Abstract
Prior to 1967, attenuation of influenza virus was achieved by gradually lowering the incubation temperature until optimal growth at 25°C was obtained. The process of attenuation of a Hong Kong strain was modified and considerably shortened. The temperature of incubation was changed abruptly from 35°C to 25°C and a cold variant was selected using the plaque-assay system.
A set of genetic markers was developed for assessing the potential virulence of cold-passaged variants. The cold variant of the Hong Kong strain was temperature-sensitive, acid-labile and produced a small plaque in primary chick kidney cells incubated at 35°C. Additional differentiating biological properties relating to the adaptation of the virus to growth at 25°C and to loss of virulence in a susceptible host are presented.
The cold-adapted variant was found to be relatively avirulent and highly antigenic for mice and ferrets, and virus was recovered from the nasopharynx of infected ferrets during the first 3 days. The virus recovered was still unable to grow well at 41°C (rct/41—), was sensitive to acid pH and produced small plaques at 35°C and larger ones at 25°C.
After a series of plaque purifications, the cold variant showed further loss of virulence to mice, more vigorous growth at 25°C, complete failure to grow at 41°C and good antigenic potency.
The genetic markers were stable in the plaque-purified cold variant after at least 10 consecutive passages either in tissue culture at 35°C, or in mice.
Cold variants of type B influenza virus have a narrower range of temperature sensitivity compared with type A strains. Reduced plaquing efficiency and reproductive capacity occurred at 35°C (rct/35—) with the attenuated type B strains instead of at 41°C as with the type A strains.
Clinical trials with the attenuated Hong Kong strain of influenza virus (A2/Aichi/2/68) have demonstrated the acceptability and immunogenicity of the strain in man.
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Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Maassab H. F. Adaptation and growth characteristics of influenza virus at 25 degrees c. Nature. 1967 Feb 11;213(5076):612–614. doi: 10.1038/213612a0. [DOI] [PubMed] [Google Scholar]
- Maassab H. F. Biologic and immunologic characteristics of cold-adapted influenza virus. J Immunol. 1969 Mar;102(3):728–732. [PubMed] [Google Scholar]
- Maassab H. F. Plaque formation of influenza virus at 25 degrees C. Nature. 1968 Aug 10;219(5154):645–646. doi: 10.1038/219645a0. [DOI] [PubMed] [Google Scholar]
- Medvedeva T. E., Alexandrova G. I., Smorodintsev A. A. Differentiation of temperature variants of influenza A2 viruses on the basis of plaque formation. J Virol. 1968 May;2(5):456–460. doi: 10.1128/jvi.2.5.456-460.1968. [DOI] [PMC free article] [PubMed] [Google Scholar]