End-point Dilution Assay Research Articles

A rapid method for titrating baculovirus stocks using the Sf-9 Easy Titer cell line

A new rapid method for titrating baculovirus stocks has been developed using a novel cell line Sf-9 Easy Titer (Sf-9ET). The Sf-9ET cell line has been transfected with plasmid DNA containing the enhanced green fluorescent protein (eGFP) gene under the control of the baculovirus polyhedrin promoter. When used in the titration assay, the Sf-9ET cells turn green when they are infected with baculovirus due to the activation of the polyhedrin promoter/eGFP complex by baculovirus gene products expressed during the infection. Using a 96-well plate format end-point dilution assay, baculovirus titers can be determined in three days using a fluorescence microscope.

Feline calicivirus에서 항바이러스 활성을 가지는 천연식물자원 탐색

노로바이러스는 인간에게 급성위장염을 일으키는 식중독 바이러스로, 최근 전세계적으로 노로바이러스 식중독 사고가 증가하고 있다. 따라서 본 연구에서는 다양한 생리활성 작용이 있다고 알려진 향신료, 차(茶), 식물성 한약재등 11종의 천연물질을 대상으로 노로바이러스의 대체모델인 feline calicivirus (FCV 에 대한 항바이러스 활성을 조사하였다. 각 추출물에 대한 항바이러스 활성은 end point dilution assay 방법인 50% tissue culture infectious dose (<TEX>$TCID_{50}$</TEX>)으로 바이러스 감염가를 측정하였다. 녹차(Camellia sinensis L.) 추출물의 경우 3.13 mg/ml의 농도에서 1 시간만에 FCV가 완전히 불활성화 되었으며, 홍화씨 (Carthamus tinctorius L.) 추출물은 6.25 mg/ml의 농도에서 5시간 만에 바이러스가 불활성화 되어 녹차추출물이 FCV에 대한 항바이러스 활성이 가장 뛰어난 것으로 조사되었으며, 농도 및 반응시간에 유의적으로 항바이러스 활성을 나타내는 것으로 관찰되었다. 녹차 및 홍화씨 추출물 등과 같은 천연식물 자원을 이용하여 FCV에 대한 항바이러스 활성을 조사한 결과는 궁극적으로는 노로바이러스에 의한 식중독 저감을 위한 기초자료에 이용될 수 있을 것으로 기대된다. In an effort to discover an antiviral substance against noroviruse (NV), which causes gastroenteritis illness world-wide, several plants including spices and herbs were evaluated for their antiviral activities against feline calicivirus (FCV) as a surrogate for NV. Among them, methanolic extract of green tea (Camellia sinensis L.) exhibited significant antiviral activity against FCV. After treatment with green tea extract (3.13 mg/ml) for 1 hr, FCV was completely inactivated. The antiviral activity of green tea extract against FCV was also determined to be dose and time- dependent. The results obtained in this study suggested that green tea will be effective in the prevention of food-borne diseases caused by NV.

Construction of a RU486 inducible recombinant adenoviral vector carrying murine interleukin-12 gene and experimental treatment of colonic carcinoma

To construct a RU486 inducible recombinant adenovirus of murine IL-12 protein and study its effect and safety on colonic cancer. The replication-defective recombinant adenovirus were produced after cotransfection of shutter vector pDC-RUmIL-12 and adenovirus DNA helper plasmid pBHGloxDeltaE1, 3Cre into HEK293 cells. The recombined adenovirus was purified by CsCl density gradient centrifugation and its titer was determined by end point dilution assay. Expression of this regulatable recombinant adenovirus vector in infected C26 colonic carcinoma cells was tested by ELISA kit in vitro. The tumor model was established by hypodermic inoculation of C26 cells. Sixty tumor-bearing mice were randomly divided into 4 groups: Ad-buffer group; Ad-RUmIL-12 group; Ad-RUmIL-12 + RU486 group and Ad-mIL-12 group, and the treatment effects and side effects were evaluated. The adenoviral vector containing murine IL-12 gene was identify by PCR. The viral titer of Ad-RUmIL-12 was 4.62 x 10(10) pfu/ml. The expression of IL-12 protein was induced by the RU486 and the highest expression (516 +/- 43) pg/ml whereas no significant IL-12 protein was detected without inducer or getting rid of the inducer [(38 +/- 3) pg/ml and (42 +/- 5) pg/ml respectively]. The tumor size increased rapidly in group Ad-buffer and group Ad-RUmIL-12 (P > 0.05). Administration of Ad-RUmIL-12 + RU486 and Ad-mIL-12 were showed to delay markedly the growth of transplanted C26 tumor (P > 0.05). Significantly necrosis was observed in both Ad-mIL-12 and Ad-RUmIL-12 + RU486 experimental groups, but the level of the serum alanine transaminase and the rate of side effect was higher in Ad-mIL-12 group (4/15 and 10/15 respectively, P < 0.05). A RU486 regulatable recombinant adenoviral vector containing IL-12 gene was successfully constructed. The expression of vector Ad-RUmIL-12, regulated by inducer RU486 in vivo, can obviously improve safety in tumor treatment and provide a good primer for further researches on in vivo gene therapy.

