Intracellular Replication-Deficient Leishmania donovani Induces Long Lasting Protective Immunity against Visceral Leishmaniasis

No vaccine is currently available for visceral leishmaniasis (VL) caused by Leishmania donovani. This study addresses whether a live attenuated centrin gene-deleted L. donovani (LdCen1(-/-)) parasite can persist and be both safe and protective in animals. LdCen1(-/-) has a defect in amastigote replication both in vitro and ex vivo in human macrophages. Safety was shown by the lack of parasites in spleen and liver in susceptible BALB/c mice, immune compromised SCID mice, and human VL model hamsters 10 wk after infection. Mice immunized with LdCen1(-/-) showed early clearance of virulent parasite challenge not seen in mice immunized with heat killed parasites. Upon virulent challenge, the immunized mice displayed in the CD4(+) T cell population a significant increase of single and multiple cytokine (IFN-gamma, IL-2, and TNF) producing cells and IFN-gamma/IL10 ratio. Immunized mice also showed increased IgG2a immunoglobulins and NO production in macrophages. These features indicated a protective Th1-type immune response. The Th1 response correlated with a significantly reduced parasite burden in the spleen and no parasites in the liver compared with naive mice 10 wk post challenge. Protection was observed, when challenged even after 16 wk post immunization, signifying a sustained immunity. Protection by immunization with attenuated parasites was also seen in hamsters. Immunization with LdCen1(-/-) also cross-protected mice against infection with L. braziliensis that causes mucocutaneous leishmaniasis. Results indicate that LdCen1(-/-) can be a safe and effective vaccine candidate against VL as well as mucocutaneous leishmaniasis causing parasites.