An integrated microfluidic device for influenza and other genetic analyses

An integrated microfluidic device capable of performing a variety of genetic assays has been developed as a step towards building systems for widespread dissemination. The device integrates fluidic and thermal components such as heaters, temperature sensors, and addressable valves to control two nanoliter reactors in series followed by an electrophoretic separation. This combination of components is suitable for a variety of genetic analyses. As an example, we have successfully identified sequence-specific hemagglutinin A subtype for the A/LA/1/87 strain of influenza virus. The device uses a compact design and mass production technologies, making it an attractive platform for a variety of widely disseminated applications.

Stable attachment for herpes simplex virus penetration into human cells requires glycoprotein D in the virion and cell receptors that are missing for entry-defective porcine cells

Clonal porcine kidney cell lines that are non-permissive for herpes simplex virus (HSV) infection produced five orders of magnitude less virus than human cells, contained heparan sulfate (HS), and are restricted only at HSV entry. By fluorescent activated cell sorting, we examined HSV attachments to porcine and human cells. Stable attachment to susceptible human embryonic lung (HEL) cells occurred with infectious wild-type virus, complemented gD or gH mutant viruses, or non-infectious virus lacking gH. On HEL cells, mutant virus lacking gD bound to heparan sulfate, but failed to stably bind. None of these viruses stably attached to SK6-A7 cells, one of the non-permissive porcine cell clones. However, HSV could replicate in these cells when entry was mediated by polyethylene glycol. These results confirm that, in neutral pH entry of HSV, (i) multiple attachments to HS and non-HS components lead to penetration, (2) stable attachment before penetration is one required function of gD, but not gH, and (3) for stable attachment, gD interacts directly, or indirectly through another viral or cellular component, with receptors that are present on human cells, but absent for entry-defective porcine cells. Easily propagated clonal porcine cells are a novel resource to investigate stable attachment, the molecular mechanisms of gD functions, and the viral and cellular components that allow HSV entry and spread.

Cell cycle dependence of retroviral transduction: An issue of overlapping time scales

Recombinant retroviruses are currently used as gene delivery vehicles for the purpose of gene therapy. It is generally believed that the efficiency of retroviral transduction depends on the cell cycle status of the target cells. However, it has been reported that this is not the case for the transduction of human and murine fibroblasts, in contrast to other cell types such as lymphocytes. The predictions of a mathematical model that we constructed, offer an explanation of this contradiction, based on the dynamics of the underlying processes of target cell growth and the intracellular decay of retroviral vectors. The model suggests that the utility of synchronization experiments, that are usually employed to study cell cycle specificity, is severely limited when the time scales of the above kinetic events are comparable to each other. The predictions of the model also suggest the use of retroviral vectors as cell cycle markers, as an alternative way to detect cell cycle dependence of retroviral transduction. This method obviates the need for cell synchronization and therefore, it does not perturb the cell cycle or interfere with the life cycle of retroviral vectors. Moreover, it does not depend on the intracellular stability of retroviral vectors. Our results show that in contrast to previously reported results, transduction of murine fibroblasts is cell cycle dependent, and they are consistent with the current notion that mitosis is the phase that confers transduction susceptibility.

Moloney murine leukemia virus-derived retroviral vectors decay intracellularly with a half-life in the range of 5.5 to 7.5 hours

Replication-incompetent recombinant retroviruses are currently used for gene delivery. The limited efficiency of gene transfer using these vectors hampers implementation of gene therapy. Successful integration of Moloney murine leukemia virus (MMuLV)-derived retroviral vectors into the host cell DNA requires cell division. The time difference between virus entry and cell division is variable and prolonged in slowly dividing cells. Therefore, the rate of intracellular decay of internalized vectors between the time of entry into the target cell and cell division may limit the probability of successful integration following viral entry. We present two methods that measure the intracellular stability of MMuLV-derived retroviral vectors in NIH 3T3 cells. The first is based on a temporary interruption of cell cycle progression by using cell detachment. This method provides an estimate, but not a direct measurement, of the half-life. The results show that the MMuLV intracellular half-life is on the order of but shorter than the total cell cycle time. The second method allows the direct measurement of the intracellular half-life by using two cell cycle-specific labels: 5-bromodeoxyuridine, a thymidine analog that labels cells in S-phase; and the viral vector that labels cells in mitosis. By varying the time between the administration of the two labels, the intracellular half-life is measured to be in the range of 5.5 to 7.5 h. Such a short intracellular half-life may restrict the efficiency of gene transfer by retroviral vectors, particularly in slowly dividing target cells.

