A simple and rapid microassay for the titration of human respiratory syncytial virus

Development and application of a higher throughput RSV plaque assay by immunofluorescent imaging

Viral plaque assays are important tools in the development and evaluation of new antiviral drugs or vaccines in both preclinical and clinical research. While plaque assays are the standard tools to measure infectious virus, the methodology is time-consuming and requires experience in recognizing plaques. The assays are also prone to variation among analysts due to plaque recognition and manual counting errors. Here we describe the development of two simplified plaque assays for measuring RSV virus titers and anti-RSV antibody neutralization titers using 96 well plate formats. First, we evaluated multiple parameters to build up a quantitative plaque assay to measure infectious RSV. We then optimized the assay conditions to assess the fundamental changes from the traditional plaque assay, which were elimination of overnight pre-seeding host cells and addition of a centrifugation step after viral infection of the cells. We designed DoE to refine four key parameters within one experiment for host cell density, host cell volume, viral inoculum volume, host cell and viral mixture incubation time to make this assay more robust. We have also adapted these conditions into a second assay, which was an automated plaque reduction neutralization assay (PRNT) to determine neutralization titers of anti-RSV antibodies. Both assays utilize immune fluorescence staining to detect viral plaques. The images of the immuno-stained wells are captured by the PerkinElmer EnSight instrument and show clear visualization of plaques harvesting on day 3. Software algorithm was specifically designed for automatic counting of these fluorescent "objects". The quantitative plaque assay provided titers of RSV similar to those obtained from the traditional plaque assay. The method has been successfully utilized to screen multiple vaccine candidates in viral shedding efficacy studies. The automated PRNT assay provided antibody neutralizing titers that matched with published data. This automated 96 well plaque assay has made it possible to screen RSV samples in a higher throughput manner, and can be extended to other infectious organisms that form plaques for vaccine or drug evaluation.

Antiviral Activities of Peptides with Sequences Related to the Cytoplasmic Domain of the Respiratory Syncytial Virus Glycoproteins

A set of peptides from 6–10 amino acids in length with sequences corresponding to the respiratory syncytial virus (RSV) attachment glycoprotein, G, has been found to inhibit secretion of virus particles from two human cell lines infected with RSV. Peptides of similar lengths with sequences related to the fusion (F) protein were not effective. A modified assay for syncytia formation was used to measure infectious particles and we found that cell-free infectious RSV was inhibited by those same peptides that blocked particle release. However, only about 5–20% of the total infectious virus formed in the cultured cells was released from the cells – the remaining was assayable only after lysis of the infected cells. Formation of this major fraction of infectious, cell-associated RSV was not affected by the peptides. The inhibitory peptides were ineffective in blocking release of infectious virus from cells infected with influenza, vesicular stomatitis and Semliki Forest viruses. Only a set of specific amino acids in these peptides were inhibitory and they consisted of a very hydrophobic sequence that also required a cysteine residue.

Antibody response and avidity of respiratory syncytial virus-specific total IgG, IgG1, and IgG3 in young children

Respiratory syncytial virus (RSV) is a major cause of acute respiratory disease in infants and young children. Considering that several aspects of the humoral immune response to RSV infection remain unclear, this study aimed to investigate the occurrence, levels, and avidity of total IgG, IgG1, and IgG3 antibodies against RSV in serum samples from children ≤5 years old. In addition, a possible association between antibody avidity and severity of illness was examined. The occurrence and levels of RSV-specific IgG depended on age, with infants <3 months old displaying high levels of antibodies, which were probably acquired from the mother. Children ≥24 months old also showed frequent occurrence and high levels of IgG, which was produced actively during infection. In addition, the avidity assay showed that the avidity of RSV-specific total IgG and IgG1 was lower in infants <3 months old who had acute respiratory disease than in age-matched controls. The avidity of RSV-specific IgG detected in children ≥24 months old with lower respiratory infection was lower than that in children with upper respiratory infection. These results indicate that the presence of high avidity RSV-specific IgG antibodies may lead to better protection against RSV infection in children <3 months old, who may have a lower probability of developing disease of increased severity. In addition, children ≥24 months old with RSV-specific IgG antibodies of low avidity tended to develop more severe RSV illness. These findings may be helpful in establishing vaccination schedules when a vaccine becomes available.

