Phagosome/lysosome fusion: a possible prerequisite for the enhancement of antibody responses in vitro by BCG, Mycobacterium leprae and Corynebacterium parvum.

Primary in vitro antibody responses to SRBC were suppressed in cultures prepared from the spleens of CBA mice injected i.v. 20 days previously with 10(8) liver BCG. In contrast, cultures prepared from mice injected with dead BCG showed enhanced responses. In vitro spleen cell responses of the mice had returned to normal levels 4--6 weeks after their injection, but if dead BCG, M. leprae or C. parvum was added to the cultures, responses were enhanced. The enhancing effect of the added bacteria could be removed by adding also suramin, a drug known to inhibit in vitro fusion of lysosomes with phagosomes. It is suggested that the different in vivo effects of live and dead BCG may relate to differences in their handling by macrophages and more especially that the enhanced antibody forming cell response seen in the restimulated cultures of spleen cells from BCG primed mice, depends upon efficient intracellular fusion of lysosomes with the phagosomes containing the added dead bacteria.