receptor clustering. Receptor clustering induced by incubation of T cells with MHC-peptide oligomers leads to the induction of T-cell activation processes, including downregulation of engaged receptors and upregulation of the cell-surface proteins CD69 and CD25. Dose-response curves for these T-cell activation markers are bell-shaped, with different maxima and midpoints, depending on the valency of the soluble oligomer used. In this study, we have analyzed the activation behavior using a mathematical model that describes the binding of multivalent ligands to cell-surface receptors. We show that a simple equilibrium binding model accurately describes the activation data for CD4(+) T cells treated with MHC-peptide oligomers of varying valency. The model can be used to predict activation and binding behavior for T cells and MHC oligomers with different properties.

Quantitative analysis of synthetic gene delivery vector design properties

As intracellular gene delivery pathways are highly complex combinations of multiple potentially rate-limiting cellular and molecular processes, approaches to the design of synthetic delivery vectors focusing on any single barrier individually will likely be suboptimal. We offer here an "integrative systems" approach to vector characterization and design, combining quantitative experiment and computational modeling studies of vector uptake and trafficking kinetics. This model is validated using data for delivery of a green fluorescent protein (GFP)-encoding plasmid by means of Lipofectamine, permitting specification of model parameter values. The model is then used to make a priori predictions on the effect of polymer length in polyplex vectors, with additional parameter values determined from previous independent experimental studies of plasmid release. Comparison with data on GFP expression via these polyplex vectors shows that the model successfully predicts an experimentally observed biphasic dependence of expression efficiency on polymer length and quantifies the contributions of competing effects yielding the optimal intermediate polymer length. Finally, we use the model to predict potential effects of incorporating nuclear localization sequences in these kinds of synthetic vectors, and find that the degree of benefit from these will depend on the values of other key system properties including the vector unpackaging rate constant. Thus, we demonstrate the usefulness of a bioengineering, integrative-systems modeling approach to improved vector design and analysis.

Adenovirus type 5 viral particles pseudotyped with mutagenized fiber proteins show diminished infectivity of coxsackie B-adenovirus receptor-bearing cells

A major limitation of adenovirus type 5 (Ad5)-based gene therapy, the inability to target therapeutic genes to selected cell types, is attributable to the natural tropism of the virus for the widely expressed coxsackievirus-adenovirus receptor (CAR) protein. Modifications of the Ad5 fiber knob domain have been shown to alter the tropism of the virus. We have developed a novel system to rapidly evaluate the function of modified fiber proteins in their most relevant context, the adenoviral capsid. This transient transfection/infection system combines transfection of cells with plasmids that express high levels of the modified fiber protein and infection with Ad5.beta gal.Delta F, an E1-, E3-, and fiber-deleted adenoviral vector encoding beta-galactosidase. We have used this system to test the adenoviral transduction efficiency mediated by a panel of fiber protein mutants that were proposed to influence CAR interaction. A series of amino acid modifications were incorporated via mutagenesis into the fiber expression plasmid, and the resulting fiber proteins were subsequently incorporated onto adenoviral particles. Mutations located in the fiber knob AB and CD loops demonstrated the greatest reduction in fiber-mediated gene transfer in HeLa cells. We also observed effects on transduction efficiency with mutations in the FG loop, indicating that the binding site may extend to the adjacent monomer in the fiber trimer and in the HI loop. These studies support the concept that modification of the fiber knob domain to diminish or ablate CAR interaction should result in a detargeted adenoviral vector that can be combined simultaneously with novel ligands for the development of a systemically administered, targeted adenoviral vector.

Baculovirus infection of nondividing mammalian cells: mechanisms of entry and nuclear transport of capsids

We have studied the infection pathway of Autographa californica multinuclear polyhedrosis virus (baculovirus) in mammalian cells. By titration with a baculovirus containing a green fluorescent protein cassette, we found that several, but not all, mammalian cell types can be infected efficiently. In contrast to previous suggestions, our data show that the asialoglycoprotein receptor is not required for efficient infection. We demonstrate for the first time that this baculovirus can infect nondividing mammalian cells, which implies that the baculovirus is able to transport its genome across the nuclear membrane of mammalian cells. Our data further show that the virus enters via endocytosis, followed by an acid-induced fusion event, which releases the nucleocapsid into the cytoplasm. Cytochalasin D strongly reduces the infection efficiency but not the delivery of nucleocapsids to the cytoplasm, suggesting involvement of actin filaments in cytoplasmic transport of the capsids. Electron microscopic analysis shows the cigar-shaped nucleocapsids located at nuclear pores of nondividing cells. Under these conditions, we observed the viral genome, major capsid protein, and electron-dense capsids inside the nucleus. This suggests that the nucleocapsid is transported through the nuclear pore. This mode of transport seems different from viruses with large spherical capsids, such as herpes simplex virus and adenovirus, which are disassembled before nuclear transport of the genome. The implications for the application of baculovirus or its capsid proteins in gene therapy are discussed.

Receptor-mediated targeting of gene delivery vectors: insights from molecular mechanisms for improved vehicle design

One way to deliver transgenes to cells in a selective manner is to target the delivery vehicles, or vectors, to specific cell-surface receptors as a first step toward ultimate transport of the gene to the nucleus for expression. While selective delivery, although often to undesired cell types, occurs naturally for some viral vectors and can be achieved for nonviral vehicles, current understanding and control of the delivery mechanism is inadequate for many therapeutic applications. The complicated nature of receptor-mediated transgene uptake and transport requires improved analysis to more effectively evaluate delivery vehicles. As receptor-mediated pathways for gene delivery typically involve vector binding, internalization, subcellular trafficking, vesicular escape, nuclear translocation, and unpackaging for transcription, each of these processes offer mechanisms that can be exploited to enhance targeted gene delivery via properly designed vehicles. For the purpose of this review, current targeted gene delivery vehicles are divided into three approaches: viral, synthetic, and hybrid vectors. Each approach possesses advantages as well as disadvantages at the present time for in vitro and in vivo application, and provides particular challenges to overcome in order to gain significantly improved targeted delivery properties. Quantitative experiments and mathematical modeling of the gene delivery pathway will serve to provide insight into molecular mechanisms and rate-limiting steps for effective gene expression. Information on molecular mechanisms obtained by such methodologies can then be applied to specific vectors, whether viral, synthetic, or hybrid, allowing for the creation of targeted, effective, and safe gene therapeutics.