Defective entry of herpes simplex virus types 1 and 2 into porcine cells and lack of infection in infant pigs indicate species tropism

We have determined if a defect at entry of the human pathogen herpes simplex virus type 1 (HSV-1) into cultured porcine cells extends to HSV-2 and if the poor susceptibility of porcine cells for these viruses is indicative of in vivo species tropism. HSV-1 replicates poorly in swine testis (ST) and other porcine cells which lack a functional non-heparan sulphate receptor(s) required for virus entry. By several criteria, ST cells resist infection by either HSV-1 or HSV-2. Infection can be restored if normal entry is bypassed by PEG-mediated virion-cell membrane fusion. Neither HSV serotype infects, replicates or produces clinical symptoms in infant pigs. No virus was isolated from any of multiple sites and seroconversion did not occur. The in vitro defect in porcine cells blocking HSV entry correlates with, and is likely to be at least partly responsible for, in vivo resistance of pigs to infection.

Swine testis cells contain functional heparan sulfate but are defective in entry of herpes simplex virus

Herpes simplex virus (HSV) enters and infects most cultured cells. We have found that swine testis cells (ST) produce yields of infectious HSV-1 up to four orders of magnitude lower than those of human embryonic lung (HEL) and HEp-2 cells because of a defect in virus entry. For ST cells, virus binding is reduced, DNA from input virus cannot be detected, and virus proteins are not synthesized. Polyethylene glycol treatment of ST cells after exposure to HSV allows viral entry, protein synthesis, and productive infection. Transfection of viral genomic DNA that bypasses the normal entry process produces similar yields of infectious virus from ST, HEL, and HEp-2 cells. Therefore, all three cell lines can support the HSV replicative cycle. Biochemical analyses and inhibition of sulfation by sodium chlorate treatment show that ST cells contain amounts and types of heparan sulfate (HS) similar to those of highly susceptible cells. HSV infection of sodium chlorate-treated HEL and ST cells indicates the presence of a second, non-HS receptor(s) on susceptible HEp-2 and HEL cells that is missing, or not functional, on poorly susceptible ST cells. We conclude that ST cells are defective in HSV entry, contain functional HS, but lack a functional non-HS receptor(s) required for efficient HSV-1 entry. Further, ST cells provide a novel resource that can be used to identify, isolate, and characterize an HSV non-HS receptor(s) and its role in the entry and tropism of this important human pathogen.

Cell-specific kinetics and efficiency of herpes simplex virus type 1 entry are determined by two distinct phases of attachment

We previously provided evidence for a model of herpes simplex virus type 1 (HSV-1) entry by a cascade of interactions between components of the virion envelope and cellular plasma membrane (Fuller and Lee, 1992, J. Virol. 66, 5002-5012). In this report we have determined entry kinetics of wild-type HSV-1 into two highly susceptible cell lines to further explore the contributions of viral or cellular factors to entry. Penetration rates of preattached virus varied among several common laboratory HSV-1 strains into one cell line. However, entry kinetics varied substantially for a single strain into highly susceptible HEp-2 or Vero cells under identical conditions. Plaquing efficiencies and sensitivity to heparin also significantly differed between these cells. Kinetics of entry that included virus attachment and penetration showed that the cell-specific effects can be explained by two distinct phases of attachment that occurred before penetration, but differed in duration on both susceptible cell lines. Initial attachment of virus is resistant to removal with phosphate-buffered saline, but sensitive to removal with buffer containing heparin. This is followed by a second type of attachment that is heparin resistant, but still sensitive to extracellular inactivation. We conclude that although undefined factors unique to individual wild-type HSV-1 laboratory strains affect entry kinetics, entry of any one strain is greatly influenced by interactions of virus with specific cell components during at least two distinct phases of attachment before penetration. Moreover, the second phase to stabilize binding seems to be the rate-limiting event in entry. Since major differences in the amounts or sulfation patterns of heparan sulfate were not detected, differences in the surfaces of HEp-2 and Vero cells that influence the kinetics and efficiency of HSV-1 entry are likely in the fine structure of heparan sulfate or in the presence and quantity of other unidentified receptors.

Herpes simplex virus type 1 and pseudorabies virus bind to a common saturable receptor on Vero cells that is not heparan sulfate

Herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV) infect different natural hosts but are very similar in structure, replicative cycle, and entry into cultured cells. We determined whether HSV-1 and PRV use the same cellular components during entry into Vero cells, which are highly susceptible to each virus but are not from native hosts for either. UV-inactivated virions of either HSV-1 or PRV could saturate cell surfaces to block infection of challenge HSV-1 or PRV. In the presence of saturating levels for infection of either virus, radiolabeled virus bound well and in a heparin-sensitive manner. This result shows that heparan sulfate proteoglycans on Vero cells are not the limiting cellular component. To identify the virus component required for blocking, we used an HSV-1 null mutant virus lacking gB, gD, or gH as blocking virus. Virions lacking gB were able to block infection of challenge virus to the same level as did virus containing gB. In contrast, virions lacking gD lost all and most of the ability to block infection of HSV-1 and PRV, respectively. HSV-1 lacking gH and PRV lacking gp50 also were less competent in blocking infection of challenge virus. We conclude that HSV-1 and PRV bind to a common receptor for infection of Vero cells. Although both viruses bind a heparin-like cell component on many cells, including Vero cells, they also attach to a different and limited cell surface component that is bound at least by HSV-1 gD and possibly gH and to some degree by PRV gp50 but not gB. These results clearly demonstrate binding of both HSV-1 and PRV to a common cell receptor that is not heparan sulfate and demonstrate that several types of attachment occur for both viruses during infectious entry.

