DEVELOPMENT OF A SAFE LIVE LEISHMANIA VACCINE LINE BY GENE REPLACEMENT

Vaccination with live Leishmania major has been shown to yield effective immunization in humans; however, this has been discontinued because of problems associated with virulence of the available vaccine lines, To circumvent this, we tested the ability of a dhfr-ts(-) null mutant of L. major, obtained by gene targeting, to infect and then to vaccinate mice against challenge with virulent L. major. Survival and replication of dhfr-ts(-) in macrophages in vitro were dependent upon thymidine, with parasites differentiating into amastigotes prior to destruction, dhfr-ts(-) parasites persisted in BALB/c mice for up to 2 months, declining with a half-life of 2-3 days, Nonetheless, dhfr-ts(-) was incapable of causing disease in both susceptible and immunodeficient (nu/nu) BALB/c mice, Animal infectivity could be partially restored by thymidine supplementation. When inoculated by the i.v., s.c., or i.m. routes into mice, dhfr-ts(-) could elicit substantial resistance to a subsequent challenge with virulent L. major. Thus, Leishmania bearing auxotrophic gene knock-outs can be safe and induce protective immunity, Potentially, dhfr-ts(-) could be used as a platform for delivery of immunogens relevant to other diseases.