Potentiation of Immunity to Murine Leukemia (LSTRA) by Bacillus Calmette-Guérin¹

LSTRA murine leukemia injected s.c. or intradermally was uniformly lethal for BALB/c × DBA/2 F₁ mice at doses of 10³ cells or greater. Mice that survived after injection of near-threshold tumor cell doses usually were not immune to the tumor. Few mice were cured by excision of the injection site tumor ≥6 days after injection, but those that were cured developed tumor immunity. Irradiation of the tumor cells with 8000 rads reliably prevented growth. Injection of 10⁶ LSTRA cells exposed to 8000 rads (Lx) immunized few animals to LSTRA challenge while 10⁷ Lx protected most of the mice against challenge with 10⁵ or 10⁶ tumor cells. LSTRA-induced immunity was effective against a tumor line induced by the murine sarcoma virus (Moloney) but not against a spontaneous mammary carcinoma or an antigenic, chemically-induced sarcoma. Lymphoid cells from immune mice were effective in the local passive transfer (Winn) test. The low level of immunity induced by 10⁶ Lx was significantly potentiated by admixture of 5 × 10³ or 5 × 10⁵ colony-forming units Bacillus Calmette-Guérin in the tumor cell vaccine. Bacillus Calmette-Guérin given alone did not protect mice against LSTRA, and admixture of a higher dose of Bacillus Calmette-Guérin (5 × 10⁷ colony-forming units) did not alter the weak immunity induced by the 10⁶ Lx. Thus, potentiation of tumor immunity by an immune stimulant may occur with a relatively low optimum dose; higher doses may evoke lesser effects.