We have examined the expression of type C RNA viruses in different Friend erythroleukaemic cell types, distinguished on the basis of increasingly malignant characteristics, which arise during the ageing of mice inoculated with the polycythaemic Friend virus complex. Most early appearing Friend cells (type I) expressed ecotropic virus, but cells of later malignant types showed decreased and variable expression. In general, the more malignant cells released less ecotropic virus. Xenotropic virus was detected in low numbers from type I, II, and IV cells. Two viruses were cloned from type II tumour cells: a xenotropic virus (II clone 1) and an N-tropic ecotropic virus (II clone 2). No pathogenic activity was found when II clone 1 was inoculated into newborn and adult DBA/2J and NIH/Swiss mice observed for up to 20 months, whereas II clone 2 caused a rapid anaemic erythroleukaemia in both N- and B-type newborn mouse strains. It caused a similar form of leukaemia in susceptible N-type adult mice, but at a lower frequency and with a longer latency (usually >5 months). This finding demonstrated a lack of NB restriction in newborn mice. The virus was much less active in DBA/2J mice from which it had been originally cloned; it also appeared to cause lymphoma or to shorten the latency of spontaneous lymphoma in DBA/2J mice.


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