HEMAGGLUTINATION BY ANIMAL VIRUSES 29 



Chapter IV by Gottschalk. For the purposes of this chapter, it will be con- 

 venient to use the term mucoid as a general one for those components of 

 cells or inhibitors which serve as substrates for the influenza virus enzyme, 

 neuraminidase (Gottschalk, 1957), or the receptor-destroying enzyme (RDE) 

 of Vibrio cliolerae. 



After complete elution of the virus, the red cells are no longer agglutinated 

 by the eluting virus and are said to be stable. The eluting virus will still 

 firmly agglutinate fresh red cells and elute from them in turn. The alteration 

 produced in the red cell is termed receptor destruction. 



Elution, or receptor destruction, has a temperature coefficient, being 

 negligible at 0°C. and rapid at 37°C. (Stone, 1949b). Cations are necessary 

 for elution, calcium being more efficient than sodium (Burnet and Edney, 

 1952). The elution of influenza proceeds best in the presence of 0.1 % calcium 

 and at a reduced rate in the presence of sodium hexametaphosphate. 

 Fluoride, phosphate, and certain resins which remove calcium do not prevent 

 elution (Porterfield, 1952). Elution of NDV proceeds best at pH 6.8 to 7.7 

 (Sagik and Levine, 1957). 



The speed and effectiveness of stabilization varies between different 

 viruses; with many influenza strains it is rare to obtain cells stable enough 

 for experimental work by simple treatment with virus. In these cases it is 

 possible to stabilize the cells by the subsequent addition of the minimal 

 amount of specific immune serum (Burnet et al., 1945). 



2. Properties of Stabilized Cells 



a. Receptor Gradient. Cells stabilized by a virus are not agglutinated by 

 fresh supplies of the same strain (Hirst, 1942b; Burnet et al., 1945). They may, 

 however, be agglutinated by some other strains of myxovirus. 



Any series of strains can be arranged in a linear order, called a receptor 

 gradient (Burnet et al., 1946), so that when red cells are stabilized by one 

 strain they are not susceptible to agglutination by any virus earlier in the 

 gradient but are agglutinated by every strain later in the gradient. The order 

 of several strains is mumps, NDV, then the influenza viruses, MEL, WS, LEE, 

 BEL, MIL, B, and swine. The order is the same for human and fowl cells, 

 but chick embryo cells stabilized by NDV are not agglutinated by certain 

 influenza viruses, including MEL and WS (Burnet et al., 1945). 



Burnet and his colleagues originally described this gradient, using in certain 

 cases cells stabilized by the combined action of virus for a standard time, 

 and then immune serum. Hirst (1950) found that certain influenza viruses, if 

 allowed to act to completion on the cells, would produce cells insusceptible 

 to all other viruses of the group. No gradient was then demonstrable. Both 

 authors seem agreed, however, that by partial treatment of red cells with 



