Studies on the specificity of killing of intracellular pathogens by macrophages.

To determine whether macrophages can discriminate in an immunologically specific manner between the intracellular pathogens which they inhibit or kill, unelicited peritoneal macrophages from mice infected with either of two related but antigenically dissimilar protozoa were challenged with these protozoa in vitro. Experimental conditions were varied in an attempt to establish a state in vivo in which macrophage specificity might be demonstrated. No differences could be discerned between the ability of macrophages from three different strains of mice infected with the protozoa to kill Besnoitia and Toxoplasma. The effect of macrophages on Toxoplasma as compared with Besnoitia did not evolve or vary during development, expression, or decline of an immune response, i.e., with varying times after infection of mice as well as with varying times after treatment of mice with irradiated Toxoplasma. The route of infection could not be shown to confer specificity on macrophages, as subcutaneous and intraperitoneal inoculation of Toxoplasma did not lead to differential ability of macrophages to inhibit or kill the protozoa. The different strains of protozoa used for infection of mice did not affect the ability of peritoneal macrophages from Besnoitia- and Toxoplasma-infected mice to inhibit multiplication of or kill Besnoitia and Toxoplasma comparably in vitro. Peritoneal macrophages of mice treated with Corynebacterium parvum kill both organisms efficiently. These macrophages were employed to determine whether stimulation of macrophages by treatment of mice with a substance unrelated to the protozoa would produce activated macrophages. Uninfected mice and mice infected with either Besnoitia or Toxoplasma were challenged with varying doses of the protozoa in parallel with examination of macrophages from the same groups of mice in vitro to determine whether the presence of stimulated macrophages in the peritoneal cavity was necessary for protection against Toxoplasma and Besnoitia, and if so if their presence was sufficient for protection. Only mice with activated peritoneal macrophages were protected. However, protection was greater when the primary infection was with the same organisms used for challenge at a time when macrophages inhibited or killed both protozoa efficiently in vitro. The possible role of other effector cells, subpopulations of macrophages of different functional abilities in various sites, and antibody or other lymphocyte products acting in concert with macrophages as factors which may explain the differences observed between in vivo protection and in vitro capacity to inhibit or kill the protozoa are discussed.