Detection of Semliki Forest virus in cell culture by use of an enzyme immunoassay with peroxidase-labeled monoclonal antibodies specific for glycoproteins E1 and E2

Neutralization enzyme immunoassay for influenza virus

A neutralization enzyme immunoassay (N-EIA) was developed for the detection of antibody titer rises in sera of patients infected with influenza A (H3N2) virus. In this N-EIA, a selected strain of influenza A (H3N2) virus was added to monolayers of LLC-MK2 cells in microtiter plates. After 24 h, the replicated virus could be demonstrated with a virus-specific enzyme-labeled monoclonal antibody. Preincubation of the influenza virus with convalescent-phase sera of patients infected with influenza A (H3N2) virus resulted 1 day later in decreased absorbance values that could be used for calculation of neutralization titers. From use of paired serum samples from 10 patients with a history of flu-like symptoms, the results obtained with N-EIA correlated well (r = 0.83) with those of the standard hemagglutination inhibition test.

Inhibition of Semliki Forest virus multiplication in L-cells by combinations of interferon and ribavirin as measured by plaque titration and direct enzyme immunoassay

Inhibition of Semliki Forest virus (SFV) multiplication in L-cell monolayers by combinations of mouse interferon (IFN) and ribavirin was measured by plaque titration and by direct enzyme immunoassay of SFV in L-cells. When critically inhibitory quantities of IFN and ribavirin were combined, an additive inhibitory effect was observed in either assay.

Competition binding assay in cell culture for identification of epitopes on enveloped and naked viruses

Virus infected monolayers, in wells of 96-well plates, could be used as antigen in competition binding assays to identify epitopes on respectively Semliki Forest virus, encephalomyocarditis virus and mumps virus.

Application of immunoassay of encephalomyocarditis virus in cell culture with enzyme-labeled virus-specific monoclonal antibodies for rapid detection of virus, neutralizing antibodies, and interferon

Encephalomyocarditis virus (EMCV)-specific monoclonal antibody UM 21.1 labeled with horseradish peroxidase was used to detect EMCV in L-cell monolayers. This direct enzyme immunoassay of EMCV, performed in wells of 96-well plates, could be applied for various purposes, such as early detection of virus multiplication, determination of 50% tissue culture infective doses, and rapid titration of interferon and EMCV-neutralizing antibodies. Multiplication of EMCV is indicated by a rapid increase of the absorbance values measured against EMCV-infected L cells starting as early as 4.5 h after virus inoculation. The early rise of absorbance (i.e., virus multiplication) is inhibited by interferon, allowing its rapid titration. Preincubation of the virus inoculum with neutralizing antibodies also yielded decreased absorbance values. With the latter enzyme immunoassay for neutralizing antibodies, performed after an infection period of 8 h, antibody titers measured were comparable to those obtained with a conventional plaque reduction test. We assume that similar assays could be developed for other picornaviruses (e.g., polioviruses).

Enzyme immunoassay of mumps virus in cell culture with peroxidase-labelled virus specific monoclonal antibodies and its application for determination of antibodies

Mumps neutralizing monoclonal antibodies (MAs) were purified and labelled with horseradish peroxidase and used to detect virus-infected Vero cells, which were seeded as monolayers in wells of 96-well plates. This direct enzyme immunoassay (EIA) in cell culture proved to be a sensitive method for detection and titration of mumps virus and it may be useful for diagnostic purposes. The EIA is also suitable for the rapid determination of neutralizing antibodies. Neutralization of mumps virus by preincubation with either monoclonal or polyclonal antibodies was indicated by inhibition of the absorbance at 450 nm as measured with a multichannelled photometer. The EIA (duration 2 days) for determination of mumps neutralizing antibodies is an attractive alternative for the plaque reduction test (duration 6 days).

Rapid determination of neutralizing antibodies to Semliki Forest virus in serum by enzyme immunoassay in cell culture with virus-specific monoclonal antibodies

We describe in this study a rapid enzyme immunoassay for the titration of neutralizing antibodies in serum against Semliki Forest virus. For this assay L cells were added to preincubated virus-antiserum mixtures to form monolayers. Six hours after infection by residual, nonneutralized virus, the monolayers were fixed, and the E2 glycoprotein of Semliki Forest virus on the surface of infected cells was quantified with an E2-specific, peroxidase-labeled monoclonal antibody (UM 5.1). The serum antibody titer was defined arbitrarily as the inverse value of that dilution of serum associated with a 25% inhibition of control absorbance values. These titers of both early and later mouse immune sera were similar to those determined in simultaneously performed 50% plaque reduction tests. The results indicate that the enzyme immunoassay (duration, 9 h) is reliable and compares favorably with the conventional plaque reduction test (duration, 25 h) in rapidity, ease of performance, and objectivity.

