Leishmania vaccines: from leishmanization to the era of DNA technology

Leishmania are obligate intracellular vector-borne parasites that cause significant morbidity and mortality in many countries worldwide. There are several species of the parasite which vary according to geographical location and cause a variety of clinical syndromes ranging from self-limiting cutaneous lesions to potentially fatal infection of the viscera. The disease manifested is dependent on both the species of the parasite and the immune response of the host. Depending on the species of the parasite, resistance to infection is generally associated with a T-helper-1 immune response that activates macrophages to kill intracellular Leishmania in a nitric oxide-dependent manner. Conversely, disease progression is generally associated with a T-helper-2 response that activates humoral immunity. Chemotherapeutic treatments for leishmaniasis exist but are expensive, toxic and ineffective against resistant strains. A vaccine against leishmaniasis is feasible since most individuals that were once infected become resistant to clinical infection when later exposed. However, despite the wealth of information regarding the genetics of the parasite and the experimental immunology of the disease, there is currently no vaccine against Leishmania. A multitude of vaccine strategies have been pursued including the use of killed and genetically modified parasites. Immunization with naked plasmid DNA encoding Leishmania antigens represents a new approach to a Leishmania vaccine and confers several advantages over the more traditional vaccination methods. In order to develop an effective vaccine against leishmaniasis, it is important to understand the mechanisms of the immune response to Leishmania infection. This review discusses such immune mechanisms in detail and also explores several of the Leishmania vaccination strategies employed to date, with particular emphasis on DNA vaccines.