Leishmania mexicana cysteine proteinase-deficient mutants have attenuated virulence for mice and potentiate a Th1 response.

Leishmania mexicana mutants lacking cysteine proteinase genes cpa (delta cpa), cpb (delta cpb), or both cpa and cpb (delta cpa/cpb) have been generated by targeted gene disruption. Delta cpa mutants produce a disease phenotype in BALB/c mice close to that of wild-type L. mexicana, but delta cpb mutants are much less infective, producing very slowly growing small lesions, and delta cpa/cpb double mutants do not induce lesion growth. Immunologic analysis of Ab isotype during infection and splenocyte IFN-gamma, IL-2, and IL-4 production following stimulation with Leishmania Ag or Con A indicates that there was a significant shift from a predominantly Th2-associated immune response in mice infected with wild-type L. mexicana to a Th1-associated response in mice inoculated with delta cpb or delta cpa/cpb. Significantly, delta cpa altered the balance of the immunologic response to a lesser extent than did the other mutants. Similar disease outcomes and switches in the Th1/Th2 balance were also observed when other L. mexicana-susceptible mouse strains were infected with the mutants. BALB/c and C57BL/6 mice vaccinated with delta cpa/cpb and CBA/Ca mice vaccinated with delta cpb or delta cpa/cpb were subsequently more resistant, to varying degrees, than were untreated mice to infection with wild-type parasites, as measured by development of lesions and parasite burden. These data implicate leishmanial cysteine proteinases not only as parasite virulence factors but also in modulation of the immune response and provide strong encouragement that cysteine proteinase-deficient L. mexicana mutants are candidate attenuated live vaccines.