LEUKAEMOGENESIS — VIRAL AND CYTOLOGICAL ASPECTS 



69 



mice was suggested by the co-leukaemogenic effects of filtrates of their 

 tissues administered conjointly with X-rays (Upton, 1959), such effects were 

 not consistently reproducible. Furthermore, electron micrographs disclosed 

 few "virus particles" in such donor mice (Parsons et al., 1962). Hence, it was 

 inferred that donors with primary leukaemia were seldom a satisfactory 

 source of Icukaemogenic virus. 



To develop passage materials with greater viral activity, efforts were made 

 to cultivate the leukaemia cells in vitro to promote growth of virus, as has 

 been noted, for example, with the fowl myeloblastosis agent (Beaudreau et al., 

 1960) and polyoma virus (Stewart et al., 1957). In spleen cell cultures from 

 mice with transplanted myeloid leukaemia, it was observed that the numbers 

 of "virus particles" (Figs. 1 and 2) increased in successive passages (Table II). 



Table II. Evidence of viral activity in radiogenic myeloid leukaemias of mice 



t Electron microscopy: Sections of spleen or cultured cells; figures denote numbers of 

 animals examined (Parsons et al., 1962). 



\ Of cell-free filtrates in non-irradiated isologous recipients: Cell-free filtrates of brain or 

 homogenates were injected intravenously into recipients less than 24-hr old (newborn) or 10 

 weeks old (adult); figures denote proportion of recipients developing leukaemia during the first 

 6 months of life (A. C. Upton and V. K. Jenkins, unpubUshed data). 



Furthermore, suspensions of serially passaged leukaemia cells yielded cell- 

 free filtrates with leukaemogenic potency. The leukaemias induced by these 

 filtrates in adult recipients were myeloid, but smaller numbers of thymic 

 lymphomas also developed in newborn recipients (Table II). Although the 

 number of mice developing leukaemia after injection of filtrates was relatively 

 small, none was observed among controls of comparable age (Table II). The 

 induction of myeloid leukaemia in newborn recipients is particularly note- 

 worthy since the disease has not been induced by irradiation alone in the 

 neonatal period (Fig. 3). 



To explore the possibility that the same agent causing myeloid leukaemia 

 also caused thymic lymphoma, depending on the constitution and physiolo- 

 gical condition of the host, spontaneous and radiogenic leukaemias in 