Error assessment in recombinant baculovirus titration: Evaluation of different methods

The success of baculovirus/insect cells system in heterologous protein expression depends on the robustness and efficiency of the production workflow. It is essential that process parameters are controlled and include as little variability as possible. The multiplicity of infection (MOI) is the most critical factor since irreproducible MOIs caused by inaccurate estimation of viral titers hinder batch consistency and process optimization. This lack of accuracy is related to intrinsic characteristics of the method such as the inability to distinguish between infectious and non-infectious baculovirus. In this study, several methods for baculovirus titration were compared. The most critical issues identified were the incubation time and cell concentration at the time of infection. These variables influence strongly the accuracy of titers and must be defined for optimal performance of the titration method. Although the standard errors of the methods varied significantly (7–36%), titers were within the same order of magnitude; thus, viral titers can be considered independent of the method of titration. A cost analysis of the baculovirus titration methods used in this study showed that the alamarblue, real time Q-PCR and plaque assays were the most expensive techniques. The remaining methods cost on average 75% less than the former methods. Based on the cost, time and error analysis undertaken in this study, the end-point dilution assay, microculture tetrazolium assay and flow cytometric assay were found to be the techniques that combine all these three main factors better. Nevertheless, it is always recommended to confirm the accuracy of the titration either by comparison with a well characterized baculovirus reference stock or by titration using two different methods and verification of the variability of results.

A Pyruvate Cycling Pathway Involving Cytosolic NADP-dependent Isocitrate Dehydrogenase Regulates Glucose-stimulated Insulin Secretion

Glucose-stimulated insulin secretion (GSIS) from pancreatic islet beta-cells is central to control of mammalian fuel homeostasis. Glucose metabolism mediates GSIS in part via ATP-regulated K+ (KATP) channels, but multiple lines of evidence suggest participation of other signals. Here we investigated the role of cytosolic NADP-dependent isocitrate dehydrogenase (ICDc) in control of GSIS in beta-cells. Delivery of small interfering RNAs specific for ICDc caused impairment of GSIS in two independent robustly glucose-responsive rat insulinoma (INS-1-derived) cell lines and in primary rat islets. Suppression of ICDc also attenuated the glucose-induced increments in pyruvate cycling activity and in NADPH levels, a predicted by-product of pyruvate cycling pathways, as well as the total cellular NADP(H) content. Metabolic profiling of eight organic acids in cell extracts revealed that suppression of ICDc caused increases in lactate production in both INS-1-derived cell lines and primary islets, consistent with the attenuation of pyruvate cycling, with no significant changes in other intermediates. Based on these studies, we propose that a pyruvate cycling pathway involving ICDc plays an important role in control of GSIS.

Flow Cytometry-Based Assay for Titrating Dengue Virus

Plaque assays for titrating dengue virus (DENV) are time-consuming and not suitable for strains that do not plaque. Fluorescence-activated cell sorting (FACS) has been used to detect DENV-infected cells. Here we describe a FACS-based assay for titrating DENV. We determined that at 24 h postinfection, the number of infected cells detected by FACS represented the first round of infection and therefore could be used as a readout of the number of infectious particles in the inoculum. When the titers of different laboratory and clinical strains of DENV were compared using FACS, plaque, and endpoint dilution assays, for most strains the FACS titers were comparable to titers obtained by plaque or endpoint dilution assays. The FACS assay is an improvement over the plaque assay because the infection period is reduced from 5 to 7 days to 24 h and the assay can be used to titrate clinical isolates that frequently do not form clear plaques on cell monolayers. The novel FACS-based methods described here will facilitate laboratory studies of dengue.

