Leishmaniae are obligatory intracellular protozoa in mononuclear phagocytes. They cause a spectrum of diseases, ranging in severity from spontaneously healing skin lesions to fatal visceral disease. Worldwide, there are 2 million new cases each year and 1/10 of the world's population is at risk of infection. To date, there are no vaccines against leishmaniasis and control measures rely on chemotherapy to alleviate disease and on vector control to reduce transmission. However, a major vaccine development program aimed initially at cutaneous leishmaniasis is under way. Studies in animal models and humans are evaluating the potential of genetically modified live attenuated vaccines, as well as a variety of recombinant antigens or the DNA encoding them. The program also focuses on new adjuvants, including cytokines, and delivery systems to target the T helper type 1 immune responses required for the elimination of this intracellular organism. The availability, in the near future, of the DNA sequences of the human and Leishmania genomes will extend the vaccine program. New vaccine candidates such as parasite virulence factors will be identified. Host susceptibility genes will be mapped to allow the vaccine to be targeted to the population most in need of protection.

/pdf/leishmaniasis-current-status-of-vaccine-development-3rp6570msi.pdf

Leishmaniasis: Current Status of Vaccine Development

Infection with Leishmania major induces a protective immune response and long-term resistance to reinfection, which is thought to depend upon persistent parasites. Here we demonstrate that although effector CD4(+) T cells are lost in the absence of parasites, central memory CD4(+) T cells are maintained. Upon secondary infection, these central memory T cells become tissue-homing effector T cells and mediate protection. Thus, immunity to L. major is mediated by at least two distinct populations of CD4(+) T cells: short-lived pathogen-dependent effector cells and long-lived pathogen-independent central memory cells. These data suggest that central memory T cells should be the targets for nonlive vaccines against infectious diseases requiring cell-mediated immunity.

/pdf/central-memory-t-cells-mediate-long-term-immunity-to-12pn0tqmwz.pdf

Central memory T cells mediate long-term immunity to Leishmania major in the absence of persistent parasites

Protozoan parasites of the genus Leishmania undergo a complex life cycle involving transmission by biting sand flies and replication within mammalian macrophage phagolysosomes A major component of the Leishmania surface coat is the glycosylphosphatidylinositol (GPI)-anchored polysaccharide called lipophosphoglycan (LPG) LPG has been proposed to play many roles in the infectious cycle, including protection against complement and oxidants, serving as the major ligand for macrophage adhesion, and as a key factor mitigating host responses by deactivation of macrophage signaling pathways However, all structural domains of LPG are shared by other major surface or secretory products, providing a biochemical redundancy that compromises the ability of in vitro tests to establish whether LPG itself is a virulence factor To study truly lpg− parasites, we generated Leishmania major lacking the gene LPG1 [encoding a putative galactofuranosyl (Galf) transferase] by targeted gene disruption The lpg1− parasites lacked LPG but contained normal levels of related glycoconjugates and GPI-anchored proteins Infections of susceptible mice and macrophages in vitro showed that these lpg− Leishmania were highly attenuated Significantly and in contrast to previous LPG mutants, reintroduction of LPG1 into the lpg− parasites restored virulence Thus, genetic approaches allow dissection of the roles of this complex family of interrelated parasite virulence factors, and definitively establish the role of LPG itself as a parasite virulence factor Because the lpg1− mutant continue to synthesize bulk GPI-anchored Galf-containing glycolipids other than LPG, a second pathway distinct from the Golgi-associated LPG synthetic compartment must exist

https://www.pnas.org/content/pnas/97/16/9258.full.pdf

Lipophosphoglycan is a virulence factor distinct from related glycoconjugates in the protozoan parasite Leishmania major

The immune response to Leishmania: mechanisms of parasite control and evasion

The increasing availability of genomic data for pathogens that cause tropical diseases has created new opportunities for drug discovery and development. However, if the potential of such data is to be fully exploited, the data must be effectively integrated and be easy to interrogate. Here, we discuss the development of the TDR Targets database (http://tdrtargets.org), which encompasses extensive genetic, biochemical and pharmacological data related to tropical disease pathogens, as well as computationally predicted druggability for potential targets and compound desirability information. By allowing the integration and weighting of this information, this database aims to facilitate the identification and prioritization of candidate drug targets for pathogens.

Genomic-scale prioritization of drug targets: the TDR Targets database

Vaccination with live Leishmania major has been shown to yield effective immunization in humans; however, this has been discontinued because of problems associated with virulence of the available vaccine lines. To circumvent this, we tested the ability of a dhfr-ts- null mutant of L. major, obtained by gene targeting, to infect and then to vaccinate mice against challenge with virulent L. major. Survival and replication of dhfr-ts- in macrophages in vitro were dependent upon thymidine, with parasites differentiating into amastigotes prior to destruction. dhfr-ts- parasites persisted in BALB/c mice for up to 2 months, declining with a half-life of 2-3 days. Nonetheless, dhfr-ts- was incapable of causing disease in both susceptible and immunodeficient (nu/nu) BALB/c mice. Animal infectivity could be partially restored by thymidine supplementation. When inoculated by the i.v., s.c., or i.m. routes into mice, dhfr-ts- could elicit substantial resistance to a subsequent challenge with virulent L. major. Thus, Leishmania bearing auxotrophic gene knockouts can be safe and induce protective immunity. Potentially, dhfr-ts- could be used as a platform for delivery of immunogens relevant to other diseases.

/pdf/development-of-a-safe-live-leishmania-vaccine-line-by-gene-33r6653582.pdf

L. A. R. De Freitas

Papers

Development of a safe live Leishmania vaccine line by gene replacement.