Inhibitory effects of BCG on adenovirus tumorigenesis: Dependence on administration schedule

The cumulative incidence of tumours was registered in CBA mice inoculated with adenovirus type 12 when newborn and treated with BCG in different ways during the latency period. A single subcutaneous dose of BCG at 3–4 weeks of age had a preventive effect, while no prevention could be recorded after a single BCG dose at 9 weeks of age or eight BCG injections at intervals of 1 week. Sera obtained during the latency period were tested for capacity to block lymph‐node cell‐mediated tumour immunity. Blocking activity was detectable in pooled serum of the mice which had been infected with BCG at 9 weeks of age, already at 10 weeks, i.e. 1 to 2 weeks before the appearance of palpable tumours. Blocking activity could not be demonstrated in a serum pool from control mice until 14 weeks after virus inoculation, at which time sera from mice inoculated with BCG at the age of 3 weeks were still negative. The results indicate that the effect of BCG upon tumour development is dependent upon the point of time of BCG treatment and further suggest that BCG treatment initiated at a stage when serum blocking activity is already present can further stimulate the production of blocking factors and tentatively promote tumour development.