Infectivity titration of the fast-replicating and cytopathic hepatitis A virus strain HM175A.2 by an in situ enzyme immunoassay

The use of RT-PCR for determination of separate end-points for the strains IB H120 and IB D274 in titration of the combination vaccine Poulvac IB® primer

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Reading of live virus titrations by PCR.

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Determination of endpoints for different viruses in one titration by PCR.

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Similarity of titration results of reading by visual examination and reading by PCR.

Poulvac IB^® Primer is a lyophilized vaccine containing two attenuated infectious bronchitis strains in one vial, IB H120 and IB D274. For quantification of the viral content of the vaccine, dilution series of the final product are inoculated in embryonated chicken eggs. After the incubation period of seven days standard practice is for the embryos to be taken from each egg and examined visually for IB specific lesions; these readings are used to determine an end-point in viral titrations. The result is a titre value to which both strains contribute. However, it is not clear what the live virus titre is for strain IB H120 and for strain IB D274. In order to determine end-points in the titration for each of the two strains, we collected the allantoic fluids from each egg after the incubation period and tested these for the presence of IB H120 and IB D274 by a strain specific reverse phase PCR. Based on the data obtained by PCR we were able to determine an end-point for each of the two strains. For a given commercial batch of Poulvac IB primer we determined titres of 10^6.31 EID₅₀ per vial for IB H120 and 10^6.59 EID₅₀ for IB D274 using PCR for end-point determination. These end-points matched well with the end-point determined for both strains cumulatively after visual examination, i.e. 10^6.67 EID₅₀ per vial. It is concluded that PCR is a suitable means to determine end-points in titrations of live viruses.

In situ enzyme immunoassay for titration of a Brazilian hepatitis A virus strain (HAF-203)

Hepatitis A virus (HAV) replicates relatively slowly in cell culture without a cytopathic effect, a fact that limits the use of tissue culture assays. The radioimmunofocus assay is the standard method for HAV titration, although it is labor intensive and requires the use of radioisotopes. A simple, rapid and objective infectivity assay based on an in situ enzyme immunoassay (EIA) is described here for a Brazilian cell culture-adapted HAV strain (HAF-203). The assay uses a peroxidase-labeled polyclonal antibody to fixed monolayers as an indicator of infection. EIA may be completed within 7 days using serial 5-fold dilutions of the virus, yielding a titer of 5.024 log 50% tissue culture infective dose (TCID50)/ml for HAF-203. This technique had a detection limit of 1.1 log TCID50/ml and the specificity was demonstrated by detecting no reaction on the columns of uninfected wells. The reproducibility (with intra- and inter-assay coefficients of variation ranging from 1.9 to 3.8% and from 3.5 to 9.9%, respectively) and quantitation of the assay were demonstrated by close agreement in virus infectivity titers among different assays of the same amount of virus and between assays of different amounts of virus. Furthermore, this assay does not require the use of radiolabeled antibodies. We describe here an efficient EIA that is highly reproducible and that could be used to monitor HAV growth in cell culture and to determine the quantity of HAV antigen needed for diagnostic assays. This is the first report of the infectious titer of the Brazilian cell culture-adapted HAV strain (HAF-203).

A novel cytopathic microtitre plate assay for hepatitis A virus and anti-hepatitis A neutralizing antibodies

The slow growth of hepatitis A virus (HAV) has made tissue culture assay for infectious virus difficult. Strains of the virus of greater cytopathogenicity have been selected for use in plaque cytopathic assays. However, in our hands, this assay has been difficult to reproduce consistently due to problems in maintaining intact cell monolayers over the long incubations involved. This report describes the development of a cytopathic TCID50 assay for HAV. From the results of repeated assay of one preparation of the virus, the coefficient of variation of the assay was calculated to be 4%. This assay has also been adapted to quantitate antibodies to HAV. Initial results of assaying the WHO standard immune serum globulin are comparable with the titre obtained by the radioimmunofocus inhibition test. Antibody titres in human and mouse sera could also be quantitated. The cytopathic TCID50 assay and the adapted inhibition assay described, may prove useful for the development and control of HAV vaccines and the validation of viral inactivation protocols.