INTERFERENCE BETWEEN ANIMAL VIRUSES 181 



influenza strains will aid in this. Up to now, most of the pertinent studies have 

 been done with homologous systems, and here interference is so interwoven 

 with the genetic potential of incomplete virus production that conclusions 

 are difficult to draw. 



C. Demonstration of Interference in Tissue Culture 



1. Interference in Surviving Tissue Fragments 



Prior to the recognition of cytopathogenic changes in growing tissue 

 cultures as an index of viral activity, minced tissues in suitable media were 

 employed occasionally for studies on interference (Andrewes, 1942; Huang, 

 1943; Lennette and Koprowski, 1946; Schlesinger, 1951). The viral combina- 

 tions involved in these investigations are included in Table I. A true fore- 

 runner of the more sophisticated work with growing cultures is the work of 

 Huang (1943), who recognized the pH change associated with actively 

 metabolizing tissue fragments and used its inhibition as an indicator of cell 

 destruction by viruses. Utilizing this tool, he observed that nondestructive 

 agents, such as SLE or EMC viruses, effectively blocked metabolic inhibition 

 due to EE virus. 



2. Interference in Growing Cell Cultures 



Actively proliferating cell cultures have provided an ideal tool for the 

 detailed study of heterologous, homologous, or autointerference. At the 

 qualitative level, i.e., in terms of inhibition of cytopathogenic effects, 

 interference has been demonstrated for the following viral combinations: 

 homotypic or heterotypic polioviruses (Le Bouvier, 1954; Sabin, 1954; Sabin 

 et al., 1954; Ledinko and Melnick, 1954; Drake, 1958), NDV and poliovirus 

 (Chanock, 1955), Japanese B encephalitis and poliovirus (Mason and Woodie, 

 1955), influenza and EE (Taylor, 1953), NDV or influenza and WEE (Levine, 

 1958), UV-NDV and active NDV (Baluda, 1957), homologous and heterologous 

 vesicular stomatitis virus (VSV) (Cooper, 1958), and a number of others for 

 which documentation at the time of this writing is inadequate (Okuno et al., 

 1956). Autointerference has found expression either in the form of depressed 

 cytopathogenicity of large as compared with small doses of WEE virus in L 

 cells (Chambers, 1957) or of inhibition of plaque formation by concentrated 

 inocula of influenza (Granoff, 1955a) or VSV (Cooper, 1958). 



Perhaps of the greatest interest and promise are observations on resistance 

 of long-term virus-carrying cells to superinfection with homologous or 

 heterologous viruses. Thus, Cieciura et al. (1957) found that less than 0.1 % 

 of NDV-infected HeLa cells in monolayers survived. Such cells were grown to 

 mass culture from which single cells were isolated. These were then permitted 

 to form pure clones, and the process was repeated for over two years through 



