Reciprocal Interference between Mouse Mammary Tumour Virus and Leukaemia Virus

THE possibility that mammary tumour virus (MTV) and lymphatic leukaemia virus (LLV) may interfere with each other in mice was suggested by the findings that: (1) RILL mice, a strain carrying MTV in which there is a high incidence of mammary tumours, also carry “inhibitors” of MTV in their milk1, show a spontaneous tendency to lose MTV2, and develop a number of lymphoid tumours when free from MTV3. (2) Factors are present in the RIII strain which delay the progression of mammary tumours. Tumours of this strain show long periods of “pregnancy dependence”, that is, a very slow progression towards autonomous growth4–6. In this respect RIII mice differ significantly from MTV-carrying C3H strain, the tumours of which rapidly progress7,8. (3) Factors which delay progression of mammary tumours in the RIII strain are extrachromosomal and are transmitted through the milk together with MTV. When low-tumour BALB/c mice, supposedly free from virus, are foster-nursed by RIII and then maintained as an inbred line for generations, this line (BALB/cf(RIII)) shows in breeding females a high incidence of mammary tumours the behaviour of which closely resembles that of RIII tumours9,10. Conversely, the behaviour of mammary tumours in BALB/c mice foster-nursed by C3H (BALB/cf (C3H)) closely approaches that of C3H tumours11. (4) A high incidence of lymphatic leukaemia has unexpectedly been observed in BALB/cf(RIII) mice, especially in males and virgin females12–14. In contrast, however, the incidence of leukaemia in BALB/c, C3H and BALB/cf(C3H) mice is low14. (5) In BALB/cf(RIII) females there is repulsion between mammary tumours and leukaemia14–15. (6) A cell-free leukaemogenic agent has been demonstrated in BALB/cf(RIII) mice15.