Macrophages and protective immunity in Mycobacterium lepraemurium infections in a 'resistant' (C57Bl) and a 'susceptible' (BALB/c) mouse strain.

The progressive low resistance form of M. lepraemurium infection in BALB/c mice and the more benign form of infection in C57Bl mice provided appropriate models for analysing the role of macrophages in the spectrum of leprosy in man. Although C57Bl mice were more resistant to both primary and challenge infections than BALB/c mice, peritoneal macrophages from infected mice of both strains were bacteriostatic in vitro. However, a diffusion chamber technique demonstrated that macrophages of BALB/c mice were usually less effective in controlling mycobacterial multiplication in vivo than those of C57Bl mice. This technique also revealed two diffusible factors in infected mice of both strains: one able to activate, the other able to suppress macrophage anti-mycobacterial activity. In C57Bl mice, the macrophage activating factor was apparently dominant; in BALB/c mice, the macrophage suppressor factor seemed to play the major role.