184 R. W. SCHLESINGER 



It is too early to define the nature of interferon. It is not a self-perpetuating 

 agent, but arises only in tissue exposed to inactivated virus. In this sense, its 

 demonstration does not negate the basic premise that interference is induced 

 by virus. But it poses a problem in the interpretation of results obtained 

 under conditions where the interfering virus is added to the cell population 

 at a multiplicity of less than 1 per cell. 



B. Quantitative Criteria 



1. Number of Interfering Particles per Cell 



Fazekas de St. Groth and Edney (1952) calculated that a single particle 

 of inactivated influenza virus per cell could induce heterologous interference. 

 The same conclusion was reached by Baluda (1957) for homologous inter- 

 ference by UV-NDV. In this system, the calculated values were put to 

 experimental test by enumerating viral particles before irradiation by 

 plaque assay. On the other hand, for heterologous interference with two 

 distinct strains of vesicular stomatitis virus, "many more than one inactivated 

 particle" were required (Cooper, 1958). 



2. Speed and Duration of Interference 



According to Baluda (1957), the rapidity with which interference by UV- 

 NDV with active NDV is established in tissue culture cells depends on the 

 number of inactive particles adsorbed per cell. At very high multiplicity 

 (140 particles/cell), the time required to induce exclusion may be as low as 

 0.1 minute. A single particle may establish interference after 6 minutes. 

 With inactivated influenza virus, interference in individual cells may not be 

 established for a much longer period of time, probably about 16 to 24 hours 

 (Fazekas de St. Groth et ah, 1952; Henle and Paucker, 1958). Cooper (1958), 

 using monolayers and plaque assays, found exclusion between heterotypic 

 VSV strains operating after 12 minutes. The situation appears to be funda- 

 mentally different in the case of interference by NDV (or PR8) with WEE virus 

 (Levine, 1958). Here, there is a mutual depressing effect of the two viruses 

 involved. That is to say that the ability of NDV-infected cells to produce 

 WEE virus decreases exponentially with time after NDV infection; at the 

 same time, WEE virus, added to cells up to 5 to 6 hours after infection with 

 NDV, can reduce the amount of NDV produced. Cells infected with PR8 

 virus lose their ability to produce WEE virus only after 4 to 5 hours. 



The duration of interference depends, of course, on the experimental 

 system used and, specifically, on the nature of the association between 

 interfering virus and cell. In the most critically studied system, that of 

 Baluda (1957), exclusion of NDV by UV-NDV-infected cells disappeared 

 after 26 to 60 hours. For influenza viruses in eggs, it was found earlier that 