Using QPCR to assign infectious potencies to adenovirus based vaccines and vectors for gene therapy: toward a universal method for the facile quantitation of virus and vector potency

The assignment of infectious potency to test articles of adenovirus has been conducted mainly using classical end-point dilution methods, which rely on virus induced cytopathology to reveal the presence of infectious virus. These assays suffer the disadvantages of labor intensity, duration, throughput restriction and variability. In the course of our development of an Ad5 based HIV vaccine for clinical evaluation, we sought a facile method for the assignment of potency to the numerous test articles generated during the development of bioprocesses for bulk manufacture, downstream purification and formulation. In this paper we describe a quantitative PCR based potency assay (QPA) which uses QPCR to quantitate adenovirus genomes replicated 24 h after the inoculation of a test article on 293 cell monolayers, and then relates that mass to potency by interpolation to a standard curve of replicated adenovirus genomes constructed with a reference adenovirus standard to which infectious potency has been previously assigned in the classical end-point dilution assay. The QPA assay for adenovirus is simple and rapid, with a throughput capacity adequate to the potency assay demands of bioprocess development, and with a precision expressed as a root variability of 16.8% R.S.D., allowing for close discriminations of the products of alternative process configurations. The adenovirus QPA principle can be applied to the quantitation of infectious potency of both RNA and DNA viruses and we report briefly on the development of QPA assays for measles and mumps. QPA assays owing to their simplicity and easy automation, rapidity, capacity and precision hold promise to become widely practiced methods for the quantitation of the potency of live virus vaccines and other recombinant virus vectors.

Essential Role of Synoviolin in Embryogenesis

We recently reported the importance of Synoviolin in quality control of proteins through the endoplasmic reticulum (ER)-associated degradation (ERAD) system and its involvement in the pathogenesis of arthropathy through its anti-apoptotic effect. For further understanding of the role of Synoviolin in vivo, we generated in this study synoviolin-deficient (syno(-/-)) mice by genetargeted disruption. Strikingly, all fetuses lacking syno died in utero around embryonic day 13.5, although Hrd1p, a yeast orthologue of Synoviolin, is non-essential for survival. Histologically, hypocellularity and aberrant apoptosis were noted in the syno(-/-) fetal liver. Moreover, definitive erythropoiesis was affected in non-cell autonomous manner in syno(-/-) embryos, causing death in utero. Cultured embryonic fibroblasts derived from syno(-/-) mice were more susceptible to endoplasmic reticulum stress-induced apoptosis than those from syno(+/+) mice, but the susceptibility was rescued by overexpression of synoviolin. Our findings emphasized the indispensable role of the Synoviolin in embryogenesis.

Trypanosome Cyclic Nucleotide Phosphodiesterase 2B Binds cAMP through Its GAF-A Domain

Trypanosoma brucei, the causative agent of sleeping sickness in humans and livestock, expresses at least three cAMP-specific class I phosphodiesterases (PDEs), all of which are essential for survival of the parasite. These PDEs have either one or two N-terminal GAF domains, which in other proteins function as signaling domains. However, neither the functional roles nor ligands for these domains in trypanosome PDEs are known. The present study shows that TbPDE2B, which contains two tandem GAF domains, binds cAMP with high affinity through its GAF-A domain. A purified recombinant N terminus + GAF-A domain binds cAMP with an affinity (Ki) of approximately 16 nM. It also binds cGMP but with a 15-fold lower affinity of approximately 275 nM. The TbPDE2B holoenzyme has a somewhat lower affinity (approximately 55 nM) for cAMP but a greatly lower affinity (approximately 10 microM) for cGMP. This suggests that both the selectivity and affinity for a ligand can be determined not only by the nature of the binding domain but also by the adjacent domains. Additionally, binding of cAMP to the holoenzyme showed positive cooperativity, with a Hill coefficient value of 1.75. However, binding of cGMP to the holoenzyme did not show any cooperativity, suggesting differences in the conformational changes caused by binding of these two cyclic nucleotides with the protein. Point mutation of a key predicted binding site residue (T317A) resulted in a complete loss of high affinity cAMP binding. This mutation increased the apparent Km of the mutant enzyme for substrate without altering the Vmax. A truncated catalytic domain construct of TbPDE2B also exhibited an increased Km, strongly suggesting that cAMP binding to the GAF-A domain can regulate TbPDE2B by allowing the full activity of the enzyme to be expressed. These properties of the GAF-A domain of TbPDE2B thus suggest that it could be a new target for anti-trypanosomal drugs.

