Lymphomas and high-level expression of murine leukemia viruses in CFW mice

Historically, Swiss Webster mice of the CFW subline, both inbred and random -bred stocks, have been considered to have a low spontaneous occurrence of hematopoietic system tumors, and previous reports of infectious expression of murine leukemia viruses (MuLVs) have been rare and unremarkable, In mark ed contrast, in the present study of CFW mice from one source observed by t wo laboratories over a 2-year period, nearly 60% developed tumors, 85% of w hich were lymphomas, the majority of B-cell origin, All tumors tested expre ssed ecotropic MuLVs, and most expressed mink cell focus-inducing (MCF) MuL Vs. Among normal mice of weanling to advanced age, over one-half were posit ive for ecotropic virus in tail or lymphoid tissues, and MCF virus was freq uently present in lymphoid tissue, less often in tail. Patterns of ecotropi c proviral integration indicated that natural infection occurred by both ge netic and exogenous routes. Lymphomas were induced in NIH Swiss mice infect ed as neonates with tissue culture-propagated MuLVs isolated from normal an d tumor tissue of CFW mice.