Management of visceral leishmaniasis poses a daunting challenge, in part due to the side effects associated with the use of the traditional treatment. However, one vaccine development strategy has recently shown promising results.
Despite their designation as "neglected", several tropical diseases have received a substantial amount of attention from governments, medical research, and mass media entities, and the public. Visceral leishmaniasis (VL) should be no different, as this disease that is transferred from female sand flies to humans is fatal if untreated,1 resulting in to 20,000 to 30,000 deaths each year around the world.2
According to the World Health Organization (WHO), “leishmaniasis is caused by a protozoa parasite from over 20 Leishmania species” and “over 90 sandfly species are known to transmit Leishmania parasites.” The three main forms of the disease are: visceral, cutaneous, and mucocutaneous, with over 90% of the new cases of VL occurring in the poorest regions of six countries: Bangladesh, Brazil, Ethiopia, India, South Sudan, and Sudan. Diagnosis of VL is made through clinical signs and parasitological, or serological tests.2
Management of VL poses a daunting challenge, in part due to the side effects associated with the use of the traditional treatment, parenteral administration of pentavalent antimonial compounds. Therefore, vaccine development is considered to be a high priority. One promising vaccine development strategy includes intravenous immunization with a live attenuated parasite vaccine comprised of Leishmania donovani parasites lacking the centrin gene (LdCen-/-). This vaccine has been used to confer protection in mice, hamsters, and dogs.3-5
Jacqueline Araújo Fiuza, PhD, of the Laboratory of Emerging Pathogens at the Center for Biologics Evaluation and Research in the US Food and Drug Administration, as well as the Laboratório de Imunologia Celular e Molecular at Centro de Pesquisas René Rachou—Fiocruz Minas, Belo Horizonte, in Minas Gerais, Brazil, and her colleagues have pioneered this new approach. The results of their latest experiments toward advancing the science of VL vaccine development were published earlier this year in PLoS Neglected Tropical Diseases.6
Fiuza et al conducted new experiments using their live attenuated LdCen-/- parasites as a vaccine against virulent L. donovani; however, another agent, sand fly salivary protein LJM19 was added as an adjuvant. This particular protein was selected due to its ability to protect hamsters against challenge with other Leishmania species. The vaccine was administered as an intradermal (ID) injection based on the investigators' assertion that this route of immunization had the potential to offer improved protective immunity, as well as simplified delivery logistics. Furthermore, the ear was chosen as the site of ID needle inoculation. The rationale for this selection was that it was already a frequently used route of infection in other studies seeking to most closely replicate the physiological ID and intra-epidermal deposition of parasites by the bite of infected sand flies. Regarding this vaccination strategy, Fiuza and colleagues stated, "In the present work, we examined the value of combining the two immunization strategies for their potential to elicit protective immune responses in a hamster model of Leishmania donovani infection."
After confirming the persistence of LdCen-/- in animals immunized by ID injection, the results of subsequent experiments demonstrated that an ID injection immunization strategy involving priming with LJM19 and boosting with genetically modified live attenuated LdCen-/- in combination with recombinant LJM19 could induce a strong anti-Leishmania antibody response. Additional experiments showed that ID immunization with LdCen-/- parasites resulted in control of the parasite load of virulent L. donovani early in the course of infection. Fiuza and colleagues also found that the prime/boost strategy of ID immunization with LJM19 and LdCen-/- plus LJM19 offered long term protection against a virulent L. donovani challenge in hamsters. When the two components of the vaccine were tested separately, LJM19 immunization alone was able to provide protection against L. donovani parasites, although the protective effect was substantially weaker than that afforded by the prime/boost ID immunization strategy.
In a summation of their results, Fiuza et al stated, "In the present study, we report for the first time the immunogenicity and protection outcome in hamsters intradermally primed with salivary protein LJM19 and boosted with genetically modified live attenuated L. donovani parasites (LdCen-/-) in combination with recombinant LJM19. Immunized hamsters demonstrated a strong immune response comparable to that of intracardial immunization with LdCen-/- and resulted in long term protection against infection with virulent L. donovani parasites." These results are no doubt encouraging for those seeking a way to prevent this potentially fatal, vector-borne disease.
William Perlman, PhD, CMPP is a former research scientist currently working as a medical/scientific content development specialist. He earned his BA in Psychology from Johns Hopkins University, his PhD in Neuroscience at UCLA, and completed three years of postdoctoral fellowship in the Neuropathology Section of the Clinical Brain Disorders Branch of the National Institute of Mental Health.