Inactivation of adenovirus types 5 and 6 by Virkon ® S

Throughout the pharmaceutical industry, adenovirus-based products are being developed for human use as vaccine vectors and gene therapy delivery vehicles. The implementation of effective decontamination procedures is critical to the successful manufacture of these products to minimize the risk of personnel exposure and prevent product cross-contamination in the manufacturing facility. In this investigation, we have conducted small-scale decontamination studies to determine the efficacy of Virkon ® S on the inactivation of adenovirus types 5 and 6 in suspension. Virkon ® S is a commercially available oxidative disinfectant used against a variety of bacteria, spores, fungi, and viruses. A cytotoxicity-based endpoint dilution assay was used to quantify adenovirus potency before and after Virkon ® S treatment. We show that the level of organic content in the inactivation sample matrix has a significant impact on Virkon ® S activity. The potency of adenovirus types 5 and 6 was reduced by greater than six logs upon a five minute exposure to the appropriate concentration of Virkon ® S. Based on these results, we propose that Virkon ® S liquid decontamination procedures for adenovirus types 5 and 6 use 0.9% Virkon ® S for contact times greater than five minutes.

Evaluation of three Copan viral transport systems for the recovery of cultivatable, clinical virus isolates

Three Copan viral transport media (VTM) were compared to the BBL Viral Culturette and Micro Test M4 systems for their ability to maintain the viability of clinical strains of enterovirus, herpes simplex virus, adenovirus and parainfluenza virus. VTM were inoculated, incubated up to 72 h at both 4°C and 22°C, and processed for cell culture. The concentration of infectious virus in VTM was determined by standard endpoint dilution assay. All VTM maintained viable enterovirus and adenovirus for 72 h but adenovirus from M4 and Copan CVM media were detected in cell culture at least one day sooner than from other media. Only M4 and Copan CVM media supported the recognition of low-titer herpes simplex virus in cell culture within 24 h. Parainfluenza virus was consistently detected in culture within six days when maintained in M4 and two of the Copan VTM media. Overall, the Copan CVM compared favorably to the M4 in maintaining concentrations of viable virus that could be recovered and identified rapidly in routine cell culture.

Candida albicansKilling by RAW 264.7 Mouse Macrophage Cells: Effects ofCandidaGenotype, Infection Ratios, and Gamma Interferon Treatment

Phagocytic cells such as neutrophils and macrophages are potential components of the immune defense that protects mammals against Candida albicans infection. We have tested the interaction between the mouse macrophage cell line RAW 264.7 and a variety of mutant strains of C. albicans. We used an end point dilution assay to monitor the killing of C. albicans at low multiplicities of infection (MOIs). Several mutants that show reduced virulence in mouse systemic-infection models show reduced colony formation in the presence of macrophage cells. To permit analysis of the macrophage-Candida interaction at higher MOIs, we introduced a luciferase reporter gene into wild-type and mutant Candida cells and used loss of the luminescence signal to quantify proliferation. This assay gave results similar to those for the end point dilution assay. Activation of the macrophages with mouse gamma interferon did not enhance anti-Candida activity. Continued coculture of the Candida and macrophage cells eventually led to death of the macrophages, but for the RAW 264.7 cell line this was not due to apoptotic pathways involving caspase-8 or -9 activation. In general Candida cells defective in the formation of hyphae were both less virulent in animal models and more sensitive to macrophage engulfment and growth inhibition. However the nonvirulent, hypha-defective cla4 mutant line was considerably more resistant to macrophage-mediated inhibition than the wild-type strain. Thus although mutants sensitive to engulfment are typically less virulent in systemic-infection models, sensitivity to phagocytic macrophage cells is not the unique determinant of C. albicans virulence.

