New Zealand black mice are immunologically resistant to high-dose, but not low-dose Leishmania mexicana infection.

The course of infection following s.c. inoculation of a wide dose range of L. mexicana stationary-phase promastigotes (SPP) was examined in sexually mature and immature NZB mice of both sexes. Infection with a high dose (greater than 10(7) SPP) was able to induce a protective in vivo response, which could be adoptively transferred with parasite-immune T cells, into naive, syngeneic recipients. In contrast, s.c. infection with a low dose (less than 10(7) SPP) induced non-healing lesions; disease susceptibility could also be transferred into naive animals with T cells from non-immune donors. When the ability to mount a delayed-type hypersensitivity (DTH) reaction was tested in these two groups, the high-parasite dose group gave a significantly higher response. The in vivo protection and high DTH response were reflected in the ability of cells derived from the high-dose resistant (but not low-dose susceptible) mice to mount an antigen-specific T cell response in vitro. The possible immunological effector mechanisms underlying high-dose resistance and low-dose susceptibility are discussed.