The Replication of Type A Influenza Viruses in the Infant Rat: A Marker for Virus Attenuation

Abstract

SUMMARYTwenty recombinant influenza virus strains bearing HSwl N I, H I N I or H3N2 surface antigens, together with their respective wild-type or laboratory-propagated parent viruses, were inoculated into 2 day-old infant rats and their replication in the turbinates and lungs of these animals observed over a period of 5 days. In addition, the ability of each of the recombinant and parent viruses to enhance a subsequent infection of these infant rats by Haemophilus influenzae type b was determined. The results showed that both parent and recombinant viruses replicated less well in the lungs than in the turbinates of infant rats, but the titres in both tissues were generally lower for the recombinant strains. The capacity of the majority of the recombinant influenza viruses to promote bacterial infection of the infant rats, as determined by the incidence of H. influenzae bacteraemia and meningitis, was also markedly less than that of their parent viruses. A correlation between virulence for man and both the replication in infant rat turbinates and the ability to enhance H. influenzae infection, was established for the virus strains studied. The data are discussed in relationship to the value of the infant rat-H, influenzae system as a laboratory marker for the determination of the virulence of influenza virus strains.

Infant rat model of attenuation for recombinant influenza viruses prepared from cold-adapted attenuated A/Ann Arbor/6/60

“…The use of infant rats as a model of virus virulence for humans was investigated for 40 strains of virus: the results indicate that for 38 strains, virulence for humans was related to virus behavior in rats. Thus, virulent strains grew to higher titers in rat turbinates and promoted subsequent systemic infection by H. influenzae type b, whereas attenuated strains grew to lower titers in infant rat turbinates and failed to promote systemic H. influenzae infection (10). The two virus strains which did not behave in this way were AIPR/8/34 (HlNl) and recombinant virus RIT4050 (H3N2), a recombinant of A/ PR/8/34 virus.…”

“…The two virus strains which did not behave in this way were AIPR/8/34 (HlNl) and recombinant virus RIT4050 (H3N2), a recombinant of A/ PR/8/34 virus. Both these virus strains are attenuated for humans, possibly owing to the long passage history of A/PR8 virus in animals, but behave as virulent strains in rats (10). Since designing laboratory tests to measure virus attenuation may be necessary for vaccine development, and since A/PR/8/34 proved an exception in other tests of virus virulence for humans (16), this may indicate that A/PR/8/34 virus recombinants would be poor candidates for vaccine development.…”

“…Thus, a laboratory test which could determine virulence in humans would be a great advantage in vaccine development. Virus infection in infant rats has been extensively investigated in this laboratory as a model for human virulence (10,13,17,28). Some 40 virus strains have been tested in this model, and for 38 strains virulence in rats correlated with the known virulence in humans (10).…”

“…Virus infection in infant rats has been extensively investigated in this laboratory as a model for human virulence (10,13,17,28). Some 40 virus strains have been tested in this model, and for 38 strains virulence in rats correlated with the known virulence in humans (10). Studies with coldadapted recombinant strains (14) showed complete correlation between attenuation in humans and in rats; however, relatively few of these strains have been tested to date (13).…”

Attenuated influenza virus vaccines

“…These reassortants have been shown in laboratory studies to be attenuated for ferret and hamster trachea organ culture^,^^^^^ and attenuated in a variety of animal models, including hamsters," ferrets," and infant rats. 82 In addition, using an intranasal inoculation dose of i06-io7 TCID,,, reassortants have been shown to be attenuated for seronegative and children;85 safe in the elderly,86 and patients with asthma;87 nontransmissible to close contacts;84r85 to induce serum HI and NI antibody, and local IgA and confer protection against challenge and natural virus infection.91892 In addition, comparative studies with inactivated and A/AA/60-ca reassortant vaccines have indicated that the latter induce a broader immunity against virus variant^,'^ and a more sustained serum and local antibody response;89 thus, these vaccines may provide immunity for 2 or more years and in the face of limited antigenic drift. Collectively, the results of these studies have been most encouraging.…”