Differences in resistance to Trypanosoma musculi infection among strains of inbred mice.

Inbred strains of mice were inoculated with Trypanosoma musculi, and the course of the ensuing parasitemia was followed. The mouse strains fell into three groups: those displaying high and moderate (fivefold less) parasitemia and C57BL/6 (B/6) mice which had exceptionally low infections. To gain insight concerning the mechanisms responsible for interstrain variations in infections, several types of experiments were performed. Comparison of the ability of spleen cells from the various strains to provide the growth-promoting substances required by T. musculi for growth in culture revealed that B/6 cells were deficient; this suggested one mechanism for regulating parasite infections. Exposure of C3H (high parasitemia) and B/6 mice to graded levels of ionizing radiation revealed that B/6 mice have much greater innate resistance to infection than do C3H mice. The effects of treating mice with silica dust or mercaptoethanol indicated that relative resistance to infection is not primarily associated with macrophage activity or limited growth-promoting substances. We conclude that variations in immune responsiveness to parasite antigens (probably not associated with the H-2 complex), possibly in concert with variations in a non-immunological mechanism, account for interstrain variation in resistance to T. musculi infections.