Antigenic Differences in Strains of Human Influenza Virus

Abstract

On the basis of earlier studies with mouse and ferret passage virus, 1 it was concluded that the Puerto Rico (PR8) and the Philadelphia (Phila) strains of human influenza virus were immunologically identical, while the swine influenza virus was serologically distinct. The 2 strains of human virus were also indistinguishable from the WS strain of the English workers. 1 2 After repeated inoculation of ferrets with human influenza virus, however, it was noted that the serum of an animal so treated developed the capacity of neutralizing the swine virus as well. 3 Moreover, the serum of rabbits (a non-susceptible animal species) vaccinated with ferret-passage human influenza virus developed antibodies against both the human and swine viruses, whereas rabbits vaccinated with swine influenza virus produced antibodies which neutralized only the swine virus. 3 Identical results were obtained with horse sera prepared by Laidlaw, Smith, Andrewes and Dunkin. 3 4 It was suggested, therefore, that the human and swine viruses, while immunologically distinct contained common antigens and that the swine antigenic components were present in the human virus as secondary antigens, 3 5 .

Since it seemed likely that the antibody response of an insusceptible animal might reflect the secondary antigens of the virus more completely than that of the susceptible animal in which antibodies to the primary antigen rather than to the secondary antigen would probably constitute the initial response, further studies were carried out in rabbits using the tissue culture virus as the source of the various strains. Rabbits were inoculated intraperitoneally with 2 cc. of culture fluid containing the PR8, Phila, and the Swine-2 strains, respectively. Blood was taken from the marginal ear vein 9 to 15 days later.