The effect of inoculum size on the immune response to BCG infection in mice.

In mice infected with BCG the rate of development of resistance to L. monocytogenes was related, and the growth of BCG in the liver and spleen was inversely related, to the size of the infecting inoculum. The rate of elimination of BCG after the onset of immunity was independent of the infecting dose. The effect of dose on the growth of BCG in vivo was abolished by immunosuppressants, and was restored by certain categories of lymphoid cells. Spleen and thymus cells, but not bone marrow cells, were able to confer immunity on X-irradiated recipients. There was no evidence of co-operation between thymus and bone marrow cells. It is concluded that the inoculum effect in BCG infections has an immunological basis. With increasing doses of BCG there is progressive reduction of the latent period prior to the induction of immunity.