Enhanced detection of viral hemorrhagic septicemia virus (a salmonid rhabdovirus) by pretreatment of the virus with a combinatorial library-selected peptide

A 17-mer peptide (SAAEASAKATAEATAKG, p5) was selected by screening a combinatorial library for its ability to enhance in vitro the infectivity of viral hemorrhagic septicemia virus (VHSV), a salmonid rhabdovirus. Preincubation of VHSV samples with p5 at micromolar concentrations led to up to 5-fold increase of viral titers compared to untreated samples, as measured by a 1-day post-infection immunochemical focus assay. Treatment with p5 also increased VHSV titers when using the more traditional plaque and end-point dilution assays. Preincubation of p5 with infectious hematopoietic necrosis virus (another rhabdovirus of salmonids), but not with infectious pancreatic necrosis virus (birnavirus) also led to a similar increase in sensitivity. These results indicate that the addition of p5 may be used to improve the sensitivity of diagnostic tests for salmonid rhabdoviruses.

Quantitation of baculovirus particles by flow cytometry

A method using flow cytometry (FCM) analysis was developed to quantitate baculovirus total particles produced in insect cell cultures. The method is a direct count of particles and involves staining of the baculovirus DNA with SYBR Green I, a highly fluorescent nucleic acid specific dye. Sample preparation of cell-free supernatant containing budded viral particles involves fixation with paraformaldehyde, freeze–thaw treatment, viral membrane permeabilization with Triton X-100, and sample heating to improve staining efficiency and enhance baculovirus particle green fluorescence intensities. In this study, the effects of the different treatment steps and medium composition on viral particle counts were examined in order to identify optimal preparation conditions. FCM analysis linearity was established over a viral concentration range of two logs with a lower detection limit at 10 5 viral particles per ml. Robustness and reproducibility of the method were assessed using samples from large-scale bioreactor cultures. The events (or virus particle counts) obtained by FCM analysis were usually higher than the titres obtained by end-point dilution assay (EPDA). Results from 16 different viral stocks showed an average ratio of 3.7 total particles (FCM) to infectious particles (EPDA). Essentially, the FCM analysis reported below shortens baculovirus quantitation time to 2 h and provides a good estimation of virus titers. It is believed that these findings will contribute to acceleration of process development in the area of baculovirus expression technology in general and specifically in process where stoichiometric multi-viral infections of cells are critical to the expression of complex products.

Replication of Chilo iridescent virus in the cotton boll weevil, Anthonomus grandis, and development of an infectivity assay.

The boll weevil, Anthonomus grandis Boheman, is a devastating pest of cotton. Chemical pesticides are problematic due to relative lack of target specificity and resistance. Microbial pesticides may provide viable alternatives because of their narrow host range. Chilo iridescent virus (CIV) is the type species for genus Iridovirus, family Iridoviridae: large, icosahedral cytoplasmic viruses containing a double-stranded DNA genome. Earlier work suggested that CIV replicated in the boll weevil; however, efficiency or production of infectious virus was not established. We showed that CIV undergoes a productive cycle in A. grandis. CIV DNA levels in boll weevil pupae increased significantly from 0 to 3 days post infection. Moreover, virogenic stromata and complete virus particles were observed in the cytoplasm by 7 days. An endpoint dilution assay using viral DNA replication as indicator suggested a 10(5)-fold increase in infectious virus titer over 7 days. This is the first such demonstration in larval infections with genus Iridovirus. Our study establishes that CIV undergoes a productive cycle in the boll weevil and provides an important and useful model system for replication at the organismal level. These results have important implications for the potential of CIV and its components in boll weevil control.

Photodynamic treatment of pooled coumarin plasma for external quality assessment of the prothrombin time

Aims—To determine the conditions of photodynamic inactivation of vesicular stomatitis virus (VSV) added to pooled coumarin plasma and the effects of the photodynamic treatment on the prothrombin times and international...

Primary targets for photoinactivation of vesicular stomatitis virus by AIPcS4 or Pc4 and red light.