Enhancement of respiratory mucosal antiviral defenses by the oxidation of iodide

Recent reports postulate that the dual oxidase (DUOX) proteins function as part of a multicomponent oxidative pathway used by the respiratory mucosa to kill bacteria. The other components include epithelial ion transporters, which mediate the secretion of the oxidizable anion thiocyanate (SCN(-)) into airway surface liquid, and lactoperoxidase (LPO), which catalyzes the H(2)O(2)-dependent oxidation of the pseudohalide SCN(-) to yield the antimicrobial molecule hypothiocyanite (OSCN(-)). We hypothesized that this oxidative host defense system is also active against respiratory viruses. We evaluated the activity of oxidized LPO substrates against encapsidated and enveloped viruses. When tested for antiviral properties, the LPO-dependent production of OSCN(-) did not inactivate adenovirus or respiratory syncytial virus (RSV). However, substituting SCN(-) with the alternative LPO substrate iodide (I(-)) resulted in a marked reduction of both adenovirus transduction and RSV titer. Importantly, well-differentiated primary airway epithelia generated sufficient H(2)O(2) to inactivate adenovirus or RSV when LPO and I(-) were supplied. The administration of a single dose of 130 mg of oral potassium iodide to human subjects increased serum I(-) concentrations, and resulted in the accumulation of I(-) in upper airway secretions. These results suggest that the LPO/I(-)/H(2)O(2) system can contribute to airway antiviral defenses. Furthermore, the delivery of I(-) to the airway mucosa may augment innate antiviral immunity.

Purification of human respiratory syncytial virus: superiority of sucrose gradient over percoll, renografin, and metrizamide gradients

A method was devised for producing and purifying human respiratory syncytial virus (HRSV) preparations with high titers. Previous attempts to obtain substantial amounts of purified human respiratory syncytial virus have been unsuccessful due to the extreme lability of this virus, its close association with the host cell membrane, and its tendency to aggregate during concentration procedures. We describe a comparative study of various purification media as well as a novel approach for obtaining high titers of HRSV. Virus was produced in HEp-2 cells grown on gelatin beads and concentrated either by precipitation with polyethylene glycol or by centrifugation over a sucrose cushion, the latter yielding 100% recovery. These procedures employed EDTA to disaggregate the virus and MgSO4 as a virus stabilizing agent. Attempts to purify HRSV over percoll, renografin and metrizamide gradients lead to loss of infectivity. Sucrose was found to represent the best purification medium with a yield of up to 60%.

Rapid titration of bovine, caprine and human RS virus by a micro-immunoperoxidase assay using a monoclonal antibody and a permissive ovine kidney cell line

An indirect immunoperoxidase micro-assay, using a continuous cell line derived from ovine kidney cells (OK) and a previously characterized monoclonal antibody (7C2), specific for an exposed and highly conserved epitope of the fusion protein of different strains of RS virus, was used advantageously to rapidly titrate bovine, caprine and human strains of RSV by either quantal (TCID50) or plaque forming assays. Virus titers, obtained in less than 36 h, were in agreement with those obtained by the conventional plaque assays which required an incubation period of 4 days or more. This assay is also applicable to micro-neutralization of fusion inhibition assays for testing serum or screening monoclonal antibodies.

Modified immunoprecipitation procedure for the identification of human respiratory syncytial virus polypeptides

When analysed by polyacrylamide gel electrophoresis, human respiratory syncytial virus harvested after a one step growth cycle and purified through a continuous sucrose density gradient was shown to be composed of nine structural proteins of 90, 68, 49, 42, 34, 28, 25, 19 an 13 kd. The 90, 49 and 19 kd polypeptides were identified as glycopolypeptides by glucosamine incorporation. A modified immunoprecipitation procedure confirmed the viral specificity of the 49, 42, 28, 25 and 19 kd polypeptides.

Concentration of human respiratory syncytial virus using ammonium sulfate, polyethylene glycol or hollow fiber ultrafiltration

Human respiratory syncytial virus was concentrated by polyethylene glycol or ammonium sulfate precipitation as well as by hollow ultrafiltration. Recoveries obtained were respectively 49.4%, 47.7%, and 75.2%; however, further analysis of these results by resuspension experiments showed that all the infectivity could be recovered from the different concentrates. The protein content of polyethylene glycol concentrates was much lower than those of ammonium sulfate or hollow fiber ultrafiltration. Electron microscopy revealed that the morphological integrity of virus particles was unaffected by the concentration methods used. Purified virus was obtained when polyethylene glycol concentrates were centrifuged through two successive density gradients.