174 R. W. SCHLESINGER 



particles (Gotlieb and Hirst, 1954; Granoff and Hirst, 1954), a criterion for 

 designating all A strains (or all B strains) as homologous in relation to one 

 another would be established. Although this assumption is premature, 

 adherence to it facilitates logical sequence of discussion. 



2. Interference and Mixed Infection with Two Active Myxoviruses 



a. Heterologous Systems. The observation that infection with a type A 

 strain (PR8) could lead to suppression of superinfecting type B (LEE) virus, 

 and vice versa, was first reported by Ziegler and Horsfall (1944). They showed 

 that (a) both viruses could multiply side by side if small amounts of each 

 were inoculated; (b) the direction and efficacy of interference depended on the 

 temporal and dosage advantage of one over the other strain. Other conditions 

 being comparable, PR8 was more effective as interfering agent than LEE, 

 and this was in line with its more rapid rate of multiplication. 



Because of the dominant interest then commanded by work on interference 

 by inactivated influenza virus, not much more was done on this problem. 

 It was tacitly assumed that a cell infected by PR8 virus could not support 

 multiplication of LEE and vice versa, and that mixed yields indicated that 

 the initial infecting dose had not sufficed to infect all cells with both viruses. 



This all-or-none concept of mutual exclusion was shattered when Sugg and 

 Magill (1948), Sugg (1951), and Liu and Henle (1951b) found that, under 

 carefully controlled conditions, mixed PR8-LEE infection could be propagated 

 serially through as many as 52 passages. Moreover, Liu and Henle (1953) 

 found that subpassage in limiting dilution (1/2-1/8 ID 50 ) of virus harvested 

 from mixedly infected eggs often yielded virus of both serotypes. Further 

 analysis of the phenomenon by Gotlieb and Hirst (1954) suggested that a 

 single doubly-antigenic particle, obtained from mixedly infected eggs, could 

 give rise to either one or the other serotype, not to both. This led them to 

 postulate that such particles were "phenotypically mixed." The same situ- 

 ation was found for the system MEL-NDV (Granoff and Hirst, 1954). The 

 occurrence of phenotypically mixed particles can be explained only if one 

 assumes that A and B or influenza and NDV particles can infect a single cell 

 and that mutual exclusion therefore is not an all-or-none effect. By denning 

 the relative concentration of each component in mixtures optimal for obtain- 

 ing such doubly antigenic progeny, Hirst and co-workers at the same time 

 crystallized more sharply the conditions for mutual interference. The relative 

 proportions favoring release of doubly antigenic progeny were different for 

 the two systems, but in both small shifts in either direction led to prepon- 

 derance in the yield of either one or the other component virus. It should be 

 noted that the PR8-LEE mixtures at ratios 2 : 1 to 8 : 1 yielded predominantly 

 LEE, in contrast to the earlier findings by Ziegler and Horsfall (1944) with 

 smaller infecting doses. This may be related to the fact that the constant 



