Photosensitization of Leishmania and its potential applications

Cell biology of photosensitized Leishmania and their potential applications were studied using either an exogenous or endogenous photosensitizer. Exogenous photosensitizer aluminum phthalocyanine chloride (AlPhCl) was effective in causing photolysis of Leishmania at both stages. However both host and parasite were killed by the phototoxicity of this compound. Endogenous photosensitizer uroporphyrin was induced in situ to form in a mutant, which was created by genetic complementation of the defective heme biosynthesis pathway of Leishmania. All species of Leishmania examined were found equally defective in heme biosynthesis and produced porphyrins following induction. Uroporphyric Leishmania was a unique model to study mechanism of phototoxicity in human congenital erythropoietic porphyria. These mutants were also used as live suicidal photo-vaccine for Leishmaniasis. This was possible because pre-porphyric mutants infected host macrophages and dendritic cells (antigen presenting cells) and established parasitism in phagolysosomes. Induction of porphyria in infected cultures resulted in persistent accumulation of uroporphyrin in Leishmania, but only transient porphyria in hosts. Light exposure of these cultures resulted in selective photolysis of Leishmania leaving host cells unscratched. It was hypothesized that such phagolysosomal lysis results in release of natural vaccine molecules of Leishmania for effective antigen processing and presentation by macrophages or dendritic cells. This was expected to induce protective immunity. In-vivo photolysis of porphyric mutants resulted in vaccination of mice, which were partially protected against homologous challenges in the cutaneous model of Leishmaniasis.