Phthalocyanines are useful sensitizers for the photodynamic sterilization of red blood cell concentrates. The mechanism of photoinactivation of lipid-enveloped viruses is not completely understood. Vesicular stomatitis virus (VSV) was used as a model virus to study the primary targets of photoinactivation by aluminum phthalocyanine tetrasulfonate (AIPcS4) or silicon phthalocyanine HOSiPcOSi(CH3)2(CH2)3N(CH3)2 (Pc4) and red light. Inactivation conditions for VSV in buffer were determined using an end point dilution assay, and viral RNA synthesis in host cells was measured to determine the loss of infectivity in a direct way. The very rapid decrease in the viral RNA synthesis after photodynamic treatment was correlated with respect to different potential primary targets that are involved in different steps of the viral replication cycle. Damage to the viral proteins, induced by treatment with AIPcS4 or Pc4 and analyzed by gel electrophoresis, could not account for the observed loss of infectivity. Binding of VSV to host cells was only slightly impaired after photodynamic treatment with both sensitizers and could therefore not be responsible for the rapid decrease in viral RNA synthesis in cells. A very strong inhibition of viral RNA polymerase activity after treatment with AIPcS4 and red light was detectable using an in vitro assay. This decrease correlated well with the loss of infectivity, indicating that either the RNA or the viral RNA polymerase is the primary target for photoinactivation of VSV with AIPcS4. Treatment with Pc4 did not cause inhibition of viral RNA polymerase activity to an extent that could account for the observed very rapid loss of infectivity. It was therefore concluded that neither the viral proteins nor the binding to the host cells nor the RNA or RNA polymerase are the primary targets for photoinactivation of VSV by Pc4.

Optimization of an assay for baculovirus titer and design of regimens for the synchronous infection of insect cells.

We have previously described a quantitative model of the trafficking of baculovirus in insect cells that considers the various infection steps such as attachment, internalization, endosomal fusion, and nuclear accumulation. Concepts from the model were used to design synchronous infection regimens for various cell lines, to analyze the inherent inefficiency of existing assays for virus titer, and to develop a modified end-point dilution assay optimized to measure more completely the concentration of intrinsically infectious particles in a virus inoculum. The titer obtained from existing assays incompletely counts infectious virus particles due primarily to the incomplete adsorption of the virus during the short, standard 1-h incubation period. For representative assays, the calculated bound virus is generally about 10% of the added virus, but could be as low as 1.4%, underestimating actual titers by 3-70-fold. A modified end-point dilution assay involving centrifugation has been developed from both quantitative and qualitative analyses. The ratio of particle to plaque-forming unit with the optimized assay was 4-6 compared to 100-300 for typical assays, representing a significant improvement in the measurement of total infectious virus concentration.

Neutralization of primary human immunodeficiency virus type 1 isolates: a study of parameters implicated in neutralization in vitro.

Various studies have reported that primary human immunodeficiency viruses seem to be more refractory to neutralization by HIV-positive sera than T cell line-adapted strains. In this study we also show that adaptation of the HIV-1SF-2 strain, produced in PBMCs, to the cell line CEM-SS renders this isolate sensitive to neutralization by almost all the sera tested. Further neutralization studies should thus focus on the development of an assay involving primary isolates in order to detect antibodies having a neutralizing activity in vivo. Neutralization protocols currently use either an antibody end-point dilution assay, which combines a fixed inoculum of virus with serial dilutions of antibody, or an infectivity reduction assay, which uses serial dilutions of virus with a single dilution of antibody. We have developed an assay designed for studying the neutralization of primary isolates that combines these two approaches. Performing the assay on PBMCs allows all primary isolates to be analyzed, not just those multiplying in T cell lines. The neutralizing titer measured on PBMCs for human HIV-positive sera is low, but reproducible and independent of the virus titer in a given experiment. It can be increased about five-fold by changing the temperature and duration of virus-serum interaction (overnight at 4 degrees C instead of 1 hr at 37 degrees C). These results emphasize the need for a relevant neutralization assay involving primary isolates and primary cells for a better understanding of the role of humoral response in HIV infection.

Titration of human-bovine rotavirus reassortants using a tetrazolium-based colorimetric end-point dilution assay.

A colorimetric end-point dilution assay was developed for the titration of rotavirus-containing samples that uses commercially available tetrazolium dyes as an indicator of virus infection. This assay offers several advantages over both plaque assays and traditional end-point dilution methods. The latter assays require manual counting of plaques or the scoring of wells for the presence of virus based on observed cytopathic effects. The colorimetric end-point dilution assay enables the scoring of wells based upon absorbance readings alone, thereby eliminating time-consuming and subjective manual screenings. This method also has the potential for automating the analysis of large numbers of samples. Virus titers of human-bovine rotavirus reassortants obtained using this method are comparable to those determined by plaque assay. The scoring of wells based on absorbance readings was also found to agree with manual scoring of cytopathic effects and with the production of viral antigen.