Killed but Metabolically Active Leishmania infantum as a Novel Whole-Cell Vaccine for Visceral Leishmaniasis

Abstract

ABSTRACTThere are currently no effective vaccines for visceral leishmaniasis, the second most deadly parasitic infection in the world. Here, we describe a novel whole-cell vaccine approach usingLeishmania infantum chagasipromastigotes treated with the psoralen compound amotosalen (S-59) and low doses of UV A radiation. This treatment generates permanent, covalent DNA cross-links within parasites and results inLeishmaniaorganisms termed killed but metabolically active (KBMA). In this report, we characterize thein vitrogrowth characteristics of both KBMAL. majorand KBMAL. infantum chagasi. Concentrations of S-59 that generate optimally attenuated parasites were identified. Like liveL. infantum chagasi, KBMAL. infantum chagasiparasites were able to initially enter liver cellsin vivoafter intravenous infection. However, whereas liveL. infantum chagasiinfection leads to hepatosplenomegaly in mice after 6 months, KBMAL. infantum chagasiparasites were undetectable in the organs of mice at this time point.In vitro, KBMAL. infantum chagasiretained the ability to enter macrophages and induce nitric oxide production. These characteristics of KBMAL. infantum chagasicorrelated with the ability to prophylactically protect mice via subcutaneous vaccination at levels similar to vaccination with live, virulent organisms. Splenocytes from mice vaccinated with either liveL. infantum chagasior KBMAL. infantum chagasidisplayed similar cytokine patternsin vitro. These results suggest that KBMA technology is a potentially safe and effective novel vaccine strategy against the intracellular protozoanL. infantum chagasi. This approach may represent a new method for whole-cell vaccination against other complex intracellular pathogens.