Protective immunity against the protozoan Leishmania chagasi is induced bysubclinical cutaneous infection with virulent but not avirulent organisms

Protective immunity against Leishmania major is provided by s.c. immunizati on with a low dose of L. major promastigotes or with dihydrofolate-thymidyl ate synthase gene locus (DHFR-TS) gene knockout L. major organisms. Whether these vaccine strategies will protect against infection with other Leishma nia species that elicit distinct immune responses and clinical syndromes is not known. Therefore, we investigated protective immunity to Leishmania ch agasi, a cause of visceral leishmaniasis. In contrast to L. major, a high d ose s.c. inoculum of L. chagasi promastigotes was required to elicit protec tive immunity. Splenocytes from mice immunized with a high dose produced si gnificantly greater amounts of IFN-gamma and lower TGF-beta than mice immun ized with a low dose of promastigotes. The development of protective immuni ty did not require the presence of NK cells. Protection was not afforded by s.c. immunization with either attenuated L. chagasi or with L. major proma stigotes, and s.c. L. chagasi did not protect against infection with L majo r. Subcutaneous immunization with DHFR-TS gene knockouts derived from L. ch agasi, L. donovani, or L. major did not protect against L. chagasi infectio n. We conclude that s.c. inoculation of high doses of live L. chagasi cause s a subclinical infection that elicits protective immune responses in susce ptible mice. However, L. chagasi that have been attenuated either by long-t erm passage or during the raising of recombinant gene knockout organisms do not elicit protective immunity, either because they fail to establish a su bclinical infection or because they no longer express critical antigenic ep itopes.