Herpes simplex virus type 1 entry through a cascade of virus-cell interactions requires different roles of gD and gH in penetration

We examined the entry process of herpes simplex virus type 1 (HSV-1) by using infectious virus and previously characterized noninfectious viruses that can bind to cells but cannot penetrate as a result of inactivation of essential viral glycoprotein D (gD) or H (gH). After contact of infectious virus with the cell plasma membrane, discernible changes of the envelope and tegument could be seen by electron microscopy. Noninfectious virions were arrested at distinct steps in interactions with cells. Viruses inactivated by anti-gD neutralizing antibodies attached to cells but were arrested prior to initiation of a visible fusion bridge between the virus and cell. As judged from its increased sensitivity to elution, virus lacking gD was less stably bound to cells than was virus containing gD. Moreover, soluble gD could substantially reduce virus attachment when added to cells prior to or with the addition of virus. Virus inactivated by anti-gH neutralizing antibodies attached and could form a fusion bridge but did not show expansion of the fusion bridge or extensive rearrangement of the envelope and tegument. We propose a model for infectious entry of HSV-1 by a series of interactions between the virion envelope and the cell plasma membrane that trigger virion disassembly, membrane fusion, and capsid penetration. In this entry process, gD mediates a stable attachment that is likely required for penetration, and gH seems to participate in fusion initiation or expansion.

Neutralizing antibodies specific for glycoprotein H of herpes simplex virus permit viral attachment to cells but prevent penetration

Monoclonal antibodies specific for gH of herpes simplex virus were shown previously to neutralize viral infectivity. Results presented here demonstrate that these antibodies (at least three of them) block viral penetration without inhibiting adsorption of virus to cells. Penetration of herpes simplex virus is by fusion of the virion envelope with the plasma membrane of a susceptible cell. Electron microscopy of thin sections of cells exposed to virus revealed that neutralized virus bound to the cell surface but did not fuse with the plasma membrane. Quantitation of virus adsorption by measuring the binding of purified radiolabeled virus to cells revealed that the anti-gH antibodies had little or no effect on adsorption. Monitoring cell and viral protein synthesis after exposure of cells to infectious and neutralized virus gave results consistent with the electron microscopic finding that the anti-gH antibodies blocked viral penetration. On the basis of the results presented here and other information published elsewhere, it is suggested that gH is one of three glycoproteins essential for penetration of herpes simplex virus into cells.

Anti-glycoprotein D antibodies that permit adsorption but block infection by herpes simplex virus 1 prevent virion-cell fusion at the cell surface

Certain monoclonal antibodies specific for glycoprotein D of herpes simplex virus have potent neutralizing activity but fail to block attachment of virus to cells. Here we have investigated the fate of neutralized and infectious virus after attachment to primate cells. Infectious virions fused with the cell surface such that naked nucleocapsids were detectable in the cytoplasm near or just under the plasma membrane. Neutralized virions did not fuse with the cell. They remained attached to the cell surface and could be rendered infectious by treatment with polyethylene glycol. We conclude that some anti-glycoprotein D neutralizing antibodies can inhibit the penetration of herpes simplex virus by blocking fusion of the virion envelope with the plasma membrane. These results identify a pathway of entry that initiates successful herpes simplex virus infection and a step in this pathway that is highly sensitive to neutralizing antibodies. A role for glycoprotein D in virion-cell fusion is indicated.

Herpes simplex virus glycoproteins associated with different morphological entities projecting from the virion envelope

Spikes of different kinds, distinct in size and appearance were detected on the surfaces of herpes simplex virions by electron microscopy of negatively stained preparations. Use of monoclonal antibodies coupled to colloidal gold permitted identification of viral glycoproteins present in different structures projecting from the virion envelope. Antibodies specific for the glycoprotein designated gB bound to the most prominent spikes, which were about 14 nm long and, in side view, had a flattened T-shaped top. Antibodies specific for gC bound to structures that, in some instances, appeared to extend as much as 24 nm from the surface of the envelope and were too thin to resolve. Antibodies specific for gD bound to structures that extended as much as 8 to 10 nm from the surface of the envelope. The gB spikes were invariably clustered, usually in protrusions of the envelope varying from small bulbous distentions to long tail-like projections. The gC components were randomly distributed and widely spaced and the gD components were irregularly clustered in patterns distinct from those of the gB spikes. These three glycoproteins therefore form structures that are different in size, morphology and distribution in the envelope.