The role of complement in monoclonal antibody-mediated protection against virulent Semliki Forest virus

Monoclonal antibodies (MAs), specific for either the E1 or E2 glycoproteins of Semliki Forest virus (SFV), and belonging to various immunoglobulin subclasses (IgM, IgG2a, IgG2b and IgG3), effected lysis of SFV-infected L cells in co-operation with guinea-pig complement. In this antibody-dependent complement-mediated cytolysis (ADCMC) test, IgG1 MAs were not effective although these antibodies recognize the viral antigens on the surface of SFV-infected L cells. The latter was shown with horseradish peroxidase (HRPO)-labelled MAs in a direct enzyme immunoassay. The binding reactivities of HRPO-labelled MAs to infected L cells at selected time-intervals after infection correlated well with the amount of cytolysis in a parallel ADCMC test. Cytolysis was dependent on the duration of incubation with antibodies: more cytolysis was measured after a 4-hr incubation period with MA, starting at 4 hr after infection, compared to a 1-hr incubation period starting after 7 hr of infection. However, in the latter case (1-hr period) the amount of cytolysis measured correlated better to neutralization and/or protection by MAs than after the extended period (4 hr) of incubation. Complement (C3) depletion by cobra venom factor treatment led to a higher mortality and viraemia of mice prophylactically injected with critically protective doses of either the neutralizing MA UM 8.4 (IgM) or the non-neutralizing MA UM 4.2 (IgG2a). The results suggest a co-operative role of MA with complement in mediating protection against SFV. Passive immunization by administration of low amounts (0.1 micrograms/mouse) of neutralizing MA UM 5.1 resulted in protection of normal mice against a lethal infection with SFV. Mice immunosuppressed by cyclophosphamide were not protected by these doses. If the doses were increased however, these mice were protected both prophylactically and therapeutically. These results indicate that, using critical doses of MAs, an intact immune system ensures survival in normal mice after infection with virulent SFV.

Antigenic differences between virulent and avirulent strains of Semliki Forest viruses detected with monoclonal antibodies. Brief report

Eleven monoclonal antibodies (MAs) reacted strongly in an enzyme immunoassay with virulent Semliki Forest virus (SFV) replicating in L cell monolayers. Three MAs showed a considerably diminished reaction with an avirulent strain of SFV both in enzyme immunoassays and plaque reduction tests.

Determination of inhibitory concentrations of antiviral agents in cell culture by use of an enzyme immunoassay with virus-specific, peroxidase-labeled monoclonal antibodies

An enzyme immunoassay (EIA) to determine 50% inhibitory concentrations of drugs which suppress Semliki Forest virus replication is described. Inhibition of virus replication was measured in L-cells, seeded as monolayers in 96-well plates by use of horseradish peroxidase-labeled monoclonal antibodies directed against the E1 glycoprotein of Semliki Forest virus. The antiviral agents tested were cycloheximide, tunicamycin, NH4Cl, and disodium cromoglycate. The 50% inhibitory concentration of these antiviral agents was arbitrarily defined as the concentration of drug, in culture medium, associated with 50% reduction of the control absorbance value measured on Semliki Forest virus-infected cells without drug in the culture fluid. Twenty-two hours after infection the 50% inhibitory concentrations of the drugs were 0.2 microgram/ml for cycloheximide, 0.8 microgram/ml for tunicamycin, 0.3 mg/ml for NH4Cl, and 4.9 mg/ml for disodium cromoglycate. These values are similar to those determined by others with conventional methods of virus quantification. This test is sensitive and easy to perform and therefore is suited for large-scale experiments.

Mechanisms of monoclonal antibody-mediated protection against virulent Semliki Forest virus

Both neutralizing and nonneutralizing immunoglobulin G2a monoclonal antibodies (MAs) directed against the E2 glycoprotein of Semliki Forest virus (SFV) protected mice prophylactically and therapeutically against virulent SFV infection. The neutralizing MAs, however, conferred protection to mice at lower doses than did nonneutralizing MAs. The antibody-dependent, complement-mediated cytolysis of SFV-infected L cells was effectuated by both kinds of antibodies, but again neutralizing MAs were more effective. Removal of the Fc part of the neutralizing MA UM 5.1 by pepsin digestion resulted in a 100-fold reduction of the neutralization titer (10(4) versus 10(6)) and a complete loss of its capacity to mediate antibody-dependent, complement-mediated cytolysis. Passive protection of infected mice occurred only after administration of relatively high doses of F(ab')2 of MA UM 5.1 (30.0 micrograms versus 0.1 microgram). F(ab')2 fragments prepared from the nonneutralizing MA UM 4.2 had lost their protective capacity completely. Surprisingly, the nonneutralizing MA UM 4.2 retarded virus growth in mouse fibroblasts (L cells), although inhibition was at much higher doses than with the neutralizing MA UM 5.1. Furthermore, both MAs promoted the uptake of virulent SFV in the Fc receptor-bearing WEHI-3 cells. The results suggest that nonneutralizing MAs protect mice not only by antibody-dependent, complement-mediated cytolysis but also by growth inhibition and enhanced uptake of SFV in the nonpermissive macrophages of BALB/c mice. This hypothesis is supported by the absence of viremia in recipients of nonneutralizing MA UM 4.2 at 24 h after infection.